Table 2.

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

While one could consider liquid crystal based sensors in a similar category to those above, the authors wish to express they fall outside the remit of this chapter. However should the reader wish to explore these technologies, Windhorn and Cain [25] provides a good starting point. Several recent works exploring the use of liquid crystal thermometry using PCFs have also been published [53–55]. Wolinski et al. presented a multi-faceted liquid crystal PCF which was capable of temperature,

The remit of this review was to address current state-of-the-art LiF-OFTSs presented in literature. The optical fibre sensors examined were based on the principles of extrinsic Fabry-Perot interferometers, Mach-Zehnder interferometers and Sagnac interferometers. While sensitivities varied to an extent, they each presented performance equivalent to, or better than, reported electrical sensors. As noted in other reviews of optical fibre sensing such as that of Lee [57] and Poeggel et al. [58] optical fibre sensors had numerous additional advantages over their electronic counterparts such as immunity to EMI, small form factor, along with distributed and multiplexed sensing capability. Perhaps encouragingly, the commercial marketplace for optical fibre sensors appears to have matured significantly since the publication of Lee's review [57]. A driving factor in this will likely have been the coupled high resolution and fast response times, which are generally sub-Kelvin and

While FBGs remain the predominant method of temperature sensing, EFPI sensors have gained momentum in the commercial arena with companies such as FISO & Opsens offering varied ranges, sensitivities and sampling rates, indicating that they are slowly gaining favour in engineering and biomedical applications. That said however, MZI based temperature sensors were the more reported sensor type in literature. From this review it was obvious that while several ultra-high resolution liquid filled temperature sensors have been developed there is still scope for significant work to be carried out to improve their performance and stimulate

Considering that a significant portion of optical fibre sensor research has been focused towards biomedical applications in recent years, it is understandable that a market for inline sensors may not exist, that said however it may bode well for the

ratio of filled to unfilled PCF increased temperature sensitivity.

electric field, and hydrostatic pressure detection [56].

4.4 Similar sensors

Applications of Optical Fibers for Sensing

5. Conclusions

sub-second respectively.

108

widespread commercial deployment.

Commercially available optical fibre temperature sensors.

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 order to potentially be commercially competitive.
