Temperature independent polarisation maintaining fibre for sensing and interferometry

Abstract

Polarisation maintaining fibres used for sensing and interferometry typically have high birefringence [1-3] and are known as HiBi fibres. Since photonic crystal fibre (PCF) was first reported [4,5], HiBi PCFs with birefringence comparable to and greater than conventional highly stressed bow-tie and PANDA fibre have been demonstrated [6-10]. Very high levels of form birefringence in PCFs have been possible due to the flexibility in geometry and the high refractive index contrast offered by making a fibre with an air silica structure (ASS). In this paper we present experimental results that show effective temperature independent, or athermal, birefringence in a HiBi-PCF [11-14]. This is expected to be beneficial for a number of sensing and interferometric applications. For example, fibre optic gyroscopes (FOG) generally use very long lengths of coiled HiBi fibre in a Sagnac configuration to attain suitable sensitivity. FOG cost is, however, a significant driving factor in limiting the expansion of FOGs into new lower cost applications. FOG performance has been primarily limited by environmental temperature sensitivity [15,16] and stabilisation routes, using temperature-stabilised packaging, add too much to their cost. The use of a passive, temperature insensitive HiBi-PCF is a much lower cost alternative that does not require active stabilisation, thereby potentially overcoming these limitations and potentially opening up a new low cost market for FOG technology whilst retaining high performance.