BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers

*Bowen Zhang, Mingjie Ding, Shuen Wei, Binbin Yan, Gang-Ding Peng, Yanhua Luo and Jianxiang Wen*

## **Abstract**

Bismuth-doped optical fiber (BDF) and bismuth/erbium co-doped optical fiber (BEDF) have attracted much attention due to their ultra-broadband luminescence in the near-infrared (NIR) region. The photobleaching effect on bismuth active centers (BACs) related to the NIR luminescence has been systematically investigated and summarized, in terms of irradiation intensity, irradiation wavelength, and temperature. All these findings not only give the deep insights into the fundamental structure of BACs but also provide an effective way to control the BACs. They play an important role for the development of BDF- and BEDF-based devices with high performance and stability under laser exposure in future.

**Keywords:** bismuth-doped optical fiber (BDF), bismuth/erbium co-doped optical fiber (BEDF), bismuth active center (BAC), laser irradiation, photobleaching, irradiation intensity, irradiation wavelength, temperature

## **1. Introduction**

Since Fujimoto et al. in 1999 first demonstrated that bismuth-doped silica glass could generate broadband luminescence covering the near-infrared (NIR) region [1], the bismuth-doped materials have attracted considerable attention due to their ultra-wide luminescence [2–8]. Especially, the bismuth-doped and bismuth/erbium co-doped optical fibers (BDFs and BEDFs) have been developed for tunable fiber laser, amplifier, and ultra-broadband light source operating in the range from 1000 to 1800 nm [9–15].

Although great endeavor has been made to improve the performance of BDFs and BEDFs, challenges still exist, and they have become obstacles to many practical applications. One of the key challenges is that the fundamental structure of bismuth active centers (BACs) and the nature of their NIR luminescence remain unclear. Based on the previous researches, it was generally accepted that the formation of BACs greatly depends on the material compositions [12, 15]. With different doping elements such as aluminum, phosphorus, silicon and germanium in the glass environment, there are four types of BACs in the BDFs, namely BAC associated with aluminum (BAC-Al), BAC associated with phosphorus (BAC-P), BAC associated with silicon (BAC-Si),

and BAC associated with germanium (BAC-Ge), respectively [16–19]. Multiple absorption peaks of these BACs have been observed in BDFs and BEDFs, for example, BAC-Al (510, 700, and 1050 nm), BAC-P (460, 750, and 1300 nm), BAC-Si (420, 830, and 1400 nm), and BAC-Ge (463, 925, and 1600 nm) [17, 18]. The typical NIR luminescence bands of these BACs locate at ~1100 nm (BAC-Al), ~1300 nm (BAC-P), ~1400 nm (BAC-Si), and ~ 1700 nm (BAC-Ge), respectively [15, 17].

To better reveal the nature of the NIR luminescence in BDFs/BEDFs and the configuration of BACs, various post treatments, such as ionizing radiation, laser irradiation, and thermal treatment, have already been applied [20–25]. Thereinto, as a result of laser irradiation, the photobleaching of BDFs/BEDFs leading to the gradual decay of the luminescence of BACs is quite obvious. In this chapter, the photobleaching effect on BACs observed in BDFs and BEDFs has been reviewed. More specially, this effect is demonstrated and analyzed in detail from the angle of BAC type, irradiation intensity, irradiation wavelength, and temperature. In addition, the photobleaching mechanism for each BAC is also discussed. The investigation of this photobleaching process gives not only the deep insights into the structure of BACs but also more information of photostability of BDFs/BEDFs. With further understanding of the BACs, it helps to develop an effective way to control the BACs and obtain better and more stable optical performance of BDF and BEDF for practical applications.

## **2. Phenomenon of photobleaching**

The photobleaching effect observed in some luminescent materials is featured by the gradual decay of luminescence after laser irradiation. This effect can be referred as a process of laser irradiation-induced luminous centers destruction and/or converting into the nonluminous center. This process is called the photobleaching effect [26].

The photobleaching effect has been found in many materials. For example, it was reported that the green fluorescent protein could be bleached under laser irradiation [27]. The Sm2+ emission in epitaxial CaF2 film could be bleached partially under 633-nm irradiation [28]. The similar photoinduced reduction of luminescence has also been observed in Nd3+:LiYF4 and YVO4:Bi3+/Eu3+ nanoparticles [29, 30]. In addition, a large number of dyes present the photobleaching characteristics [31–34]. In general, the photobleaching effect is caused by breaking of covalent bonds or nonspecific reactions between the luminous center and surrounding molecules. Especially, some photobleaching-based techniques such as fluorescence loss in photobleaching and fluorescence recovery after photobleaching have been developed and used for molecular marker, in vivo cell tracking, and investigation of molecule diffusion in biology [27, 35–37].

The photobleaching effect also exists in the glass materials. Exposure to 977 nm light irradiation led to the absorption decrease of Yb3+ in ytterbium-doped silica fiber [38]. In addition, the darkened Yb-doped fiber could be photobleached when irradiated by 355 and 633 nm laser [39, 40]. The similar photobleaching effect has also been observed in the thulium-doped fibers [41].

As for the bismuth-doped materials, the BAC-related NIR luminescence was reduced under laser irradiation in bismuth-doped silica-based glasses, TlCdCl3 crystal and Sr2B5O9Cl:Bi crystal [42–44]. In the case of BDFs/BEDFs, a number of studies on the photobleaching of various BACs have been reported [19, 21, 45–54]. Herein, the photobleaching effect on BAC-Si, BAC-Ge, BAC-Al, and BAC-P in BDFs and BEDFs is demonstrated in relation to irradiation intensity, irradiation wavelength, and temperature, respectively. The underlying mechanisms of these photobleaching effects in BDFs and BEDFs are also discussed below.

**33**

**Figure 1.**

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers*

The photobleaching of BAC-Si in BEDF under 830-nm laser irradiation has been reported in [46]. By the 830 nm laser irradiation with an intensity of 0.12 MW/cm2

To characterize the photobleaching effect on BAC-Si, the stretched exponential

β

*It I I I e* (1)

τ

830-nm laser, which is well fitted by SEF in Eq. (1).

*t*

( ) ( ) − =+ −

1 01

where *I*1 and *I*0 are the luminescence intensities at bleaching saturated time and initial time, *τ* represents the bleaching time and *β* stands for the stretched parameter. In addition, the bleaching ratio *r*B, defined by bleached part of the luminescence (*I*0*−I*1) divided by the initial luminescence before irradiation (*I*0), is used to quantify the photobleaching degree [47]. **Figure 2** shows the typical luminescence variation at 1420 nm

It has been found that the irradiation intensity (power) has a significant effect on the photobleaching of BACs. Higher irradiation intensity provides more photons, which may lead to stronger photobleaching effect. The irradiation intensity dependence of photobleaching of BAC-Si in BEDF was investigated under 830-nm laser irradiation, and the variation of irradiation intensity was achieved by changing the incident power ranged from 0.39 to 35 mW [47]. **Figure 3a** demonstrates the time

*(a) Luminescence spectra of the BEDF under 830-nm irradiation in 60 minutes of exposure time; (b) BEDF* 

*insertion absorption spectra before and after irradiation (20 minutes to 48 hours) [46].*

the luminescence spectra of 50 cm BEDF at different irradiation time were shown in **Figure 1a**. It is evident that the NIR luminescence of BAC-Si peaking at ~1420 nm decreases continuously with the longer exposure time. In addition, the absorption of BAC-Si peaking at ~816 nm demonstrates the similar trend, gradually decaying when exposed to the 830 nm laser as shown in **Figure 1b**. The decrease of both the NIR luminescence and absorption clearly indicates the degradation of BAC-Si. Interestingly, after the irradiation, both the absorption and luminescence of BAC-Si gradually recover to the initial value in 48 hours at room temperature (RT). This recovery behavior implies that the photobleaching of BAC-Si under 830-nm irradia-

,

*DOI: http://dx.doi.org/10.5772/intechopen.93414*

tion in BEDF is reversible in the mild condition.

function (SEF) has been used, which is expressed as [55]:

**3. Photobleaching of BAC-Si**

irradiated by 0.12 MW/cm2

**3.1 Irradiation intensity dependence**

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93414*

## **3. Photobleaching of BAC-Si**

*Bismuth - Fundamentals and Optoelectronic Applications*

and BEDF for practical applications.

**2. Phenomenon of photobleaching**

molecule diffusion in biology [27, 35–37].

also been observed in the thulium-doped fibers [41].

and BAC associated with germanium (BAC-Ge), respectively [16–19]. Multiple absorption peaks of these BACs have been observed in BDFs and BEDFs, for example, BAC-Al (510, 700, and 1050 nm), BAC-P (460, 750, and 1300 nm), BAC-Si (420, 830, and 1400 nm), and BAC-Ge (463, 925, and 1600 nm) [17, 18]. The typical NIR luminescence bands of these BACs locate at ~1100 nm (BAC-Al), ~1300 nm (BAC-P),

To better reveal the nature of the NIR luminescence in BDFs/BEDFs and the configuration of BACs, various post treatments, such as ionizing radiation, laser irradiation, and thermal treatment, have already been applied [20–25]. Thereinto, as a result of laser irradiation, the photobleaching of BDFs/BEDFs leading to the gradual decay of the luminescence of BACs is quite obvious. In this chapter, the photobleaching effect on BACs observed in BDFs and BEDFs has been reviewed. More specially, this effect is demonstrated and analyzed in detail from the angle of BAC type, irradiation intensity, irradiation wavelength, and temperature. In addition, the photobleaching mechanism for each BAC is also discussed. The investigation of this photobleaching process gives not only the deep insights into the structure of BACs but also more information of photostability of BDFs/BEDFs. With further understanding of the BACs, it helps to develop an effective way to control the BACs and obtain better and more stable optical performance of BDF

The photobleaching effect observed in some luminescent materials is featured by the gradual decay of luminescence after laser irradiation. This effect can be referred as a process of laser irradiation-induced luminous centers destruction and/or converting into the nonluminous center. This process is called the photobleaching effect [26]. The photobleaching effect has been found in many materials. For example, it was reported that the green fluorescent protein could be bleached under laser irradiation [27]. The Sm2+ emission in epitaxial CaF2 film could be bleached partially under 633-nm irradiation [28]. The similar photoinduced reduction of luminescence has also been observed in Nd3+:LiYF4 and YVO4:Bi3+/Eu3+ nanoparticles [29, 30]. In addition, a large number of dyes present the photobleaching characteristics [31–34]. In general, the photobleaching effect is caused by breaking of covalent bonds or nonspecific reactions between the luminous center and surrounding molecules. Especially, some photobleaching-based techniques such as fluorescence loss in photobleaching and fluorescence recovery after photobleaching have been developed and used for molecular marker, in vivo cell tracking, and investigation of

The photobleaching effect also exists in the glass materials. Exposure to 977 nm light irradiation led to the absorption decrease of Yb3+ in ytterbium-doped silica fiber [38]. In addition, the darkened Yb-doped fiber could be photobleached when irradiated by 355 and 633 nm laser [39, 40]. The similar photobleaching effect has

As for the bismuth-doped materials, the BAC-related NIR luminescence was reduced under laser irradiation in bismuth-doped silica-based glasses, TlCdCl3 crystal and Sr2B5O9Cl:Bi crystal [42–44]. In the case of BDFs/BEDFs, a number of studies on the photobleaching of various BACs have been reported [19, 21, 45–54]. Herein, the photobleaching effect on BAC-Si, BAC-Ge, BAC-Al, and BAC-P in BDFs and BEDFs is demonstrated in relation to irradiation intensity, irradiation wavelength, and temperature, respectively. The underlying mechanisms of these

photobleaching effects in BDFs and BEDFs are also discussed below.

~1400 nm (BAC-Si), and ~ 1700 nm (BAC-Ge), respectively [15, 17].

**32**

The photobleaching of BAC-Si in BEDF under 830-nm laser irradiation has been reported in [46]. By the 830 nm laser irradiation with an intensity of 0.12 MW/cm2 , the luminescence spectra of 50 cm BEDF at different irradiation time were shown in **Figure 1a**. It is evident that the NIR luminescence of BAC-Si peaking at ~1420 nm decreases continuously with the longer exposure time. In addition, the absorption of BAC-Si peaking at ~816 nm demonstrates the similar trend, gradually decaying when exposed to the 830 nm laser as shown in **Figure 1b**. The decrease of both the NIR luminescence and absorption clearly indicates the degradation of BAC-Si. Interestingly, after the irradiation, both the absorption and luminescence of BAC-Si gradually recover to the initial value in 48 hours at room temperature (RT). This recovery behavior implies that the photobleaching of BAC-Si under 830-nm irradiation in BEDF is reversible in the mild condition.

To characterize the photobleaching effect on BAC-Si, the stretched exponential function (SEF) has been used, which is expressed as [55]:

$$I(t) = I\_\text{\tiny\tiny\} + \left(I\_\text{\tiny\tiny\raisebox{0.2ex}{0.1em}{ $\raisebox{0.2em}{1em}{$ \raisebox{0.2em}{1em}{1em}}}\right)e^{-\left(\frac{t}{t}\right)^\rho} \tag{1}$$

where *I*1 and *I*0 are the luminescence intensities at bleaching saturated time and initial time, *τ* represents the bleaching time and *β* stands for the stretched parameter. In addition, the bleaching ratio *r*B, defined by bleached part of the luminescence (*I*0*−I*1) divided by the initial luminescence before irradiation (*I*0), is used to quantify the photobleaching degree [47]. **Figure 2** shows the typical luminescence variation at 1420 nm irradiated by 0.12 MW/cm2 830-nm laser, which is well fitted by SEF in Eq. (1).

### **3.1 Irradiation intensity dependence**

It has been found that the irradiation intensity (power) has a significant effect on the photobleaching of BACs. Higher irradiation intensity provides more photons, which may lead to stronger photobleaching effect. The irradiation intensity dependence of photobleaching of BAC-Si in BEDF was investigated under 830-nm laser irradiation, and the variation of irradiation intensity was achieved by changing the incident power ranged from 0.39 to 35 mW [47]. **Figure 3a** demonstrates the time

#### **Figure 1.**

*(a) Luminescence spectra of the BEDF under 830-nm irradiation in 60 minutes of exposure time; (b) BEDF insertion absorption spectra before and after irradiation (20 minutes to 48 hours) [46].*

**Figure 2.** *Variation of luminescence intensity at 1420 nm as a function of irradiation time [46].*

evolution of BAC-Si luminescence at 1420 nm under different irradiation power. It is obvious that the luminescence of BAC-Si decays more severely as the irradiation power increases. Furthermore, the luminescence at 1420 nm, time constant *1/τ*, and bleaching ratio *r*B vs. irradiation power is plotted as **Figure 3b**. It is worth noting that both the bleaching ratio and bleaching rate tend to be saturated as the irradiation power increases. Such trend is similar with the variation trend of luminescence at 1420 nm. It hints that the excitation of BAC-Si may participate in the photobleaching process.

## **3.2 Irradiation wavelength dependence**

The photobleaching of BAC-Si was observed in BEDF under 710 and 1380 nm irradiation. As shown in **Figure 4**, the luminescence of BAC-Si is significantly bleached under irradiation of 710 nm, but almost no change under 980-nm irradiation. The inset of **Figure 4** indicates that the BAC-Si luminescence can be slightly bleached under 1380-nm irradiation, which is much weaker than that 710 nm irradiation. In addition, the luminescence of BAC-Si in BDF can be bleached under 532- and 407-nm irradiation [45]. The photobleaching effect on BAC-Si with different irradiation wavelengths is further summarized and listed in **Table 1**. Seen from **Table 1**, except for 980 nm, the photobleaching of BAC-Si can be obtained under all the other applied irradiation wavelengths even the irradiation intensity for some wavelengths is quite small. Generally, it is believed that the shorter wavelength provides larger photon energy. With the increasing photon energy, a greater number of BACs are degraded, resulting in the stronger photobleaching effect. However, even though light at 980 nm has more photon energy than that at 1380 nm, no obvious photobleaching effect is observed under 980-nm irradiation. Considering the fact that 980 nm is unable to excite BAC-Si to the upper energy level, therefore, it is suggested that the photobleaching effect on BAC-Si can only happen when the irradiation wavelength is capable of pumping BAC-Si to excited state. The detailed mechanism will be discussed in Section 3.4.

## **3.3 Temperature dependence**

The temperature dependence of photobleaching effect on BAC-Si in BEDF was studied in the range from 77 to 673 K [47]. The bleaching ratio of BAC-Si under

**35**

**Table 1.**

**Figure 4.**

**Figure 3.**

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers*

830-nm irradiation as a function of temperature is plotted as **Figure 5**.

As the temperature rises from 77 to 673 K, the bleaching ratio of BAC-Si increases from 34 to 66% and then tends to be saturated. This result indicates that the heating of BEDF

*(a) Evolution of BAC-Si luminescence at 1420 nm under different irradiation power; (b) luminescence, time* 

*Luminescence spectra excited by 0.2 mW 830-nm laser before and after 710- and 980-nm irradiation. The inset presents the variation of luminescence at 1420 nm with irradiation wavelengths of 710 and 1380 nm [47].*

BDF 407 1 0.85 [45] BDF 532 1.5 0.8 [45] BEDF 710 0.36 0.55 [47] BEDF 830 0.36 0.57 [47] BEDF 980 0.36 — [47] BEDF 1380 0.005 0.06 [47]

**) rB Ref.**

**Sample Irradiation λ (nm) Intensity (MW/cm2**

*Summary of irradiation wavelength influence upon photobleaching of BAC-Si.*

*DOI: http://dx.doi.org/10.5772/intechopen.93414*

*constant, and bleaching ratio vs. irradiation power [47].*

0.36 MW/cm2

0.36 MW/cm2 830-nm irradiation as a function of temperature is plotted as **Figure 5**. As the temperature rises from 77 to 673 K, the bleaching ratio of BAC-Si increases from 34 to 66% and then tends to be saturated. This result indicates that the heating of BEDF

#### **Figure 3.**

*Bismuth - Fundamentals and Optoelectronic Applications*

**3.2 Irradiation wavelength dependence**

**Figure 2.**

evolution of BAC-Si luminescence at 1420 nm under different irradiation power. It is obvious that the luminescence of BAC-Si decays more severely as the irradiation power increases. Furthermore, the luminescence at 1420 nm, time constant *1/τ*, and bleaching ratio *r*B vs. irradiation power is plotted as **Figure 3b**. It is worth noting that both the bleaching ratio and bleaching rate tend to be saturated as the irradiation power increases. Such trend is similar with the variation trend of luminescence at 1420 nm. It hints that the excitation of BAC-Si may participate in the photobleaching process.

*Variation of luminescence intensity at 1420 nm as a function of irradiation time [46].*

The photobleaching of BAC-Si was observed in BEDF under 710 and 1380 nm irradiation. As shown in **Figure 4**, the luminescence of BAC-Si is significantly bleached under irradiation of 710 nm, but almost no change under 980-nm irradiation. The inset of **Figure 4** indicates that the BAC-Si luminescence can be slightly bleached under 1380-nm irradiation, which is much weaker than that 710 nm irradiation. In addition, the luminescence of BAC-Si in BDF can be bleached under 532- and 407-nm irradiation [45]. The photobleaching effect on BAC-Si with different irradiation wavelengths is further summarized and listed in **Table 1**. Seen from **Table 1**, except for 980 nm, the photobleaching of BAC-Si can be obtained under all the other applied irradiation wavelengths even the irradiation intensity for some wavelengths is quite small. Generally, it is believed that the shorter wavelength provides larger photon energy. With the increasing photon energy, a greater number of BACs are degraded, resulting in the stronger photobleaching effect. However, even though light at 980 nm has more photon energy than that at 1380 nm, no obvious photobleaching effect is observed under 980-nm irradiation. Considering the fact that 980 nm is unable to excite BAC-Si to the upper energy level, therefore, it is suggested that the photobleaching effect on BAC-Si can only happen when the irradiation wavelength is capable of pumping BAC-Si to excited state. The detailed mechanism will be discussed in Section 3.4.

The temperature dependence of photobleaching effect on BAC-Si in BEDF was studied in the range from 77 to 673 K [47]. The bleaching ratio of BAC-Si under

**34**

**3.3 Temperature dependence**

*(a) Evolution of BAC-Si luminescence at 1420 nm under different irradiation power; (b) luminescence, time constant, and bleaching ratio vs. irradiation power [47].*

#### **Figure 4.**

*Luminescence spectra excited by 0.2 mW 830-nm laser before and after 710- and 980-nm irradiation. The inset presents the variation of luminescence at 1420 nm with irradiation wavelengths of 710 and 1380 nm [47].*


#### **Table 1.**

*Summary of irradiation wavelength influence upon photobleaching of BAC-Si.*

**Figure 5.** *Temperature dependence of bleaching ratio of BAC-Si under 0.36 MW/cm2 830-nm irradiation [47].*

to the high temperature can accelerate the electron movement, leading to the stronger photobleaching of BAC-Si.

### **3.4 Photobleaching mechanism of BAC-Si**

Based on the fact that the excitation of BAC-Si is an essential condition for the photobleaching [47], the photobleaching process is deduced to be expressed as:

$$\text{Bi}^{\ast \times \text{x}}\_{\text{BAC}-\text{Si}} \xrightarrow{h\nu} \text{"}\text{Bi}^{\ast \times \text{x}}\_{\text{BAC}-\text{Si}} \xrightarrow{kT} \text{Bi}^{\ast \times (\text{x}\ast \text{4})}\_{\text{BAC}-\text{Si}} + \text{e}^-,\tag{2}$$

where *<sup>x</sup> BiBAC Si* + <sup>−</sup> stands for the Bi ion in a BAC-Si with a valence state of +x, '\*' symbolizes the excited state of the active center, and *hν* and *kT* are the photon energy and thermal energy, respectively. As shown by Eq. (2), the photobleaching mechanism of BAC-Si can be described as: first, the Bi ions absorb the 830 nm photons and are pumped to the upper excited state corresponding to 816 nm; second, with the help of thermal energy, some Bi ions release the electrons which are seized by the surrounding material defects (e.g., self-trapped holes, STH), arousing the decay of luminescence and absorption. As seen from **Table 1**, the BAC-Si could also be bleached under 1380-nm irradiation with a slight bleaching ratio of 0.06. The Bi ions absorb 1380 nm photons and then are excited to the lower excited state, and then the electrons can transfer from a small part of Bi ions at the lower excited state to the nearby defects. Compared with Bi ions at the upper excited state, this electron movement pathway at the lower excited state is more difficult.

## **4. Photobleaching of BAC-Ge**

Photobleaching of BAC-Ge has been found in bismuth-doped optical germanosilicate fibers [45]. After 30 minutes' irradiation of 532-nm laser with an intensity of 1.2 MW/cm2 , both the absorption and the luminescence related to BAC-Ge significantly change as shown by the absorption and luminescence spectra of BDF before and

**37**

**Figure 6.**

*annealed [19, 53].*

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers*

to the recovery of irradiated fibers but also activate the new BACs.

after the irradiation. The dramatical decrease of absorption at 925 and 1650 nm and luminescence at 1700 nm clearly indicate the destruction (photobleaching) of BAC-Ge. The photobleaching of BAC-Ge can also be fitted well by SEF in Eq. (1). The variation of luminescence at 1700 nm excited by 1550 nm under 532-nm irradiation has been demonstrated in [19, 45]. The decay curve of BAC-Ge luminescence at 1700 nm shows the exponential trend. Moreover, there is no recovery behavior after the photobleaching at RT. However, by thermal annealing of the bleached BDF, BAC-Ge could be recovered [19, 45, 53]. After 532-nm laser irradiation, the luminescence of BAC-Ge almost disappears. Annealing the bleached BDF at 600°C, the luminescence of BAC-Ge at ~1700 nm significantly increases, even more than the pristine value, as shown in **Figure 6**. The evident increase of the luminescence of BACs by thermal treatment has been observed in unirradiated BDFs and BEDFs [22, 24, 25, 56–59]. All these results indicate that the thermal treatment can not only lead

The irradiation intensity dependence of photobleaching of BAC-Ge has been studied under 532-nm irradiation with intensity ranged from 0.5 to 2 MW/cm2

The bleaching rate *1/τ* calculated by the SEF as a function of irradiation power under 532-nm irradiation is plotted as **Figure 7**. Seen from **Figure 7**, the relationship between the bleaching rate and irradiation power in log-log scale is almost linear with a slope of ~1.7. The fitting slope is close to 2, which suggests that the photobleaching of BAC-Ge under 532-nm irradiation is likely to be a two-photon process. In addition, the bleaching rate increases with the irradiation intensity, indicating that more photons obtained

The irradiation wavelength dependence of photobleaching of BAC-Ge differs from that of BAC-Si. As mentioned above, the photobleaching of BAC-Si only happens when the irradiation photon is able to excite BAC-Si. The photobleaching

*Evolution of photoluminescence spectra of a BDF at different stages of the experiment: Pristine, irradiated, and* 

by higher irradiation intensity results in faster photobleaching of BAC-Ge.

[48].

*DOI: http://dx.doi.org/10.5772/intechopen.93414*

**4.1 Irradiation intensity dependence**

**4.2 Irradiation wavelength dependence**

### *BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93414*

after the irradiation. The dramatical decrease of absorption at 925 and 1650 nm and luminescence at 1700 nm clearly indicate the destruction (photobleaching) of BAC-Ge.

The photobleaching of BAC-Ge can also be fitted well by SEF in Eq. (1). The variation of luminescence at 1700 nm excited by 1550 nm under 532-nm irradiation has been demonstrated in [19, 45]. The decay curve of BAC-Ge luminescence at 1700 nm shows the exponential trend. Moreover, there is no recovery behavior after the photobleaching at RT. However, by thermal annealing of the bleached BDF, BAC-Ge could be recovered [19, 45, 53]. After 532-nm laser irradiation, the luminescence of BAC-Ge almost disappears. Annealing the bleached BDF at 600°C, the luminescence of BAC-Ge at ~1700 nm significantly increases, even more than the pristine value, as shown in **Figure 6**. The evident increase of the luminescence of BACs by thermal treatment has been observed in unirradiated BDFs and BEDFs [22, 24, 25, 56–59]. All these results indicate that the thermal treatment can not only lead to the recovery of irradiated fibers but also activate the new BACs.

## **4.1 Irradiation intensity dependence**

*Bismuth - Fundamentals and Optoelectronic Applications*

photobleaching of BAC-Si.

**Figure 5.**

where *<sup>x</sup> BiBAC Si* +

**4. Photobleaching of BAC-Ge**

**3.4 Photobleaching mechanism of BAC-Si**

*Temperature dependence of bleaching ratio of BAC-Si under 0.36 MW/cm2*

to the high temperature can accelerate the electron movement, leading to the stronger

Based on the fact that the excitation of BAC-Si is an essential condition for the photobleaching [47], the photobleaching process is deduced to be expressed as:

> + ∗+ + + ( ) − − −− → →+

symbolizes the excited state of the active center, and *hν* and *kT* are the photon energy and thermal energy, respectively. As shown by Eq. (2), the photobleaching mechanism of BAC-Si can be described as: first, the Bi ions absorb the 830 nm photons and are pumped to the upper excited state corresponding to 816 nm; second, with the help of thermal energy, some Bi ions release the electrons which are seized by the surrounding material defects (e.g., self-trapped holes, STH), arousing the decay of luminescence and absorption. As seen from **Table 1**, the BAC-Si could also be bleached under 1380-nm irradiation with a slight bleaching ratio of 0.06. The Bi ions absorb 1380 nm photons and then are excited to the lower excited state, and then the electrons can transfer from a small part of Bi ions at the lower excited state to the nearby defects. Compared with Bi ions at the upper excited state, this electron

Photobleaching of BAC-Ge has been found in bismuth-doped optical germanosilicate fibers [45]. After 30 minutes' irradiation of 532-nm laser with an intensity of

, both the absorption and the luminescence related to BAC-Ge significantly change as shown by the absorption and luminescence spectra of BDF before and

<sup>−</sup> stands for the Bi ion in a BAC-Si with a valence state of +x, '\*'

<sup>1</sup> , *<sup>h</sup> kT x x <sup>x</sup> Bi Bi Bi e BAC Si BAC Si BAC Si* (2)

 *830-nm irradiation [47].*

ν

movement pathway at the lower excited state is more difficult.

**36**

1.2 MW/cm2

The irradiation intensity dependence of photobleaching of BAC-Ge has been studied under 532-nm irradiation with intensity ranged from 0.5 to 2 MW/cm2 [48]. The bleaching rate *1/τ* calculated by the SEF as a function of irradiation power under 532-nm irradiation is plotted as **Figure 7**. Seen from **Figure 7**, the relationship between the bleaching rate and irradiation power in log-log scale is almost linear with a slope of ~1.7. The fitting slope is close to 2, which suggests that the photobleaching of BAC-Ge under 532-nm irradiation is likely to be a two-photon process. In addition, the bleaching rate increases with the irradiation intensity, indicating that more photons obtained by higher irradiation intensity results in faster photobleaching of BAC-Ge.

## **4.2 Irradiation wavelength dependence**

The irradiation wavelength dependence of photobleaching of BAC-Ge differs from that of BAC-Si. As mentioned above, the photobleaching of BAC-Si only happens when the irradiation photon is able to excite BAC-Si. The photobleaching

#### **Figure 6.**

*Evolution of photoluminescence spectra of a BDF at different stages of the experiment: Pristine, irradiated, and annealed [19, 53].*

**Figure 7.** *Bleaching rate as a function of irradiation power under 532-nm irradiation (log-log scale) [48].*

of BAC-Ge in BDFs has been observed when irradiated by 244, 407, 532, 639, 975, and 1460 nm [21, 45, 48, 51, 53]. In [48], the variation of BAC-Ge luminescence at 1700 nm in BDF under irradiation at different wavelengths with the irradiation intensity kept at ~0.5 MW/cm<sup>2</sup> has been demonstrated. Clearly, shorter irradiation wavelength provides higher photon energy, leading to stronger photobleaching of BAC-Ge. From this result, the photobleaching of BAC-Ge is almost unrelated to the resonant radiation wavelength. The mechanism of photobleaching of BAC-Ge will be discussed in detail in Section 4.4, which may be different from that of BAC-Si.

## **4.3 Temperature dependence**

The temperature also has an influence upon the photobleaching of BAC-Ge. The BDF was irradiated under 532 nm with an intensity of 0.5 MW/cm<sup>2</sup> at room temperature (300 K) and liquid nitrogen temperature (77 K), respectively [48]. Seen from **Figure 8**, the photobleaching of BAC-Ge is suppressed significantly when the temperature is cooled down to 77 K. It is noticeable that the photoionization of the germanium-related oxygen deficient center (GeODC) decreases at low temperature [60]. Especially, it has already been confirmed that the BAC in bismuth-doped materials should be some cluster making up of Bi ion with the oxygen deficiency center (ODC) rather than Bi ions themselves [61]. Therefore, it is reasonable to deduce that the photobleaching of BAC-Ge may be related to the photoionization of GeODC. It was also found that the 1550-nm laser had no photobleaching effect upon BAC-Ge under 80°C but was able to bleach BAC-Ge when the temperature of BDF was elevated to hundreds of degrees [54]. Such combined effect of thermal treatment and laser irradiation on BAC-Al has also been observed in BEDF [52].

## **4.4 Photobleaching mechanism of BAC-Ge**

It is known that the most convincing model of the nature of BAC is a Bi ion close to a structural defect, and the defect is most probably to be an ODC [62]. Furthermore, the mechanism of photobleaching of BAC-Ge is assumed to be the photoionization of GeODC. The photobleaching process of BAC-Ge induced by the destruction of GeODC by laser irradiation can be expressed as follows:

**39**

should exist.

**Figure 8.**

**5. Photobleaching of BAC-Al**

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers*

( )

*Variation of BAC-Ge luminescence at 1700 nm in BDF irradiated by 532 nm at 300 and 77 K [48].*

to the photoionization of GeODC at low temperature [48].

apply to the photobleaching of BAC-Si, BAC-Al, and BAC-P.

tures; there remains no observable recovery of BAC-Al.

<sup>2</sup>*<sup>h</sup> n n BAC Bi ODC Bi e E center* ν

The hypothesis is supported by the following facts: (1) the irradiation intensity dependence of bleaching rate demonstrates that photobleaching of BAC-Ge is likely to be a two-photon process [48]; (2) the GeODC can be photoionized under UV light irradiation [63]; and (3) the behavior of photobleaching of BAC-Ge is similar

These results also suggest that the Bi ion adjacent to the ODC is the credible structure of BAC. According to this mechanism, it is believed that the number of the GeODC in the fiber should have an impact on the photobleaching of BAC-Ge. In this case, the doping concentration of Ge may affect the formation of GeODC, resulting in different photobleaching phenomena with various core compositions in [21]. This doping concentration dependence of photobleaching effect may also

In addition, it is worth noting that the photobleaching of BAC-Si under 532-nm irradiation can be explained utilizing this hypothesis [45]. Therefore, more than one mechanism of photobleaching of BACs under different irradiation wavelengths

Compared with BAC-Si and BAC-Ge, the understanding of the fundamental structure of BAC-Al is still limited. A number of studies on photobleaching of BAC-Al have been taken to explore the nature of BAC-Al. It has been reported that the BAC-Al can be bleached with various irradiation conditions in bismuth/erbium co-doped aluminosilicate fibers [49]. **Figure 9** shows the absorption spectra of BEDF in the range from 650 to 750 nm before and after the irradiation. It is clear that the absorption of BAC-Al at ~700 nm decreases after the irradiation of 532, 633, 710, and 830 nm laser (for irradiation at 980 nm, the bleaching effect is not that obvious.). Unlike BAC-Si, after the irradiation, there is no obvious recovery behavior of BAC-Al at RT. Even the bleached BEDF is annealed at high tempera-

<sup>+</sup> + −′ ≡ + → ++ (3)

*DOI: http://dx.doi.org/10.5772/intechopen.93414*

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93414*

**Figure 8.** *Variation of BAC-Ge luminescence at 1700 nm in BDF irradiated by 532 nm at 300 and 77 K [48].*

$$BAC \equiv \left( Bi^{\ast n} + ODC \right) \stackrel{zh\nu}{\to} Bi^{\ast n} + e^- + E^\prime center \tag{3}$$

The hypothesis is supported by the following facts: (1) the irradiation intensity dependence of bleaching rate demonstrates that photobleaching of BAC-Ge is likely to be a two-photon process [48]; (2) the GeODC can be photoionized under UV light irradiation [63]; and (3) the behavior of photobleaching of BAC-Ge is similar to the photoionization of GeODC at low temperature [48].

These results also suggest that the Bi ion adjacent to the ODC is the credible structure of BAC. According to this mechanism, it is believed that the number of the GeODC in the fiber should have an impact on the photobleaching of BAC-Ge. In this case, the doping concentration of Ge may affect the formation of GeODC, resulting in different photobleaching phenomena with various core compositions in [21]. This doping concentration dependence of photobleaching effect may also apply to the photobleaching of BAC-Si, BAC-Al, and BAC-P.

In addition, it is worth noting that the photobleaching of BAC-Si under 532-nm irradiation can be explained utilizing this hypothesis [45]. Therefore, more than one mechanism of photobleaching of BACs under different irradiation wavelengths should exist.

## **5. Photobleaching of BAC-Al**

Compared with BAC-Si and BAC-Ge, the understanding of the fundamental structure of BAC-Al is still limited. A number of studies on photobleaching of BAC-Al have been taken to explore the nature of BAC-Al. It has been reported that the BAC-Al can be bleached with various irradiation conditions in bismuth/erbium co-doped aluminosilicate fibers [49]. **Figure 9** shows the absorption spectra of BEDF in the range from 650 to 750 nm before and after the irradiation. It is clear that the absorption of BAC-Al at ~700 nm decreases after the irradiation of 532, 633, 710, and 830 nm laser (for irradiation at 980 nm, the bleaching effect is not that obvious.). Unlike BAC-Si, after the irradiation, there is no obvious recovery behavior of BAC-Al at RT. Even the bleached BEDF is annealed at high temperatures; there remains no observable recovery of BAC-Al.

*Bismuth - Fundamentals and Optoelectronic Applications*

intensity kept at ~0.5 MW/cm<sup>2</sup>

**Figure 7.**

**4.3 Temperature dependence**

**4.4 Photobleaching mechanism of BAC-Ge**

of BAC-Ge in BDFs has been observed when irradiated by 244, 407, 532, 639, 975, and 1460 nm [21, 45, 48, 51, 53]. In [48], the variation of BAC-Ge luminescence at 1700 nm in BDF under irradiation at different wavelengths with the irradiation

*Bleaching rate as a function of irradiation power under 532-nm irradiation (log-log scale) [48].*

wavelength provides higher photon energy, leading to stronger photobleaching of BAC-Ge. From this result, the photobleaching of BAC-Ge is almost unrelated to the resonant radiation wavelength. The mechanism of photobleaching of BAC-Ge will be discussed in detail in Section 4.4, which may be different from that of BAC-Si.

The temperature also has an influence upon the photobleaching of BAC-Ge. The

perature (300 K) and liquid nitrogen temperature (77 K), respectively [48]. Seen from **Figure 8**, the photobleaching of BAC-Ge is suppressed significantly when the temperature is cooled down to 77 K. It is noticeable that the photoionization of the germanium-related oxygen deficient center (GeODC) decreases at low temperature [60]. Especially, it has already been confirmed that the BAC in bismuth-doped materials should be some cluster making up of Bi ion with the oxygen deficiency center (ODC) rather than Bi ions themselves [61]. Therefore, it is reasonable to deduce that the photobleaching of BAC-Ge may be related to the photoionization of GeODC. It was also found that the 1550-nm laser had no photobleaching effect upon BAC-Ge under 80°C but was able to bleach BAC-Ge when the temperature of BDF was elevated to hundreds of degrees [54]. Such combined effect of thermal treatment and laser irradiation on BAC-Al has also been observed in BEDF [52].

It is known that the most convincing model of the nature of BAC is a Bi ion close to a structural defect, and the defect is most probably to be an ODC [62]. Furthermore, the mechanism of photobleaching of BAC-Ge is assumed to be the photoionization of GeODC. The photobleaching process of BAC-Ge induced by the

destruction of GeODC by laser irradiation can be expressed as follows:

BDF was irradiated under 532 nm with an intensity of 0.5 MW/cm<sup>2</sup>

has been demonstrated. Clearly, shorter irradiation

at room tem-

**38**

**Figure 9.** *Absorption of BEDF before and after irradiation at various wavelengths [49].*

## **5.1 Irradiation intensity dependence**

The photobleaching of BAC-Al also largely depends on the irradiation intensity [49, 52]. The intensity dependence of photobleaching of BAC-Al under 532-nm irradiation has been demonstrated in [49]. As irradiation intensity increases from 0.06 to 0.16 MW/cm2 , both the bleaching rate and bleaching ratio increase, indicating a faster and stronger photobleaching process. Moreover, the irradiation power dependence of bleaching rate in log-log scale shows a linear trend with a slope of ~1.8, as shown in **Figure 10**. The fitting slope close to 2 also demonstrates that the photobleaching of BAC-Al under 532-nm irradiation is likely to be a two-photon process.

## **5.2 Irradiation wavelength dependence**

The BAC-Al can be bleached under irradiation at both resonant and nonresonant wavelengths. The different irradiation wavelengths (532, 633, 710, 830, and 980 nm) have been applied for the investigation of the photobleaching of Al-doped BEDF [49]. At RT, there is no obvious reduction of BAC-Al luminescence under 980-nm irradiation while the photobleaching of BAC-Al takes place under irradiation of all the other wavelengths. Especially, the stronger photobleaching of BAC-Al can be obtained by increasing the photon energy (reducing the irradiation wavelength) with a growth of the bleaching rate and a reduction of the unbleached ratio as shown in **Figure 11**. This irradiation wavelength dependence suggests that the Al-related ODC (AlODC) may take part in the photobleaching process instead of the Bi ion.

## **5.3 Temperature dependence**

Similar to the BAC-Si and BAC-Ge, the photobleaching of BAC-Al is suppressed at low temperature. It has been reported that the bleaching ratio of BAC-Al under 0.12 MW/cm2 532-nm irradiation is reduced from 10 to 5% as the temperature falls down from RT to 77 K [49]. In addition, the temperature aggravated photobleaching of BAC-Al has been observed in BEDF [52]. Under 0.34 MW/cm2 980-nm irradiation, the BAC-Al luminescence at 1191 nm has little change at 293 K but decreases obviously at higher temperatures (423–623 K). The bleaching ratio of BAC-Al dramatically increases with the rising temperature, especially at 623 K, up to 35% of luminescence is bleached, as shown in **Figure 12**. The increasing temperature provides more

**41**

**Figure 12.**

*temperature [52].*

*Bleaching ratio of BAC-Al under 1 hour of 0.3 MW/cm<sup>2</sup>*

 *980-nm irradiation as a function of the* 

**Figure 10.**

**Figure 11.**

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers*

*The bleaching rate 1/τ of BAC-Al in BEDF under 532-nm irradiation versus irradiation power [49].*

*Unbleached ratio and bleaching rate as a function of the photon energy [49].*

*DOI: http://dx.doi.org/10.5772/intechopen.93414*

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93414*

#### **Figure 10.**

*Bismuth - Fundamentals and Optoelectronic Applications*

**5.1 Irradiation intensity dependence**

**5.2 Irradiation wavelength dependence**

**5.3 Temperature dependence**

0.16 MW/cm2

**Figure 9.**

The photobleaching of BAC-Al also largely depends on the irradiation intensity [49, 52]. The intensity dependence of photobleaching of BAC-Al under 532-nm irradiation has been demonstrated in [49]. As irradiation intensity increases from 0.06 to

and stronger photobleaching process. Moreover, the irradiation power dependence of bleaching rate in log-log scale shows a linear trend with a slope of ~1.8, as shown in **Figure 10**. The fitting slope close to 2 also demonstrates that the photobleaching of

The BAC-Al can be bleached under irradiation at both resonant and nonresonant wavelengths. The different irradiation wavelengths (532, 633, 710, 830, and 980 nm) have been applied for the investigation of the photobleaching of Al-doped BEDF [49]. At RT, there is no obvious reduction of BAC-Al luminescence under 980-nm irradiation while the photobleaching of BAC-Al takes place under irradiation of all the other wavelengths. Especially, the stronger photobleaching of BAC-Al can be obtained by increasing the photon energy (reducing the irradiation wavelength) with a growth of the bleaching rate and a reduction of the unbleached ratio as shown in **Figure 11**. This irradiation wavelength dependence suggests that the Al-related ODC (AlODC) may take part in the photobleaching process instead of the Bi ion.

Similar to the BAC-Si and BAC-Ge, the photobleaching of BAC-Al is suppressed at low temperature. It has been reported that the bleaching ratio of BAC-Al under

down from RT to 77 K [49]. In addition, the temperature aggravated photobleaching

the BAC-Al luminescence at 1191 nm has little change at 293 K but decreases obviously at higher temperatures (423–623 K). The bleaching ratio of BAC-Al dramatically increases with the rising temperature, especially at 623 K, up to 35% of luminescence is bleached, as shown in **Figure 12**. The increasing temperature provides more

of BAC-Al has been observed in BEDF [52]. Under 0.34 MW/cm2

532-nm irradiation is reduced from 10 to 5% as the temperature falls

980-nm irradiation,

BAC-Al under 532-nm irradiation is likely to be a two-photon process.

*Absorption of BEDF before and after irradiation at various wavelengths [49].*

, both the bleaching rate and bleaching ratio increase, indicating a faster

**40**

0.12 MW/cm2

*The bleaching rate 1/τ of BAC-Al in BEDF under 532-nm irradiation versus irradiation power [49].*

**Figure 11.** *Unbleached ratio and bleaching rate as a function of the photon energy [49].*

**Figure 12.**

*Bleaching ratio of BAC-Al under 1 hour of 0.3 MW/cm<sup>2</sup> 980-nm irradiation as a function of the temperature [52].*

thermal energy, making it possible for the electron to escape from the BAC-Al. The results imply the strong temperature dependence of photobleaching of BAC-Al, indicating the important role of thermal energy in the photobleaching process.

## **5.4 Photobleaching mechanism of BAC-Al**

Considering that the BAC-Al can be bleached under irradiation of both resonant and nonresonant wavelengths, it is believed that the degradation of BAC-Al under irradiation is due to the photoinduced effects on AlODCs [49], which is much similar with BAC-Ge. Subsequently, the electron released from the AlODC is captured by the nearby defects, arousing the destruction of BAC-Al. In addition, the suppression of photobleaching of BAC-Al at the low temperature arising from the decreasing phononassisted rate for relaxation of AlODC, and the electron movement may support the view of participation of AlODC. However, unlike GeODC, the information of photoionization of AlODC is still limited, and there may exist more than one mechanism of photobleaching of BAC-Al. Therefore, the mechanism of photobleaching of BAC-Al still needs further investigation for deep understanding of the origin of BAC-Al.

## **6. Photobleaching of BAC-P**

Only one photobleaching relevant study on the BAC-P has been reported so far. The study demonstrates that the luminescence of BAC-P at 1300 nm in BDF can be bleached under 1 MW/cm2 407-nm irradiation [45]. The evolution of the luminescence of BAC-P is plotted as **Figure 13**, which is possibly linked with P-related ODC (PODC). Of course, more investigation on photobleaching of BAC-P needs to be taken in terms of irradiation intensity, irradiation wavelength, and temperature dependences to get in-depth knowledge of the photobleaching of BAC-P and its structure.

#### **Figure 13.**

*Evolution of BAC-P luminescence at 1300 nm in BDF under 407-nm irradiation with an intensity of ~1 MW/cm2 at room temperature [45].*

## **7. Inductive analysis**

The photobleaching effect exists in all four types of BACs. Since photobleaching effect varies case by case and is largely dependent on the irradiation conditions,

**43**

**Fiber** BEDF

BDF BEDF BEDF

BDF BDF BDF BDF BDF BEDF BEDF BEDF

BDF **Table 2.**

*Summary of photobleaching of BACs in BDFs and BEDFs.*

SiO2-Al

O2

3-GeO2-P

5P

*Note: T, temperature of the fiber when it is irradiated by the laser.*

O2

5–95SiO2: Bi

O2 5-Er

O2 3-Bi

O2

3 (~0.1 at%)

BAC-Al BAC-P

SiO2-Al

O2 3-Er

O2 3-Bi

O2

3 (~0.1 at%)

SiO2-Al

O2 3-Er

O2 3-Bi

O2

3 (~0.1 at%)

SiO2-Al

O2

3-GeO2-P

O2 5-Er 50GeO2–50SiO2: Bi (<0.1 mol%)

95GeO2–5SiO2: Bi (<0.1 mol%)

50GeO2–50SiO2: Bi (<0.1 mol%)

50GeO2–50SiO2: Bi (~100 ppm)

50GeO2–50SiO2: Bi (~100 ppm)

O2 3-Bi

O2

3 (~0.1 at%)

BAC-Si BAC-Ge BAC-Ge BAC-Ge BAC-Ge BAC-Ge BAC-Al BAC-Al

830 532 532 244

1460, 975, 639, 532, 407

532 532 980, 830, 710, 633, 532

980 407

0.12 0.34

1

300

30

0.53

[45]

293–623

60

0–0.35

[52]

300

55

0–0.1

[49]

0.5 0.5 0.06–0.16

300

55

0.05–0.15

[49]

77, 300

40

0.35, 0.6

[48]

300

60

0.05–0.98

[48]

SiO2-Al

O2

3-GeO2-P

O2 5-Er

O2 3-Bi

O2

3 (~0.1 at%)

BAC-Si

SiO2-Al

O2

3-GeO2-P

O2 5-Er 10GeO2– 90SiO2: Bi

O2 3-Bi

O2

3 (~0.1 at%)

BAC-Si BAC-Si

**Core composition**

**BAC type**

**Irradiation λ**

**nm** 830 407

532

980, 830, 710, 1380

0.36

300

—

0–0.66

[47]

0.06

0.005

0.36 0.6–1.2 0.6–1.2

—

300

—

1

[51]

300

60

—

[21]

300

60

—

[21]

77–673

—

0.34–0.66

[47]

0.004–0.36

1

300

30

0.85

[45]

0.8

1.5

300

360

0.2–0.57

[47]

**Intensity**

**MW/cm2**

**K**

**minutes**

*T*

**Time**

*r***B**

**Ref.**

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers*

*DOI: http://dx.doi.org/10.5772/intechopen.93414*


### *BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93414*

**Table 2.**

*Summary of photobleaching of BACs in BDFs and BEDFs.*

*Bismuth - Fundamentals and Optoelectronic Applications*

**5.4 Photobleaching mechanism of BAC-Al**

**6. Photobleaching of BAC-P**

be bleached under 1 MW/cm2

thermal energy, making it possible for the electron to escape from the BAC-Al. The results imply the strong temperature dependence of photobleaching of BAC-Al, indicating the important role of thermal energy in the photobleaching process.

Considering that the BAC-Al can be bleached under irradiation of both resonant and nonresonant wavelengths, it is believed that the degradation of BAC-Al under irradiation is due to the photoinduced effects on AlODCs [49], which is much similar with BAC-Ge. Subsequently, the electron released from the AlODC is captured by the nearby defects, arousing the destruction of BAC-Al. In addition, the suppression of photobleaching of BAC-Al at the low temperature arising from the decreasing phononassisted rate for relaxation of AlODC, and the electron movement may support the view of participation of AlODC. However, unlike GeODC, the information of photoionization of AlODC is still limited, and there may exist more than one mechanism of photobleaching of BAC-Al. Therefore, the mechanism of photobleaching of BAC-Al still needs further investigation for deep understanding of the origin of BAC-Al.

Only one photobleaching relevant study on the BAC-P has been reported so far. The study demonstrates that the luminescence of BAC-P at 1300 nm in BDF can

cence of BAC-P is plotted as **Figure 13**, which is possibly linked with P-related ODC (PODC). Of course, more investigation on photobleaching of BAC-P needs to be taken in terms of irradiation intensity, irradiation wavelength, and temperature dependences

The photobleaching effect exists in all four types of BACs. Since photobleaching effect varies case by case and is largely dependent on the irradiation conditions,

*Evolution of BAC-P luminescence at 1300 nm in BDF under 407-nm irradiation with an intensity of* 

to get in-depth knowledge of the photobleaching of BAC-P and its structure.

407-nm irradiation [45]. The evolution of the lumines-

**42**

**Figure 13.**

*~1 MW/cm2*

**7. Inductive analysis**

 *at room temperature [45].*

here, **Table 2** summarizes a series of photobleaching of BACs observed in BDFs and BEDFs, along with their fiber compositions, BAC type, and irradiation conditions. The samples are doped with different compositions, such as Si, Ge, Al, P, Bi, and Er. The irradiation wavelengths are from 244 to 1460 nm, and the irradiation power varies from 0.005 to 1.5 MW/cm2 . The exposure temperature of the sample is in the range of 77–673 K.

In general, the bleaching ratio increases with irradiation intensity. Higher irradiation intensity provides more photons, leading to stronger photobleaching effect. It is worth noting that for both BAC-Si and BAC-Al, the bleaching ratio tends to saturate as the irradiation intensity increases [47, 52]. More interestingly, the photobleaching effect could happen under irradiation of some wavelengths even the irradiation intensity is quite small. However, for some irradiation wavelengths, even the irradiation intensity is large, there is still no obvious photobleaching phenomenon. For example, the luminescence of BAC-Si decays 20% after 830-nm irradiation with an intensity of 0.005 MW/cm2 but has no change when exposed to 0.36 MW/ cm2 980-nm irradiation [47]. This indicates that the irradiation wavelength affects more on the photobleaching than the irradiation intensity.

The photobleaching effect significantly depends on the irradiation wavelength. As for BAC-Ge and BAC-Al, the photobleaching effect could happen by the irradiations at both resonant and nonresonant wavelengths, and larger bleaching ratio can be achieved by shorter irradiation wavelength [48, 49]. However, as for BAC-Si, only the wavelengths that are able to excite BAC-Si to the upper level could lead to the photobleaching [47]. According to **Table 2**, it is worth noting that irradiation wavelengths that can cause the photobleaching effect are always shorter than the luminescence peak wavelength of BACs. Therefore, it is supposed that the premise of photobleaching is that the photon energy for the photobleaching is larger than the excitation energy between the ground state and the first excited state of BACs.

Higher temperature evidently accelerates the electron movement rate. It is believed that the photobleaching of BACs is due to the electron escape. Therefore, the photobleaching of BACs becomes stronger at high temperatures, as demonstrated in [47, 48, 52, 54]. It is remarkable that for some irradiation wavelengths, the luminescence has little change at room temperature but is bleached significantly at higher temperatures [52, 54]. The increasing temperature provides enough thermal energy and assists the electron to flee from the BACs; however, the phonon energy is not sufficient for electron escape at room temperature.

## **8. Summary**

Since the first observation of NIR luminescence in bismuth-doped glass, the bismuth-doped materials have attracted great attention due to their ultra-broadband luminescence. Especially, the bismuth-doped and bismuth/erbium co-doped optical fibers have been developed for the potential applications as optical amplifier and fiber laser. A number of researches have been taken focusing on the photostability of bismuth active center (BAC) in these BDFs/BEDFs. The results have demonstrated that the laser radiation can evidently cause the photobleaching of all types of BACs (BAC-Si, BAC-Ge, BAC-Al, and BAC-P), leading to the change of optical characteristics of the fiber. For BAC-Si, BAC-Ge, and BAC-Al, the photobleaching is much dependent upon the irradiation intensity, irradiation wavelength, and temperature. The recovery behavior of bleached BAC-Si and BAC-Ge can be achieved with the aid of phonon-assisted relaxation after the irradiation. It is noted that the BAC-Ge and BAC-Al can be bleached under irradiation at both resonant and nonresonant wavelengths; however, the photobleaching of BAC-Si can only

**45**

**Author details**

**Acknowledgements**

, Mingjie Ding1

\* and Jianxiang Wen3

, Shuen Wei1

Beijing University of Posts and Telecommunications, Beijing, China

Communication, Shanghai University, Shanghai, China

provided the original work is properly cited.

\*Address all correspondence to: yanhua.luo1@unsw.edu.au

1 Photonics and Optical Communications, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW, Australia

2 State Key Laboratory of Information Photonics and Optical Communications,

3 Key laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Binbin Yan<sup>2</sup>

, Gang-Ding Peng1

,

Bowen Zhang1

Yanhua Luo1

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers*

the practical applications as an optical amplifier and fiber laser.

happen when the irradiation photon is able to excite the BAC-Si to the upper energy level. These differences indicate that the photobleaching of BACs is driven by multiple possible mechanisms. In bismuth-doped germanosilicate fiber, the possible mechanism of the photobleaching effect is the photoionization of GeODC, which participate in the formation of BAC-Ge. In addition, the fabrication process, material compositions, treatment conditions, as well as doping concentration of Si/Ge/ Al/P have a great impact on the formation of their related ODCs (GeODC, AlODC, PODC, etc.) and ultimately affect the photobleaching of related BACs. All these investigations on photobleaching of BACs in these BDFs/BEDFs not only provide their photostability information but also give an insight to reveal the fundamental structure of BACs, which can be utilized to control the BACs in BDFs and BEDFs for

The authors are thankful for the support of National Natural Science Foundation

of China (61520106014 and 61675032), Science and Technology Commission of Shanghai Municipality, China (SKLSFO2018-02) and 111 Project (D20031) and wish to express their thanks to other collaborators for their contributions.

*DOI: http://dx.doi.org/10.5772/intechopen.93414*

*BAC Photobleaching in Bismuth-Doped and Bismuth/Erbium Co-Doped Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93414*

happen when the irradiation photon is able to excite the BAC-Si to the upper energy level. These differences indicate that the photobleaching of BACs is driven by multiple possible mechanisms. In bismuth-doped germanosilicate fiber, the possible mechanism of the photobleaching effect is the photoionization of GeODC, which participate in the formation of BAC-Ge. In addition, the fabrication process, material compositions, treatment conditions, as well as doping concentration of Si/Ge/ Al/P have a great impact on the formation of their related ODCs (GeODC, AlODC, PODC, etc.) and ultimately affect the photobleaching of related BACs. All these investigations on photobleaching of BACs in these BDFs/BEDFs not only provide their photostability information but also give an insight to reveal the fundamental structure of BACs, which can be utilized to control the BACs in BDFs and BEDFs for the practical applications as an optical amplifier and fiber laser.

## **Acknowledgements**

*Bismuth - Fundamentals and Optoelectronic Applications*

varies from 0.005 to 1.5 MW/cm2

with an intensity of 0.005 MW/cm2

more on the photobleaching than the irradiation intensity.

is not sufficient for electron escape at room temperature.

range of 77–673 K.

cm2

here, **Table 2** summarizes a series of photobleaching of BACs observed in BDFs and BEDFs, along with their fiber compositions, BAC type, and irradiation conditions. The samples are doped with different compositions, such as Si, Ge, Al, P, Bi, and Er. The irradiation wavelengths are from 244 to 1460 nm, and the irradiation power

In general, the bleaching ratio increases with irradiation intensity. Higher irradiation intensity provides more photons, leading to stronger photobleaching effect. It is worth noting that for both BAC-Si and BAC-Al, the bleaching ratio tends to saturate as the irradiation intensity increases [47, 52]. More interestingly, the photobleaching effect could happen under irradiation of some wavelengths even the irradiation intensity is quite small. However, for some irradiation wavelengths, even the irradiation intensity is large, there is still no obvious photobleaching phenomenon. For example, the luminescence of BAC-Si decays 20% after 830-nm irradiation

980-nm irradiation [47]. This indicates that the irradiation wavelength affects

The photobleaching effect significantly depends on the irradiation wavelength. As for BAC-Ge and BAC-Al, the photobleaching effect could happen by the irradiations at both resonant and nonresonant wavelengths, and larger bleaching ratio can be achieved by shorter irradiation wavelength [48, 49]. However, as for BAC-Si, only the wavelengths that are able to excite BAC-Si to the upper level could lead to the photobleaching [47]. According to **Table 2**, it is worth noting that irradiation wavelengths that can cause the photobleaching effect are always shorter than the luminescence peak wavelength of BACs. Therefore, it is supposed that the premise of photobleaching is that the photon energy for the photobleaching is larger than the excitation energy between the ground state and the first excited state of BACs. Higher temperature evidently accelerates the electron movement rate. It is believed that the photobleaching of BACs is due to the electron escape. Therefore, the photobleaching of BACs becomes stronger at high temperatures, as demonstrated in [47, 48, 52, 54]. It is remarkable that for some irradiation wavelengths, the luminescence has little change at room temperature but is bleached significantly at higher temperatures [52, 54]. The increasing temperature provides enough thermal energy and assists the electron to flee from the BACs; however, the phonon energy

Since the first observation of NIR luminescence in bismuth-doped glass, the bismuth-doped materials have attracted great attention due to their ultra-broadband luminescence. Especially, the bismuth-doped and bismuth/erbium co-doped optical fibers have been developed for the potential applications as optical amplifier and fiber laser. A number of researches have been taken focusing on the photostability of bismuth active center (BAC) in these BDFs/BEDFs. The results have demonstrated that the laser radiation can evidently cause the photobleaching of all types of BACs (BAC-Si, BAC-Ge, BAC-Al, and BAC-P), leading to the change of optical characteristics of the fiber. For BAC-Si, BAC-Ge, and BAC-Al, the photobleaching is much dependent upon the irradiation intensity, irradiation wavelength, and temperature. The recovery behavior of bleached BAC-Si and BAC-Ge can be achieved with the aid of phonon-assisted relaxation after the irradiation. It is noted that the BAC-Ge and BAC-Al can be bleached under irradiation at both resonant and nonresonant wavelengths; however, the photobleaching of BAC-Si can only

. The exposure temperature of the sample is in the

but has no change when exposed to 0.36 MW/

**44**

**8. Summary**

The authors are thankful for the support of National Natural Science Foundation of China (61520106014 and 61675032), Science and Technology Commission of Shanghai Municipality, China (SKLSFO2018-02) and 111 Project (D20031) and wish to express their thanks to other collaborators for their contributions.

## **Author details**

Bowen Zhang1 , Mingjie Ding1 , Shuen Wei1 , Binbin Yan<sup>2</sup> , Gang-Ding Peng1 , Yanhua Luo1 \* and Jianxiang Wen3

1 Photonics and Optical Communications, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW, Australia

2 State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing, China

3 Key laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai University, Shanghai, China

\*Address all correspondence to: yanhua.luo1@unsw.edu.au

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[46] Ding M, Wei S, Luo Y, Peng G-D. Reversible photo-bleaching effect in a bismuth/erbium co-doped optical fiber under 830 nm irradiation. Optics Letters. 2016;**41**(20):4688-4691

[47] Ding M, Fang J, Luo Y, Wang W, Peng G-D. Photo-bleaching mechanism of the BAC-Si in bismuth/erbium co-doped optical fibers. Optics Letters.

[48] Firstov SV, Alyshev SV, Firstova EG, Melkumov MA, Khegay AM, Khopin VF, et al. Dependence of the photobleaching on laser radiation wavelength in bismuthdoped germanosilicate fibers. Journal of

[49] Zhao Q, Luo Y, Tian Y, Peng G-D. Pump wavelength dependence and thermal effect of photobleaching of BAC-Al in bismuth/erbium codoped aluminosilicate fibers. Optics Letters.

[50] Ding M, Luo Y, Wen J, Peng G-D, editors. Dynamic behavior of pump

[44] Wang X, Xu S, Yang Z,

[45] Firstov SV, Alyshev SV, Kharakhordin AV, Riumkin KE, Dianov EM. Laser-induced bleaching and thermo-stimulated recovery of luminescent centers in bismuth-doped optical fibers. Optical Materials Express.

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Campbell RE, Remington SJ. Structural basis for reversible photobleaching of a green fluorescent protein homologue. Proceedings of the National Academy of Sciences. 2007;**104**(16):6672-6677

[36] Sinnecker D, Voigt P, Hellwig N, Schaefer M. Reversible photobleaching of enhanced green fluorescent proteins. Biochemistry. 2005;**44**(18):7085-7094

[37] White J, Stelzer E. Photobleaching GFP reveals protein dynamics inside live cells. Trends in Cell Biology.

Barmenkov YO, Il'Ichev N. Reversible photo-darkening and resonant photobleaching of ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation. Laser Physics Letters.

[39] Manek-Hönninger I, Boullet J, Cardinal T, Guillen F, Ermeneux S, Podgorski M, et al. Photodarkening and photobleaching of an ytterbium-doped silica double-clad LMA fiber. Optics Express. 2007;**15**(4):1606-1611

[40] Gebavi H, Taccheo S, Tregoat D, Monteville A, Robin T. Photobleaching of photodarkening in ytterbium doped aluminosilicate fibers with 633 nm irradiation. Optical Materials Express.

[41] Laperle P, Chandonnet A, Vallée R. Photobleaching of thulium-doped ZBLAN fibers with visible light. Optics

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[53] Firstov S, Firstova E, Alyshev S, Khopin V, Riumkin K, Melkumov M, et al. Recovery of IR luminescence in photobleached bismuth-doped fibers by thermal annealing. Laser Physics. 2016;**26**(8):084007

[54] Alyshev S, Kharakhordin A, Firstova E, Khegai A, Melkumov M, Khopin V, et al. Photostability of laseractive centers in bismuth-doped GeO2–SiO2 glass fibers under pumping at 1550 nm. Optics Express. 2019;**27**(22):31542-31552

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**51**

**Chapter 4**

**Abstract**

the optical properties of BRDFs.

**1. Background and meaning**

materials for silica optical fibers remains unclear.

silica optical fiber

Radiation Effect on Optical

Properties of Bi-Related Materials

*Yanhua Luo, Gang-Ding Peng, Fufei Pang and Tingyun Wang*

Three kinds of Bi-related materials co-doped silica optical fibers (BRDFs), including Bi/Al, Bi/Pb, and Bi/Er co-doped fibers, were fabricated using atomic layer deposition (ALD) and modified chemical vapor deposition (MCVD). Then, the effect of irradiation on the optical properties of BRDFs was investigated. The experimental results showed that the fluorescence intensity, the fluorescence lifetime of BRDFs at the 1150 nm band, increased significantly with low-dose treatment, whereas it decreased with a further increase in the radiation dose. In addition, the merit Mα values of the BRDFs, a ratio of useful pump absorption to total pump absorption, decreased with an increase of the radiation doses. The Verdet constants of different doped fibers increased up to saturation level with increases in the radiation dose. However, for a Bi-doped fiber, its Verdet constant decreased and the direction of Faraday's rotation changed under low-dose radiation treatment. In addition, the Verdet constant increase of the Bi-doped silica fiber was much faster than that of other single mode fiber (SMF) and Pb-doped silica fibers treated with high-dose radiation. All of these findings are of great significance for the study of

**Keywords:** Bi-related materials, optical properties, gamma-ray radiation,

Bismuth oxide is an important material with many promising applications [1, 2]. In particular, bismuth-doped optical fibers, as a promising active medium for amplifying and lasing in the 1.1–1.8 μm range [3, 4], have been extensively studied, ever since their broadband near-infrared (NIR) fluorescence properties were first reported by Fujimoto et al. [1]. Thereafter, an amplification at the 1300 nm band in Bi-doped silica glass was realized [2] and an optical amplifier and fiber laser were achieved [4–6]. Previous investigations have demonstrated that the valence state of Bi ions varies in glass materials [7–11]. However, the valence conversion mechanism of Bi-related

In addition, the effects of radiation on the fluorescence properties of Bi-doped glass or optical fibers have been previously studied [12–18]. In Refs. [16–18], the

Co-Doped Silica Optical Fibers

*Jianxiang Wen, Ying Wan, Yanhua Dong, Yi Huang,* 

## **Chapter 4**

## Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers

*Jianxiang Wen, Ying Wan, Yanhua Dong, Yi Huang, Yanhua Luo, Gang-Ding Peng, Fufei Pang and Tingyun Wang*

## **Abstract**

Three kinds of Bi-related materials co-doped silica optical fibers (BRDFs), including Bi/Al, Bi/Pb, and Bi/Er co-doped fibers, were fabricated using atomic layer deposition (ALD) and modified chemical vapor deposition (MCVD). Then, the effect of irradiation on the optical properties of BRDFs was investigated. The experimental results showed that the fluorescence intensity, the fluorescence lifetime of BRDFs at the 1150 nm band, increased significantly with low-dose treatment, whereas it decreased with a further increase in the radiation dose. In addition, the merit Mα values of the BRDFs, a ratio of useful pump absorption to total pump absorption, decreased with an increase of the radiation doses. The Verdet constants of different doped fibers increased up to saturation level with increases in the radiation dose. However, for a Bi-doped fiber, its Verdet constant decreased and the direction of Faraday's rotation changed under low-dose radiation treatment. In addition, the Verdet constant increase of the Bi-doped silica fiber was much faster than that of other single mode fiber (SMF) and Pb-doped silica fibers treated with high-dose radiation. All of these findings are of great significance for the study of the optical properties of BRDFs.

**Keywords:** Bi-related materials, optical properties, gamma-ray radiation, silica optical fiber

## **1. Background and meaning**

Bismuth oxide is an important material with many promising applications [1, 2]. In particular, bismuth-doped optical fibers, as a promising active medium for amplifying and lasing in the 1.1–1.8 μm range [3, 4], have been extensively studied, ever since their broadband near-infrared (NIR) fluorescence properties were first reported by Fujimoto et al. [1]. Thereafter, an amplification at the 1300 nm band in Bi-doped silica glass was realized [2] and an optical amplifier and fiber laser were achieved [4–6]. Previous investigations have demonstrated that the valence state of Bi ions varies in glass materials [7–11]. However, the valence conversion mechanism of Bi-related materials for silica optical fibers remains unclear.

In addition, the effects of radiation on the fluorescence properties of Bi-doped glass or optical fibers have been previously studied [12–18]. In Refs. [16–18], the

radiation-induced photoluminescence (PL) effect of Bi-doped silica optical fibers was investigated and the relationships between the radiation-induced optical properties and defect centers in Bi-doped silica fibers (BDFs) were reported. Moreover, the fluorescence intensity was enhanced by UV irradiation [11, 12]. Shen et al. [14] also reported fluorescence enhancement from exposing Bi-doped borosilicate glass to a radiation environment. The photo-bleaching effect on Bi-doped glass fiber with a 532-nm laser treatment was studied [15]. These results provide deeper insight into the nature and formation mechanism of PL [19]. Furthermore, the magneto-optical properties of the Bi-doped silica fibers were studied before and after irradiation, and radiation-induced magneto-optical phenomena were found [20–22]. Finally, thermal effects on the luminescence properties of Bi co-doped silica fibers were studied [23–25].

However, the nature of the NIR fluorescence properties in Bi-doped glass or silica optical fibers is still unclear. Although many studies have reported the luminescent properties of Bi co-doped fibers, there are few reports on the effect of irradiation on the optical properties of Bi-related co-doped silica optical fibers.

In this chapter, three kinds of Bi-related co-doped silica optical fibers, including Bi/Al, Bi/Pb, and Bi/Er co-doped fibers, are fabricated using atomic layer deposition (ALD) combined with a modified chemical vapor deposition (MCVD) process. The optical properties of bi-related materials co-doped silica optical fibers (BRDFs) that are influenced by irradiation are investigated, including luminescence, lifetime decay, magnetic-optical, and unsaturable absorption, and the changes in these optical properties are compared.

## **2. Fabrication of Bi-related co-doped silica optical fibers**

Currently, the fabrication technologies of different doped fibers such as rare earth-doped fibers mainly use a solution-doping chemical vapor deposition technique. However, the technology lacks uniformity and consistency, and doping materials are easily volatilized and form clusters in a high-temperature environment, which limits the excellent performance of the fabricated doped fibers. Recently, a novel doping method, ALD technology, has been developed. It is not only an advanced deposition technique [5, 26–30] that allows for ultrasmall dopants of a few nanometers to be deposited in a precisely controlled way but also a chemical vapor deposition technique based on the sequential use of self-terminating gas– solid reactions. In particular, the novel technology involves a self-limiting surface reaction, whose advantages include a low-temperature process, good uniformity, favorable dispersibility, high doping concentration, and wide range of materials used. To date, there have been only a few reports [30–34] regarding the preparation of rare earth optical fibers by ALD.

ALD technology typically involves a cycle of four steps that is repeated as many times as necessary to achieve the required doping concentrations. As an example, we perform ALD on Al2O3, using Al2(CH3)3 (Trimethylaluminum, TMA) and H2O as the reactants. The detailed deposition process is shown in **Figure 1**. **Step 1:** A pulse precursor vapor of TMA reacts with the inner surface of the substrate tube. With the optimized choice of precursors and reaction conditions, the reaction of this step is self-limiting. **Step 2:** Purging all residual precursors and reaction products. **Step 3:** Low damage by remote exposure of the surface to reactive oxygen radicals, where these radicals oxidize the inner surface and remove surface ligands. This reaction is also self-limiting because of the limited number of surface ligands. **Step 4:** Reaction products are purged from the chamber. Only Step 3 varies, between H2O and O2 plasma by thermal processing. As each cycle of the ALD process deposits

**53**

**Figure 2.**

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers*

a layer of sub-angstrom thickness, the atomic scale ranges of deposition can be

The reaction of X(thd)3 (X: metal ions, such as Bi, Pb, and Er; thd: 2,2,6,6-tetramethyl-3,5-heptanedionato) and H2O can be described by Eqs. (1)–(3) [35]. The

which involves two processes: process A in Eq. (2) is the hydroxyl on silicon reacting with the X source to obtain Si-O-X(thd)2; process B is obtaining Si-O-X(OH)2 by the reaction in Eq. (3) of H2O and Si-O-X(thd)2 with the termination of ▬OH groups. On repeating the ABAB (A and B represent different reaction processes, respectively) operations, an X-doped layer with the desired thickness is obtained. Similarly, Al2O3 can be deposited using these following analogous reactions.

( )<sup>3</sup> 2 23 2X thd 3H O X O 6H thd + → +− (1)

( ) ( ) 3 2 A : Si OH X thd Si O X thd H thd − + → −− +− (2)

( ) <sup>2</sup> ( ) 2 2 B : Si O X thd 2H O Si O X OH 2H thd −− + → −− + − (3)

The fabrication process of the BRDFs can be divided into four steps, as shown in **Figure 2**. First, a porous soot layer is deposited inside the silica substrate tube using

*DOI: http://dx.doi.org/10.5772/intechopen.93495*

controlled through the process.

*Specific process of ALD technology deposition.*

**Figure 1.**

whole reaction can be written as follows:

*Fabrication process of the BRDFs based on ALD + MCVD technology.*

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93495*

**Figure 1.** *Specific process of ALD technology deposition.*

*Bismuth - Fundamentals and Optoelectronic Applications*

studied [23–25].

cal properties are compared.

of rare earth optical fibers by ALD.

radiation-induced photoluminescence (PL) effect of Bi-doped silica optical fibers was investigated and the relationships between the radiation-induced optical properties and defect centers in Bi-doped silica fibers (BDFs) were reported. Moreover, the fluorescence intensity was enhanced by UV irradiation [11, 12]. Shen et al. [14] also reported fluorescence enhancement from exposing Bi-doped borosilicate glass to a radiation environment. The photo-bleaching effect on Bi-doped glass fiber with a 532-nm laser treatment was studied [15]. These results provide deeper insight into the nature and formation mechanism of PL [19]. Furthermore, the magneto-optical properties of the Bi-doped silica fibers were studied before and after irradiation, and radiation-induced magneto-optical phenomena were found [20–22]. Finally, thermal effects on the luminescence properties of Bi co-doped silica fibers were

However, the nature of the NIR fluorescence properties in Bi-doped glass or silica optical fibers is still unclear. Although many studies have reported the luminescent properties of Bi co-doped fibers, there are few reports on the effect of irradiation on the optical properties of Bi-related co-doped silica optical fibers.

**2. Fabrication of Bi-related co-doped silica optical fibers**

In this chapter, three kinds of Bi-related co-doped silica optical fibers, including Bi/Al, Bi/Pb, and Bi/Er co-doped fibers, are fabricated using atomic layer deposition (ALD) combined with a modified chemical vapor deposition (MCVD) process. The optical properties of bi-related materials co-doped silica optical fibers (BRDFs) that are influenced by irradiation are investigated, including luminescence, lifetime decay, magnetic-optical, and unsaturable absorption, and the changes in these opti-

Currently, the fabrication technologies of different doped fibers such as rare earth-doped fibers mainly use a solution-doping chemical vapor deposition technique. However, the technology lacks uniformity and consistency, and doping materials are easily volatilized and form clusters in a high-temperature environment, which limits the excellent performance of the fabricated doped fibers. Recently, a novel doping method, ALD technology, has been developed. It is not only an advanced deposition technique [5, 26–30] that allows for ultrasmall dopants of a few nanometers to be deposited in a precisely controlled way but also a chemical vapor deposition technique based on the sequential use of self-terminating gas– solid reactions. In particular, the novel technology involves a self-limiting surface reaction, whose advantages include a low-temperature process, good uniformity, favorable dispersibility, high doping concentration, and wide range of materials used. To date, there have been only a few reports [30–34] regarding the preparation

ALD technology typically involves a cycle of four steps that is repeated as many times as necessary to achieve the required doping concentrations. As an example, we perform ALD on Al2O3, using Al2(CH3)3 (Trimethylaluminum, TMA) and H2O as the reactants. The detailed deposition process is shown in **Figure 1**. **Step 1:** A pulse precursor vapor of TMA reacts with the inner surface of the substrate tube. With the optimized choice of precursors and reaction conditions, the reaction of this step is self-limiting. **Step 2:** Purging all residual precursors and reaction products. **Step 3:** Low damage by remote exposure of the surface to reactive oxygen radicals, where these radicals oxidize the inner surface and remove surface ligands. This reaction is also self-limiting because of the limited number of surface ligands. **Step 4:** Reaction products are purged from the chamber. Only Step 3 varies, between H2O and O2 plasma by thermal processing. As each cycle of the ALD process deposits

**52**

a layer of sub-angstrom thickness, the atomic scale ranges of deposition can be controlled through the process.

The reaction of X(thd)3 (X: metal ions, such as Bi, Pb, and Er; thd: 2,2,6,6-tetramethyl-3,5-heptanedionato) and H2O can be described by Eqs. (1)–(3) [35]. The whole reaction can be written as follows:

$$\text{2X(thd)}\_{\text{\textsuperscript{\text{\tiny}}}} + \text{3H}\_{\text{z}}\text{O} \rightarrow \text{X}\_{\text{z}}\text{O}\_{\text{\tiny}} + \text{6H} - \text{thd} \tag{1}$$

which involves two processes: process A in Eq. (2) is the hydroxyl on silicon reacting with the X source to obtain Si-O-X(thd)2; process B is obtaining Si-O-X(OH)2 by the reaction in Eq. (3) of H2O and Si-O-X(thd)2 with the termination of ▬OH groups. On repeating the ABAB (A and B represent different reaction processes, respectively) operations, an X-doped layer with the desired thickness is obtained. Similarly, Al2O3 can be deposited using these following analogous reactions.

$$\text{A} \colon \text{Si}-\text{OH} + \text{X} \text{(thd)}\_{\text{\textquotedblleft}} \rightarrow \text{Si}-\text{O}-\text{X} \text{(thd)}\_{\text{\textquotedblright}} + \text{H}-\text{thd} \tag{2}$$

$$\text{B: Si}-\text{O}-\text{X(thd)}\_{\text{z}} + 2\text{H}\_{\text{z}}\text{O} \rightarrow \text{Si}-\text{O}-\text{X(OH)}\_{\text{z}} + 2\text{H}-\text{thd} \tag{3}$$

The fabrication process of the BRDFs can be divided into four steps, as shown in **Figure 2**. First, a porous soot layer is deposited inside the silica substrate tube using

**Figure 2.** *Fabrication process of the BRDFs based on ALD + MCVD technology.*

the MCVD method. In this process, chemical reactions in the gas phase generate a fine soot of silica that coats the inner surface of the substrate tube, which is then sintered into a semi-clear soot layer. Second, Bi, Pb, or Er ions are introduced on the surface of the porous soot layer using the ALD technique (TFS-200, Beneq, Finland). This results in the formation of bismuth oxide, lead oxide, and erbium oxide with the precursors of bis (2, 2, 6, 6-tetra-methyl-3, 5-heptanedionato) bismuth (III) (Bi(thd)3), bis (2, 2, 6, 6-tetra-methyl-3, 5-heptanedionato) lead (III) (Pb(thd))3, and bis (2, 2, 6, 6-tetra-methyl-3, 5-heptanedionato) erbium (III) (Er(thd))3 (supplied by Shanghai J&K Scientific Ltd), respectively. They mainly react with water or ozone to form the metal oxidation layer, the O3 that originated from the O2. Third, germanium oxide is doped into the fiber preform core by the MCVD process, and then a Bi-related co-doped optical fiber preforms with a Ge-doped higher index core that is formed by collapsing on an MCVD lathe heated by a high-temperature oxyhydrogen flame. Finally, the preform is drawn into a doped optical fiber with a Bi-related material.

## **3. Effect of radiation on optical properties**

For optical fiber material, a perfect structure is visualized as a co-doped ion random network of SiO4 tetrahedrons joined at the corner, and different ions are doped into irregular vitreous silica, forming a stable network structure [36]. It is important to accumulate further knowledge regarding the influence of radiation on optical fiber materials, including material network structures, defect centers, and optical properties. Radiation as an effective method can induce changes in the optical properties of materials. It mainly involves the process of high-energy particles interacting with fiber materials, including the photoelectric effect, the Compton effect, the electron pair effect, and more. For BRDFs, irradiation significantly improves their optical properties, which mainly accounts for the variation in the valence states of Bi (Bi5+, Bi2+, Bi+ , Bi0 , defect centers, Bi clusters, Bi2− 2 dimers, or Bi atoms). Here, gamma rays are selected as the irradiation source, mainly due to their short wavelength and strong penetrating ability. The effects of gamma ray irradiation on the optical properties of BRDFs, including Bi/Al co-doped silica fibers (BADFs), Bi/Er co-doped silica fibers (BEDFs), and Bi/Pb co-doped silica fibers (BPDFs), are investigated.

## **3.1 Radiation effect on luminescence characteristics**

The radiation-induced PL properties of BADFs were investigated in [19]. The PL spectra in the inset of **Figure 3** reveal two emission bands at approximately ~1150 and ~ 1410 nm, corresponding to the aluminum-related Bi active center (BAC-Al) and the silicon-related Bi active center (BAC-Si), respectively. **Figure 3** illustrates that the fluorescence intensities of BAC-Al increased by 0.73, 2.25, and 1.35 dB at 1150 nm with 1.0, 2.0, and 3.0 kGy of irradiation, respectively. The fluorescence intensities of BAC-Al in the BADF samples increased with the increase in radiation dose (0–2.0 kGy) and then decreased when the radiation dose exceeded 2 kGy. Moreover, the change in the fluorescence intensity of BAC-Si trended similar to that of BAC-Al; however, the fluorescence intensity of BAC-Si increased considerably more. Furthermore, the fluorescence intensity of BAC-Si was approximately four times stronger than that of the unirradiated fiber sample.

For BEDF, five BEDF samples were irradiated with cumulative doses of approximately 0.3, 0.5, 0.8, 1.5, and 3.0 kGy at room temperature. The radiation dose rate was 800 Gy/h. Under excitation at 980 nm (pump power is 1.8 mW),

**55**

**Figure 4.**

**Figure 3.**

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers*

the fluorescence spectra of BEDF samples were measured, as shown in **Figure 4**. For BAC-Al, as the radiation dose was increased, the fluorescence intensity first increased and then decreased. With a 0.3 kGy dose of irradiation, the fluorescence intensity of BAC-Al in the BEDF sample is slightly higher than that of the pristine fiber, as shown in **Figure 4(a)**. However, when the radiation dose was less than

*Fluorescence spectra of BEDF samples at different bands with different radiation doses. (a) 1100, (b) 1550 nm,* 

*and (c) the variations of the fluorescence intensity at 1100, 1450, and 1550 nm.*

*Fluorescence intensity of BAC-Al and BAC-Si as a function of radiation dose; inset are the PL spectra of the* 

*DOI: http://dx.doi.org/10.5772/intechopen.93495*

*BADF samples before and after* γ*-ray irradiation.*

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93495*

#### **Figure 3.**

*Bismuth - Fundamentals and Optoelectronic Applications*

doped optical fiber with a Bi-related material.

**3. Effect of radiation on optical properties**

valence states of Bi (Bi5+, Bi2+, Bi+

(BPDFs), are investigated.

the MCVD method. In this process, chemical reactions in the gas phase generate a fine soot of silica that coats the inner surface of the substrate tube, which is then sintered into a semi-clear soot layer. Second, Bi, Pb, or Er ions are introduced on the surface of the porous soot layer using the ALD technique (TFS-200, Beneq, Finland). This results in the formation of bismuth oxide, lead oxide, and erbium oxide with the precursors of bis (2, 2, 6, 6-tetra-methyl-3, 5-heptanedionato) bismuth (III) (Bi(thd)3), bis (2, 2, 6, 6-tetra-methyl-3, 5-heptanedionato) lead (III) (Pb(thd))3, and bis (2, 2, 6, 6-tetra-methyl-3, 5-heptanedionato) erbium (III) (Er(thd))3 (supplied by Shanghai J&K Scientific Ltd), respectively. They mainly react with water or ozone to form the metal oxidation layer, the O3 that originated from the O2. Third, germanium oxide is doped into the fiber preform core by the MCVD process, and then a Bi-related co-doped optical fiber preforms with a Ge-doped higher index core that is formed by collapsing on an MCVD lathe heated by a high-temperature oxyhydrogen flame. Finally, the preform is drawn into a

For optical fiber material, a perfect structure is visualized as a co-doped ion random network of SiO4 tetrahedrons joined at the corner, and different ions are doped into irregular vitreous silica, forming a stable network structure [36]. It is important to accumulate further knowledge regarding the influence of radiation on optical fiber materials, including material network structures, defect centers, and optical properties. Radiation as an effective method can induce changes in the optical properties of materials. It mainly involves the process of high-energy particles interacting with fiber materials, including the photoelectric effect, the Compton effect, the electron pair effect, and more. For BRDFs, irradiation significantly improves their optical properties, which mainly accounts for the variation in the

atoms). Here, gamma rays are selected as the irradiation source, mainly due to their short wavelength and strong penetrating ability. The effects of gamma ray irradiation on the optical properties of BRDFs, including Bi/Al co-doped silica fibers (BADFs), Bi/Er co-doped silica fibers (BEDFs), and Bi/Pb co-doped silica fibers

The radiation-induced PL properties of BADFs were investigated in [19]. The PL spectra in the inset of **Figure 3** reveal two emission bands at approximately ~1150 and ~ 1410 nm, corresponding to the aluminum-related Bi active center (BAC-Al) and the silicon-related Bi active center (BAC-Si), respectively. **Figure 3** illustrates that the fluorescence intensities of BAC-Al increased by 0.73, 2.25, and 1.35 dB at 1150 nm with 1.0, 2.0, and 3.0 kGy of irradiation, respectively. The fluorescence intensities of BAC-Al in the BADF samples increased with the increase in radiation dose (0–2.0 kGy) and then decreased when the radiation dose exceeded 2 kGy. Moreover, the change in the fluorescence intensity of BAC-Si trended similar to that of BAC-Al; however, the fluorescence intensity of BAC-Si increased considerably more. Furthermore, the fluorescence intensity of BAC-Si was approximately four

, defect centers, Bi clusters, Bi2−

2 dimers, or Bi

, Bi0

**3.1 Radiation effect on luminescence characteristics**

times stronger than that of the unirradiated fiber sample.

For BEDF, five BEDF samples were irradiated with cumulative doses of approximately 0.3, 0.5, 0.8, 1.5, and 3.0 kGy at room temperature. The radiation dose rate was 800 Gy/h. Under excitation at 980 nm (pump power is 1.8 mW),

**54**

*Fluorescence intensity of BAC-Al and BAC-Si as a function of radiation dose; inset are the PL spectra of the BADF samples before and after* γ*-ray irradiation.*

the fluorescence spectra of BEDF samples were measured, as shown in **Figure 4**. For BAC-Al, as the radiation dose was increased, the fluorescence intensity first increased and then decreased. With a 0.3 kGy dose of irradiation, the fluorescence intensity of BAC-Al in the BEDF sample is slightly higher than that of the pristine fiber, as shown in **Figure 4(a)**. However, when the radiation dose was less than

#### **Figure 4.**

*Fluorescence spectra of BEDF samples at different bands with different radiation doses. (a) 1100, (b) 1550 nm, and (c) the variations of the fluorescence intensity at 1100, 1450, and 1550 nm.*

0.5 kGy, the fluorescence intensity of BAC-Al was significantly lower than that of the pristine fiber. In addition, the fluorescence intensity of BAC-Si in BEDF showed the same trend in **Figure 4(c)** (red curve). The fluorescence of Er ions at 1550 nm was also observed, as shown in **Figure 4(b)**. For Er ions, the fluorescence intensity decreased with an increase in the radiation dose and fluorescence enhancement at low-dose radiation (<0.5 kGy) such as Bi ions did not appear..

For BPDF, five BPDF samples were irradiated with cumulative doses of approximately 0.3, 1.0, 1.5, 2.0, and 3.0 kGy at room temperature. The radiation dose rate was 800 Gy/h, which is the same as in the other experiment. The fluorescence spectra of BPDFs at different doses under 830 nm pumping are shown in **Figure 5(a)**. Comparing the PL spectra before and after irradiation, the shape did not change significantly. The fluorescence spectra of the fiber samples range from 1100 to 1600 nm with a peak at 1420 nm, which is derived from BAC-Si. The change in the fluorescence peak of BAC-Si is shown in **Figure 5(b)**. With an increase in the radiation dose, the fluorescence intensities of BAC-Si first increased and then decreased with a further increase in the radiation dose. Moreover, when the radiation dose was 1.5 kGy, the fluorescence intensity of BAC-Si was two times that of the unirradiated BPDF. That is to say, low-dose irradiation can promote the formation of BAC-Si, enhancing the fluorescence intensity. For radiation doses up to 3.0 kGy, the fluorescence intensity of BAC-Si was still higher than that of untreated fiber. This indicated that the BPDF samples had a certain degree of radiation resistance, which has great potential for photonic applications of optical fiber amplification devices in harsh radiation environments.

### **3.2 Radiation effect on fluorescence lifetime**

The luminescence decay curves of the Bi-related active centers in BEDFs and BPDFs were measured using a fluorescence spectrophotometer (Edinburgh FLS-980, England) equipped with an nF900 flash lamp. The fluorescence lifetime decay curves of BAC-Al in BEDF samples before and after radiation are shown in **Figure 6(a)**. In order to compare the fluorescence decay curves of the BEDF samples with different radiation doses, a single exponential function was used to fit them. The relationship between fluorescence lifetime and radiation dose is shown in **Figure 6(b)**. When the radiation doses were 0, 0.3, 0.5, 0.8, 1.5, and 3 kGy, the fluorescence lifetimes of BAC-Al were 564, 599, 585, 560, 559, and 553 μs, respectively. These results demonstrated that their lifetimes increased at low radiation doses (0–0.3 kGy) that were increasing, whereas at higher radiation doses (0.5–3 kGy), their lifetimes were decreased.

**Figure 5.**

*(a) PL spectra of BPDF samples with different radiation doses and (b) variation of the fluorescence intensity at 1420 nm.*

**57**

**Figure 7.**

*of the fluorescence lifetime.*

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers*

For comparative analysis, the fluorescence lifetime of the Er3+ ions at 1534 nm was also measured, as shown in **Figure 7(a)**; when the radiation doses were 0, 0.3, 0.5, 0.8, 1.5, and 3 kGy, the fluorescence lifetimes of the Er3+ ions were 11.26, 11.13, 11.11, 11.10, 10.73, and 10.23 ms, respectively. The fluorescence lifetimes of Er3+ ions

*(a) Luminescence decay curves with different radiation doses and (b) variation in the fluorescence lifetime.*

For the BPDF samples, the luminescence decay curves of BAC-Al are presented in **Figure 8(a)**. The single exponential function is a close fit. The luminescence lifetimes of BAC-Al were 740, 699, 573, and 500 μs for radiation doses of 0, 0.3, 1.0, and 3.0 kGy, respectively. Further, under the radiation conditions, the lifetimes of BAC-Al decreased rapidly, as shown in **Figure 8(b)**. It is inferred that the radiation increases the probability of the non-radiative transition, which may be attributed to the faster process whereby the electron in the excited state returns to the ground state or to the role of lead ions. To confirm this hypothesis, a more detailed experi-

Unsaturable pump absorption (αus) is ideally determined by the direct measurement of the remaining absorption of pump light. The saturable pump absorption (αs), which is a measure of the effective pump absorption of the fiber used for the radiative emission, decreases with the increasing pump power. The pump absorption

*(a) Luminescence decay curves of Er3+ active center in BEDF with different radiation doses and (b) variation* 

decreased with increasing of radiation doses, as shown in **Figure 7(b)**.

**3.3 Effect of radiation on unsaturable absorption characteristics**

consists of αus and αs. In fact, we focus more on the merit Mα, defined as

*DOI: http://dx.doi.org/10.5772/intechopen.93495*

ment is required in the future.

**Figure 6.**

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93495*

**Figure 6.** *(a) Luminescence decay curves with different radiation doses and (b) variation in the fluorescence lifetime.*

For comparative analysis, the fluorescence lifetime of the Er3+ ions at 1534 nm was also measured, as shown in **Figure 7(a)**; when the radiation doses were 0, 0.3, 0.5, 0.8, 1.5, and 3 kGy, the fluorescence lifetimes of the Er3+ ions were 11.26, 11.13, 11.11, 11.10, 10.73, and 10.23 ms, respectively. The fluorescence lifetimes of Er3+ ions decreased with increasing of radiation doses, as shown in **Figure 7(b)**.

For the BPDF samples, the luminescence decay curves of BAC-Al are presented in **Figure 8(a)**. The single exponential function is a close fit. The luminescence lifetimes of BAC-Al were 740, 699, 573, and 500 μs for radiation doses of 0, 0.3, 1.0, and 3.0 kGy, respectively. Further, under the radiation conditions, the lifetimes of BAC-Al decreased rapidly, as shown in **Figure 8(b)**. It is inferred that the radiation increases the probability of the non-radiative transition, which may be attributed to the faster process whereby the electron in the excited state returns to the ground state or to the role of lead ions. To confirm this hypothesis, a more detailed experiment is required in the future.

## **3.3 Effect of radiation on unsaturable absorption characteristics**

Unsaturable pump absorption (αus) is ideally determined by the direct measurement of the remaining absorption of pump light. The saturable pump absorption (αs), which is a measure of the effective pump absorption of the fiber used for the radiative emission, decreases with the increasing pump power. The pump absorption consists of αus and αs. In fact, we focus more on the merit Mα, defined as

**Figure 7.**

*(a) Luminescence decay curves of Er3+ active center in BEDF with different radiation doses and (b) variation of the fluorescence lifetime.*

*Bismuth - Fundamentals and Optoelectronic Applications*

harsh radiation environments.

**3.2 Radiation effect on fluorescence lifetime**

doses (0.5–3 kGy), their lifetimes were decreased.

low-dose radiation (<0.5 kGy) such as Bi ions did not appear..

0.5 kGy, the fluorescence intensity of BAC-Al was significantly lower than that of the pristine fiber. In addition, the fluorescence intensity of BAC-Si in BEDF showed the same trend in **Figure 4(c)** (red curve). The fluorescence of Er ions at 1550 nm was also observed, as shown in **Figure 4(b)**. For Er ions, the fluorescence intensity decreased with an increase in the radiation dose and fluorescence enhancement at

For BPDF, five BPDF samples were irradiated with cumulative doses of approximately 0.3, 1.0, 1.5, 2.0, and 3.0 kGy at room temperature. The radiation dose rate was 800 Gy/h, which is the same as in the other experiment. The fluorescence spectra of BPDFs at different doses under 830 nm pumping are shown in **Figure 5(a)**. Comparing the PL spectra before and after irradiation, the shape did not change significantly. The fluorescence spectra of the fiber samples range from 1100 to 1600 nm with a peak at 1420 nm, which is derived from BAC-Si. The change in the fluorescence peak of BAC-Si is shown in **Figure 5(b)**. With an increase in the radiation dose, the fluorescence intensities of BAC-Si first increased and then decreased with a further increase in the radiation dose. Moreover, when the radiation dose was 1.5 kGy, the fluorescence intensity of BAC-Si was two times that of the unirradiated BPDF. That is to say, low-dose irradiation can promote the formation of BAC-Si, enhancing the fluorescence intensity. For radiation doses up to 3.0 kGy, the fluorescence intensity of BAC-Si was still higher than that of untreated fiber. This indicated that the BPDF samples had a certain degree of radiation resistance, which has great potential for photonic applications of optical fiber amplification devices in

The luminescence decay curves of the Bi-related active centers in BEDFs and BPDFs were measured using a fluorescence spectrophotometer (Edinburgh FLS-980, England) equipped with an nF900 flash lamp. The fluorescence lifetime decay curves of BAC-Al in BEDF samples before and after radiation are shown in **Figure 6(a)**. In order to compare the fluorescence decay curves of the BEDF samples with different radiation doses, a single exponential function was used to fit them. The relationship between fluorescence lifetime and radiation dose is shown in **Figure 6(b)**. When the radiation doses were 0, 0.3, 0.5, 0.8, 1.5, and 3 kGy, the fluorescence lifetimes of BAC-Al were 564, 599, 585, 560, 559, and 553 μs, respectively. These results demonstrated that their lifetimes increased at low radiation doses (0–0.3 kGy) that were increasing, whereas at higher radiation

*(a) PL spectra of BPDF samples with different radiation doses and (b) variation of the fluorescence intensity* 

**56**

**Figure 5.**

*at 1420 nm.*

#### **Figure 8.**

*(a) Luminescence decay curves of BAC-Al in BPDF samples with different radiation doses and (b) variation of the fluorescence lifetime.*

Mα = αs / (αs + αus), which represents the ratio of useful pump absorption, αs, to the total pump absorption at the pump wavelength. This fraction is a key indicator of useful pump absorption and has a direct correlation to laser efficiency. Here, the unsaturable absorption characteristics of BEDFs at 980 nm before and after irradiation were investigated, as shown in **Figure 9(a)**. When the radiation doses were 0, 0.3, 0.5, 0.8, 1.5, and 3 kGy, the αus values of the BEDF were 40.6, 37.0, 40.7, 43.5, 46.8, and 49.6 dB/m, respectively. As the radiation dose increased, αus first decreased and then increased, as shown in **Figure 9(b)**. According to the relationship between αμs and the radiation dose, the decrease of αμs in the sample at a low radiation dose (0.3 kGy) may be attributed to the local structural change of Bi ions. Moreover, when the radiation dose was below 3.0 kGy, the αs of the BEDF (3.6 dB/m) was smaller than that of the unirradiated BEDF. At the same time, their corresponding Mα values were also calculated as 58.6%, 57.5%, 54.8%, 53.3%, 52.2%, and 50.2%. Hence, the Mα of BEDF continuously decreased with an increase in the radiation dose.

The unsaturable absorption characteristics of the BPDF and Bi-doped silica fibers are shown in **Figure 10**. The unsaturable absorption of the Bi-doped silica fiber (αus1) and the Pb/Bi co-doped silica fiber (αus2) at 830 nm were approximately 18 and 8 dB/m, respectively, and their corresponding saturable absorptions were 72 dB/m (αs1) and 45 dB/m (αs2), respectively.

#### **Figure 9.**

*(a) Unsaturable absorption characteristic of BEDF samples with different radiation doses and (b) variation of the* α*us and M*α*.*

**59**

**Figure 11.**

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers*

The derived merit Mα of the Pb/Bi co-doped silica fiber was approximately 85.1%,

*Unsaturable absorption characteristic of Pb/Bi co-doped fiber (black curve) and Bi-doped fiber (red curve) at* 

To further study the influence of radiation on the characteristics of Bi ions, the effect of radiation on the magnetic-optical properties of the Bi-doped silica fiber (BDF) was investigated by comparing it with other silica fibers, such as SMF and

*(a) Unsaturable absorption characteristics of BPDF at 830 nm with different radiation doses and (b)* 

*unsaturated absorption coefficient and M*α *value with the function of radiation dose.*

which was larger than that of the Bi-doped silica fiber (80.0%). A high merit M<sup>α</sup> meant that a large proportion of the pump photons would participate in the excitation of the active ions, promoting the desirable luminescence process at the corresponding bands. As such, the larger the Mα value, the higher the laser efficiency. Compared with the fiber-doped Bi ions only, the Pb/Bi co-doped silica fiber exhibited improved unsaturable characteristics. This would be beneficial for fiber lasers and amplifiers. After the BPDF samples were treated with different radiation doses, the unsaturable absorption characteristics were measured as shown in **Figure 11(a)**, and both αus and αs changed significantly. With an increase in radiation dose, αus trended with a gradual increase, whereas αs decreased and exhibited a small fluctuation. Furthermore, Ma trended similar to αs, as shown in **Figure 11(b)**. For the BPDF, the radiation effect on αus was small, similar to the effect of radiation on the fluores-

*DOI: http://dx.doi.org/10.5772/intechopen.93495*

cence lifetime of the Er3+ ions.

Pb-doped silica fiber.

**Figure 10.**

*830 nm.*

**3.4 Effect of radiation on magnetic-optical property**

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93495*

**Figure 10.**

*Bismuth - Fundamentals and Optoelectronic Applications*

Mα = αs / (αs + αus), which represents the ratio of useful pump absorption, αs, to the total pump absorption at the pump wavelength. This fraction is a key indicator of useful pump absorption and has a direct correlation to laser efficiency. Here, the unsaturable absorption characteristics of BEDFs at 980 nm before and after irradiation were investigated, as shown in **Figure 9(a)**. When the radiation doses were 0, 0.3, 0.5, 0.8, 1.5, and 3 kGy, the αus values of the BEDF were 40.6, 37.0, 40.7, 43.5, 46.8, and 49.6 dB/m, respectively. As the radiation dose increased, αus first decreased and then increased, as shown in **Figure 9(b)**. According to the relationship between αμs and the radiation dose, the decrease of αμs in the sample at a low radiation dose (0.3 kGy) may be attributed to the local structural change of Bi ions. Moreover, when the radiation dose was below 3.0 kGy, the αs of the BEDF (3.6 dB/m) was smaller than that of the unirradiated BEDF. At the same time, their corresponding Mα values were also calculated as 58.6%, 57.5%, 54.8%, 53.3%, 52.2%, and 50.2%. Hence, the Mα of

*(a) Luminescence decay curves of BAC-Al in BPDF samples with different radiation doses and (b) variation of* 

BEDF continuously decreased with an increase in the radiation dose.

72 dB/m (αs1) and 45 dB/m (αs2), respectively.

The unsaturable absorption characteristics of the BPDF and Bi-doped silica fibers are shown in **Figure 10**. The unsaturable absorption of the Bi-doped silica fiber (αus1) and the Pb/Bi co-doped silica fiber (αus2) at 830 nm were approximately 18 and 8 dB/m, respectively, and their corresponding saturable absorptions were

*(a) Unsaturable absorption characteristic of BEDF samples with different radiation doses and (b) variation of* 

**58**

**Figure 9.**

*the* α*us and M*α*.*

**Figure 8.**

*the fluorescence lifetime.*

*Unsaturable absorption characteristic of Pb/Bi co-doped fiber (black curve) and Bi-doped fiber (red curve) at 830 nm.*

The derived merit Mα of the Pb/Bi co-doped silica fiber was approximately 85.1%, which was larger than that of the Bi-doped silica fiber (80.0%). A high merit M<sup>α</sup> meant that a large proportion of the pump photons would participate in the excitation of the active ions, promoting the desirable luminescence process at the corresponding bands. As such, the larger the Mα value, the higher the laser efficiency. Compared with the fiber-doped Bi ions only, the Pb/Bi co-doped silica fiber exhibited improved unsaturable characteristics. This would be beneficial for fiber lasers and amplifiers.

After the BPDF samples were treated with different radiation doses, the unsaturable absorption characteristics were measured as shown in **Figure 11(a)**, and both αus and αs changed significantly. With an increase in radiation dose, αus trended with a gradual increase, whereas αs decreased and exhibited a small fluctuation. Furthermore, Ma trended similar to αs, as shown in **Figure 11(b)**. For the BPDF, the radiation effect on αus was small, similar to the effect of radiation on the fluorescence lifetime of the Er3+ ions.

### **3.4 Effect of radiation on magnetic-optical property**

To further study the influence of radiation on the characteristics of Bi ions, the effect of radiation on the magnetic-optical properties of the Bi-doped silica fiber (BDF) was investigated by comparing it with other silica fibers, such as SMF and Pb-doped silica fiber.

**Figure 11.**

*(a) Unsaturable absorption characteristics of BPDF at 830 nm with different radiation doses and (b) unsaturated absorption coefficient and M*α *value with the function of radiation dose.*

The Faraday rotation degree of the BDF in different magnetic fields ranging from 0 to 118 mT was measured. The slope of the Faraday rotation curve, marked as βi, where i = 1–7, in **Figure 12(a)**, determined the Verdet constants of the corresponding fiber samples. The Faraday rotations of the fiber samples were proportional to the intensity of the applied magnetic field. The slope of the rotation angle of BDF (β2) before irradiation was larger than that of SMF (β1). After the irradiation, the trend of the slope of the rotation angle changed from β2 to β4 clockwise, and then from β5 to β7 anticlockwise. The Verdet constant (1.64 rad/(Tm)) of the BDF before irradiation is 26.0% larger than that of SMF (1.29 rad/(Tm)), and the Verdet constant value is positive, indicating that the BDF material has diamagnetic properties. After radiation, the Verdet constant of the SMF increased with increasing radiation doses, as shown in **Figure 12(b)**; however, those of the BDF decreased at low radiation doses (<0.3 kGy). In particular, after 0.3 kGy of irradiation, the Verdet constant of the BDF became negative, showing that the BDF material has a paramagnetic property. Its Verdet constant value was positive and increased with the increase in radiation doses from 0.5 to 3 kGy. The Verdet constant of the BDF after 3.0 kGy of irradiation became 1.87 rad/(Tm), which is 23.84% larger than that of SMF with 1.51 rad/(Tm) and 44.96% larger than that of SMF without radiation.

For the irradiated SMF and Pb-doped silica fibers, their Verdet constants always increased with an increase in the radiation dose, as shown by the red and black curves in **Figure 13**. With a further increase in radiation doses, the Verdet constant of the SMF became essentially constant, which may be due to the fact that the concentration of Ge-related defect centers induced by radiation tended to be saturated. For the Pb-doped silica fiber, the Verdet constant also increased with an increase in the radiation dose (0–1.5 kGy). The Verdet constant of the Pb-doped silica fiber was higher than that of the SMF. This result indicated that gamma-ray radiation enhanced the Verdet constants of the fiber samples, especially for Pb-doped silica fibers. Irradiation not only induced Ge- and Si-related defect centers such as Si′, Ge′ color centers, but also led to new Pb-related defect centers in the Pb-doped silica fibers. These defect centers increased the electron transition probability of Pb2+ in 1 S0 → 1 P1 and contributed further to the orbital electron spin. This may be why the increase of the Verdet constant for Pb-doped silica fiber is faster than that for the SMF with an increase in the radiation dose (1.5–2.5 kGy). Therefore, it is supposed that gamma rays improve the magneto-optical properties of fibers.

For the BDF irradiation, with the increase in the radiation dose, the Verdet constant of the BDF decreased first and then increased. In particular, under

**61**

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers*

0.3 kGy, the Verdet constant had a negative value, as shown by the blue curve in **Figure 11**. The change in the Verdet constant may mainly result from Bi ions, which

*Verdet constants of Bi-doped silica fiber, Pb-doped silica fiber, and SMF with different radiation doses.*

Bi3+, and Bi5+. Furthermore, among various valence states, the conversion may be possible under radiation treatment. These different valence states have differ-

unpaired electrons in their outer electronic shells, showed diamagnetic properties.

erties because of unpaired electrons in the 6p layer, contributing to the intrinsic magnetic moment. These detailed results have already been reported in [22, 37]. Furthermore, the Verdet constant increase of the Bi-doped silica fiber was faster than that of the SMF and Pb-doped silica fiber with the increase in the radiation dose (1.5–2.5 kGy). Therefore, it is believed that gamma rays clearly improve the

In this chapter, certain types of BRDFs, including Bi/Al, Bi/Pb, and Bi/Er co-doped optical fibers, were fabricated using the ALD and MCVD process. Then, the radiation effects on their optical properties were investigated. The fluorescence intensity and fluorescence lifetimes of the BRDFs at 1150 nm with low-dose radiation increased significantly, whereas they decreased with a further increase in the radiation dose. The merit Mα values of the BRDFs, a ratio of useful pump absorption to total pump absorption, decreased with an increase in the radiation doses. However, the Verdet constants in different doped fibers increased and reached saturation with the increasing radiation dose. The incremental increases of the Verdet constants for the Pb-doped and Bi-doped fibers were faster than those for the SMF with an increase in the radiation dose (1.5–2.5 kGy). Moreover, the Verdet constant decreased and the direction of Faraday's rotation changed at low radiation doses. Hence, the increase in the Verdet constant increase for BDF is much faster than that of other fiber samples treated with high-dose radiation. All these results are of great significance for the

), and Bi2+ (6s2

6p1

, Bi1+, Bi2+,

) and Bi5+ (5d10) ions, which have no

) showed paramagnetic prop-

present the formation of multiple valence states in the fiber, such as Bi0

 (6s2 6p2

ent outer electronic shell structures. Bi3+ (6s2

 (6s2 6p3 ), Bi+

magneto-optical properties of the BDF.

study of the optical properties of BRDFs.

In contrast, Bi0

**Figure 13.**

**4. Conclusion**

*DOI: http://dx.doi.org/10.5772/intechopen.93495*

**Figure 12.** *Relationship between Faraday rotation and (a) magnetic field density and (b) radiation doses.*

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93495*

**Figure 13.** *Verdet constants of Bi-doped silica fiber, Pb-doped silica fiber, and SMF with different radiation doses.*

0.3 kGy, the Verdet constant had a negative value, as shown by the blue curve in **Figure 11**. The change in the Verdet constant may mainly result from Bi ions, which present the formation of multiple valence states in the fiber, such as Bi0 , Bi1+, Bi2+, Bi3+, and Bi5+. Furthermore, among various valence states, the conversion may be possible under radiation treatment. These different valence states have different outer electronic shell structures. Bi3+ (6s2 ) and Bi5+ (5d10) ions, which have no unpaired electrons in their outer electronic shells, showed diamagnetic properties. In contrast, Bi0 (6s2 6p3 ), Bi+ (6s2 6p2 ), and Bi2+ (6s2 6p1 ) showed paramagnetic properties because of unpaired electrons in the 6p layer, contributing to the intrinsic magnetic moment. These detailed results have already been reported in [22, 37]. Furthermore, the Verdet constant increase of the Bi-doped silica fiber was faster than that of the SMF and Pb-doped silica fiber with the increase in the radiation dose (1.5–2.5 kGy). Therefore, it is believed that gamma rays clearly improve the magneto-optical properties of the BDF.

## **4. Conclusion**

*Bismuth - Fundamentals and Optoelectronic Applications*

SMF without radiation.

The Faraday rotation degree of the BDF in different magnetic fields ranging from 0 to 118 mT was measured. The slope of the Faraday rotation curve, marked as βi, where i = 1–7, in **Figure 12(a)**, determined the Verdet constants of the corresponding fiber samples. The Faraday rotations of the fiber samples were proportional to the intensity of the applied magnetic field. The slope of the rotation angle of BDF (β2) before irradiation was larger than that of SMF (β1). After the irradiation, the trend of the slope of the rotation angle changed from β2 to β4 clockwise, and then from β5 to β7 anticlockwise. The Verdet constant (1.64 rad/(Tm)) of the BDF before irradiation is 26.0% larger than that of SMF (1.29 rad/(Tm)), and the Verdet constant value is positive, indicating that the BDF material has diamagnetic properties. After radiation, the Verdet constant of the SMF increased with increasing radiation doses, as shown in **Figure 12(b)**; however, those of the BDF decreased at low radiation doses (<0.3 kGy). In particular, after 0.3 kGy of irradiation, the Verdet constant of the BDF became negative, showing that the BDF material has a paramagnetic property. Its Verdet constant value was positive and increased with the increase in radiation doses from 0.5 to 3 kGy. The Verdet constant of the BDF after 3.0 kGy of irradiation became 1.87 rad/(Tm), which is 23.84% larger than that of SMF with 1.51 rad/(Tm) and 44.96% larger than that of

For the irradiated SMF and Pb-doped silica fibers, their Verdet constants always

P1 and contributed further to the orbital electron spin. This may be why the

increase of the Verdet constant for Pb-doped silica fiber is faster than that for the SMF with an increase in the radiation dose (1.5–2.5 kGy). Therefore, it is supposed

For the BDF irradiation, with the increase in the radiation dose, the Verdet constant of the BDF decreased first and then increased. In particular, under

that gamma rays improve the magneto-optical properties of fibers.

*Relationship between Faraday rotation and (a) magnetic field density and (b) radiation doses.*

increased with an increase in the radiation dose, as shown by the red and black curves in **Figure 13**. With a further increase in radiation doses, the Verdet constant of the SMF became essentially constant, which may be due to the fact that the concentration of Ge-related defect centers induced by radiation tended to be saturated. For the Pb-doped silica fiber, the Verdet constant also increased with an increase in the radiation dose (0–1.5 kGy). The Verdet constant of the Pb-doped silica fiber was higher than that of the SMF. This result indicated that gamma-ray radiation enhanced the Verdet constants of the fiber samples, especially for Pb-doped silica fibers. Irradiation not only induced Ge- and Si-related defect centers such as Si′, Ge′ color centers, but also led to new Pb-related defect centers in the Pb-doped silica fibers. These defect centers increased the electron transition probability of Pb2+ in

**60**

**Figure 12.**

1 S0 → 1

> In this chapter, certain types of BRDFs, including Bi/Al, Bi/Pb, and Bi/Er co-doped optical fibers, were fabricated using the ALD and MCVD process. Then, the radiation effects on their optical properties were investigated. The fluorescence intensity and fluorescence lifetimes of the BRDFs at 1150 nm with low-dose radiation increased significantly, whereas they decreased with a further increase in the radiation dose. The merit Mα values of the BRDFs, a ratio of useful pump absorption to total pump absorption, decreased with an increase in the radiation doses. However, the Verdet constants in different doped fibers increased and reached saturation with the increasing radiation dose. The incremental increases of the Verdet constants for the Pb-doped and Bi-doped fibers were faster than those for the SMF with an increase in the radiation dose (1.5–2.5 kGy). Moreover, the Verdet constant decreased and the direction of Faraday's rotation changed at low radiation doses. Hence, the increase in the Verdet constant increase for BDF is much faster than that of other fiber samples treated with high-dose radiation. All these results are of great significance for the study of the optical properties of BRDFs.

## **Funding**

This work is supported by Natural Science Foundation of China (Grant Nos. 61520106014, 61975113, 61935002, and 61675125) and the Pre-Research Fund Project (6140414030203).

## **Author details**

Jianxiang Wen1 \*, Ying Wan1 , Yanhua Dong1 , Yi Huang1 , Yanhua Luo2 , Gang-Ding Peng2 , Fufei Pang1 and Tingyun Wang1

1 Key Lab of Specialty Fiber Optics and Optical Access Networks, School of Communication and Information Engineering, Shanghai Institute for Advanced Communication and Data Science, Shanghai University, Shanghai, China

2 Photonics and Optical Communications, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW, Australia

\*Address all correspondence to: wenjx@shu.edu.cn

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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*DOI: http://dx.doi.org/10.5772/intechopen.93495*

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[2] Fujimoto Y. Local structure of the infrared bismuth luminescent center in bismuth-doped silica glass. Journal of the American Ceramic Society.

[3] Firstov S, Alyshev S, Melkumov M, R

iumkin K, Shubin A, Dianov E. Bismuth-doped optical fibers and fiber lasers for a spectral region of 1600-1800 nm. Optics Letters.

[4] Dianov EM. Amplification in extended transmission bands using bismuth-doped optical fibers. Journal of Lightwave Technology.

[5] Razdobreev I, Bigot L, Pureur V, Favre A, Bouwmans G, Douay M. Efficient all-fiber bismuth-doped laser. Applied Physics Letters.

[6] Bufetov IA, Dianov EM. Bi-doped fiber lasers. Laser Physics Letters.

[7] Zheng JY, Peng MY, Kang FW, Cao RP, Ma ZJ, Dong GP, et al. Broadband NIR luminescence from a new bismuth doped Ba2B5O9Cl crystal:

Express. 2012;**20**(20):22569-22578

wavelength-dependent near-infrared luminescence from Bi-doped silica glass. Journal of Alloys and Compounds.

[9] Sokolov VO, Plotnichenko VG, Dianov EM. Origin of broadband near-infrared luminescence in

[8] Zhang LL, Dong GP, Wu JD, Peng MY, Qiu JR. Excitation

model. Optics

**References**

2010;**93**(2):581-589

2014;**39**(24):6927-6930

2013;**31**(4):681-688

2007;**90**(3):031103

2009;**6**(7):487-504

evidence for the Bi0

2012;**531**:10-13

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93495*

## **References**

*Bismuth - Fundamentals and Optoelectronic Applications*

**Funding**

Project (6140414030203).

**62**

**Author details**

Jianxiang Wen1

Gang-Ding Peng2

\*, Ying Wan1

, Fufei Pang1

\*Address all correspondence to: wenjx@shu.edu.cn

provided the original work is properly cited.

, Yanhua Dong1

and Tingyun Wang1

This work is supported by Natural Science Foundation of China (Grant Nos. 61520106014, 61975113, 61935002, and 61675125) and the Pre-Research Fund

2 Photonics and Optical Communications, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW, Australia

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

1 Key Lab of Specialty Fiber Optics and Optical Access Networks, School of Communication and Information Engineering, Shanghai Institute for Advanced Communication and Data Science, Shanghai University, Shanghai, China

, Yi Huang1

, Yanhua Luo2

,

[1] Fujimoto Y, Nakatsuka M. Infrared luminescence from bismuth-doped silica glass. Japanese Journal of Applied Physics. 2001;**40**(3B):L279-L281

[2] Fujimoto Y. Local structure of the infrared bismuth luminescent center in bismuth-doped silica glass. Journal of the American Ceramic Society. 2010;**93**(2):581-589

[3] Firstov S, Alyshev S, Melkumov M, R iumkin K, Shubin A, Dianov E. Bismuth-doped optical fibers and fiber lasers for a spectral region of 1600-1800 nm. Optics Letters. 2014;**39**(24):6927-6930

[4] Dianov EM. Amplification in extended transmission bands using bismuth-doped optical fibers. Journal of Lightwave Technology. 2013;**31**(4):681-688

[5] Razdobreev I, Bigot L, Pureur V, Favre A, Bouwmans G, Douay M. Efficient all-fiber bismuth-doped laser. Applied Physics Letters. 2007;**90**(3):031103

[6] Bufetov IA, Dianov EM. Bi-doped fiber lasers. Laser Physics Letters. 2009;**6**(7):487-504

[7] Zheng JY, Peng MY, Kang FW, Cao RP, Ma ZJ, Dong GP, et al. Broadband NIR luminescence from a new bismuth doped Ba2B5O9Cl crystal: evidence for the Bi0 model. Optics Express. 2012;**20**(20):22569-22578

[8] Zhang LL, Dong GP, Wu JD, Peng MY, Qiu JR. Excitation wavelength-dependent near-infrared luminescence from Bi-doped silica glass. Journal of Alloys and Compounds. 2012;**531**:10-13

[9] Sokolov VO, Plotnichenko VG, Dianov EM. Origin of broadband near-infrared luminescence in

bismuth-doped glasses. Optics Letters. 2008;**33**(13):1488-1490

[10] Ren JJ, Qiu JR, Chen DP, Wang C, Jiang XW, Zhu CS. Infrared luminescence properties of bismuthdoped barium silicate glasses. Journal of Materials Research. 2007;**22**(7):1954-1958

[11] Xia H, Wang X. Near infrared broadband emission from Bi5+-doped Al2O3-GeO2-X (X=Na2O,BaO,Y2O3) glasses. Applied Physics Letters. 2006;**89**:051917

[12] Ban C, Bulatov LI, Dvoyrin VV, Mashinsky VM, Limberger HG, Dianov EM. Infrared luminescence enhancement by UV-irradiation of H2 loaded Bi-Al-doped fiber. In: European Conference on Optical Communication, 35th ECOC. Vienna, Austria: IEEE; 2009. p. 2

[13] Violakis G, Limberger HG, Mashinsky VM, Dianov EM. Dose dependence of luminescence increase in H2-loaded Bi-Al co-doped optical fibers by cw 244-nm and pulsed 193 nm laser irradiation. In: Optical Fiber Communication Conference, OFC. California, United States: IEEE; 2013. P. OTh4C.2

[14] Shen W, Ren J, Baccaro S, Cemmi A, Chen GR. Broadband infrared luminescence in γ-ray irradiated bismuth borosilicate glasses. Optics Letters. 2013;**38**(4):516-518

[15] Firstov S, Alyshev S, Khopin V, Melkumov M, Guryanov A, Dianov E. Photobleaching effect in bismuth-doped germanosilicate fibers. Optics Express. 2015;**23**(15):19226-19233

[16] Ou Y, Baccaro S, Zhang Y, Yang Y, Chen G. Effect of gamma-ray irradiation on the optical properties of PbO-B2O3- SiO2 and Bi2O3-B2O3-SiO2 glasses. Journal of the American Ceramic Society. 2010;**93**(2):338-341

[17] Elbatal FH, Marzouk MA, Abdel-Ghany AM. Gamma rays interaction with bismuth borate glasses doped by transition metal ions. Journal of Materials Science. 2011;**46**(15):5140-5152

[18] Girard S, Kuhnhenn J, Gusarov A, Brichard B, Van Uffelen M, Ouerdane Y, et al. Radiation effects on silica-based optical fibers: Recent advances and future challenges. IEEE Transactions on Nuclear Science. 2013;**60**(3):2015-2036

[19] Wen JX, Liu WJ, Dong YH, Luo YH, Peng GD, Chen N, et al. Radiationinduced photoluminescence enhancement of Bi/Al-codoped silica optical fibers via atomic layer deposition. Optics Express. 2015;**23**:29004-29013

[20] Guo Q , Wen JX, Huang Y, Wang WN, Pang FF, Chen ZY, et al. Magneto-optical properties and measurement of the novel doping silica optical fibers. Measurement. 2018;**127**:63-67

[21] Kim Y, Ju S, Jeong S, Jang MJ, Kim JY, Lee NH, et al. Influence of gamma-ray irradiation on Faraday effect of Cu-doped germano-silicate optical fiber. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 2015;**344**:39-43

[22] Wen JX, Wang WN, Guo Q , Huang Y, Dong YH, Pang FF, et al. Gamma-ray radiation on magnetooptical property of Pb-doped silica fiber. Inorganic Materials. 2018;**33**(4):416-420

[23] Wei S, Luo YH, Ding MJ, Cai FF, Xiao G, Fan DS, et al. Thermal effect on attenuation and luminescence of Bi/Er Co-doped fiber. IEEE Photonics Technology Letters. 2017;**29**(1):43-46

[24] Yang G, Chen DP, Wang W, Xu YS, Zeng HD, Yang YX, et al. Effects of thermal treatment on broadband near-infrared emission from Bi-doped chalcohalide glasses. Journal of the European Ceramic Society. 2008;**28**:3189-3191

[25] Yang G, Chen DP, Ren J, Xu YS, Zeng HD, Yang YX, et al. Effects of melting temperature on the broadband infrared luminescence of Bi-doped and Bi/Dy co-doped chalcohalide glasses. Journal of the American Ceramic Society. 2007;**90**(11):3670-3672

[26] Dianov EM, Dvoyrin VV, Mashinsky VM, Umnikov AA, Yashkov MV, Gur'yanov AN. CW bismuth fibre laser. Quantum Electronics. 2005;**35**:1083-1084

[27] Puurunen RL. Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/ water process. Applied Physics Reviews. 2005;**97**:121301

[28] George SM. Atomic layer deposition: An overview. Chemical Reviews. 2010;**110**:111-131

[29] Sneck S, Soininen P, Putkonen M, Norin L. A new way to utilize atomic layer deposition-case study: Optical fiber manufacturing. In: Proceedings of AVS - 6th International Conference on Atomic Layer Deposition. 2006

[30] Montiel i Ponsoda JJ, Norin L, Ye C, Bosund M, Söderlund MJ, Tervonen A, et al. Ytterbium-doped fibers fabricated with atomic layer deposition method. Optics Express. 2012;**20**:25085-25095

[31] Dong YH, Wen JX, Pang FF, Chen ZY, Wang J, Luo YH, et al. Optical properties of PbS-doped silica optical fiber materials based on atomic layer deposition. Applied Surface Science. 2014;**320**:372-378

[32] Shang YN, Wen JX, Dong YH, Zhan HH, Luo YH, Peng GD, et al.

**65**

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers*

*DOI: http://dx.doi.org/10.5772/intechopen.93495*

Luminescence properties of PbS quantum-dot-doped silica optical fibre produced via atomic layer deposition. Journal of Luminescence.

[33] Dong YH, Wen JX, Guo Q , Pang FF, Wang TY. Formation and photoluminescence property of PbS quantum dots in silica optical fiber based on atomic layer deposition. Optical Materials Express.

[34] Wang Q , Wen JX, Luo YH, Peng G-D, Pang FF, Chen ZY, et al. Enhancement of lifetime in Er-doped silica optical fiber by doping Yb ions via atomic layer deposition. Optical Materials Express.

[35] Shen YD, Li YW, Li WM, Zhang JZ,

Hu ZG, Chu JH. Growth of Bi2O3 ultrathin films by atomic layer deposition. Journal of Physical Chemistry C. 2012;**116**:3449-3456

[36] Wang TY, Wen JX, Luo WY, Xiao ZY, Chen ZY. Influences of irradiation on network microstructure of low water peak optical fiber material. Journal of Non-Crystalline Solids.

[37] Wen JX, Che QQ , Dong YH, Guo Q , Pang FF, Chen ZY, et al. Irradiation effect on the magnetooptical properties of Bi-doped silica optical fiber based on valence state change. Optical Materials Express.

2017;**187**:201-204

2015;**5**(4):712-719

2020;**10**:2397-2407

2010;**356**:1332-1336

2020;**10**(1):88-98

*Radiation Effect on Optical Properties of Bi-Related Materials Co-Doped Silica Optical Fibers DOI: http://dx.doi.org/10.5772/intechopen.93495*

Luminescence properties of PbS quantum-dot-doped silica optical fibre produced via atomic layer deposition. Journal of Luminescence. 2017;**187**:201-204

*Bismuth - Fundamentals and Optoelectronic Applications*

[24] Yang G, Chen DP, Wang W, Xu YS, Zeng HD, Yang YX, et al. Effects of thermal treatment on broadband near-infrared emission from Bi-doped chalcohalide glasses. Journal of the European Ceramic Society.

[25] Yang G, Chen DP, Ren J, Xu YS, Zeng HD, Yang YX, et al. Effects of melting temperature on the broadband infrared luminescence of Bi-doped and Bi/Dy co-doped chalcohalide glasses. Journal of the American Ceramic Society. 2007;**90**(11):3670-3672

[26] Dianov EM, Dvoyrin VV, Mashinsky VM, Umnikov AA, Yashkov MV, Gur'yanov AN. CW bismuth fibre laser. Quantum Electronics. 2005;**35**:1083-1084

2005;**97**:121301

2010;**110**:111-131

[27] Puurunen RL. Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/ water process. Applied Physics Reviews.

[28] George SM. Atomic layer deposition:

[29] Sneck S, Soininen P, Putkonen M, Norin L. A new way to utilize atomic layer deposition-case study: Optical fiber manufacturing. In: Proceedings of AVS - 6th International Conference on

[30] Montiel i Ponsoda JJ, Norin L, Ye C, Bosund M, Söderlund MJ, Tervonen A, et al. Ytterbium-doped fibers fabricated with atomic layer deposition method. Optics Express. 2012;**20**:25085-25095

Chen ZY, Wang J, Luo YH, et al. Optical properties of PbS-doped silica optical fiber materials based on atomic layer deposition. Applied Surface Science.

An overview. Chemical Reviews.

Atomic Layer Deposition. 2006

[31] Dong YH, Wen JX, Pang FF,

[32] Shang YN, Wen JX, Dong YH, Zhan HH, Luo YH, Peng GD, et al.

2014;**320**:372-378

2008;**28**:3189-3191

of the American Ceramic Society.

[18] Girard S, Kuhnhenn J, Gusarov A, Brichard B, Van Uffelen M, Ouerdane Y, et al. Radiation effects on silica-based optical fibers: Recent advances and future challenges. IEEE Transactions on Nuclear Science. 2013;**60**(3):2015-2036

[19] Wen JX, Liu WJ, Dong YH, Luo YH, Peng GD, Chen N, et al. Radiationinduced photoluminescence enhancement of Bi/Al-codoped silica optical fibers via atomic layer deposition. Optics Express.

[17] Elbatal FH, Marzouk MA, Abdel-Ghany AM. Gamma rays interaction with bismuth borate glasses doped by transition metal ions. Journal of Materials Science.

2010;**93**(2):338-341

2011;**46**(15):5140-5152

2015;**23**:29004-29013

2018;**127**:63-67

2015;**344**:39-43

2018;**33**(4):416-420

[20] Guo Q , Wen JX, Huang Y, Wang WN, Pang FF, Chen ZY, et al. Magneto-optical properties and measurement of the novel doping silica optical fibers. Measurement.

[21] Kim Y, Ju S, Jeong S, Jang MJ, Kim JY, Lee NH, et al. Influence of gamma-ray irradiation on Faraday effect of Cu-doped germano-silicate optical fiber. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms.

[22] Wen JX, Wang WN, Guo Q , Huang Y, Dong YH, Pang FF, et al. Gamma-ray radiation on magnetooptical property of Pb-doped silica fiber. Inorganic Materials.

[23] Wei S, Luo YH, Ding MJ, Cai FF, Xiao G, Fan DS, et al. Thermal effect on attenuation and luminescence of Bi/Er Co-doped fiber. IEEE Photonics Technology Letters. 2017;**29**(1):43-46

**64**

[33] Dong YH, Wen JX, Guo Q , Pang FF, Wang TY. Formation and photoluminescence property of PbS quantum dots in silica optical fiber based on atomic layer deposition. Optical Materials Express. 2015;**5**(4):712-719

[34] Wang Q , Wen JX, Luo YH, Peng G-D, Pang FF, Chen ZY, et al. Enhancement of lifetime in Er-doped silica optical fiber by doping Yb ions via atomic layer deposition. Optical Materials Express. 2020;**10**:2397-2407

[35] Shen YD, Li YW, Li WM, Zhang JZ, Hu ZG, Chu JH. Growth of Bi2O3 ultrathin films by atomic layer deposition. Journal of Physical Chemistry C. 2012;**116**:3449-3456

[36] Wang TY, Wen JX, Luo WY, Xiao ZY, Chen ZY. Influences of irradiation on network microstructure of low water peak optical fiber material. Journal of Non-Crystalline Solids. 2010;**356**:1332-1336

[37] Wen JX, Che QQ , Dong YH, Guo Q , Pang FF, Chen ZY, et al. Irradiation effect on the magnetooptical properties of Bi-doped silica optical fiber based on valence state change. Optical Materials Express. 2020;**10**(1):88-98

**67**

**Chapter 5**

Fiber

**Abstract**

properties, cutoff wavelength

**1. Introduction**

Effects of Electron Irradiation

Bismuth-Doped Phosphosilicate

The basic optical properties of yttrium-phosphosilicate fiber doped with bismuth (Bi) are assessed in both pristine state and that established after bombardment by a beam of high-energy electrons. The fiber has been developed and fabricated with a target to use it for laser applications in visible/near-infrared (VIS/NIR) domain. In this chapter, the main attention is paid to the dramatic changes in absorption spectra of the fiber under electron irradiation. Meanwhile, we reveal its overall resistance to irradiation in terms of emissive potential and bleaching contrast at excitation into the absorption bands of bismuth-related active centers. Besides, we report a new effect of large dose-dependent Stokes shift, experienced by the fiber's cutoff wavelength, which arises due to refractive index rise in its core area. The laws obeyed by the fiber's

characteristics vs. dose are examined for possible applications in dosimetry.

**Keywords:** bismuth, yttrium-phosphosilicate fiber, electron irradiation, optical

Silica-based fibers doped with semi-metals, e.g., bismuth (Bi), have become a popular object for sensing ionizing radiation (dosimetry), including electron beams: recently some of the Bi-doped fibers (BDFs) were primarily examined in this regard [1–8]. Sensing dose of ionizing radiation using optical fiber is an important issue: dose-induced changes in its characteristics are worth assessing as they may serve a base for making cost-effective, remotely monitorable sensors, easily located in harsh environments such as proximity to a nuclear reactor [9]; optical fibers can be also used for plasma diagnostics in fusion reactors [10]. The relevance of choosing BDFs for sensing relies on the understanding that Bi is highly susceptible to interaction with nuclear particles, e.g., energetic (further "β") electrons. Since high-energy "primary" β-electrons are virtually non-dissipating at propagation through silica fiber, the basic effect behind the transformations arising in its material and optical properties is creating β-induced carriers, viz., "secondary" free holes/electrons. Given that Bi is a heavy semi-metal, it becomes—when used as dopant—a powerful source of secondary carriers, eventually stabilized in core glass network generating plenty of defect centers. Such centers (a part of which is associated with the presence of Bi ions) are

on Optical Properties of

*Alexander V. Kir'yanov and Arindam Halder*

## **Chapter 5**
