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Advancements in silicon photonics technology have resulted in significant progress toward tactical-grade chip-scale optical gyroscopes for applications such as inertial navigation for a range of self-driving vehicles. Our first generation of gyro, reported a year ago, was a resonant ring gyro fabricated with an ultra-low-loss silicon-nitride waveguide in a racetrack shape with a perimeter of 37 mm and a finesse of 1270. When the laser frequency was tuned to interrogate the resonance with the lowest backscattering coefficient, and balanced detection was implemented to reduce common noise in the two output signals, the angular random walk (ARW) was measured to be 80 deg/h/√Hz, and the gyro output was dominated by backscattering noise. The second-generation reported here utilizes a longer ring to further reduce backscattering noise. The ring resonator is a circular spiral with 33 turns, a length of 1.2 m, and a finesse of 29. When interrogated with a narrow-linewidth laser like the racetrack gyro, it has a measured ARW of 210 deg/h/√Hz dominated by laser-frequency noise. The ARW is higher than that of the racetrack gyro because the balanced detection was not as effective (13.2 dB of common noise rejection compared to 18 dB in the racetrack gyro). Tests in a vacuum indicate that environmental fluctuations do not contribute to the noise, and that most of the measured drift (3,500 deg/h) has an optical and/or electronic origin. We also report the noise performance of the racetrack gyro interrogated in a Sagnac interferometer probed with broadband light. This configuration was inspired by a recent publication from Shanghai Jiao Tong University that reports a resonant fiber optic gyroscope interrogated with broadband light with a measured ARW that meets tactical-grade requirements. The advantages of this interrogation technique are that it eliminates the need to stabilize the resonator, it reduces the component count, and by making use of incoherent light, it reduces the backscattering noise. The measured ARW of the racetrack gyro interrogated with broadband light was dominated by excess noise at large detected powers, and it was a factor of ~900 larger than the ARW of the same racetrack gyro interrogated with the laser. The reason for this increase in ARW is that the advantage of having a high-finesse resonator is lost when the ring is interrogated with broadband light, and the sensitivity is reduced by a factor of the finesse compared to the same ring resonator interrogated with a laser. This reduction in sensitivity is demonstrated experimentally. Achieving tactical-grade requirements will require returning to a laser interrogation, improving the balanced detection scheme to achieve a noise cancellation of 25 dB or better, and optimizing the laser linewidth to minimize both laser frequency noise and backscattering noise.
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