This thesis describes a signal processing technique of digital phase tracking for open-loop fiber optic gyroscopes, and resolution enhancement of the digital phase tracking approach. A fiber optic gyroscope is an interferometric instrument which measures the rotation rate utilizing the Sagnac effect. In general, the output of an interferometer is cosinusoidal function of phase difference and has the 2π ambiguity, leading to a narrow dynamic range and low sensitivity when rotation rate is very small. The digital phase tracking signal processing is a technique for open loop fiber optic gyroscopes to solve the problems mentioned above.
In digital phase tracking technique the detector signal is multiplied by a digital waveform which has an adjustable pulse spacing δ. The digital waveform maintains the dc component of the multiplied signal at zero by adjusting δ. When the phase modulation amplitude $\Phi_m$ is 2.77 radians, the pulse spacing of the digital waveform is linearly proportional to the rotation induced Sagnac phase shift. The parameter δ of the digital waveform is monitored to read the Sagnac phase shift. In our experiment, 9 bits were used for the digital waveform generation, and the least significant bit(LSB) of the digital circuit corresponds to the Sagnac phase shift of 0.352 degrees.
The limited resolution problem was solved by reading and integrating the error signal. The magnitude of the error signal of the feedback loop can be read by an Analog/Digital converter. The error signal provides the information of uncompensated rotation rate by the digital waveform, enhancing the resolution. With an 8 bit A/D converter the LSB of the electronic processing circuit corresponds to 0.00034 deg of the Sagnac phase shift.
The gyroscope was composed of a Super-Luminescent Diode(SLD) as a light source, two polished directional couplers, a polarizer, 1 km - long Sagnac loop, and an avalanche photodiode. The center wave-length of the SLD was 823 nm and FWHM was 15 nm. After enhancing the resolution, the gyroscope could resolve the earth's rotation rate. The electronic processing circuit has short term rms noise of 2.5 deg/hour/$\sqrt{Hz}$ and long term drift of 10 deg/hour.
