Mode-locked fiber laser gyroscopes (ML-FLaG) with a semiconductor optical amplifier and an erbium-doped fiber (EDF) as a gain medium have been constructed, and the output characteristics of the ML-FLaG's have been investigated. Significant improvements in the performance of the ML-FLaG's have been made by applying a lock-in detection method for signal processing.
The ML-FLaG is a fiber laser employing a Sagnac interferometer both as a rotation sensing element and a feedback reflector in a laser cavity. It consists of a laser cavity formed by a conventional mirror and a Sagnac interferometer, a gain medium, and a phase modulator inside the Sagnac loop interferometer. The output of a ML-FLaG is an optical pulse train having two pulses per every round-trip time of light in the laser cavity. The time interval between the two consecutive pulses varies as a function of rotation rate, that can be measured by using a time interval counter or a lock-in amplifier as described in this work.
The ML-FLaG built with a semiconductor optical amplifier as a gain medium exhibited stable pulse train. However each output pulse was too wide and limited the dynamic range of the gyroscope, that originated from the rapid gain recovery time of the semiconductor optical amplifier compared to pulse intervals.
To make optical pulses narrower, we have constructed a ML-FLaG with an EDF (ML-FLaG:EDF) as the gain medium, which shows a much slower gain recovery. The output pulse train exhibited significant instability due to gain competition, that could be suppressed by detuning the modulation frequency from the original longitudinal mode spacing of the laser
However, the remaining fluctuation in the relative intensity of the two pulses, though very small, gave rise to non-negligible errors in the direct time interval measurement. Using a lock-in detection technique, the error terms could be easily removed by phase sensitive detection mechanism. Moreover, the direct output of the lock-in amplifier has a desirable linear dependence on the rotation rate, that is a great advantage over a sinusoidal dependence in conventional fiber-optic gyroscope. The dependence of the scale factor on the total output power can be removed by normalizing the output of the lock-in amplifier with respect to the average laser output power.
The ML-FLaG:EDF using the lock-in detection showed the bias drift of about 0.8 deg/hr, corresponding to 10 μrad of phase error that was about 1/20 of that from the time interval measurement. The rms noise equivalent random walk was also improved to 0.018 deg/$\sqrt {hr}}$ which corresponded to 9 μrad/$\sqrt{Hz}}$.
In summary, the work described in this thesis demonstrated ways to improve the performance of a ML-FLaG significantly by employing EDF as a gain medium, detuned modulation frequency, and a lock-in detection method.
