Crystalline Waveguides for Optical Gyroscopes

Abstract

We demonstrate ultra low loss and ultra low scattering ring waveguide optical resonators made of calcium fluoride crystal and illustrate their usefulness in optical metrology. The resonators are fabricated from crystalline preforms by mechanical polishing and subsequent thermal annealing. The feasibility of long delay lines based on the crystalline waveguides of similar morphology is discussed. A proof of principle chip-scale model of a Raman laser gyroscope using a calcium fluoride ring optical waveguide microcavity pumped with a C-band diode laser is demonstrated. The high quality of the optical waveguides results in symmetric clockwise (cw) and counterclockwise (ccw) Raman emission in the cavity. The counter-propagating Raman emission is utilized to measure rotation revealing itself as a Sagnac-induced frequency shift. The relative instability of the cw and ccw laser emission does not exceed 1 Hz for 0.1–10 $^4$ second averaging time and the frequency noise of the signal is less than 0.8 Hz/Hz $^{\text{1/2}}$ enabling a 3.8-mm ring cavity gyroscope prototype with better than 100 deg/hr sensitivity. We argue that the measurement sensitivity can be improved by four orders of magnitude by optimization of the Raman laser configuration.