Multi-parameter sensing system based on a polarimetric fiber ring laser sensor and beat frequency demodulation

In this paper, a multi-parameter optical fiber system is developed and demonstrated by using a single polarimetric fiber ring laser sensor and beat frequency demodulation technique. The polarimetric fiber ring laser is formed by a fiber Bragg grating, a piece of Erbium doped fiber and a 3 dB fiber coupler. Since the fiber laser cavity is long enough, there are many longitudinal modes in the laser cavity. For each order longitudinal mode, two orthogonal polarization modes will experience slightly mode splitting due to the intrinsic fiber birefringence. As a result, in this polarimetric fiber ring laser, there exists two kinds of beat frequency signals, i.e. longitudinal mode beat frequency and polarization mode beat frequency signals. When the fiber laser cavity suffers from external perturbations, such as temperature change, strain change, and the fiber birefringence change, these two kinds of beat frequency signals will experience different response and can be used to measure multiple parameters simultaneously. We have theoretically analyzed the principle of measurement temperature, strain and fiber birefringence and experimentally measured these parameters in the test. The proposed multi-parameter sensing system just uses a single polarimetric fiber ring laser sensor and one beat frequency demodulation equipment. It will be promising in many application fields due to its advantages of simple structure, portability, high sensitivity, and low cost.