Fiber Loop Mirror Based on Optical Fiber Circulator for Sensing Applications

In this paper, a different Fiber Loop Mirror (FLM) configuration with two circulators is presented. This configuration is demonstrated and characterized for sensing applications. This new design concept was used for strain and torsion discrimination. For strain measurement, the interference fringe displacement has a sensitivity of (0.576 ± 0.009) pm‧με^-1. When the FFT (Fast Fourier Transformer) is calculated and the frequency shift and signal amplitude are monitored, the sensitivities are (-2.1 ± 0.3) × 10^-4 nm^-1 με^-1 and (4.9 ± 0.3) × 10^-7 με^-1, respectively. For the characterization in torsion, an FFT peaks variation of (-2.177 ± 0.002) × 10^-12 nm^-1/° and an amplitude variation of (1.02 ± 0.06) × 10^-3/° are achieved. This configuration allows the use of a wide range of fiber lengths and with different refractive indices for controlling the free spectral range (FSR) and achieving refractive index differences, i.e., birefringence, higher than 10^-2, which is essential for the development of high sensitivity physical parameter sensors, such as operating on the Vernier effect. Furthermore, this FLM configuration allows the system to be balanced, which is not possible with traditional FLMs.