Thermal Sensitivity of the Birefringence of Air-Core Fibers and Implications for the RFOG

We present an analytical model of the drift in the resonant fiber optic gyroscope (RFOG) caused by thermally induced changes in the fiber beat length L_B(T) and find that the relevant metric for thermal-drift performance is not dL_B/dT but rather (1/L_B²)dL_B/dT. This disfavors conventional solid-core PM fibers for which L_B is very short and dL_B/dT is large. Because the air core of hollow-core fibers does not support the strains associated with differential thermal expansion in conventional PM fibers, the birefringence of hollow-core fibers should be more thermally stable than conventional fibers with a correspondingly smaller (1/L_B²)dL_B/dT. To identify the relative merits of different types of fibers in the RFOG, we measure (1/L_B²)dL_B/dT for a variety of air-core and solid-core fibers by recording the shift in the resonance frequencies of an all-fiber ring resonator when its temperature is changed. We find that the PM air-core fiber offers a 270-fold improvement in (1/L_B²)dL_B/dT coefficient compared to a Panda fiber.