**2.2 PCFI fabrication**

Fusion splicing of the PCF to the SMF is undertaken using the electric arc discharge of a conventional arc fusion splicer. During the splicing process the voids of the PCF collapse through surface tension within a microscopic region close to the splice point. In fabricating such an interferometer, one critical condition for good sensor performance is achieving a regular interference pattern and good interference fringe visibility. The visibility of the interferometer depends on the power in the excited modes, which in turn depends on the length of the collapsed region [Barrera et al., 2010]. However a long collapsed region length causes activation of many cladding modes and therefore degrades the sinusoidal nature of the interference patterns and furthermore increases the splice loss. Therefore for an improved sensor performance, only one cladding mode is preferred due to its simple interference with the core mode. The collapsed region length can be controlled by the arc power and duration [Barrera et al., 2010]. In our experiments, PCF (LMA10, NKT Photonics) designed for an endless single-mode operation was used. It has four layers of air holes arranged in a hexagonal pattern around a solid silica core. The light guidance mechanism in such a fibre is by means of modified total internal reflection. The dimensions of the LMA-10 PCF simplify alignment and splicing with the SMF with a standard splicing machine and minimize the loss due to mode field diameter mismatch compared to other PCFs. For the interferometer fabricated in our study the total length of the collapsed region was 200 μm. After fusion splicing, the PCF was cleaved using a standard fibre cleaving machine so that the end surface of the PCF acts as a reflecting surface.

### **2.3 PCFI fringe spacing vs length of PCF**

Initially to investigate the influence of the length of the PCFI on the fringe spacing thirteen PCFIs were fabricated with lengths ranging from 3.5 mm to circa 100 mm. As an example Fig. 2 shows the measured reflection spectra of three PCFIs in the 1500-1600 nm wavelength range with lengths of 92, 10.5 and 3.5 mm. The reflection spectra of the interferometers exhibit regular interference patterns with a period or fringe spacing inversely proportional to the length of the PCF section. A modulation of the expected sinusoidal pattern is observed for the spectra shown in Fig. 2, which might be due to the excitation of more than one cladding mode or possibly due to the polarization dependence of the intermodal interference [Bock et al., 2009]. Fig. 3 shows the measured fringe spacing or periods of the fabricated PCFIs as a function of length of the PCF section. The measured periods agree well with the expected ones for a two-mode interferometer given by the expression P ≈ λ2/(2∆nL). The value of ∆n obtained based on the experimental data is ~4.2x10-3.

## **3. Relative humidity sensor based on PCFI**

Humidity refers to the water vapour content in air or other gases and its measurements can be stated in a variety of terms and units. The three commonly used terms are absolute humidity, relative humidity (RH) and dew point. Absolute humidity is the ratio of the mass of water vapour to the volume of air or gas. It is commonly expressed in grams per cubic

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

Fusion splicing of the PCF to the SMF is undertaken using the electric arc discharge of a conventional arc fusion splicer. During the splicing process the voids of the PCF collapse through surface tension within a microscopic region close to the splice point. In fabricating such an interferometer, one critical condition for good sensor performance is achieving a regular interference pattern and good interference fringe visibility. The visibility of the interferometer depends on the power in the excited modes, which in turn depends on the length of the collapsed region [Barrera et al., 2010]. However a long collapsed region length causes activation of many cladding modes and therefore degrades the sinusoidal nature of the interference patterns and furthermore increases the splice loss. Therefore for an improved sensor performance, only one cladding mode is preferred due to its simple interference with the core mode. The collapsed region length can be controlled by the arc power and duration [Barrera et al., 2010]. In our experiments, PCF (LMA10, NKT Photonics) designed for an endless single-mode operation was used. It has four layers of air holes arranged in a hexagonal pattern around a solid silica core. The light guidance mechanism in such a fibre is by means of modified total internal reflection. The dimensions of the LMA-10 PCF simplify alignment and splicing with the SMF with a standard splicing machine and minimize the loss due to mode field diameter mismatch compared to other PCFs. For the interferometer fabricated in our study the total length of the collapsed region was 200 μm. After fusion splicing, the PCF was cleaved using a standard fibre cleaving machine so that

Initially to investigate the influence of the length of the PCFI on the fringe spacing thirteen PCFIs were fabricated with lengths ranging from 3.5 mm to circa 100 mm. As an example Fig. 2 shows the measured reflection spectra of three PCFIs in the 1500-1600 nm wavelength range with lengths of 92, 10.5 and 3.5 mm. The reflection spectra of the interferometers exhibit regular interference patterns with a period or fringe spacing inversely proportional to the length of the PCF section. A modulation of the expected sinusoidal pattern is observed for the spectra shown in Fig. 2, which might be due to the excitation of more than one cladding mode or possibly due to the polarization dependence of the intermodal interference [Bock et al., 2009]. Fig. 3 shows the measured fringe spacing or periods of the fabricated PCFIs as a function of length of the PCF section. The measured periods agree well with the expected ones for a two-mode interferometer given by the expression P ≈

λ2/(2∆nL). The value of ∆n obtained based on the experimental data is ~4.2x10-3.

Humidity refers to the water vapour content in air or other gases and its measurements can be stated in a variety of terms and units. The three commonly used terms are absolute humidity, relative humidity (RH) and dew point. Absolute humidity is the ratio of the mass of water vapour to the volume of air or gas. It is commonly expressed in grams per cubic

to be used for sensing.

**2.2 PCFI fabrication** 

the end surface of the PCF acts as a reflecting surface.

**3. Relative humidity sensor based on PCFI** 

**2.3 PCFI fringe spacing vs length of PCF** 

Fig. 2. The reflection spectra of interferometers with L = 92 mm, 10.5 mm and 3.5 mm in the wavelength range of 1500-1600 nm.

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

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 hold at that temperature and pressure is the relative humidity.

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

Photonic Crystal Fibre Interferometer for Humidity Sensing 165

with respect to the ambient humidity values which in turn change the position of the interference pattern accordingly. An increase in humidity causes the shift of the interference pattern of a PCFI toward longer wavelengths and the value of this interference peak shift is exponential with respect to relative humidity [Mathew, 2010]. This shift of the interference peak is mainly due to the adsorption and desorption of H2O molecules along the surface of holes within the PCF, at the interface between air and silica glass. Since the whole device is exposed to humidity the adsorption and desorption of water vapour on the PCF outer surface and on the end face also contribute to the shift of the interference pattern. But considering the field distribution of the interfering cladding mode shown in [Cárdenas-Sevilla et al., 2011; Uranus, 2010] and below the dew point temperature the main contribution to the interference shift is considered to be due to the adsorption of water molecules within the voids of the PCF. The adsorption on the end face mainly causes a shift

Fig. 4. Schematic representation of water vapor adsorption mechanisms on an SiO2 surface.

The sensor system is composed of a broadband light source (SLED), a fibre coupler/circulator (FOC), the PCF interferometer or sensor head, and an optical spectrum analyser (OSA) as shown in Fig. 5. The sensor head, as the main part of the sensor system, is composed of a small stub of PCF fusion spliced to the end of a standard SMF. The PCF in the sensor head has a microhole collapsed region near the splicing point and the free end of the PCF is exposed to ambient air. The humidity response of the device was studied at a temperature (25 OC) and at normal atmospheric pressure by placing it in a controlled environmental chamber as shown in Fig. 5. Fig. 6 shows the changes in the reflection spectrum with respect to ambient humidity for a device with L=40.5 mm. The change in the adsorption with respect to ambient humidity changes the effective refractive index of the cladding mode (ncl). The resulting phase change in turn results in a shift of the interference pattern. The curves in Fig. 6 show the position of a zoomed section of the device spectrum at relative humidity values of 30, 60, 80 and 90 %RH. When humidity increases the interference pattern shifts to longer wavelengths and this shift is more significant at higher humidity values. To study the effect of reducing the length of the PCFI a second PCFI was fabricated with a shorter length of 17 mm. Fig. 7 shows the peak shift of the interferometer with respect

It is observed from the Fig. 7 that the sensitivity of the device to humidity decreases as the length of the device decreases. This is due to the fact that for a small device the fibre length

in the overall power level of the interference pattern.

**3.2 Experimental characterization of the sensor** 

to humidity obtained for two devices with L=17 mm and 40.5 mm.

counterparts, optical fibre humidity sensors offer specific advantages, such as small size and weight, immunity to electromagnetic interference, corrosion resistance and remote operation. A wide range of optical fibre humidity sensors have been reported in the literature. Most of these fibre optic humidity sensors work on the basis of a hygroscopic material coated over the optical fibre to modulate the light propagating through the fibre [Yeo et al., 2008; Mathew et al. 2007, 2011]. A polymer optical fibre has been adapted for humidity sensing [Zhang et al., 2010] without the use of a hygroscopic coating but the fibre is highly temperature dependent and is not suitable for high-temperature applications. An all-glass fibre-optic relative humidity sensor which does not require any special coatings to measure humidity using a reflection-type two-mode photonic crystal fibre interferometer is presented in this section. The spectrum of it exhibits good sensitivity to humidity variations.
