**2.1 PCFI working principle**

In a PCFI the excitation and recombination of modes can be carried out by the hole collapsed region of the PCF [Choi et al., 2007; Villatoro et al., 2007]. A microscopic image of the PCFI and a schematic of the excitation and recombination of modes in the PCFI are shown in Fig. 1. The fundamental SMF mode begins to diffract when it enters the collapsed section of the PCF. Because of diffraction, the mode broadens; depending on the modal characteristics of the PCF and the hole collapsed region, the power in the input beam can be coupled to the fundamental core mode and to higher order core modes [Villatoro et al., 2007, 2009b; Barrera et al., 2010] or to cladding modes [Cárdenas-Sevilla et al., 2011; Choi et al., 2007; Jha et al., 2008] of the PCF. The modes propagate through the PCF until they reach the cleaved end from where they are reflected. Since the modes propagate at different phase

[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 high-

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

Photonic crystal fibre interferometers based on micro-hole collapse have attained great importance in recent times due to the simple fabrication process involved and excellent sensing performance [Villatoro et al., 2007, 2009a, 2009b]. A reflection-type PCFI consists of a stub of PCF fusion spliced at the distal end of a single mode fibre [Mathew et al., 2010]. The key element of the device is the hole collapsed region close to the splice point. Some advantages of the PCF interferometers fabricated using microhole collapse are that since interferometers are fabricated by fusion splicing the splice is highly stable even at high

In a PCFI the excitation and recombination of modes can be carried out by the hole collapsed region of the PCF [Choi et al., 2007; Villatoro et al., 2007]. A microscopic image of the PCFI and a schematic of the excitation and recombination of modes in the PCFI are shown in Fig. 1. The fundamental SMF mode begins to diffract when it enters the collapsed section of the PCF. Because of diffraction, the mode broadens; depending on the modal characteristics of the PCF and the hole collapsed region, the power in the input beam can be coupled to the fundamental core mode and to higher order core modes [Villatoro et al., 2007, 2009b; Barrera et al., 2010] or to cladding modes [Cárdenas-Sevilla et al., 2011; Choi et al., 2007; Jha et al., 2008] of the PCF. The modes propagate through the PCF until they reach the cleaved end from where they are reflected. Since the modes propagate at different phase

temperature environments to monitor humidity.

**2. Photonic crystal fibre interferometer** 

temperatures and also its characteristics will not degrade over time.

is also proposed in this section.

**2.1 PCFI working principle** 

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 pattern in the PCF.

Fig. 1. Microscope image of the PCFI (upper) & a schematic of the excitation/recombination of modes in the hole collapsed region (lower).

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

Photonic Crystal Fibre Interferometer for Humidity Sensing 163

Fig. 2. The reflection spectra of interferometers with L = 92 mm, 10.5 mm and 3.5 mm in the

Fig. 3. The fringe spacing as a function of length of PCF observed for a reflection type

hold at that temperature and pressure is the relative humidity.

meter. Dew point, expressed in °C or °F, is the temperature and pressure at which a gas begins to condense into a liquid. The ratio of the percentage of water vapour present in air at a particular temperature and pressure to the maximum amount of water vapour the air can

The measurement of humidity is required in a range of areas, including meteorological services, the chemical and food processing industries, civil engineering, air-conditioning, horticulture and electronic processing. Compared with their conventional electronic

wavelength range of 1500-1600 nm.

interferometer.

interference occurs when λm = (2∆nL/m). If some external stimulus changes ∆n (while L is fixed) the position of each interference peak will change, a principle which allows the device to be used for sensing.
