**1. Introduction**

Photonic crystal fibres (PCFs), which are also called microstructured optical fibres or holey fibres, have been extensively investigated and have considerably altered the traditional fibre optics since they appeared in the mid 1990s [Knight et al., 1996; Knight, 2003; Russell, 2003]. PCFs have a periodic array of microholes that run along the entire fibre length. They typically have two kinds of cross sections: an air–silica cladding surrounding a solid silica core or an air–silica cladding surrounding a hollow core. The light-guiding mechanism of the former is provided by means of a modified total internal reflection (index guiding), while the light-guiding mechanism of the latter is based on the photonic band gap effect (PBG guiding). The number, size, shape, and the separation between the air-holes as well as the air-hole arrangement are what confer PCFs unique guiding mechanism and modal properties [Russell, 2006]. This gives PCF many unique properties such as single mode operation over a wide wavelength range [Birks et al., 1997], very large mode area [Knight et al., 1998], and unusual dispersion [Renversez et al., 2003]. Because of their freedom in design and novel wave-guiding properties, PCFs have been used for a number of novel fibre-optic devices and fibre-sensing applications that are difficult to be realized by the use of conventional fibres.

While optical interferometers offer high resolution in metrology applications, the fibre optic technology additionally offers many degrees of freedom and some advantages such as stability, compactness, and absence of moving parts for the construction of interferometers. The two commonly followed approaches to build fibre optic interferometer are: two arm interferometer and modal interferometer. Two- arm interferometer involves splitting and recombining two monochromatic optical beams that propagate in different fibres which requires several meters of optical fibre and one or two couplers. Modal interferometer exploits the relative phase displacement between two modes of the fibre. In modal interferometers compared to their two-arm counterparts the susceptibility to environmental fluctuations is reduced because the modes propagate in the same path or fibre. Recently the unique properties of the photonic crystal fibre have attracted the sensor community. Design of PCF based interferometers in particular is interesting owing to their proven high sensitivity and wide range of applications. Photonic crystal fibre based modal interferometers include PCFs in a fibre loop mirror [Zhao et al., 2004], interferometer built with long period gratings [Lim et al., 2004], interferometers built with tapered PCFs

Photonic Crystal Fibre Interferometer for Humidity Sensing 161

velocities, thus in a certain length of PCF the modes accumulate a differential phase shift. Therefore constructive or destructive interference occurs along the length of PCF. The phase velocities and phase difference are also wavelength dependent; therefore the optical power reflected by the device will be a maximum at certain wavelengths and minimum at others [Villatoro et al., 2009b]. When the reflected modes re-enter the collapsed region they will further diffract and because the mode field of the SMF is smaller, the core acts as a spatial filter and picks up only a part of the resultant intensity distribution of the interference

Fig. 1. Microscope image of the PCFI (upper) & a schematic of the excitation/recombination

A regular interference pattern in the reflection spectrum of the PCFI suggests that only two modes are interfering in the device. In our reported work [Mathew et al., 2010] on a PCFI using LMA 10 fibre, based on the fact that higher order modes can exist in the core of a PCF with a short length [Káčik et al., 2004; Uranus et al., 2010], the interfering modes in the PCF are considered as two core modes. However in a later experiment, which involved varying the refractive index surrounding the cladding of a PCFI, good ambient refractive index sensitivity is observed for a PCFI fabricated using the same LMA 10 fibre. This suggests that the interfering modes are a core mode and a cladding mode of the PCF, a conclusion that is supported by [Choi et al., 2007; Cárdenas-Sevilla et al., 2011] for an LMA10 fibre. Thus considering a core mode and a cladding mode as the interfering modes of the PCFI and designating the effective refractive indexes of the core mode as nc and cladding mode as ncl, the accumulated phase difference is 2π∆n(2L)/λ, where ∆n=nc-ncl, λ the wavelength of the optical source, and L the physical length of the PCFI [Villatoro et al., 2009a]. The power reflection spectrum of this interferometer will be proportional to cos(4π∆nL/λ). The wavelengths at which the reflection spectrum shows maxima are those that satisfy the condition 4π∆nL/λ=2mπ, with m being an integer. This means that a periodic constructive

of modes in the hole collapsed region (lower).

pattern in the PCF.

[Monzón-Hernández et al., 2008], and interferometers fabricated via micro-hole collapse [Choi et al., 2007; Villatoro et al., 2007a]. The latter technique is really simple since it only involves cleaving and splicing. The different configurations reported so far are a PCF with two collapsed regions separated by a few centimetres [Choi et al., 2007], a short section of a PCF longitudinally sandwiched between standard single mode fibres by fusion splicing (transmission type) [Villatoro et al., 2007] and a stub of PCF with cleaved end fusion spliced at the distal end of a single mode fibre (reflection type) [Jha et al., 2008]. The advantage of the last two configurations is that the modal properties of the PCF are exploited but the interrogation is carried out with conventional optical fibres, thus leading to more costeffective interferometers. The interferometer with the latter configuration is demonstrated in this chapter as a relative humidity or dew sensor. The sensor presented has the unique advantages such as it does not require any special coatings to measure humidity. Also since the sensor head is made of single material (silica) it can be used in harsh and hightemperature environments to monitor humidity.

In section 2 of the chapter the operating principle of a reflection type photonic crystal fibre interferometer (PCFI), its fabrication and the dependence of the interferometer's fringe spacing on the length of the PCF are presented. Section 3 explains the water vapor adsorption/desorption phenomena on a silica surface, the working principle of a relative humidity sensor based on PCF interferometer and the humidity response of the PCF interferometer. Section 4 demonstrates the use of the PCFI as a dew sensor. The section presents the basic sensing principle of the dew sensor, the temperature dependence and the dew response of the PCF interferometer. A dew point hygrometer using PCF interferometer is also proposed in this section.
