EDF laser displacement sensor based on bending characteristics of polarization-independent double-pass cascaded-chirped long-period fiber grating

Abstract

In this study, we propose an Erbium-doped fiber (EDF) laser displacement sensor using bending characteristics of a cascaded-chirped long-period fiber grating (C-CLPG), in which the C-CLPG assisted by a Faraday rotator mirror (FRM), namely double-pass C-CLPG, shows a channeled spectrum and plays both the roles of wavelength tunable filter and sensor element. In the sensing scheme, the spectral shift of the channeled spectrum due to the displacement-induced bending deformation is utilized. Due to the FRM reflection, the double-pass C-CLPG has characteristics suitable for sensing: the polarization-independent channeled spectrum and the compensation effect for polarization variation in the sensor section. In the experiment, it is successfully demonstrated that the EDF laser in a sigma-type cavity configuration (EDFσL) using the double-pass C-CLPG enables precise displacement measurement based on the oscillation wavelength shift corresponding to the spectral shift of the double-pass C-CLPG. Further, the advantage of this EDFσL is confirmed by comparison with the conventional EDF ring laser using the single-pass C-CLPG.