Mode-locked fiber laser gyroscope.

We propose and demonstrate a novel fiber-optic gyroscope in the form of a mode-locked laser consisting of a laser cavity formed by a planar mirror at one end, a gain medium, and a Sagnac interferometer at the other end. The Sagnac interferometer serves as a reflector as well as a rotation-sensing element. The output of the gyroscope is a series of short optical pulses. The separation of two sets of optical pulses in the time domain changes as a function of rotation rate, providing a much simplified signal processing compared with that in conventional fiberoptic gyroscopes. The Sagnac effect introduces differential phase shift between the countercirculating optical waves. In a ring-laser gyroscope,' the differential phase shift is translated into beat frequency of the laser lines oscillating in the opposite directions, which could be measured with high accuracy. For interferometric fiber-optic gyroscopes, however, the basic intensity output from the Sagnac interferometer has a sinusoidal dependence on the differential phase shift. Relatively complicated electronic/optical signal processing is required in order to recover the differential phase information that is linearly proportional to the rotation rate. A number of closed-loop24 and open-loop5 -8 signal-processing techniques have been developed for this purpose. It is, however, still felt that a simpler signal processing than the existing ones is desirable for better performance and/or lower cost. The novel gyroscope configuration described in this Letter combines the characteristics of the ring-laser gyroscope and the interferometric fiberoptic gyroscope. The entire optical circuit operates as a laser without gain competition between the counterpropagating waves in the sensing loop. The rotation-induced Sagnac phase shift is translated into spacing of the mode-locked pulses instead of the beat frequency. A schematic of the simplified mode-locked fiber laser gyroscope (MLFLG) is shown in Fig. 1(a). It consists of a laser cavity formed by a planar mirror at one end and a Sagnac interferometer at the other end, with an optical amplifier in between. When the optical gain provided by the amplifier is greater than the round-trip loss, the system operates as a cw laser since the Sagnac interferometer acts as a loop reflector.9 The reflection coefficient of the loop reflector is a function of the rotation rate or any nonreciprocal phase shift introduced between the counterpropagating waves in the Sagnac interferometer. A fiber-optic phase modulator located near one end of the fiber coil, as shown in Fig. 1(a), can be used to modulate the optical loss in the cavity by modulating the phase difference between the counterpropagating waves. When the frequency of the loss modulation is the same as the frequency spacing of the longitudinal modes of the laser [i.e., Af = c/n(L. + 2Le), where n is the refractive index], mode locking takes place, and the output of the laser become a series of short pulses. The timing of the pulses is determined such that the oscillating pulses in the cavity pass through the loss modulator at the time of minimum loss. On the other hand, the depth of optical loss modulation for the system in Fig. 1(a) becomes maximum when the modulation frequency is fm = c/2Lcn. At this frequency, the modulation provided by the loop