An AM(Amplitude Modulation) mode-locked bidirectional $Er^{3+}$-doped fiber(EDF) ring laser was constructed and characterized. The EDF used in the experiments had the absorption coefficients of 5.0dB/m at the wavelength of 1.53㎛ and 1.9dB/m at 514.5nm band.
The ring laser was constructed with a 4.7m long EDF, additional single mode fiber, polished type directional couplers, and an AM mode-locker. The total cavity length was ~59m. The pump beam from an $Ar^+$ ion laser was launched through the directional coupler which served as a wavelength division multiplexer(WDM).
A fiber Mach-Zehnder interferometer type AM modulator or a pigtailed $LiNbO_3$ AM modulator was used as the AM mode-locker. The stable mode-locked pulses were generated in both directions and minimum pulse duration was ~8 nsec. The spectral linewidth (Full Width at Half Maximum) of the mode-locked ring laser output was ~63MHz, which implies that eighteen longitudinal modes were locked. When the two counter-propagating pulses were combined to interfere at the output coupler, the fringe visibility of over 0.9 was obtained. When the ring cavity was rotating, the rotation rate dependent beat signal (gyro signal) was observed through a RF spectrum analyzer. The gyro signal was more stable and its amplitude was increased over 30 times compared with the continuous-wave operation of the laser with the mode-locker off.
Due to the backreflection at the electro-optic modulator, the frequencies of the two counter-propagating pulses were locked-in. When a dither was applied to the laser cavity by using two PZT phase modulators located at the symmetric positions in the cavity, the lock-in dead band was reduced remarkably. The beat frequency of the gyro signal was linearly proportional to the rotation rate. The scale factor was measured to be 0.858 (kHz/deg/sec), and agreed well with the theoretical value.
The amplitude of the gyro signal was smaller than that of the longitudinal mode beat signal by 1/30. The linewidth of the gyro signal was increased when the pump power was increased, which is not understood yet. In order to realize a useful AM mode-locked $Er^{3+}$-doped fiber ring laser as a gyroscope, these phenomena should be analyzed.
