A nonlinear mirror structure to increase the performance of the distributed fiber temperature sensor based on Raman backscattering

We propose a novel nonlinear mirror structure that can increase the optical signal-to-noise ratio of a distributed fiber Raman temperature sensor. The nonlinear mirror is constructed by a four-port 3 dB coupler, which links two sensing optical fibers. The two sensing fibers make a bunch, and sense the same circumstance. When the quasi-continuous wave optical light is injected, the 3 dB coupler forms a backscattering nonlinear mirror, and reflects the whole scattering signals from the two sensing fibers into the receiver. Due to the constructive inference of the nonlinear mirror, the signal received by the detector is as four times higher as the case that uses a single sensing optical fiber. Thus, 6 dB improvement of the optical signal-to-noise ratio is obtained. The theoretical analysis and the experimental data are presented, which shows an agreement between each other. With the assistance of the nonlinear mirror, we demonstrate the spatial resolution of the sensor is improved from 3 meter to 1 meter, and the temperature accuracy is improved from plusmn0.6degC to plusmn0.2degC. The proposed nonlinear mirror structure can be extended to employ more sensing fibers, which can further improve the system performance.