Mode-locked fiber laser gyroscope (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. The output of the 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 is a function of rotation rate, which can be measured by using a time interval counter or a lock-in amplifier. The time interval detection has advantages of independence from the total power and simplicity of signal processing. But it has strong dependence on the peak power difference between mode-locked pulses. On the other hand, the lock-in detection has merits of linear proportionality to the rotation rate and immunity from the peak power difference. However, this method also has a disadvantage of strong dependence on the total power.
The ML-FLaG with an erbium-doped fiber (EDF) as a gain medium has been constructed and the output characteristics of the ML-FLaG has been investigated. The ML-FLaG built with an EDF exhibited very strong gain competition between two mode-locked pulses when modulation frequency was tuned exactly to the resonant frequency of the laser cavity. However, we could achieve stable mode-locking, when the modulation frequency was slightly detuned from the resonant frequency, where the pulse-duration (a few tens of nsec) and spectral width (a few nm) was fairly large.
The ML-FLaGs based on a polarization-maintaining fiber (PMF:ML-FLaG) and a conventional single-mode fiber (SMF:ML-FLaG) were composed separately and characterized using both a time interval counter and a lock-in amplifier. The long term drift and the short term noise of PMF:ML-FlaG were 500 μrad and 18 μrad/$\sqrt{Hz}$ with time interval detection. They were 13 μrad and 6.8 μrad/$\sqrt{Hz}$ with lock-in detection. Significant improvements by the lock-in detection method, means that fairy large peak power difference existed in the case of PMF:ML-FlaG. It was proved that the peak power difference came from the loss modulation in the LiNbO3 integrated-optic gyrochip.
The optimum configuration of SMF:ML-FLaG was found out, by which nonreciprocal pulses could not build up, no signal fading took place, and polarization modulation was suppressed. The long term drift and the short term noise of SMF:ML-FlaG were 52 μrad and 18 μrad/$\sqrt{Hz}$ , with time interval detection. Even though SMF:ML-FLaG consisted of cheap components, its performance was comparable to that of PMF:ML-FlaG, which composed of expensive components.
