A system for detecting the eigen frequency of a sensing coil in a fiber optic gyro (FOG) that includes a fiber coupler connected to the light source, an integrated optics chip (IOC) capable of modulating light received from the light source via the fiber coupler, a sensing coil in communication with the IOC, a first modulation generator for imparting a first modulation signal to the light, and a photodetector for receiving light returning from the sensing coil that is representative of a rotation rate of the sensing coil. Along with the foregoing there is provided a second modulation generator for imparting a second, preferably sinusoidal, modulation signal to the light, a high-frequency demodulator in communication with a signal produced, at least indirectly, by the photodector, and a low-frequency demodulator in communication with the high-frequency demodulator. The high-frequency demodulator receives the first modulation signal as a reference frequency and the low-frequency demodulator receives the second modulation signal as a reference frequency. An output of the low-frequency demodulator represents a magnitude and sign of a frequency difference between a frequency of the second modulation signal and an even-harmonic of the eigen frequency.

A fiber optic gyroscope having polarization error-correcting modulation. Polarization errors originate from the interference of primary waves and secondary cross-coupled waves. Although these error signals are very difficult to remove modulation signals may be applied to the integrated optics chip that minimize the total error over a period of time. By providing distinct but inter-related modulation signals to opposing inputs of the sensing coil, the various polarization errors can be averaged to zero over the period of the modulation signals.

A method for reducing bias error in a Vibrating Structure Gyroscope having a vibrating structure, a primary drive for putting the vibrating structure into carrier mode resonance, a primary pick-off device for sensing carrier mode motion, a secondary pick-off for sensing response mode vibration of the vibrating structure in response to applied rotation rate, a secondary drive for applying a force to control the response mode motion, closed loop primary control loops for maintaining a fixed amplitude of motion at the primary pick-off device, for maintaining the drive frequency at the resonance maximum, and secondary control loops for maintaining a null at the secondary pick-off device. In the method the ratio SF.sub.QUAD over SF.sub.IN-PHASE is measured from the secondary control loop to provide a direct measurement of Sin (.phi..sub.SD+.phi..sub.PPO), according to the relationship SF.sub.QUAD=SF.sub.IN-PHASE.times.Sin (.phi..sub.SD+.sub.PPO) where SF.sub.QUAD is the quadrature scalefactor SF.sub.IN-PHASE is the in-phase scalefactor, .phi..sub.SD is the phase error in the secondary drive and .phi..sub.PPO is the phase error in the primary pick-off device. The total phase error .phi..sub.E is obtained directly from the measured Sin (.phi..sub.SD+.phi..sub.PPO) according to the relationship; .phi..sub.E=.phi..sub.SD+.phi..sub.PPO and phase corrections applied to the secondary drive and/or primary pick-off device to reduce the phase error .phi..sub.E, and hence the quadrature bias error, to enhance the performance of the gyroscope.

An optical communications system comprises, among other things, a laser source; an optical waveguide interconnected to the laser source to carry an optical signal from the source to an optical receiver; an optical receiver interconnected to the optical waveguide for decoding the signal; and a mechanical modulator adapted to substantially continuously mechanically perturb a portion of the optical waveguide so as to reduce Rayleigh backscattering from the optical waveguide.

A carrier suppressed information signal modulation arrangement employs a plurality of serially coupled modulators. Each of the serially coupled modulators has a carrier input, a modulating signal input and a modulated carrier output. A carrier is applied to the carrier input of the first modulator and the suppressed modulated carrier output of each modulator is coupled to the carrier input of a succeeding modulator. An encoder responsive to the information signal generates modulating signals to be applied to the modulating signal inputs of the serially coupled modulators, such that a product of the modulating signals forms the information signal. The modulating signals applied to each of the serially coupled modulators modifies the modulated carrier output received at the carrier input of the respective modulator to produce a carrier suppressed information signal modulated output of the last of the serially coupled modulators.