**2. Photonic crystal fibre 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 temperatures and also its characteristics will not degrade over time.
