4. Measurement of temperature profiles in doped fibers and fiber gratings

When a silica fiber is heated up, two effects occur. First, the expansion of the fiber leads to a change of the diameter. Second, the thermo-optic effect induces a change in the refractive index of the material due to a variation of temperature. This variation modifies the spectral position of the WGM. From Eq. (1) it is possible to evaluate the shift of the resonant wavelength, ΔλR, of a WGM due to a variation of temperature, ΔT:

$$\frac{\Delta\lambda\_R}{\lambda\_R} = \left(\frac{1}{\mathbf{a}}\frac{d\mathbf{a}}{dT} + \frac{1}{n\_{\rm eff}}\frac{dn\_{\rm eff}}{dT}\right) \cdot \Delta T \tag{5}$$

In the case of optical fibers as MRs, it is a good approximation to assume that the thermo-optic coefficient (i.e., the second term in Eq. (5)) can be replaced by that of the pure silica, since the optical field of the WGMs is mainly localized in the fiber cladding (see Figure 3). The high sensitivity of WGMs to variations of temperature has been demonstrated for different geometries of the MR, such as microspheres [19, 20] or cylinders [21]. Moreover, the propagation of an optical signal of moderate power (� 1 W or higher) in a fiber generally induces a variation of temperature of the material. Due to the variation of temperature, the optical response of the fibers, or fiber components, may change when they are in operation. Thus, a detailed characterization of this effect is of interest to design properly the fiberbased optical systems. The use of WGMs allows achieving a very low detection limit: Rivera et al. claimed a detection limit of two thousandths of degree [21].

Here, we will present the characterization of temperature variations in two different examples: (i) rare-earth doped active fibers and (ii) fiber gratings inscribed in commercial photosensitive fibers.

### 4.1 Measurement of temperature in rare-earth doped fibers

Heating of rare-earth doped fibers can be an issue in fiber-based lasers and amplifiers. For example, thermal effects can be a limit to the maximum output

### Whispering Gallery Modes for Accurate Characterization of Optical Fibers' Parameters DOI: http://dx.doi.org/10.5772/intechopen.81259

power that these systems can provide [22]. Another example is the shift in wavelength observed in distributed Bragg reflectors (DBR) and distributed feedback (DFB) lasers due to a pump-induced increment of temperature [23]. The heat is due to the non-radiative processes related to the electronic relaxation of some dopants: for example, this effect is less important in ytterbium-doped fibers, while Er/Ybcodoped and erbium-doped fibers exhibited a high increase of temperature with pump, due to its specific electronic-level system [24]. Thus, it is an intrinsic characteristic of the doped fibers that one needs to evaluate in order to design the proper optical system.

In the experiments presented here, several commercially available single-mode, core-pumped doped fibers from Fibercore were investigated. Specifically, the FUTs were three Er-doped fibers (DF-1500-F-980, M12-980/125, and I25-980/125), a Ybdoped fiber (DF-1100), and an Er/Yb-codoped fiber (DF-1500 Y). The values for absorption coefficients at the pump wavelength were 5.5 dB/m (DF-1500-F-980), 12 dB/m (M12-980/125), 21.9 dB/m (I25-980/125), 1000 dB/m (DF-1100), and 1700 dB/m (DF-1100). Short sections of 2 cm in length of each FUT were used as the MR where the WGMs were excited. The FUTs were pumped with a singlemode, fiber-pigtailed laser diode that emitted a maximum power of 380 mW at 976 nm. As the pump launched to the FUT was increased, the WGM resonance shifted toward longer wavelengths in all cases, as it was expected, since the thermooptic and the thermal expansion coefficients of silica are both positives. As an example, Figure 5 shows the shift in wavelength of a resonance as a function of the pump launched to the fiber DF-1500-F-980. In our experiments, we did not investigate in detail the temporal response of the phenomenon, which will be ruled by the mechanisms that convert the pump power to heat, the heat conduction in silica, and the transfer of heat to the air. Typically, it will be on the range of a few tens of microseconds [25].

At this point, several features of this technique must be clarified. First, it is worth to point out that the shift in wavelength is virtually independent of the particular resonance used for the measurements, that is, it does not depend on its radial and azimuthal order nor on its polarization. The sensitivity to thermal variations of different WGM resonances was theoretically calculated around 1.53 μm, taking into account both the thermal expansion of the fiber and the thermo-optic effect. The results showed that the difference in sensitivity between different resonances differs in less than 1/10000 per each ºC of temperature increase. This simplifies the utility of this technique.

The second aspect to highlight is related to the fact that the dopants in the active fibers are located in their core, while WGMs are highly confined in the outer region of the cladding (see Figure 3). From the study of heat conduction in doped fibers

Figure 5.

Wavelength shift of the resonant wavelength as the pump power is increased. From left to right: pump power 0 mW, 40 mW, 110 mW, 180 mW, 270 mW, and 370 mW.

telecom fiber [17]. The microtaper and the MR are placed perpendicularly (see in the inset Figure 4a). Since the optical field of the WGMs is not axially localized (its extension is around 200 μm in length [7]), this setup allows exciting the WGM at different positions along the MR: by sweeping the microtaper along the MR, it is possible to detect variations of the parameters of the MR in the axial direction by measuring the shift of the resonances—radius [13, 18], temperature, or strain.

The transmission of the taper was measured using a photodetector, and the signal was registered by an oscilloscope synchronized with the TDL. A typical transmission trace consists on a signal that will present a series of notches at the resonant wavelengths. For MRs of 125 μm in diameter, the free spectral range between two consecutive azimuthal orders m is � 4 nm at 1550 nm, and it is the same for both polarizations. Figure 4b shows the reflection spectrum of a resonance in an optical fiber (a ¼ 62:5 μm): its linewidth is 36 fm, which corresponds to a

As it was mentioned before, the position of the resonances will depend on the value of the refractive index of the material. In the next sections, we will study the characterization of different fibers and fiber components by means of the measurement of the shift of WGM resonances as the effective index of the MR is modified.

When a silica fiber is heated up, two effects occur. First, the expansion of the fiber leads to a change of the diameter. Second, the thermo-optic effect induces a change in the refractive index of the material due to a variation of temperature. This variation modifies the spectral position of the WGM. From Eq. (1) it is possible to evaluate the shift of the resonant wavelength, ΔλR, of a WGM due to a variation of

> 1 neff

In the case of optical fibers as MRs, it is a good approximation to assume that the thermo-optic coefficient (i.e., the second term in Eq. (5)) can be replaced by that of the pure silica, since the optical field of the WGMs is mainly localized in the fiber cladding (see Figure 3). The high sensitivity of WGMs to variations of temperature has been demonstrated for different geometries of the MR, such as microspheres [19, 20] or cylinders [21]. Moreover, the propagation of an optical signal of moderate power (� 1 W or higher) in a fiber generally induces a variation of temperature of the material. Due to the variation of temperature, the optical response of the fibers, or fiber components, may change when they are in operation. Thus, a detailed characterization of this effect is of interest to design properly the fiberbased optical systems. The use of WGMs allows achieving a very low detection limit: Rivera et al. claimed a detection limit of two thousandths of degree [21]. Here, we will present the characterization of temperature variations in two different examples: (i) rare-earth doped active fibers and (ii) fiber gratings

dneff dT

� ΔT (5)

4. Measurement of temperature profiles in doped fibers and fiber

Variations can be characterized along several centimeters of the MR.

.

Δλ<sup>R</sup> λR

inscribed in commercial photosensitive fibers.

4.1 Measurement of temperature in rare-earth doped fibers

Heating of rare-earth doped fibers can be an issue in fiber-based lasers and amplifiers. For example, thermal effects can be a limit to the maximum output

<sup>¼</sup> <sup>1</sup> a da dT þ

<sup>Q</sup>-loaded factor of 4 � <sup>10</sup><sup>7</sup>

Applications of Optical Fibers for Sensing

gratings

temperature, ΔT:

142

carried out by Davis et al. [25], it is possible to calculate that, at the steady state, the increase of temperature at the core of the fiber is just 1.5% larger than at the outer surface.

reflection band but in the vicinities of the Bragg wavelength. This illumination signal was provided by an amplified tunable laser (range, 1520–1560 nm) that

Whispering Gallery Modes for Accurate Characterization of Optical Fibers' Parameters

As a preliminary experiment, a section of fiber Fibercore PS980 was uniformly irradiated (i.e., there was no grating inscribed). The length was 5 mm, and the UV

resonances was measured as the MR was illuminated with a 1550 nm optical signal, compared to the original position of the resonances, with no illumination along the FUT. Figure 7a shows the results. The data show a clear difference between the irradiated length (z < 3 mm) and the non-irradiated length (z>3 mm). A temperature gradient in an intermediate region due to the heat conduction in silica and the transfer of heat to the air can be observed. It should be noted that this section is far larger than the length of the focused UV beam ( 700 μm); thus, the beam size is not the cause of this transition length. In the irradiated section, the temperature increases at a rate higher than 10 ºC=W, for this sample, while the pristine fiber heats up at a rate lower than 1 ºC=W. The increment of temperature was linear with power in the available power range. This experiment avails that this technique allows characterizing the variations of temperature along the components with a resolution of tenths of a millimeter. This feature is useful when one needs to detect, evaluate, and correct smooth undesired non-homogeneities that may occur during the fabrication of FBG and LPG, which are usually short components. As an example, Figure 7b shows the measurement of the temperature profile of a section of an irradiated fiber (length, 5 mm) that suffered from some misalignment during the UV irradiation process. For this sample, a variation of 4 ºC is measured in such a

The temperature profile along a FBG with strong reflectivity was measured using this technique. The FBG had a reflectivity higher than 99.9%; the Bragg wavelength was 1556 nm, its length was 12 mm, and it was fabricated in PS1250 fiber (Fibercore). First, the illumination signal was tuned well outside the reflection band, at 1540 nm; in this case, there is no reflection of the optical signal; it just propagates through the FBG. The power launched to the MR was 800 mW. Curve (i) in Figure 8 shows the obtained results. As expected, a similar result to the case shown in Figure 7a was obtained: the heating over the length of the FBG was fairly constant, 5:5 ºC. It should be noted that the axial resolution of the technique will be larger than the grating period. Then, the average increment of temperature should be similar to that introduced in the case of the uniformly irradiated fiber, for

Temperature profile of irradiated FUTs, (a) uniformly and (b) nonuniformly.

. The wavelength shift of the

provided up to 1 W of CW light.

DOI: http://dx.doi.org/10.5772/intechopen.81259

short irradiated length.

Figure 7.

145

fluence power used in the irradiation was 150 J/mm<sup>2</sup>

In order to calibrate the shift in wavelength of the WGM resonances with the heating, a FBG inscribed in the core of a doped fiber was used for comparison. The procedure is described in [21]. The WGM resonances shift at a rate of 8.2 pm/ºC. With this calibration, it is possible to correlate the shifts in wavelength with the increase of temperature in the core of the fiber. For the example shown in Figure 5, the maximum increment of temperature achieved for a pump of 370 mW was 3.7ºC.

Figure 6 summarizes the measurements performed for the different doped fibers. A similar trend can be observed in all the cases; the resonances shift fast in wavelength for low pump powers, and, beyond certain pump, heating tends to saturate. It can be observed that the Yb fiber DF 1100 shows a similar increase of temperature to those of the Er-doped fibers, although the concentration of the dopants in the Yb fiber is much larger (note the absorption coefficient around 975 nm). Also, the highest temperature increment corresponds to the Er/Yb-doped fiber (DF 1500 Y), despite that it shows a lower absorption coefficient than its equivalent Yb-doped fiber (DF 1100). These results are in accordance to the fact that the heating is related to the existence of non-radiative transitions for the relaxation of electrons in the active medium.

### 4.2 Measurement of temperature profiles in fiber components

As it was mentioned before, WGMs are axially localized: their extension along the fiber is 200 μm, typically, for a MR of 62:5 μm. Thus, this technique provides spatial resolution. The taper can be swept along the MR in order to characterize the parameters of the FUT point to point. This feature was used in order to characterize the temperature profile along fiber components [26].

The FBGs used in the experiments were written in germanium-silicate boron codoped, photosensitive fibers from Fibercore, using a doubled-argon UV laser and a uniform phase mask. The length of all the gratings was 10 mm. The WGMs were excited at different positions along the FBG, and, simultaneously, it was illuminated by optical signals of moderate powers, within or outside of the

Figure 6. Heating of the doped fibers as a function of pump power.

Whispering Gallery Modes for Accurate Characterization of Optical Fibers' Parameters DOI: http://dx.doi.org/10.5772/intechopen.81259

reflection band but in the vicinities of the Bragg wavelength. This illumination signal was provided by an amplified tunable laser (range, 1520–1560 nm) that provided up to 1 W of CW light.

As a preliminary experiment, a section of fiber Fibercore PS980 was uniformly irradiated (i.e., there was no grating inscribed). The length was 5 mm, and the UV fluence power used in the irradiation was 150 J/mm<sup>2</sup> . The wavelength shift of the resonances was measured as the MR was illuminated with a 1550 nm optical signal, compared to the original position of the resonances, with no illumination along the FUT. Figure 7a shows the results. The data show a clear difference between the irradiated length (z < 3 mm) and the non-irradiated length (z>3 mm). A temperature gradient in an intermediate region due to the heat conduction in silica and the transfer of heat to the air can be observed. It should be noted that this section is far larger than the length of the focused UV beam ( 700 μm); thus, the beam size is not the cause of this transition length. In the irradiated section, the temperature increases at a rate higher than 10 ºC=W, for this sample, while the pristine fiber heats up at a rate lower than 1 ºC=W. The increment of temperature was linear with power in the available power range. This experiment avails that this technique allows characterizing the variations of temperature along the components with a resolution of tenths of a millimeter. This feature is useful when one needs to detect, evaluate, and correct smooth undesired non-homogeneities that may occur during the fabrication of FBG and LPG, which are usually short components. As an example, Figure 7b shows the measurement of the temperature profile of a section of an irradiated fiber (length, 5 mm) that suffered from some misalignment during the UV irradiation process. For this sample, a variation of 4 ºC is measured in such a short irradiated length.

The temperature profile along a FBG with strong reflectivity was measured using this technique. The FBG had a reflectivity higher than 99.9%; the Bragg wavelength was 1556 nm, its length was 12 mm, and it was fabricated in PS1250 fiber (Fibercore). First, the illumination signal was tuned well outside the reflection band, at 1540 nm; in this case, there is no reflection of the optical signal; it just propagates through the FBG. The power launched to the MR was 800 mW. Curve (i) in Figure 8 shows the obtained results. As expected, a similar result to the case shown in Figure 7a was obtained: the heating over the length of the FBG was fairly constant, 5:5 ºC. It should be noted that the axial resolution of the technique will be larger than the grating period. Then, the average increment of temperature should be similar to that introduced in the case of the uniformly irradiated fiber, for

Figure 7. Temperature profile of irradiated FUTs, (a) uniformly and (b) nonuniformly.

carried out by Davis et al. [25], it is possible to calculate that, at the steady state, the increase of temperature at the core of the fiber is just 1.5% larger than at the outer

In order to calibrate the shift in wavelength of the WGM resonances with the heating, a FBG inscribed in the core of a doped fiber was used for comparison. The procedure is described in [21]. The WGM resonances shift at a rate of

8.2 pm/ºC. With this calibration, it is possible to correlate the shifts in wavelength with the increase of temperature in the core of the fiber. For the example shown in Figure 5, the maximum increment of temperature achieved for a pump of 370 mW

Figure 6 summarizes the measurements performed for the different doped fibers. A similar trend can be observed in all the cases; the resonances shift fast in wavelength for low pump powers, and, beyond certain pump, heating tends to saturate. It can be observed that the Yb fiber DF 1100 shows a similar increase of temperature to those of the Er-doped fibers, although the concentration of the dopants in the Yb fiber is much larger (note the absorption coefficient around 975 nm). Also, the highest temperature increment corresponds to the Er/Yb-doped fiber (DF 1500 Y), despite that it shows a lower absorption coefficient than its equivalent Yb-doped fiber (DF 1100). These results are in accordance to the fact that the heating is related to the existence of non-radiative transitions for the

As it was mentioned before, WGMs are axially localized: their extension along the fiber is 200 μm, typically, for a MR of 62:5 μm. Thus, this technique provides spatial resolution. The taper can be swept along the MR in order to characterize the parameters of the FUT point to point. This feature was used in order to characterize

The FBGs used in the experiments were written in germanium-silicate boron codoped, photosensitive fibers from Fibercore, using a doubled-argon UV laser and a uniform phase mask. The length of all the gratings was 10 mm. The WGMs were excited at different positions along the FBG, and, simultaneously, it was illuminated by optical signals of moderate powers, within or outside of the

surface.

Applications of Optical Fibers for Sensing

was 3.7ºC.

Figure 6.

144

Heating of the doped fibers as a function of pump power.

relaxation of electrons in the active medium.

the temperature profile along fiber components [26].

4.2 Measurement of temperature profiles in fiber components

fiber is UV irradiated, its loss, α, increases due to two causes: absorption that will be

Whispering Gallery Modes for Accurate Characterization of Optical Fibers' Parameters

The increase of α introduced by UV irradiation has been measured before [28], since this is a parameter of interest to optimize the fabrication of FBGs, especially in the case of long or superimposed gratings with many reflection bands [29, 30]. This measurement provides information about an averaged value of the attenuation loss along the irradiated section, which includes both the absorption and the scattering contributions. The technique based on the measurement of the shift of WGM resonances will only measure the absorption coefficient; thus, by combining the two types of measurements, it is possible to evaluate both contributions separately. Different types of photosensitive fibers were studied [11]: (i) Fibercore PS980, (ii) Fibercore PS1250, (iii) Fibercore SM1500, and (iv) Corning SMF28; this fiber was hydrogenated for 15 days (pressure: 30 bar) to increase its photosensitivity. The setup used in the experiments was the same than in the previous experiments shown in this chapter. In this case, the FUTs were short sections of the different fibers, which were

Figure 7a were obtained for all of them, but with different temperature increments,

The different increases of temperature between the irradiated fiber and the pristine fiber will provide us information to quantify the variation in the αabs due to UV irradiation. It will be assumed that the heating over the transversal section of the fiber, at a given axial position, is set by the absorption coefficient, αabs. According to the analysis reported by Davis et al. [25], the heating at the steady

where <sup>h</sup> is the heat transfer coefficient (81.4 <sup>W</sup> � <sup>m</sup>�<sup>2</sup> � <sup>K</sup>�<sup>1</sup> for a silica fiber). Then, the ratio of ΔT between two different points along the FUT, 1 and 2, is

> <sup>¼</sup> <sup>α</sup>abs 2 αabs 1

Thus, with this analysis and the experimental data obtained from the measure-

Direct measurements of transmission loss variation as the fibers were irradiated were carried out for a PS980 fiber. First, the value of the loss of the pristine fiber was measured at 1550 nm by means of the cutback method: the obtained value was 120:0 � 0:5 dB/km. Then, the UV laser was swept back and forth along a 5-cm-long section of the fiber, repeatedly. The full description of the procedure is described in [11]. Figure 9a shows the data obtained in this experiment. The final loss was 6:2 � 0:4 dB/m; thus the ratio between the loss coefficients, α2=α1, increased 52 � 3 times. Please remember that this loss coefficient includes both absorption and

The contribution to the loss by means of the absorption mechanism was measured using the WGM technique (see Figure 9b). In this case, a 1550 nm laser (maximum power, 1 W) was launched to the FUT, and the thermal shift of the

ΔT<sup>2</sup> ΔT<sup>1</sup>

ment of the wavelength shift of WGM resonances in irradiated points (1) and pristine points (2) of the FUT, this ratio between the respective αabs can be

ΔT <sup>P</sup> <sup>¼</sup> <sup>1</sup> 2πah

since the photosensitivity was also different for each of them.

. Similar temperature profiles to that shown in

αabs (6)

(7)

quantified by αabs and scattering, αscat.

DOI: http://dx.doi.org/10.5772/intechopen.81259

exposed to a UV fluence of 150 J/mm2

scattering contributions (<sup>α</sup> <sup>¼</sup> <sup>α</sup>abs <sup>þ</sup> <sup>α</sup>scat).

state, ΔT, will be given by

given by

calculated.

147

Figure 8. Temperature profile of a FBG illuminated (i) outside and (ii) within the reflexion band.

the same UV fluence and fiber characteristics. Two transition zones were clearly observed at both ends of the grating.

Finally, the temperature profile was measured when the optical signal was tuned to the Bragg wavelength (power, 1 W) (see curve (ii) in Figure 8). In this case, one should take into account that the UV irradiation is constant over its length, and the gradient temperature is due to the fact that the optical signal is reflected as it penetrates into the grating. A sharp increment of temperature at the beginning of the grating, at the extreme that is illuminated, can be observed. The maximum is located at the vicinities of the point where the FBG begins. The decay of temperature extends over a length of � 5 mm, which is shorter than the length of the fiber itself (12 mm). This is consistent with the high reflectivity of this FBG. Moreover it should be noted that, at the beginning of the curve, that is, z ¼ 0 � 3 mm, the temperature increase is � 2 ºC, that is, roughly twice the value obtained for a pristine fiber. On the contrary, in the section after the grating (and even at the last millimeters of the FBG), the increment of temperature is below the detection limit of the technique. The origin of this asymmetry is the reflection of the optical signal: the amount of light that reaches the last millimeters of the FBG is very small. This technique, then, provides information about the effective length of gratings of different reflectivity, information that could be relevant for the design of optical systems that require of short cavities, or cavities that require of a very precise length, as in the case of mode-locked fiber lasers.

## 5. Measurement of absorption coefficients in photosensitive fibers

In the previous section, the gradient of temperature induced in fiber-optic components by means of an illumination signal has been characterized and discussed. It has been shown that there is a difference in temperature between the sections that have been irradiated with UV light compared to the pristine fibers. It is well known that the UV irradiation induces a change in the index of photosensitive fibers, which is employed to fabricate FBGs and LPGs. According to Kramers-Kronig relations, the change in the refractive index is associated with a variation of the absorption coefficient. In addition, the exposure of the fiber to the levels of UV light usually employed in the grating fabrication induces mechanical deformations in the fiber [27]. This leads to an increase of the loss due to scattering. Thus, when a

### Whispering Gallery Modes for Accurate Characterization of Optical Fibers' Parameters DOI: http://dx.doi.org/10.5772/intechopen.81259

fiber is UV irradiated, its loss, α, increases due to two causes: absorption that will be quantified by αabs and scattering, αscat.

The increase of α introduced by UV irradiation has been measured before [28], since this is a parameter of interest to optimize the fabrication of FBGs, especially in the case of long or superimposed gratings with many reflection bands [29, 30]. This measurement provides information about an averaged value of the attenuation loss along the irradiated section, which includes both the absorption and the scattering contributions. The technique based on the measurement of the shift of WGM resonances will only measure the absorption coefficient; thus, by combining the two types of measurements, it is possible to evaluate both contributions separately.

Different types of photosensitive fibers were studied [11]: (i) Fibercore PS980, (ii) Fibercore PS1250, (iii) Fibercore SM1500, and (iv) Corning SMF28; this fiber was hydrogenated for 15 days (pressure: 30 bar) to increase its photosensitivity. The setup used in the experiments was the same than in the previous experiments shown in this chapter. In this case, the FUTs were short sections of the different fibers, which were exposed to a UV fluence of 150 J/mm2 . Similar temperature profiles to that shown in Figure 7a were obtained for all of them, but with different temperature increments, since the photosensitivity was also different for each of them.

The different increases of temperature between the irradiated fiber and the pristine fiber will provide us information to quantify the variation in the αabs due to UV irradiation. It will be assumed that the heating over the transversal section of the fiber, at a given axial position, is set by the absorption coefficient, αabs. According to the analysis reported by Davis et al. [25], the heating at the steady state, ΔT, will be given by

$$\frac{\Delta T}{P} = \frac{1}{2\pi\omega h} a^{\text{abs}}\tag{6}$$

where <sup>h</sup> is the heat transfer coefficient (81.4 <sup>W</sup> � <sup>m</sup>�<sup>2</sup> � <sup>K</sup>�<sup>1</sup> for a silica fiber). Then, the ratio of ΔT between two different points along the FUT, 1 and 2, is given by

$$\frac{\Delta T\_2}{\Delta T\_1} = \frac{a\_2^{\text{abs}}}{a\_1^{\text{abs}}} \tag{7}$$

Thus, with this analysis and the experimental data obtained from the measurement of the wavelength shift of WGM resonances in irradiated points (1) and pristine points (2) of the FUT, this ratio between the respective αabs can be calculated.

Direct measurements of transmission loss variation as the fibers were irradiated were carried out for a PS980 fiber. First, the value of the loss of the pristine fiber was measured at 1550 nm by means of the cutback method: the obtained value was 120:0 � 0:5 dB/km. Then, the UV laser was swept back and forth along a 5-cm-long section of the fiber, repeatedly. The full description of the procedure is described in [11]. Figure 9a shows the data obtained in this experiment. The final loss was 6:2 � 0:4 dB/m; thus the ratio between the loss coefficients, α2=α1, increased 52 � 3 times. Please remember that this loss coefficient includes both absorption and scattering contributions (<sup>α</sup> <sup>¼</sup> <sup>α</sup>abs <sup>þ</sup> <sup>α</sup>scat).

The contribution to the loss by means of the absorption mechanism was measured using the WGM technique (see Figure 9b). In this case, a 1550 nm laser (maximum power, 1 W) was launched to the FUT, and the thermal shift of the

the same UV fluence and fiber characteristics. Two transition zones were clearly

Temperature profile of a FBG illuminated (i) outside and (ii) within the reflexion band.

5. Measurement of absorption coefficients in photosensitive fibers

In the previous section, the gradient of temperature induced in fiber-optic components by means of an illumination signal has been characterized and discussed. It has been shown that there is a difference in temperature between the sections that have been irradiated with UV light compared to the pristine fibers. It is well known that the UV irradiation induces a change in the index of photosensitive fibers, which is employed to fabricate FBGs and LPGs. According to Kramers-Kronig relations, the change in the refractive index is associated with a variation of the absorption coefficient. In addition, the exposure of the fiber to the levels of UV light usually employed in the grating fabrication induces mechanical deformations in the fiber [27]. This leads to an increase of the loss due to scattering. Thus, when a

Finally, the temperature profile was measured when the optical signal was tuned to the Bragg wavelength (power, 1 W) (see curve (ii) in Figure 8). In this case, one should take into account that the UV irradiation is constant over its length, and the gradient temperature is due to the fact that the optical signal is reflected as it penetrates into the grating. A sharp increment of temperature at the beginning of the grating, at the extreme that is illuminated, can be observed. The maximum is located at the vicinities of the point where the FBG begins. The decay of temperature extends over a length of � 5 mm, which is shorter than the length of the fiber itself (12 mm). This is consistent with the high reflectivity of this FBG. Moreover it should be noted that, at the beginning of the curve, that is, z ¼ 0 � 3 mm, the temperature increase is � 2 ºC, that is, roughly twice the value obtained for a pristine fiber. On the contrary, in the section after the grating (and even at the last millimeters of the FBG), the increment of temperature is below the detection limit of the technique. The origin of this asymmetry is the reflection of the optical signal: the amount of light that reaches the last millimeters of the FBG is very small. This technique, then, provides information about the effective length of gratings of different reflectivity, information that could be relevant for the design of optical systems that require of short cavities, or cavities that require of a very precise

observed at both ends of the grating.

Applications of Optical Fibers for Sensing

Figure 8.

146

length, as in the case of mode-locked fiber lasers.

### Figure 9.

(a) Direct measurement of the loss as the PS980 fiber is irradiated. (b) Heating of the PS980 fiber as a function of the illumination power.

fiber [25] and the measurements of α. The results of the contributions are compiled in Table 2. Both contributions are in the same order of magnitude, but αscat is smaller for three of the four FUTs. These values confirm that scattering loss

PS980 3680 20 99:7 1:9 2500 400 20 3 PS1250 4280 20 106:6 1:9 2300 400 24:5 1:9 SM1500 <sup>167</sup> <sup>4</sup> < 1:951 <sup>200</sup> <sup>90</sup> < 1:95<sup>1</sup> H2-SMF28 <sup>3260</sup> <sup>18</sup> <sup>113</sup>:<sup>1</sup> <sup>1</sup>:<sup>7</sup> <sup>2300</sup> <sup>400</sup> n/a2

Whispering Gallery Modes for Accurate Characterization of Optical Fibers' Parameters

αabs (dB/km) αscat (dB/km) Irradiated Pristine Irradiated Pristine

Thus, by means of the combination of both techniques, it is possible to quantify the different contributions to the loss, even for short sections of fiber. This information might be useful, for example, in the design of novel-active doped fibers, since it is possible to evaluate if the doping technique increases the scattering loss

The elasto-optic effect consists on the variation in the refractive index generated by any strain applied to the fiber. The correspondent elasto-optic coefficients are usually determined by measuring the optical activity induced by a mechanical twist and the phase change induced by longitudinal strain [32, 33]. This technique relies on the use of the conventional axial modes propagating through the fiber. Since these modes are essentially transverse to the axis of the fiber [34], the anisotropy of the elasto-optic effect does not show up. On the contrary, WGMs have a significant longitudinal component; hence, their optical fields experience the anisotropy of the elasto-optic effect intrinsically. In the last years, researchers have demonstrated a number of fiber devices in which the longitudinal components of the electromagnetic modes are significant, such as microfibers [35] and microstructured optical fibers with a high air-filling fraction [36]. For these cases, the measurement and characterization of the anisotropy of the elasto-optic effect and its Pockels coefficients are of high interest. Roselló-Mechó et al. reported a technique based on the different wavelength shifts of TE- and TM-WGM resonances in a fiber under axial strain, to measure these coefficients [37]. This technique has the additional advantage that, since it does not involve the conventional modes of the fiber, there is no need that the FUTs are single mode in order to carry out the measurements. Then, the coefficients can be measured at different wavelengths to determine their dispersion; this is a limitation of the usual technique based on the optical

increases faster than absorption loss.

DOI: http://dx.doi.org/10.5772/intechopen.81259

1

2

149

Table 2.

Nominal value.

Nonavailable, hydrogenated fiber.

unnecessarily, but not so much the absorption.

6. Measurement of Pockels coefficients in optical fibers

Absorption and scattering contributions to the overall attenuation coefficient.

activity which is overcome by means of WGM technique [38].

According to Eq. (1), a variation in the refractive index will tune the WGM resonances in wavelength. In this case, an axial strain will be applied to the FUT in order to induce this variation in the index, due to the elasto-optic effect. This feature was applied in different works in order to tune the WGM resonances


### Table 1.

Measurement of thermal heating and loss coefficient of different fibers.

resonances was measured as the laser power was increased, at two different points, one within the irradiated section and one outside it. The data does not show any sign of saturation of the heating, at this range of power. The temperature of the irradiated section increased linearly, at a rate of 26:48 � 0:15 ºC=W, and at 0:718 � 0:014 ºC=W in the pristine region. The ratio between these values, that is, the ratio αabs <sup>2</sup> =αabs <sup>1</sup> , is 36:9 � 0:7 ºC=W.

This process was repeated for all the different fibers mentioned before: PS1250, SM1500, and hydrogenated SMF28, at 1550. Table 1 includes the results from the measurements and the corresponding analysis: α2=α<sup>1</sup> was obtained for each of them from the direct measurement of the loss, while αabs <sup>2</sup> =αabs <sup>1</sup> was calculated from the technique based in WGMs.

The results, compiled in Table 1, allow establishing several conclusions of interest. First, as expected, the absorption coefficient is substantially increased due to the UV irradiation. As a consequence, even for signals of moderate powers, FBGs might experience shifts and chirps that should be taken into account [31]. Second, the results show that α2=α<sup>1</sup> is systematically higher than αabs <sup>2</sup> =αabs <sup>1</sup> . Roselló-Mechó et al. analyzed the measurements to demonstrate that these results lead to the conclusion that scattering loss increases at a higher rate than absorption loss [11].

Finally, Eq. (6) can be used to calculate the absolute value of the absorption and scattering coefficients by taking into account the values of h and a for a silica Whispering Gallery Modes for Accurate Characterization of Optical Fibers' Parameters DOI: http://dx.doi.org/10.5772/intechopen.81259


Table 2.

Absorption and scattering contributions to the overall attenuation coefficient.

fiber [25] and the measurements of α. The results of the contributions are compiled in Table 2. Both contributions are in the same order of magnitude, but αscat is smaller for three of the four FUTs. These values confirm that scattering loss increases faster than absorption loss.

Thus, by means of the combination of both techniques, it is possible to quantify the different contributions to the loss, even for short sections of fiber. This information might be useful, for example, in the design of novel-active doped fibers, since it is possible to evaluate if the doping technique increases the scattering loss unnecessarily, but not so much the absorption.
