4.1 EFPI type sensors

Chen et al. presented an FPI construction which contained an air micro-bubble encased in cured PDMS [46]. The sensor was manufactured by splicing an SMF to a hollow core fibre (HCF) 282 μm in length. The PDMS was subsequently introduced into the HCF via capillary effect. As PDMS entered the HCF it formed the air microbubble with the SMF, where the length of air micro-bubble was controlled by the period of time PDMS was allowed to enter the HCF. Online monitoring was conducted to establish the free spectral range (FSR) best suited to the desired application. A schematic of the sensor is presented in Figure 7.

The PDMS was cured for 45 min at 338.15 K, fixing the length of the air microbubble. Testing was conducted between 324.35–343.65 K and compared to a PT100 thermometer with a resolution of 0.01 K. Results indicated sensitivity was quite high with a value of 2.7035 nm/K and a highly linear response where R2 = 0.992. In addition to the reported sensitivity, reference was made to the benefit of using a double FPI in the sensor. This consisted of a thin FPI (air micro-bubble) and thick FPI (PDMS filling). The thin FPI allowed for a large FSR and the thick FPI offered high temperature resolution.

Poeggel et al. [34] presented a novel ultra-high resolution temperature sensor (UHRTS). The sensor was comprised of an existing optical fibre pressure temperature sensor (OFPTS) [47] which was further encased in an outer oil-filled capillary. The sensor was noted to have an external diameter of less than 1 mm lending to its capability to be used in volume restricted areas, a schematic of the sensor is provided in Figure 8. By combining an FBG with the highly sensitive EFPI, the UHRTS behaved similar to that of Chen et al. [46] in that the FBG and air cavity allowed for a wide spectral range to be utilised with the diaphragm element providing high resolution. Dissimilar to Chen et al. however, the construction of the OFTS was reliant on the thin diaphragm element to transduce volume changes in the oil to a temperature measurement. This being in demonstrated by Eqs. (4) and (5) in Section 3. It was claimed that the high ratio of oil in the outer capillary compared to air in the EFPI cavity was what resulted in high sensitivity. The sensor presented had a reported, theoretical, sensitivity of 52.7 <sup>10</sup><sup>3</sup> nm/K. Comparison of the UHRTS to a Bosch BMP085 [48] was carried out over a temperature range of 7 K. While both sensors detected the 7 K variation, measured EFPI sensitivity was much lower than predicted, at 8.77 nm/K. It was suggested that the error was likely caused by inconsistencies in the manufacturing process of the sensor, and the presence of micro air bubbles in the oil which resulted in response damping. Considering oil was introduced into the capillary via a micro-syringe rather than by capillary effect as utilised by several authors such as Chen et al. [46] and Xu et al. [39] this may have

Figure 7. Schematic of PDMS FPI sensor based on Chen et al. [46].

respect to each other before being recombined at the detector. This type of interferometer is the basis of fibre-optic gyroscopes [43]. While it has been utilised in thermometry [44, 45] the sensors reported have not seen a similar level of research compared to their MZI counterparts. Reported sensors used a source beam counterpropagated by a 3 coupler. Birefringence in the PCF resulted in the optical path difference which was detected by the OSA [44, 45]. Figure 6 above highlights how

As evident from Sections 1 and 3 temperature sensors have a long history. With respect to LiF-OFTSs however, while several types of sensor and base technologies have been presented in the literature, the authors have not encountered a concise review of said sensors. The materials utilised have included; alcohol [44] motor oil

[34], silicone (polydimethylsiloxane) [46], and immersion oils [39]. While polydimethylsiloxane (PDMS) has only been presented in its cured form, it is not inconceivable that the liquid form be used given its optically transparent nature. The following section is arranged by the base interferometry principle of each fluid

an SI based sensor may be constructed using optical fibre equipment.

4. Liquid filled optical fibre temperature sensors

filled sensor.

102

Figure 5.

Figure 6.

Transverse section of a photonic crystal fibre.

Applications of Optical Fibers for Sensing

Schematic of potential Sagnac interferometer based sensor.

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

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.

highly beneficial in the development of inline PCF sensors.

Review of Liquid-Filled Optical Fibre-Based Temperature Sensing

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

ΔT Δε

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

Figure 10.

105

<sup>¼</sup> ST,A <sup>S</sup>ε,A ST,B Sε,B

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

" #�<sup>1</sup> Δλ<sup>A</sup>

Δλ<sup>B</sup>

" #

(7)

" #

Figure 8. Schematic of oil-filled EFPI sensor based on Poeggel et al. [34].

Figure 9. Schematic of an inline PCF sensor based on Qiu et al. [37].

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.
