**Part 3**

**Photonic Crystal Fiber** 

156 Photonic Crystals – Introduction, Applications and Theory

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**8** 

*Ireland* 

**Photonic Crystal Fibre Interferometer** 

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

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

**1. Introduction** 

conventional fibres.

Jinesh Mathew, Yuliya Semenova and Gerald Farrell *Photonics Research Centre, Dublin Institute of Technology* 

**for Humidity Sensing** 
