Abstract

In this paper we describe an optical strain sensing system using a long-period grating interrogating a FBG (fibre Bragg grating) strain sensor to achieve both large dynamic sensing range and high resolution. The complete system is shown in figure 1. The reflected light from the FBG is incident on a long-period grating which acts as an edge filter. The light transmitted by the long-period grating increases (or decreases depending on which side of the long-period grating is in filtering) with the strain-induced wavelength shifting of the FBG. The reference arm is required to linearise the power-strain response as the spectral distribution of the Er fibre light source is non-uniform, and compensates any fluctuation in light source. The FBG had its peak wavelength at 1519nm. The long-period grating had a 20nm transmission peak centred at ~1529nm with a rejection ratio of 22dB. The filter shows a near-linear response of ~ 1.8dB/nm allowing an unambiguous sensing range of 10nm, equivalent to ~10 000µε.

© 1998 IEEE