Low-Crosstalk and Polarization-Independent Inline Interferometric Fiber Sensor Array Based on Fiber Bragg Gratings

Abstract

In this paper, a low-crosstalk and polarization-independent inline interferometric optical fiber sensor array with four time division multiplexing (TDM) channels was demonstrated. The low level of the multireflection (MR) induced crosstalk was realized by the ultra-weak ( $ 1%) fiber Bragg gratings (FBGs). The FBGs were fabricated in a single-mode fiber without any fusion points with a pulling force and a phase mask matching method. A polarization switching (PS) method based on the phase-generated carrier (PGC) scheme was proposed to eliminate the polarization-induced signal fading and the input-polarization-induced phase noise simultaneously. The PGC-based PS method was validated and the MR-induced crosstalk was tested in the experiment. The equivalent visibilities of the 4-TDM sensors separately range from 0.9 to 0.92 for sensor 1, from 0.95 to 0.96 for sensor 2, from 0.97 to 0.98 for sensor 3, and from 0.87 to 0.88 for sensor 4 for multiple measurements and the phase noise induced by the polarization fluctuation of the leading fiber was eliminated effectively. The background noise could reach $-$ 97 dB/ $\sqrt{\text{Hz}}$ at 1 kHz. The crosstalk level lower than $-$ 45 dB was achieved for the 4-TDM sensor array. The PGC demodulation scheme is first combined with the PS method using a complexing-exponent synthesis skill which can be used in the heterodyne interrogation scheme as well. Besides, though there are only four TDM channels in the study, the low crosstalk about $-$ 40 dB can be achieved if the TDM channels are expanded to five using the ultra-weak FBGs. The ultra-weak FBGs to reduce the crosstalk is especially suitable for the high-resolution applications of short-distance fiber sensing system such as the towed-array.