Optical fibre Bragg grating cladding mode sensors

Abstract

In this paper we propose and demonstrate two forms of a device for sensing cladding modes efficiently. The cladding modes, generated by an untilted and tilted fibre Bragg grating (TFBG) written in SMF28 fibre are captured by splicing it to an in-line double-clad fibre coupler (DCFC). A comaprison is made of the capture efficiency of the cladding modes in two configurations; one in which the TFBG is taper spliced to the DCF, or in the other in which an FBG in an SMF28 is etched down to match the outer core of the DCF. In both cases the cladding modes are captured efficiently, but with significantly improved results for the former configuration. We demonstrate surrounding refractive index sensing using a new signal analysis scheme based on the extinction of each cladding mode resolved over a bandwidth of over ~60nm. This robust device has the advantage of faithfully transmitting the cladding modes over a long distance and is therefore suitable for remote sensing over long distances. The sensitivity of the device is discussed. This device may be used in a variety of applications such as for bend, strain, temperature and surrounding refractive index sensing.