**2. Fiber lasers and fiber amplifier based on tellurite glass fibers**

### **2.1 Tellurite glass-based fiber lasers**

Since the discovery of the first ruby laser (Maiman in 1960), the laser has attracted worldwide attention for its excellent collimation, high brightness, and monochromaticity [46]. Since then, the development of the laser has accelerated. In 1961, Javan et al. developed the helium-neon gas laser [47], and in 1962, Hall et al. created the GaAs coherent light emission [48]. In 1963, Koester et al. first proposed the idea of fiber lasers and amplifiers [49]. However, due to the shortcomings of the optical fiber at that time, the development of the optical fiber laser was slow during this period. In 1966, Gao et al. proposed the basic concept of optical fiber communication [50]. Subsequently, optical fiber communication underwent a major research and development stage (1966–1976), a practical application stage (1976– 1986), and a large-scale optical fiber communication infrastructure construction stage after 1986. With the rapid development of optical communication, optical fiber manufacturing technology and semiconductor laser production technology have matured, which formed the foundation for the subsequent development of doped fibers, optical fiber lasers, and fiber amplifiers.

The tellurite fiber laser is based on a tellurite glass fiber which acts as a gain medium. The first significant characterization of the optical properties of tellurite glass in fiber form was reported in 1994 [12]. In 1997, Mori et al. realized that Er3+ doped tellurite glass fiber could be used for broadband optical amplifiers [13]. Further research on tellurite fiber was initiated worldwide driven by the development of the communication industry. Over the next few years, Japan's NTT led the research in this field. They fabricated tellurite fiber with a loss of 0.02 dB/m and developed the first erbium-doped tellurite fiber amplifier (EDTFA) module for use in commercial WDM systems [51]. Further subsequent significant contributions to tellurite fiber laser development have been used by several university groups as well as industry-based research institutions including American Corning corporation, Fujitsu and Nippon of Japan, Korea's ETRI, etc.

With the above advantages coupled with excellent thermal stability, tellurite glass preforms could be handled with relative ease for casting [24, 53], drilling [54],

*The relationship between the laser output power and the pump power (R1 = R2 = 11.9%) in Nd3+-doped tellurite fiber. Considering that the laser output power at both ends of the fiber is the same, the total slope*

In 1994, Wang et al. successfully prepared a Nd3+-doped tellurite single-mode fiber for the first time. The numerical aperture of the fiber was determined as 0.21. The laser resonator was formed as a consequence of multiple Fresnel reflection (11.9%) from the end surfaces of the fiber. A laser with a wavelength of 0.818 μm was used as the pump source. A laser output with a wavelength of 1.061 μm was obtained from a 0.6 m long fiber, with a laser threshold of 27 mW. When only single ended output is considered, the slope efficiency of the laser was 23%, as

In 1998, Ohishi et al. used a 0.9-m-long Er3+-doped tellurite fiber as the gain medium to construct a ring laser cavity. When the pump power was 300 mW, a continuous tunable laser output covering 1529–1623 nm was obtained using a tunable filter. A 2.4-m-long Er3+-doped tellurite fiber was used as the gain medium to obtain laser output at 1624.5 nm, with a slope efficiency of 3.6% as shown in

In 2011, Dong et al. demonstrated a high-performance Er3+/Ce3+ co-doped tellurite fiber amplifier and tunable fiber laser using a dual-pumping scheme.

and extrusion techniques [55], providing precursors for tellurite fiber-based

*Transmission spectra of two kinds of tellurite glasses before and after dipping in water [52].*

nonlinear optical processing [56] and fiber lasers.

shown in **Figure 4** [12].

*efficiency should be 46% [12].*

**Figure 5** [57].

**267**

**Figure 3.**

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

**Figure 4.**

Tellurite glass has a broad transmission window in the infrared wavelength range which extends up to 6 μm, a relatively low phonon energy of about 700 cm<sup>1</sup> , and high solubility of rare earth ions. It is therefore an excellent host material for constructing single and high repetition frequency fiber lasers. Yao et al. measured the transmission spectrum of 2-mm-thick glass samples after they were immersed in deionized water for 12 days [52], and the results are shown in **Figure 3**. There was no obvious change in the transmission spectra, and no hydrated layer was formed at the end face of the tellurite glass, which proves its great resistance to water. Several researchers have studied the reduction of water molecules and hydroxyl groups, in order to further improve the performance of tellurite glass materials used in midinfrared fiber lasers. Specific test procedures included melting the glass in a dry atmosphere, raw material dehydration, and the use of fluoride [52] or chloride raw materials [53], as eliminating hydroxyl groups diminishes loss (at specific wavelengths) and is therefore favorable for the commercialization of tellurite glass fibers.

**Figure 3.**

75TeO2-20ZnO-5Na2O [43] system increases the ΔT value from 118 to 150°C, while 1 wt% Er2O3 introduced to 90TeO2-10P2O5 [44] system decreases the ΔT value from 147 to 101°C. Furthermore, the concentration of the rare earth ion has a significant influences on the ΔT value of the 75TeO2-20ZnO-5Na2O [45] glass

**2. Fiber lasers and fiber amplifier based on tellurite glass fibers**

Since the discovery of the first ruby laser (Maiman in 1960), the laser has attracted worldwide attention for its excellent collimation, high brightness, and monochromaticity [46]. Since then, the development of the laser has accelerated. In 1961, Javan et al. developed the helium-neon gas laser [47], and in 1962, Hall et al. created the GaAs coherent light emission [48]. In 1963, Koester et al. first proposed the idea of fiber lasers and amplifiers [49]. However, due to the shortcomings of the optical fiber at that time, the development of the optical fiber laser was slow during this period. In 1966, Gao et al. proposed the basic concept of optical fiber communication [50]. Subsequently, optical fiber communication underwent a major research and development stage (1966–1976), a practical application stage (1976– 1986), and a large-scale optical fiber communication infrastructure construction stage after 1986. With the rapid development of optical communication, optical fiber manufacturing technology and semiconductor laser production technology have matured, which formed the foundation for the subsequent development of

The tellurite fiber laser is based on a tellurite glass fiber which acts as a gain medium. The first significant characterization of the optical properties of tellurite glass in fiber form was reported in 1994 [12]. In 1997, Mori et al. realized that Er3+ doped tellurite glass fiber could be used for broadband optical amplifiers [13]. Further research on tellurite fiber was initiated worldwide driven by the development of the communication industry. Over the next few years, Japan's NTT led the research in this field. They fabricated tellurite fiber with a loss of 0.02 dB/m and developed the first erbium-doped tellurite fiber amplifier (EDTFA) module for use in commercial WDM systems [51]. Further subsequent significant contributions to tellurite fiber laser development have been used by several university groups as well as industry-based research institutions including American Corning corporation,

Tellurite glass has a broad transmission window in the infrared wavelength range which extends up to 6 μm, a relatively low phonon energy of about 700 cm<sup>1</sup>

and high solubility of rare earth ions. It is therefore an excellent host material for constructing single and high repetition frequency fiber lasers. Yao et al. measured the transmission spectrum of 2-mm-thick glass samples after they were immersed in deionized water for 12 days [52], and the results are shown in **Figure 3**. There was no obvious change in the transmission spectra, and no hydrated layer was formed at the end face of the tellurite glass, which proves its great resistance to water. Several researchers have studied the reduction of water molecules and hydroxyl groups, in order to further improve the performance of tellurite glass materials used in midinfrared fiber lasers. Specific test procedures included melting the glass in a dry atmosphere, raw material dehydration, and the use of fluoride [52] or chloride raw materials [53], as eliminating hydroxyl groups diminishes loss (at specific wavelengths) and is therefore favorable for the commercialization of tellurite glass

,

system.

fibers.

**266**

**2.1 Tellurite glass-based fiber lasers**

*Advanced Functional Materials*

doped fibers, optical fiber lasers, and fiber amplifiers.

Fujitsu and Nippon of Japan, Korea's ETRI, etc.

*Transmission spectra of two kinds of tellurite glasses before and after dipping in water [52].*

#### **Figure 4.**

*The relationship between the laser output power and the pump power (R1 = R2 = 11.9%) in Nd3+-doped tellurite fiber. Considering that the laser output power at both ends of the fiber is the same, the total slope efficiency should be 46% [12].*

With the above advantages coupled with excellent thermal stability, tellurite glass preforms could be handled with relative ease for casting [24, 53], drilling [54], and extrusion techniques [55], providing precursors for tellurite fiber-based nonlinear optical processing [56] and fiber lasers.

In 1994, Wang et al. successfully prepared a Nd3+-doped tellurite single-mode fiber for the first time. The numerical aperture of the fiber was determined as 0.21. The laser resonator was formed as a consequence of multiple Fresnel reflection (11.9%) from the end surfaces of the fiber. A laser with a wavelength of 0.818 μm was used as the pump source. A laser output with a wavelength of 1.061 μm was obtained from a 0.6 m long fiber, with a laser threshold of 27 mW. When only single ended output is considered, the slope efficiency of the laser was 23%, as shown in **Figure 4** [12].

In 1998, Ohishi et al. used a 0.9-m-long Er3+-doped tellurite fiber as the gain medium to construct a ring laser cavity. When the pump power was 300 mW, a continuous tunable laser output covering 1529–1623 nm was obtained using a tunable filter. A 2.4-m-long Er3+-doped tellurite fiber was used as the gain medium to obtain laser output at 1624.5 nm, with a slope efficiency of 3.6% as shown in **Figure 5** [57].

In 2011, Dong et al. demonstrated a high-performance Er3+/Ce3+ co-doped tellurite fiber amplifier and tunable fiber laser using a dual-pumping scheme.

**Figure 5.**

*Laser characteristics of a tellurite-based fiber laser operating in the 1625 nm band. The inset shows the lasing spectrum of the fiber laser [57].*

The short 22 cm fiber exhibits a net gain of 28 dB at 1558 μm, a wide positive net gain bandwidth of 122 μm, and a noise figure of 4.1 dB. As shown in **Figure 6**, a widely tunable Er3+/Ce3+ co-doped tellurite fiber ring laser with a tuning range of 83 μm was demonstrated [58].

In 2012, M. Oermann et al. fabricated Er3+-doped tellurite microstructured fibers with three air holes using an extruding method. The resulting fibers are shown in cross section in **Figure 7**. The core diameter of the fiber was about 1.5 μm, the loss was 1.3 dB/m, and the doping concentration of Er3+ was 0.022 mol %. A 2.2-m-long Er3+-doped tellurite microstructure fiber was used as the gain medium to construct the laser cavity and was pumped using a 976 nm laser source. As shown in **Figure 8**, its threshold power is only 1.5 mW, and its slope efficiency reaches 13% [59].

In the same year, Chillcce et al. fabricated an Er3+-doped tellurite microstructured fiber using the stack-and-draw technique. They demonstrated laser emission using a simple double-pump configuration with two sources at 980 μm. The fiber core had a hexagonal structure as shown in **Figure 9**, the Er2O3 doping concentration was 7500 ppm, and the background loss of the resulting microstructured fiber was 0.2 dB/ cm at 1117 μm. Two short segments of fiber of 5 and 12 cm generated laser emissions at 1532.3, 1536.3, and 1558.5 μm, as shown in **Figure 10**. The maximum optical signal-to-noise ratio (OSNR) obtained was 21.2 dB [60].

In 2014, Yao et al. fabricated microstructured fibers consisting of a solid core surrounded by six air holes using a rod-in-tube method. A maximum unsaturated power of 9 mW laser operating at <sup>1872</sup> <sup>μ</sup>m was obtained in a Tm3+-doped 2.8 cm

long microstructure fiber with a slope efficiency of 6.53% and a threshold power of 200 mW. The results shown in **Figure 11** indicate that the Tm3+-doped tellurite microstructure fiber is a promising material for achieving a compact 2 μm output

*Fiber laser output plotted against the coupled pump power for a fiber length of 2.2 m (circles). The figure inset is a plot of the laser output spectrum for 5 mW of coupled pump power into the 1 m (dashed) and 2.2 m (solid)*

*Photographs of (a) stainless steel die exit used for the extrusion of the structured preform and (b and c) the extruded structured and jacket preforms, respectively. SEM images of the (d) fabricated fiber cross section, (e) enlarged SEM image of the fiber's core and cladding, and (f) beam profile of the laser mode emitted from the*

In 2015, Meng et al. used a 22-cm-long Tm3+/Ho3+ co-doped tellurite fiber to obtain a continuous laser output with a maximum output power of 8.34 mW and a wavelength of 2065 nm when the pump power was 507 mW, as shown in **Figure 12**.

fiber laser [61].

**269**

*lengths of fiber [59].*

**Figure 8.**

**Figure 7.**

*output of the fiber [59].*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

The slope efficiency was 2.97% [62].

**Figure 6.** *Output spectra of the E Er3+/Ce3+ co-doped tellurite fiber ring laser [58].*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

#### **Figure 7.**

The short 22 cm fiber exhibits a net gain of 28 dB at 1558 μm, a wide positive net gain bandwidth of 122 μm, and a noise figure of 4.1 dB. As shown in **Figure 6**, a widely tunable Er3+/Ce3+ co-doped tellurite fiber ring laser with a tuning range of

*Laser characteristics of a tellurite-based fiber laser operating in the 1625 nm band. The inset shows the lasing*

In 2012, M. Oermann et al. fabricated Er3+-doped tellurite microstructured fibers with three air holes using an extruding method. The resulting fibers are shown in cross section in **Figure 7**. The core diameter of the fiber was about 1.5 μm, the loss was 1.3 dB/m, and the doping concentration of Er3+ was 0.022 mol %. A 2.2-m-long Er3+-doped tellurite microstructure fiber was used as the gain medium to construct the laser cavity and was pumped using a 976 nm laser source. As shown in **Figure 8**, its threshold power is only 1.5 mW, and its slope efficiency reaches 13% [59].

In the same year, Chillcce et al. fabricated an Er3+-doped tellurite microstructured fiber using the stack-and-draw technique. They demonstrated laser emission using a simple double-pump configuration with two sources at 980 μm. The fiber core had a hexagonal structure as shown in **Figure 9**, the Er2O3 doping concentration was 7500 ppm, and the background loss of the resulting microstructured fiber was 0.2 dB/ cm at 1117 μm. Two short segments of fiber of 5 and 12 cm generated laser emissions at 1532.3, 1536.3, and 1558.5 μm, as shown in **Figure 10**. The maximum optical

In 2014, Yao et al. fabricated microstructured fibers consisting of a solid core surrounded by six air holes using a rod-in-tube method. A maximum unsaturated power of 9 mW laser operating at <sup>1872</sup> <sup>μ</sup>m was obtained in a Tm3+-doped 2.8 cm

signal-to-noise ratio (OSNR) obtained was 21.2 dB [60].

*Output spectra of the E Er3+/Ce3+ co-doped tellurite fiber ring laser [58].*

83 μm was demonstrated [58].

*spectrum of the fiber laser [57].*

*Advanced Functional Materials*

**Figure 5.**

**Figure 6.**

**268**

*Photographs of (a) stainless steel die exit used for the extrusion of the structured preform and (b and c) the extruded structured and jacket preforms, respectively. SEM images of the (d) fabricated fiber cross section, (e) enlarged SEM image of the fiber's core and cladding, and (f) beam profile of the laser mode emitted from the output of the fiber [59].*

#### **Figure 8.**

*Fiber laser output plotted against the coupled pump power for a fiber length of 2.2 m (circles). The figure inset is a plot of the laser output spectrum for 5 mW of coupled pump power into the 1 m (dashed) and 2.2 m (solid) lengths of fiber [59].*

long microstructure fiber with a slope efficiency of 6.53% and a threshold power of 200 mW. The results shown in **Figure 11** indicate that the Tm3+-doped tellurite microstructure fiber is a promising material for achieving a compact 2 μm output fiber laser [61].

In 2015, Meng et al. used a 22-cm-long Tm3+/Ho3+ co-doped tellurite fiber to obtain a continuous laser output with a maximum output power of 8.34 mW and a wavelength of 2065 nm when the pump power was 507 mW, as shown in **Figure 12**. The slope efficiency was 2.97% [62].

**Figure 9.**

*(a) Preform with the first clad before eliminating the air trapped. The air regions are indicted with a white "a". (b) Preform without the air trapped. (c) Scanning electron microscope image of the microstructure fiber [60].*

**Figure 10.**

*Laser emission spectra. (a) 5 cm fiber segment. (b) A zoom of the emission region observed in (a). (c) 12 cm fiber segment. (d) A zoom of the emission region observed in (c) [60].*

that can accumulate the power intensity of pump sources and provide adequate interaction length to facilitate the occurrence of the nonlinear processes. In 2005, half the Nobel Prize in Physics were awarded for the development of optical frequency combs that was generated from the SC coherent light source employing microstructured silica fiber. SC light sources based on silica microstructured fiber with outputs spanning from the ultraviolet to the near infrared spectral regions have been widely commercialized by major optics firms, such as American Corning

*Laser spectrum of fiber laser pumped by 1560 μm band fiber laser. The figure inset shows cross section of*

The 2–5-μm-mid-infrared region is the typical wavelength range corresponding

to the "atmospheric optics window," the "molecular fingerprint region," and "strong absorption band of hydroxyl and amino groups." Therefore, SC light sources in this region offer great possibilities for optical telecommunication, remote sensing, atmospheric pollution monitoring, molecular spectroscopy, medical

corporation, Fujitsu and Nippon of Japan, Korea's ETRI and so on.

*Spectrum of the fiber laser pumped by 1560 μm band fiber laser [62].*

**Figure 11.**

**Figure 12.**

**271**

*Tm3+-doped TZNB microstructure fiber [61].*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

#### **2.2 Tellurite fiber-based supercontinuum light source**

The supercontinuum (SC) light source is defined as a broadband laser source whose output spectrum is greatly broadened through the interaction of nonlinear effects and dispersion when a high peak power pulsed laser output (e.g., a soliton pulse) propagates in nonlinear optical medium. The SC spectra generated in transparent materials do not usually originate from a single nonlinear process—typically the initiated self-phase modulation (SPM) modulates the phase of the input laser, and then other nonlinear effects including cross-phase modulation (XPM), stimulated Raman scattering (SRS), four-wave mixing (FWM), soliton self-frequency shifting (SSFS), etc. broaden the output frequency (wavelength) spectrum [63]. The first observation and application of SC spectra were obtained in solids and liquids [64–66], but recent investigations and applications of SC light sources have utilized optical fibers including single-mode and microstructured fibers [67]. The latter is a widely used medium due to its unique geometry and low transmission loss *Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

#### **Figure 11.**

*Laser spectrum of fiber laser pumped by 1560 μm band fiber laser. The figure inset shows cross section of Tm3+-doped TZNB microstructure fiber [61].*

**Figure 12.** *Spectrum of the fiber laser pumped by 1560 μm band fiber laser [62].*

that can accumulate the power intensity of pump sources and provide adequate interaction length to facilitate the occurrence of the nonlinear processes. In 2005, half the Nobel Prize in Physics were awarded for the development of optical frequency combs that was generated from the SC coherent light source employing microstructured silica fiber. SC light sources based on silica microstructured fiber with outputs spanning from the ultraviolet to the near infrared spectral regions have been widely commercialized by major optics firms, such as American Corning corporation, Fujitsu and Nippon of Japan, Korea's ETRI and so on.

The 2–5-μm-mid-infrared region is the typical wavelength range corresponding to the "atmospheric optics window," the "molecular fingerprint region," and "strong absorption band of hydroxyl and amino groups." Therefore, SC light sources in this region offer great possibilities for optical telecommunication, remote sensing, atmospheric pollution monitoring, molecular spectroscopy, medical

**2.2 Tellurite fiber-based supercontinuum light source**

*fiber segment. (d) A zoom of the emission region observed in (c) [60].*

**Figure 9.**

*Advanced Functional Materials*

**Figure 10.**

**270**

The supercontinuum (SC) light source is defined as a broadband laser source whose output spectrum is greatly broadened through the interaction of nonlinear effects and dispersion when a high peak power pulsed laser output (e.g., a soliton pulse) propagates in nonlinear optical medium. The SC spectra generated in transparent materials do not usually originate from a single nonlinear process—typically the initiated self-phase modulation (SPM) modulates the phase of the input laser, and then other nonlinear effects including cross-phase modulation (XPM), stimulated Raman scattering (SRS), four-wave mixing (FWM), soliton self-frequency shifting (SSFS), etc. broaden the output frequency (wavelength) spectrum [63]. The first observation and application of SC spectra were obtained in solids and liquids [64–66], but recent investigations and applications of SC light sources have utilized optical fibers including single-mode and microstructured fibers [67]. The latter is a widely used medium due to its unique geometry and low transmission loss

*Laser emission spectra. (a) 5 cm fiber segment. (b) A zoom of the emission region observed in (a). (c) 12 cm*

*(a) Preform with the first clad before eliminating the air trapped. The air regions are indicted with a white "a". (b) Preform without the air trapped. (c) Scanning electron microscope image of the microstructure fiber [60].*

diagnosis, hyperspectral imaging, laser surgery, and IR opto-electric countermeasures [63, 68], all of which greatly attracted intense worldwide research interest over the past two decades [69–71]. There are several requirements of nonlinear fibers used for 2–5 μm SC light sources, e.g., they must be transparent within the 2– 5 μm window, they must have a relatively high laser damage threshold for potentially high-power light transmission, they should have a high nonlinear refractive index, and they need to be fabricated based on mature processing technology. Silica is not a candidate material for generating SC spectra at wavelengths longer than 2.2 μm, due to its high intrinsic loss and relatively low nonlinear parameters. Alternatively, soft glass fibers, mainly including fluoride, tellurite, and chalcogenide glass fibers, are being investigated to develop SC light sources in the 2–5 μm spectral region and have achieved remarkable progress to date with their broad IR transparency range as well as prominent optical nonlinearity.

Among the soft glass materials investigated, tellurite glass provides many several attractive features for use in high-power SC light sources. These include a broad IR transmission window (0.3–7 μm) that can be matched with fluoride glass while possessing lower intrinsic losses than chalcogenide glass and possessing the highest optical damage threshold than other soft glass materials. Moreover, with outstanding thermal and chemical stability, tellurite glass can be drawn as microstructured fiber from a preform constructed using the rod-in-tube method or extrusion technique. The dispersion profile and nonlinearity of the fabricated fiber can be readily optimized. In the past two decades, much effort has been concentrated on fabricating a microstructured tellurite fiber for SC generation.

which broadened the SC spectrum spanning two and a half optical octaves in the fiber having only a length 0.8. Such a short fiber length results in flatter SC spectra,

*Picture as seen in optical microscopy (a and c) and cross section profile of the tellurite PCF in electron*

In 2008, Feng et al. fabricated a large-mode-area tellurite holey fiber from an extruded preform, with a core diameter of 80 μm, attenuation of 2.9 dB/m at 1.55 μm, and zero-dispersion wavelength (ZDW) at 2.15 μm (**Figure 15**) [74]. Using

In 2009, Liao et al. fabricated the hexagonal core fiber (**Figure 16**) for the first time [75]. They studied the SC generation in such a fiber of 6 cm length pumped by a 1557 nm femtosecond laser and with a 30-cm-long fiber pumped using a 1064 nm picosecond fiber laser. Additionally, they demonstrated that the holey region has an important influence on the dispersion, nonlinear coefficient, and SC generation. In 2010, a 36-cm-length tellurite microstructured fiber with four holes [76] was

(**Figure 17**) and was pumped using a 1550 nm pulsed laser. The calculated nonlinear

In 2012, Savelii et al. prepared a low-loss suspended-core tellurite fiber, from which they generated a 0.75–2.8 μm SC spectrum when pumped at 1745 nm [77]. And in 2015, Belal et al. generated SC spectra extending to 3 μm in a suspended-core tellurite fiber. Their numerical study show that the structure of the fiber can have a significant impact on the dispersion profile and hence the nonlinear processes and

In 2013, Klimczak et al. reported a breakthrough in the design of optical fiber transverse structure. They produced a novel, regular hexagonal-lattice tellurite photonic crystal fiber (PCF) as shown in **Figure 18** [79]. Pumping the 2-cm-long

*Optical photographs of the cross-sectional views of (a) the extruded tellurite preform and (b) the resulting*

lower dispersion, and reduced material absorption at longer wavelengths.

such microstructured fiber with a 9 cm length, a broadband SC spanning of

used to generate a flattened SC spectrum spanning from 900 to 2800 nm

coefficient at 1550 nm was 539 km<sup>1</sup> W<sup>1</sup> [76].

*tellurite holey fiber with 410 μm outer diameter [74].*

0.9–2.5 μm was achieved.

*microscopy (b). Scale bar in (b) is 1 μm [73].*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

**Figure 14.**

SC broadening [78].

**Figure 15.**

**273**

Kumar et al. prepared low-loss tellurite microstructured fiber for the first time using an extrusion and rod-in-tube method, whose minimum loss was 2.3 dB/m at 1055 nm [72]. Photographs of the fiber are shown in **Figure 13**. In such a microstructured fiber with 1.02 m length, they studied the stimulated Raman scattering generation pumped using a 1064 nm pulsed laser.

In 2008, Domachuk et al. generated a SC spectrum with a broad bandwidth covering the spectral range 789–4870 nm in tellurite microstructured fiber pumped using a1550 nm pulsed laser [73]. As shown in **Figure 14**, the fiber core was surrounded by six large diameter air holes to achieve strong light confinement, and the calculated nonlinear waveguide coefficient at 1550 nm was 596 km<sup>1</sup> W<sup>1</sup> ,

#### **Figure 13.**

*(a) The cross section of the die used for extrusion. (b) Electron micrograph of an extruded tellurite preform, with outer diameter 1 mm. (c) Electron micrograph of tellurite PCF. (d) Transmission view of a tellurite PCF as seen in microscope [72].*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

#### **Figure 14.**

diagnosis, hyperspectral imaging, laser surgery, and IR opto-electric countermeasures [63, 68], all of which greatly attracted intense worldwide research interest over the past two decades [69–71]. There are several requirements of nonlinear fibers used for 2–5 μm SC light sources, e.g., they must be transparent within the 2– 5 μm window, they must have a relatively high laser damage threshold for potentially high-power light transmission, they should have a high nonlinear refractive index, and they need to be fabricated based on mature processing technology. Silica is not a candidate material for generating SC spectra at wavelengths longer than 2.2 μm, due to its high intrinsic loss and relatively low nonlinear parameters. Alternatively, soft glass fibers, mainly including fluoride, tellurite, and chalcogenide glass fibers, are being investigated to develop SC light sources in the 2–5 μm spectral region and have achieved remarkable progress to date with their broad IR transpar-

Among the soft glass materials investigated, tellurite glass provides many several attractive features for use in high-power SC light sources. These include a broad IR transmission window (0.3–7 μm) that can be matched with fluoride glass while possessing lower intrinsic losses than chalcogenide glass and possessing the highest optical damage threshold than other soft glass materials. Moreover, with outstanding thermal and chemical stability, tellurite glass can be drawn as microstructured fiber from a preform constructed using the rod-in-tube method or extrusion technique. The dispersion profile and nonlinearity of the fabricated fiber can be readily optimized. In the past two decades, much effort has been concentrated on fabricat-

Kumar et al. prepared low-loss tellurite microstructured fiber for the first time using an extrusion and rod-in-tube method, whose minimum loss was 2.3 dB/m at

microstructured fiber with 1.02 m length, they studied the stimulated Raman scat-

In 2008, Domachuk et al. generated a SC spectrum with a broad bandwidth covering the spectral range 789–4870 nm in tellurite microstructured fiber pumped

surrounded by six large diameter air holes to achieve strong light confinement, and the calculated nonlinear waveguide coefficient at 1550 nm was 596 km<sup>1</sup> W<sup>1</sup>

*(a) The cross section of the die used for extrusion. (b) Electron micrograph of an extruded tellurite preform, with outer diameter 1 mm. (c) Electron micrograph of tellurite PCF. (d) Transmission view of a tellurite PCF*

,

1055 nm [72]. Photographs of the fiber are shown in **Figure 13**. In such a

using a1550 nm pulsed laser [73]. As shown in **Figure 14**, the fiber core was

ency range as well as prominent optical nonlinearity.

*Advanced Functional Materials*

ing a microstructured tellurite fiber for SC generation.

tering generation pumped using a 1064 nm pulsed laser.

**Figure 13.**

**272**

*as seen in microscope [72].*

*Picture as seen in optical microscopy (a and c) and cross section profile of the tellurite PCF in electron microscopy (b). Scale bar in (b) is 1 μm [73].*

which broadened the SC spectrum spanning two and a half optical octaves in the fiber having only a length 0.8. Such a short fiber length results in flatter SC spectra, lower dispersion, and reduced material absorption at longer wavelengths.

In 2008, Feng et al. fabricated a large-mode-area tellurite holey fiber from an extruded preform, with a core diameter of 80 μm, attenuation of 2.9 dB/m at 1.55 μm, and zero-dispersion wavelength (ZDW) at 2.15 μm (**Figure 15**) [74]. Using such microstructured fiber with a 9 cm length, a broadband SC spanning of 0.9–2.5 μm was achieved.

In 2009, Liao et al. fabricated the hexagonal core fiber (**Figure 16**) for the first time [75]. They studied the SC generation in such a fiber of 6 cm length pumped by a 1557 nm femtosecond laser and with a 30-cm-long fiber pumped using a 1064 nm picosecond fiber laser. Additionally, they demonstrated that the holey region has an important influence on the dispersion, nonlinear coefficient, and SC generation.

In 2010, a 36-cm-length tellurite microstructured fiber with four holes [76] was used to generate a flattened SC spectrum spanning from 900 to 2800 nm (**Figure 17**) and was pumped using a 1550 nm pulsed laser. The calculated nonlinear coefficient at 1550 nm was 539 km<sup>1</sup> W<sup>1</sup> [76].

In 2012, Savelii et al. prepared a low-loss suspended-core tellurite fiber, from which they generated a 0.75–2.8 μm SC spectrum when pumped at 1745 nm [77]. And in 2015, Belal et al. generated SC spectra extending to 3 μm in a suspended-core tellurite fiber. Their numerical study show that the structure of the fiber can have a significant impact on the dispersion profile and hence the nonlinear processes and SC broadening [78].

In 2013, Klimczak et al. reported a breakthrough in the design of optical fiber transverse structure. They produced a novel, regular hexagonal-lattice tellurite photonic crystal fiber (PCF) as shown in **Figure 18** [79]. Pumping the 2-cm-long

#### **Figure 15.**

*Optical photographs of the cross-sectional views of (a) the extruded tellurite preform and (b) the resulting tellurite holey fiber with 410 μm outer diameter [74].*

performance of tellurite fiber-based MIR laser sources. BaF2 was included for the purpose of reducing the OHd content, and the introduction of Y2O3 was for better thermal stability in fiber drawing process as well as providing a higher glass transition temperature raised by the high melting temperature of Y2O3. In 2016, Wang et al. achieved SC generation extending from 0.47 μm to 2.77 μm (zero-dispersion

Tellurite microstructured fibers with dispersion modification and nonlinear coefficient enhancement have been widely studied and applied for SC generation, and significant progress has been achieved over the last 10 years. However, the air holes present in microstructured fibers readily accommodate moisture and dust particles from the atmosphere, which lead to incremental losses, which act to deteriorate the SC output. In addition, the performance of tellurite microstructured fiber for high-power output in the mid-IR SC is not satisfactory, because the thermal conductivity of the air holes and the core of microstructure fiber greatly differ, and hence means that heat dissipation is a significant problem of high-power light transmission in the fiber. Solid-state tellurite fibers (comprising a solid core and cladding with no air holes) have therefore become the nonlinear medium of choice

Hydroxyl ions have deleterious broad absorption peaks centered at 3.3 μm and 4.3 μm, which hinder the tellurite fiber from extending its spectrum to the multiphonon edge (5 μm). In 2013, Thapa et al. of NP Photonics Incorporation developed ultra-low-loss solid-core tellurite fibers which eliminate almost all molecular species, especially hydroxyl ions [83]. Using a 1922-nm all-fiber-based mode-locked fiber laser oscillator, a 1–5 μm SC spectrum shown in **Figure 19b** was generated in a tellurite fiber with a W-type (**Figure 19a**) index profile for strong light confinement, and the ZDW shifted from 2.5 μm to 1.9 μm. It was argued that the broadened anti-Stokes wavelength portion originated from self-phase modulation (SPM) and the long wavelength portion with increased power originated from the generation of a Raman soliton because of the self-frequency Raman shift. In the same year, Savelii et al. reported SC generation extending from 840 nm to 3000 nm in a low-loss suspended-core tellurite fiber with different lengths (**Figure 20**), pumped at its anomalous dispersion regime at 1745 nm [54]. It was found that the introduction of fluoride ions into the tellurite glass reduced the OHd content and

The W-type index profile makes it possible to tailor the ZDW, and this fiber can

*(a) Cross section of the W-type tellurite fiber. (b) SC spectra in W-type proprietary tellurite fiber pumped by 3 W of 20 ps pulses from a 32 MHz repetition rate amplified mode-locked laser at 1.92 μm. (Note: dotted line*

*is the transmission measured in the corresponding 1-cm-thick tellurite glass sample.) [83].*

be fusion spliced to robust step-index silica fiber with relative ease. In 2016

wavelength at 1730 nm) using a tapered TeO2-BaF2-Y2O3 (TBY)-based microstructured fiber whose core was surrounded by six air holes [81].

for high-power mid-infrared SC light sources [82].

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

resulted in a fiber that was still transparent at 4.1 μm.

**Figure 19.**

**275**

**Figure 16.**

*Scanning electron microscope (SEM) images of the fibers [75].*

*SC spectrum generated from the tellurite fiber when the peak power of the pump laser is fixed at 3.9 kW [76].*

**Figure 18.**

*Microstructure of tellurite PCF: close-up picture of the structure with propagating mode as seen in a CCD camera, and SEM images of photonic structure [79].*

PCF with 150 fs/36 nJ/1580 nm pulses, they achieved an output of 800–2500 nm SC spectrum that is comparable to that generated in suspended-core tellurite PCF pumped at wavelengths over 1800 nm.

Yao et al. proposed a novel fluorotellurite fiber with the composition 65TeO2- 25BaF2-10Y2O3 (TBY) [80]; the authors claimed further improvement in the

#### *Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

performance of tellurite fiber-based MIR laser sources. BaF2 was included for the purpose of reducing the OHd content, and the introduction of Y2O3 was for better thermal stability in fiber drawing process as well as providing a higher glass transition temperature raised by the high melting temperature of Y2O3. In 2016, Wang et al. achieved SC generation extending from 0.47 μm to 2.77 μm (zero-dispersion wavelength at 1730 nm) using a tapered TeO2-BaF2-Y2O3 (TBY)-based microstructured fiber whose core was surrounded by six air holes [81].

Tellurite microstructured fibers with dispersion modification and nonlinear coefficient enhancement have been widely studied and applied for SC generation, and significant progress has been achieved over the last 10 years. However, the air holes present in microstructured fibers readily accommodate moisture and dust particles from the atmosphere, which lead to incremental losses, which act to deteriorate the SC output. In addition, the performance of tellurite microstructured fiber for high-power output in the mid-IR SC is not satisfactory, because the thermal conductivity of the air holes and the core of microstructure fiber greatly differ, and hence means that heat dissipation is a significant problem of high-power light transmission in the fiber. Solid-state tellurite fibers (comprising a solid core and cladding with no air holes) have therefore become the nonlinear medium of choice for high-power mid-infrared SC light sources [82].

Hydroxyl ions have deleterious broad absorption peaks centered at 3.3 μm and 4.3 μm, which hinder the tellurite fiber from extending its spectrum to the multiphonon edge (5 μm). In 2013, Thapa et al. of NP Photonics Incorporation developed ultra-low-loss solid-core tellurite fibers which eliminate almost all molecular species, especially hydroxyl ions [83]. Using a 1922-nm all-fiber-based mode-locked fiber laser oscillator, a 1–5 μm SC spectrum shown in **Figure 19b** was generated in a tellurite fiber with a W-type (**Figure 19a**) index profile for strong light confinement, and the ZDW shifted from 2.5 μm to 1.9 μm. It was argued that the broadened anti-Stokes wavelength portion originated from self-phase modulation (SPM) and the long wavelength portion with increased power originated from the generation of a Raman soliton because of the self-frequency Raman shift. In the same year, Savelii et al. reported SC generation extending from 840 nm to 3000 nm in a low-loss suspended-core tellurite fiber with different lengths (**Figure 20**), pumped at its anomalous dispersion regime at 1745 nm [54]. It was found that the introduction of fluoride ions into the tellurite glass reduced the OHd content and resulted in a fiber that was still transparent at 4.1 μm.

The W-type index profile makes it possible to tailor the ZDW, and this fiber can be fusion spliced to robust step-index silica fiber with relative ease. In 2016

#### **Figure 19.**

*(a) Cross section of the W-type tellurite fiber. (b) SC spectra in W-type proprietary tellurite fiber pumped by 3 W of 20 ps pulses from a 32 MHz repetition rate amplified mode-locked laser at 1.92 μm. (Note: dotted line is the transmission measured in the corresponding 1-cm-thick tellurite glass sample.) [83].*

PCF with 150 fs/36 nJ/1580 nm pulses, they achieved an output of 800–2500 nm SC spectrum that is comparable to that generated in suspended-core tellurite PCF

*Microstructure of tellurite PCF: close-up picture of the structure with propagating mode as seen in a CCD*

*SC spectrum generated from the tellurite fiber when the peak power of the pump laser is fixed at 3.9 kW [76].*

Yao et al. proposed a novel fluorotellurite fiber with the composition 65TeO2-

25BaF2-10Y2O3 (TBY) [80]; the authors claimed further improvement in the

pumped at wavelengths over 1800 nm.

*camera, and SEM images of photonic structure [79].*

**Figure 16.**

*Advanced Functional Materials*

**Figure 17.**

**Figure 18.**

**274**

*Scanning electron microscope (SEM) images of the fibers [75].*

**Figure 20.**

*(a) SEM picture of the cross section of the fiber. (b) SC spectra generated from the suspended-core tellurite fiber with different lengths [54].*

Kedenburg et al. studied SC generation spanning 2.6–4.6 μm in low-loss W-type index tellurite fiber with a length of 15 cm [84]. Additionally, they studied the variation of spectral bandwidth with core diameter, pump wavelength, length of fibers, and pump power. In 2017, Kedenburg et al. studied the effects of the core size, pump wavelength, and fiber length on SC generation in a robust step-index tellurite fiber, and they achieved broadband SC generation spanning 1.3–5.3 μm in the fiber with a length of 9 cm and a core diameter of 3.5 μm, when pumped using a 2.4 μm femtosecond pulsed laser [85].

use as high-power mid-IR SC light sources [87]. The same authors used a tapered allsolid fluorotellurite fiber with ultra-high NA to generate an SC output spectrum covering the entire 0.4–5 μm transmission window and pumped using a 1560-nm mode-locked fiber laser [88]. Yao et al. achieved stable 10.4 W SC generation in the wavelength ranging from 947 to 3934 nm from a TBY-based all-solid fluorotellurite fiber when pumped using a high-power 1980 nm femtosecond fiber laser [52]; when the average pump power was increased to 1.1 W, large spectral broadening occurred as shown in **Figure 22(a)**. Because the fiber was pumped at anomalous dispersion regime, the spectral broadening for a pump power of ≥1.1 W originated from the SPM, the formation of higher-order soliton, soliton fission, soliton self-frequency shift (SSFS), and the generation of blue-shifted dispersive waves. The average output

*(a) Dependence of the measured SC spectra generated from 60-cm-long fluorotellurite fiber on the average power of the 1980 nm femtosecond fiber laser. (b) The dependence of the SC average power on the pump power. Inset: photograph of the power meter when the mid-IR SC laser source is operating at the output power of*

power of the SC laser source increases linearly with the average pump power (**Figure 22(b)**), and the corresponding optical-to-optical conversion efficiency was measured to be as high as 65%. The successful achievement of a 10 W output power

level represented a significant breakthrough in all-solid fluorotellurite fiber, demonstrating its bright future for high-power MIR SC light sources.

Over the past few decades, research interest in microsphere resonators has grown rapidly. For a microsphere resonator, the pump light can be coupled into the microsphere through a tapered optical fiber or via free space. Most current microsphere resonators are fabricated from the silica optical fiber, but it is also possible to fabricate microsphere resonators from compound glass materials (such as tellurite glass) other than silica. At present, the principal method used for making microsphere cavities is based on melting of the glass materials, which uses the surface tension of molten glass to form the microsphere when suspended at the tip of a fiber. There are two common methods for the preparation of tellurite glass microsphere cavities, one is to melt glass fiber and the other is a powder floating method.

Most glass microsphere cavities are prepared using a CO2 laser, arc discharge, or

high temperature ceramics to melt glass fibers. These methods have also been

**3. Tellurite glass-based microcavity lasers**

*3.1.1 Melting glass fiber method*

**277**

**Figure 22.**

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

*10.4 W [52].*

**3.1 Experimental preparation of tellurite glass microcavities**

In 2016, Shi et al. prepared a solid-state tellurite optical fiber with a numerical aperture (NA) of 0.21 and a core diameter of 12 μm [86]. **Figure 21(a)** shows a micrograph of the end face of the fiber. They studied the SC generation in the fiber which was 0.8 m-in length. **Figure 21(b)** shows the spectrum of the pump laser and SC output in the fiber when different pump powers were used. When the pump power was 9.8 W, the power spectral density of the SC spectrum in the wavelength range of 1975–3000 nm is above 5 dBm/nm. In this investigation, the maximum output power of the SC light source was 5.1 W, and the power of the spectrum at wavelengths longer than 2.5 μm was about 2.1 W.

In 2017 Jia et al. obtained a stable 4.5 W SC output spanning 1017–3438 nm, using a TBY-based 60-cm-long all-solid fluorotellurite fiber fabricated using the rod-intube method. The fiber was pumped using a 2 μm femtosecond fiber 10.48 W output power laser and thus demonstrated the capability of all-solid fluorotellurite fibers for

#### **Figure 21.**

*(a) Photograph of the tellurite fiber. (b) Spectrum of thulium-doped fiber amplifier (TDFA) and the SC spectrum generated from the tellurite fiber, pumped by various power: 5.2 W, 7.1 W, and 9.8 W [86].*

**Figure 22.**

Kedenburg et al. studied SC generation spanning 2.6–4.6 μm in low-loss W-type index tellurite fiber with a length of 15 cm [84]. Additionally, they studied the variation of spectral bandwidth with core diameter, pump wavelength, length of fibers, and pump power. In 2017, Kedenburg et al. studied the effects of the core size, pump wavelength, and fiber length on SC generation in a robust step-index tellurite fiber, and they achieved broadband SC generation spanning 1.3–5.3 μm in the fiber with a length of 9 cm and a core diameter of 3.5 μm, when pumped using a

*(a) SEM picture of the cross section of the fiber. (b) SC spectra generated from the suspended-core tellurite fiber*

In 2016, Shi et al. prepared a solid-state tellurite optical fiber with a numerical aperture (NA) of 0.21 and a core diameter of 12 μm [86]. **Figure 21(a)** shows a micrograph of the end face of the fiber. They studied the SC generation in the fiber which was 0.8 m-in length. **Figure 21(b)** shows the spectrum of the pump laser and SC output in the fiber when different pump powers were used. When the pump power was 9.8 W, the power spectral density of the SC spectrum in the wavelength range of 1975–3000 nm is above 5 dBm/nm. In this investigation, the maximum output power of the SC light source was 5.1 W, and the power of the spectrum at

In 2017 Jia et al. obtained a stable 4.5 W SC output spanning 1017–3438 nm, using a TBY-based 60-cm-long all-solid fluorotellurite fiber fabricated using the rod-intube method. The fiber was pumped using a 2 μm femtosecond fiber 10.48 W output power laser and thus demonstrated the capability of all-solid fluorotellurite fibers for

*(a) Photograph of the tellurite fiber. (b) Spectrum of thulium-doped fiber amplifier (TDFA) and the SC spectrum generated from the tellurite fiber, pumped by various power: 5.2 W, 7.1 W, and 9.8 W [86].*

2.4 μm femtosecond pulsed laser [85].

**Figure 20.**

**Figure 21.**

**276**

*with different lengths [54].*

*Advanced Functional Materials*

wavelengths longer than 2.5 μm was about 2.1 W.

*(a) Dependence of the measured SC spectra generated from 60-cm-long fluorotellurite fiber on the average power of the 1980 nm femtosecond fiber laser. (b) The dependence of the SC average power on the pump power. Inset: photograph of the power meter when the mid-IR SC laser source is operating at the output power of 10.4 W [52].*

use as high-power mid-IR SC light sources [87]. The same authors used a tapered allsolid fluorotellurite fiber with ultra-high NA to generate an SC output spectrum covering the entire 0.4–5 μm transmission window and pumped using a 1560-nm mode-locked fiber laser [88]. Yao et al. achieved stable 10.4 W SC generation in the wavelength ranging from 947 to 3934 nm from a TBY-based all-solid fluorotellurite fiber when pumped using a high-power 1980 nm femtosecond fiber laser [52]; when the average pump power was increased to 1.1 W, large spectral broadening occurred as shown in **Figure 22(a)**. Because the fiber was pumped at anomalous dispersion regime, the spectral broadening for a pump power of ≥1.1 W originated from the SPM, the formation of higher-order soliton, soliton fission, soliton self-frequency shift (SSFS), and the generation of blue-shifted dispersive waves. The average output power of the SC laser source increases linearly with the average pump power (**Figure 22(b)**), and the corresponding optical-to-optical conversion efficiency was measured to be as high as 65%. The successful achievement of a 10 W output power level represented a significant breakthrough in all-solid fluorotellurite fiber, demonstrating its bright future for high-power MIR SC light sources.
