Reduction of the Fresnel Reflection Effect in the Hybrid PBF-PMF Resonator for RFOG

This paper proposes a fiber length matching method that reduces the influence of Fresnel reflection in a hybrid fiber resonator for the resonant fiber optic gyroscope (RFOG). A transfer model of the fused points inside and outside the fiber resonator is first built. Using the transfer model, we find that when the optical path ratio of the photonic bandgap fiber (PBF) is half of the entire fiber ring, the secondary resonant frequency points caused by Fresnel reflection coincide with the main resonant frequency points of the resonator, and the resonant depth becomes equal. The simulation result of the resonance curve demonstrates this theory. To validate our deduction, two hybrid resonators are obtained with PBF and polarization-maintaining fiber (PMF). The optical path ratio of one resonator is 0.5, and the other is approximately 0.2. The results show that after the fiber length matching, the resonance depth difference of the resonator is less than 1.8%, which is the best reported index of a fused PBF fiber resonator to our knowledge and 24.9% less than that of the other resonator. The experimental result also shows that the total loss of the two PBF-PMF fused points is less than 1.3 dB. Our scheme effectively reduces the effect of the Fresnel reflection at the fusing point in the PBF-PMF resonator, which is expected to achieve accurate resonance frequency detection of the RFOG.