Fiber optic sensors have been developed for various purpose in many different forms. Among the sensors, the fiber optic gyroscope(FOG) technology has made remarkable progress over the past 17 years, and it is about to be mass produced for inertial navigation systems.
This thesis describes a fiber optic gyroscope with broadband source and polarization maintaining fiber(PMF), and a novel mode locked fiber laser gyroscope using a Nd-doped fiber source.
The optical source for the PMF gyroscope is a superluminescent laser diode with λ = 823 nm and Δλ = 15 nm. We have fabricated polished tunable couplers with polarization maintaining fiber. The polarization extinction ratio of the couplers were about 20 dB. Polarization maintaining fiber optic gyroscope was constructed by splicing the fiber optic components. Long term stability at earth rotation rate of better than 3˚/hr was observed with 100-sec integration time. This value depended on the temperature fluctuation of the environment. The rums short term noise was 0.6˚/hr/$\sqrt{Hz}$. Also, a digital signal processing(DSP) for the demodulation of the direct gyro output was demonstrated with a linear scale factor. The rms noise of the DSP was 28˚/hr with 1-sec integration time.
The source for the fiber optic gyroscope must be temporally incoherent with high power, and should have stable wavelength. Rare-earth doped fiber sources can produce such conditions for the FOG. We have proposed and demonstrated a mode-locked fiber laser gyroscope(MLFLG) using Nd-doped fiber sources. 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. The output of the MLFLG is a series of short optical pulses. The Sagnac interferometer used in the conventional FOG functions as a loop reflector. Without any rotation input, the optical pulses are equally spaced in time. With rotation input, the two sets of optical pulses are shifted in time by the same amount but in opposite directions. Thus, the rotation induced phase shift is obtained from the timing shift of the pulses. This new configuration provides the possibilities of a simpler fiber gyroscope with direct digital output.
We have investigated the polarization properties of MLFLG. The non-reciprocal phase shift is observed when we apply polarization change to the sensing coil. For a stable reciprocal operation of the MLFLG, a polarizer must be place in the system. We also observed the nonlinear Kerr effect in the Sagnac interferometer that causes a self-maintained passive mode locking of the fiber laser. An efficient pulse shortening by the nonlinear effect is obtained when the two counter-propagating optical pulses interfere with proper phase offset. The full width at half maximum of the passive modelocked pulse by the nonlinear effect obtained in the MLFLG was less than 3.6 ns.
