Sagnac interferometer based on a suspended twin-core fibre

Abstract

ABSTRACT In this work, the authors present an alternative Sagnac interferometer based on a suspended twin-core fibre. Using the suspended twin-core fibre, the pattern fringe is due to the diffe rent optical paths of the light in the two cores. In this case , the value of the refractive index difference between the two cores is ~10 -3 , which indicates an advantage of this approach, namely the possibility to use a small length of the suspended twin-core fibre. The sensing configuration was characterized for strain, temperature, curvature and torsion, respectively. Keywords: Sagnac interferometer, suspended co re fibre, physical parameters. 1. INTRODUCTION An all-fibre Sagnac interferometer can be implemented in different configura tions. The traditional Sagnac interferometer has been used as gyroscopes, hydrophones, geophones and current measuring systems [1]. The other type of Sagnac interferometer is when the loop contains a section of a high bi refringent (Hi-Bi) fibre. In this case, the pattern fringe is controlled by the length and by the birefringence of the Hi-Bi fibre [2]. In optical sensing, besides the gyroscope application, it has been used in strain [3], temperature [4], liquid level [5] and displacement [6] sensing as well as a spectral filter for fibre Bragg gratings demodulation [7]. Moreover, the Hi-Bi fi bre loop mirror combined with a Bragg grating [8] or with a long period grating [9] was also demonstrated for simultaneous measurement of strain and temperature. The first demonstration of the suspended core fibre was proposed and published in 2001 [10]. The core with 1 P m was single-mode and it was predicted to have 17% of the mode located in the air at 1550 nm. This high degree of overlap between the fundamental mode and the holes suggests that this fibre was suitable as evanescent field device sensor. Several authors have studied this novel geometry for gas sensing and biosensing [11]. Due to its geometry, this type of fibre presents high birefringence and was also demonstrated in a Sagnac interferometer when a suspended core fibre section with three holes was spliced inside the loop [12]. In this work, the authors pres ent a suspended twin-core fibre in a Sagnac interferometer. Due to the geometry design, the refractive index difference between the two cores is high and it is possible to use a small section (few centimetres) as an element sensing device.