A Mode Locked Fiber Laser Gyroscope (MLFLG), having a form of a linear-cavity laser with a Sagnac loop mirror, is constructed, characterized and analyzed. The Sagnac loop is built with a circular-core single-mode fiber of 190 meters, and does not contain a polarizer. The optical amplification is provided by a Nd-doped single-mode fiber pumped by a laser diode. A phase modulator in the Sagnac loop is used to modulate the loop reflectivity at the frequency matched to the laser cavity length, which generates the AM-mode locking. Under an optimized condition, two reciprocal pulses are produced in one round-trip time, which are equally spaced in the time domain when the Sagnac loop is at rest. When the loop is rotating, however, the Sagnac phase shift induced by the rotation makes the two pulses closer (or farther) in time domain, in proportion to the rotation rate. By measuring the pulse timing shift, the MLFLG can be used to sense the rotation rate.
With arbitrary birefringence in the cavity, however, six pulses can be produced in one round-trip time. Unlike the two reciprocal pulses, the other nonreciprocal pulses experience the nonreciprocal phase shift even in the stationary Sagnac loop. The nonreciprocal pulses have well defined polarizations, and the polarization states are dependent on the birefringence of the Sagnac loop. These experimental observations are analyzed by solving the laser cavity equation. One important result of the analysis is that the reciprocal pulses satisfy automatically the reciprocity condition without the help of a polarizer located in the laser. This is a great advantage over the conventional interferometric fiber-optic gyroscope that requires a high performance polarizer.
The phase modulator located in the Sagnac loop is implemented with a cylindrical PZT transducer around which the optical fiber is wound. When an electrical signal of a few hundred kilohertz is applied to the PZT, an unexpected birefringence modulation takes place, as well as the phase modulation. The magnitude of the polarization modulation is proportional to, roughly, the square of the modulation frequency. This phenomenon is explained by considering the acoustic wave, generated from the cylinder wall and oscillating transversely in the optical fiber. This novel principle can be utilized for implementing an all-fiber polarization scrambler.
