4.2 Mach-Zehnder interferometer type sensors

Qiu et al. [37] presented a temperature sensor based on a non-polarimetric PCF, where the sensor was created by splicing a length of commercial single mode (LMA-8) [49] between two lengths of SMF-28. During construction, isopropanol was infused into the non-polarimetric PCF micro-holes by capillary effect, with the PCF subsequently fused between the SMFs. LMA-8 PCF was chosen due to having a diameter equal to that of the SMF-28, this reduced potential complications involved in the splicing process, care was also taken to minimise collapsing of the fibre air holes. Thermo-optic coefficients (TOCs) for the isopropanol and silica are 4.5 <sup>10</sup>4/K and 8.6 <sup>10</sup>6/K respectively. A schematic for the sensor is provided in Figure 9, where an amplified spontaneous emission source (ASE) was used. As light entered from the SMF into the PCF, cladding and core modes propagated at different rates before recombining at the second splice point, this process introduced a phase difference which was observed to be temperature dependant. Akin to the designs of several other fluid-filled inline sensors, this sensor also relied on modifying the core and cladding mode TOCs in order to maximise temperature sensitivity. The sensor was tested in a temperature controlled device over temperatures ranging from 296.85–339.25 K. Blue-shifting of the two tracked waveform dips was

Review of Liquid-Filled Optical Fibre-Based Temperature Sensing DOI: http://dx.doi.org/10.5772/intechopen.80653

observed. A sample of how these waveforms may appear is presented in Figure 10. Temperature sensitivities were reported as �133 and � 166 pm/K respectively. While this was an order of magnitude improvement over previously published works [50–52] it was significantly less sensitive when compared to the design of Qian et al. [44], indicating that exploiting the birefringent nature of PCFs may be highly beneficial in the development of inline PCF sensors.

Wang et al. [38] presented a fluid-filled PCF-based modal interferometer (PCFMI). The air holes of the PCF were filled with an oil provided by Cargille Laboratories Inc. (Cedar Grove, NJ, USA) (TOC = �3.37 � <sup>10</sup>�4/K). The system design was similar to that of Qiu et al. [37]. That said however, the operation has similarities to that of Wang et al. [41] with the interference of LP01 and LP11 modes at the second splicing (recombination) point. Simulation suggested that temperature sensitivity increased proportionally with the ratio of filled to unfilled PCF, and that for constant filling ratio the sensitivity increased with increasing wavelength. It was reported that the latter was due to larger mode field areas of the longer wavelengths. Validation experiments were carried out at three filling ratios (k = 0.256, 0.282, 0.476) over a temperature range of 298.15–355.15 K. Results were in agreement with theoretical prediction, transmission spectra blue-shifted with increasing temperature, and the largest filling ratio (k = 0.476) resulted in the highest temperature sensitivity. Similarly, longer wavelengths resulted in increased sensitivity with a peak value of �340 pm/K at 1480 nm. Another benefit of the proposed sensor type is the linear response to straining, once matrix values were determined, the wavelength shifts may be used to produce temperature and strain measurements simultaneously. The matrix for the sensor presented by Wang et al. is provided by Eq. (7) where S<sup>T</sup> and S<sup>ε</sup> are the temperature and strain sensitivities respectively. A and B are the two waveform dips which were monitored.

$$
\begin{bmatrix}
\Delta T\\\\
\Delta \varepsilon
\end{bmatrix} = \begin{bmatrix}
\mathbb{S}\_{T,A} & \mathbb{S}\_{\varepsilon,A} \\
\mathbb{S}\_{T,B} & \mathbb{S}\_{\varepsilon,B}
\end{bmatrix}^{-1} \begin{bmatrix}
\Delta \lambda\_A\\\\
\Delta \lambda\_B
\end{bmatrix} \tag{7}
$$

Wang et al. [41] presented an ultra-high resolution PCF sensor which had a single liquid filled cladding hole. In the precision filling of the hole, an initial 10 μm end cap was placed on the PCF, after which a hole was precision drilled into the desired PCF hole using a femtosecond laser (FSL). A Cargille Laboratories Inc. immersion oil with a TOC of �3.89 � <sup>10</sup>�<sup>4</sup> was introduced via capillary effect. The filled region of PCF was subsequently reduced incrementally by �1 cm until a coherent resonant dip was present. Experimental results between 307.15 and 308.55 K indicated exceptional sensitivity at 54.3 nm/K. Linearity of the results was not provided. Numerical comparison was carried out in Comsol Multiphysics where

Figure 10. Representation of how two waveform dips may appear in a transmission spectrum.

been a contributing factor to bubble formation. Another potential application of the sensor proposed by Poeggel et al. [34] is use of an ionising radiation sensitive fluid whereby the temperature response varies with exposure to ionising radiation.

Qiu et al. [37] presented a temperature sensor based on a non-polarimetric PCF,

where the sensor was created by splicing a length of commercial single mode (LMA-8) [49] between two lengths of SMF-28. During construction, isopropanol was infused into the non-polarimetric PCF micro-holes by capillary effect, with the PCF subsequently fused between the SMFs. LMA-8 PCF was chosen due to having a diameter equal to that of the SMF-28, this reduced potential complications involved in the splicing process, care was also taken to minimise collapsing of the fibre air holes. Thermo-optic coefficients (TOCs) for the isopropanol and silica are 4.5 <sup>10</sup>4/K and 8.6 <sup>10</sup>6/K respectively. A schematic for the sensor is provided in Figure 9, where an amplified spontaneous emission source (ASE) was used. As light entered from the SMF into the PCF, cladding and core modes propagated at different rates before recombining at the second splice point, this process introduced a phase difference which was observed to be temperature dependant. Akin to the designs of several other fluid-filled inline sensors, this sensor also relied on modifying the core and cladding mode TOCs in order to maximise temperature sensitivity. The sensor was tested in a temperature controlled device over temperatures ranging

from 296.85–339.25 K. Blue-shifting of the two tracked waveform dips was

4.2 Mach-Zehnder interferometer type sensors

Schematic of an inline PCF sensor based on Qiu et al. [37].

Schematic of oil-filled EFPI sensor based on Poeggel et al. [34].

Applications of Optical Fibers for Sensing

Figure 8.

Figure 9.

104

the fibre and liquid rod core modes were compared. Results indicated coupling only occurred between the respective LP01 modes, where the coupling wavelength shifted by 292 nm between 303.15 and 308.15 K this giving a projected sensitivity of 58.4 nm/K. While predicted sensitivity largely agreed with experimental observations, the location of the dip did not. It was determined that this was predominantly due to uncertainty surrounding the refractive index of the immersion oil, as it had a tolerance of 0.002, where an error of 0.001 led to a 150 nm wavelength shift.

as a sealing fluid increased the temperature sensitivity of the interferometer >250 compared to the baseline at 2.3 nm/K. Heating and cooling the sensor showed repeatable results with the fitted polynomial having an R2 = 0.999. It should be noted however at higher temperatures, the sensitivity decreased and the fitting error increased. Peak resolution was reported at 283.15 K as 0.008 K with an OSA

Review of Liquid-Filled Optical Fibre-Based Temperature Sensing

DOI: http://dx.doi.org/10.5772/intechopen.80653

Xu et al. [39] went further to quantify the influence of strain on the sensor by measuring the temperature responses of a mechanically strained bare TCFMI, and an air-sealed TCFMI. Results indicated that the resonance-dip blue shifted with strain on the bare TCFMI, and the air sealed TCFMI appeared relatively temperature insensitive. The work concluded on the point of the sensitivity being primarily driven by the sealing fluid's thermo-optic coefficient. Again, however, the sensor required transmission of the light in order to be used, thus eliminating its potential

Qian et al. [44] presented an alcohol filled temperature sensor based on a highly birefringent photonic crystal fibre (HiBi-PCF) within a fibre loop mirror (FLM) as illustrated in Figure 12, the light source used was a super-luminescent light emitting diode (SLED). Owing to the birefringence of the HiBi-PCF, the counter-propagating waves caused by the 3 dB coupler have an optical path difference at recombination. Two resonant dips manifested in the transmission spectrum at 293.15 K, these being present at 1455.8 and 1549.8 nm. The sensor was tested in two conditions within an unspecified controlled temperature chamber. The first of these increasing from 293.15 to 307.15 K and the latter reducing the temperature from 293.15 to 281.15 K with the two resonant dips' responses being monitored. Measurement linearities were R<sup>2</sup> = 0.9995 and R<sup>2</sup> = 0.9997 respectively. Measured sensitivities were 6.2 and

Cui et al. [45] proposed an SI type sensor similar in construction to that of Qian et al. [44]. The study conducted, however, went further in an effort to quantify the influence of selective hole filing in the PCF versus non-selective filling. Further to this, the length of PCF and hole fill ratio were explored. Simulations of no infiltration, all holes filled, small holes filled, and big holes filled were carried out; with the birefringence sensitivity to infiltrating liquid being monitored. While all three liquid filled cases indicated a reduced PCF birefringence, the 'big holes filled'

6.6 nm/K compared to the theoretical values of 6.1 and 6.5 nm/K.

resolution of 0.2 pm.

to be used as a point sensor.

Figure 12.

107

Schematic of HiBi-PCF sensor based on Qian et al. [44].

4.3 Sagnac interferometer type sensors

Liang et al. [42] reported the first double-filled PCF sensor, with the two fluid rods having varied optical properties, both immersion oils were provided by Cargille Laboratories Inc. The first had a refractive index (RI) of 1.466 with a TOC of 3.91 <sup>10</sup><sup>4</sup> and the second 1.500 with a TOC of 4.01 <sup>10</sup><sup>4</sup> respectively. Owing to energy differences the PCF LP11 core mode was neglected. Finite element analysis indicated an interaction existed between LP01(core)–LP01(rod 1), and LP01 (core)–LP11(rod 2). This indicated two waveform dips would be present in the transmission spectrum. Furthermore rod 1 displayed red-shift with increasing temperature with the converse being true of rod 2. As the two liquid rods were relatively far apart geometrically, there was no reported interaction. Temperature response was recorded between 325.15 and 327.15 K in increments of 0.2 K. Dip sensitivities were recorded as being 42.818 and 11.343 nm/K with linearities of R2 = 0.99951 and 0.99935, thus indicating the sensor had extremely high sensitivity. A highly sensitive strain response was also reported. Force on the fibre was increased from 0.218 to 0.855 N in increments of 0.049 N, with strain sensitivities being 38.041 and 8.702 pm=μ<sup>ε</sup> Linearities were R2 = 0.99869 and 0.99495.

Xu et al. [39] utilised a thin core fibre (TCF) rather than PCF in the development of their sensor. By immersing the TCF in a Cargille Laboratories Inc. immersion oil and sealing the PCF within a capillary, the influences of external refractive indices was eliminated. The TCF was approximately 20 mm long with the protective capillary 40 mm long. A schematic of the sensor is given in Figure 11. Similar to PCF style temperature sensors, there was a resonant dip in the transmission spectrum, which was located at 1561.7 nm for the unfilled sensor. Temperature was modulated on an unspecified thermoelectric cooler, which could be controlled to 0.1 K resolution in the temperature range 288.15–318.15 K. Results were in good agreement with other published works, where a temperature sensitivity of 9.0 pm/K was recorded with a sensor linearity of R2 = 0.9957.

The introduction of immersion oil (TOC = 3.95 <sup>10</sup>4/K) and capillary resulted in higher temperatures moving the resonance dip to a shorter wavelength, i.e. blue-shift, this being the converse of the bare TCF Mach-Zehnder Interferometer (TCFMI). Experimental results indicated that introduction of the immersion oil

Figure 11. Schematic of sensor based on Xu et al. [39].

### Review of Liquid-Filled Optical Fibre-Based Temperature Sensing DOI: http://dx.doi.org/10.5772/intechopen.80653

as a sealing fluid increased the temperature sensitivity of the interferometer >250 compared to the baseline at 2.3 nm/K. Heating and cooling the sensor showed repeatable results with the fitted polynomial having an R2 = 0.999. It should be noted however at higher temperatures, the sensitivity decreased and the fitting error increased. Peak resolution was reported at 283.15 K as 0.008 K with an OSA resolution of 0.2 pm.

Xu et al. [39] went further to quantify the influence of strain on the sensor by measuring the temperature responses of a mechanically strained bare TCFMI, and an air-sealed TCFMI. Results indicated that the resonance-dip blue shifted with strain on the bare TCFMI, and the air sealed TCFMI appeared relatively temperature insensitive. The work concluded on the point of the sensitivity being primarily driven by the sealing fluid's thermo-optic coefficient. Again, however, the sensor required transmission of the light in order to be used, thus eliminating its potential to be used as a point sensor.

### 4.3 Sagnac interferometer type sensors

the fibre and liquid rod core modes were compared. Results indicated coupling only occurred between the respective LP01 modes, where the coupling wavelength shifted by 292 nm between 303.15 and 308.15 K this giving a projected sensitivity of 58.4 nm/K. While predicted sensitivity largely agreed with experimental observations, the location of the dip did not. It was determined that this was predominantly due to uncertainty surrounding the refractive index of the immersion oil, as it had a tolerance of 0.002, where an error of 0.001 led to a 150 nm wavelength shift. Liang et al. [42] reported the first double-filled PCF sensor, with the two fluid

rods having varied optical properties, both immersion oils were provided by Cargille Laboratories Inc. The first had a refractive index (RI) of 1.466 with a TOC of 3.91 <sup>10</sup><sup>4</sup> and the second 1.500 with a TOC of 4.01 <sup>10</sup><sup>4</sup> respectively. Owing to energy differences the PCF LP11 core mode was neglected. Finite element analysis indicated an interaction existed between LP01(core)–LP01(rod 1), and LP01 (core)–LP11(rod 2). This indicated two waveform dips would be present in the transmission spectrum. Furthermore rod 1 displayed red-shift with increasing temperature with the converse being true of rod 2. As the two liquid rods were relatively far apart geometrically, there was no reported interaction. Temperature response was recorded between 325.15 and 327.15 K in increments of 0.2 K. Dip sensitivities were recorded as being 42.818 and 11.343 nm/K with linearities of R2 = 0.99951 and 0.99935, thus indicating the sensor had extremely high sensitivity.

A highly sensitive strain response was also reported. Force on the fibre was increased from 0.218 to 0.855 N in increments of 0.049 N, with strain sensitivities being 38.041 and 8.702 pm=μ<sup>ε</sup> Linearities were R2 = 0.99869 and 0.99495.

with a sensor linearity of R2 = 0.9957.

Applications of Optical Fibers for Sensing

Figure 11.

106

Schematic of sensor based on Xu et al. [39].

Xu et al. [39] utilised a thin core fibre (TCF) rather than PCF in the development of their sensor. By immersing the TCF in a Cargille Laboratories Inc. immersion oil and sealing the PCF within a capillary, the influences of external refractive indices was eliminated. The TCF was approximately 20 mm long with the protective capillary 40 mm long. A schematic of the sensor is given in Figure 11. Similar to PCF style temperature sensors, there was a resonant dip in the transmission spectrum, which was located at 1561.7 nm for the unfilled sensor. Temperature was modulated on an unspecified thermoelectric cooler, which could be controlled to 0.1 K resolution in the temperature range 288.15–318.15 K. Results were in good agreement with other published works, where a temperature sensitivity of 9.0 pm/K was recorded

The introduction of immersion oil (TOC = 3.95 <sup>10</sup>4/K) and capillary resulted in higher temperatures moving the resonance dip to a shorter wavelength, i.e. blue-shift, this being the converse of the bare TCF Mach-Zehnder Interferometer (TCFMI). Experimental results indicated that introduction of the immersion oil

Qian et al. [44] presented an alcohol filled temperature sensor based on a highly birefringent photonic crystal fibre (HiBi-PCF) within a fibre loop mirror (FLM) as illustrated in Figure 12, the light source used was a super-luminescent light emitting diode (SLED). Owing to the birefringence of the HiBi-PCF, the counter-propagating waves caused by the 3 dB coupler have an optical path difference at recombination. Two resonant dips manifested in the transmission spectrum at 293.15 K, these being present at 1455.8 and 1549.8 nm. The sensor was tested in two conditions within an unspecified controlled temperature chamber. The first of these increasing from 293.15 to 307.15 K and the latter reducing the temperature from 293.15 to 281.15 K with the two resonant dips' responses being monitored. Measurement linearities were R<sup>2</sup> = 0.9995 and R<sup>2</sup> = 0.9997 respectively. Measured sensitivities were 6.2 and 6.6 nm/K compared to the theoretical values of 6.1 and 6.5 nm/K.

Cui et al. [45] proposed an SI type sensor similar in construction to that of Qian et al. [44]. The study conducted, however, went further in an effort to quantify the influence of selective hole filing in the PCF versus non-selective filling. Further to this, the length of PCF and hole fill ratio were explored. Simulations of no infiltration, all holes filled, small holes filled, and big holes filled were carried out; with the birefringence sensitivity to infiltrating liquid being monitored. While all three liquid filled cases indicated a reduced PCF birefringence, the 'big holes filled'

Figure 12. Schematic of HiBi-PCF sensor based on Qian et al. [44].

condition resulted in the highest temperature sensitivity, with a birefringence change of 27% as liquid refractive index was varied from 1.33 to 1.36. Dissimilar to the selective collapsing and cleaving method employed by Peng et al. [40] Cui et al. [45] offered a simplistic method of sealing the outer holes by introducing a microdroplet of glue to the fibre face while monitoring with a microscope. It was claimed that the process could be conducted in under a minute with repeatable results after minimal training. During experimentation, water was used in place of ethanol due to the high coefficient of thermal expansion, and reduced tendency to evaporate. The sensor indicated a good sensitivity of 2.58 nm/K with a linearity of R<sup>2</sup> <sup>=</sup> 0.9991. The OSA used to conduct the experiment had a resolution of 0.02 nm thus giving the sensor a resolution in the region of 7.75 mK. Accounting for the length of liquid filled PCF, results similar to that of Wang et al. [38] indicated increasing the ratio of filled to unfilled PCF increased temperature sensitivity.

designs of Chen et al. [46] and Poeggel et al. [34]. Another factor which has often been overlooked is the filling liquid properties, the majority of MZI sensors used an immersion oil provided by Cargille Laboratories Inc. and one reported using isopropanol. While the isopropanol may act as an irritant, the immersion oil used may be toxic if swallowed or inhaled, such as that of the Series AA [59]. It is also known to be damaging to waterways, thus indicating strict environmental controls require consideration. That said however, monitoring of industrial equipment using this method is more than plausible with the potential to use multiple PCFs on a single fibre to provide distributed sensing, Table 2 lists example commercially available OFTSs. While the sensors are not of a liquid filled construction, they indicate the minimum required performance of any potential liquid filled sensor in

Company Proximion OPSENS RJC Enterprises Sensor name WISTHEAT [60] OTG-MPK5 [61] N/A [62] Sensor type FBG GaAs Crystal EFPI Min. temperature 228.15 K 293.15 K 288.15 K Max. temperature 523.15 K 318.15 K 328.15 K Resolution 70 mK(0.5 pm OSA) 10 mK 100 mK

This work was supported by the Science Foundation Ireland grant number:

Symbol Name Unit D Flexural rigidity Pa:m<sup>3</sup> E Young's modulus N=m<sup>2</sup> P Pressure N=m<sup>2</sup> Q Heat J R Outer radius m S<sup>T</sup> Temperature sensitivity /K S<sup>ε</sup> Strain sensitivity - T Temperature K T0 Initial temperature K c Specific heat capacity J/kg.K h Thickness m neff Effective refractive index r Reference radius m

order to potentially be commercially competitive.

Commercially available optical fibre temperature sensors.

DOI: http://dx.doi.org/10.5772/intechopen.80653

Review of Liquid-Filled Optical Fibre-Based Temperature Sensing

The authors wish to declare no conflict of interest.

Acknowledgements

Conflict of interest

Nomenclature

109

15/CDA/3598

Table 2.
