The productionization of fiber gyros at Litton in the 1.0 to 0.1 deg/hr acuracy range will be reviewed. The LN-200 is an AHRS/missile IMU system, utilizing these gyros and silicon accelerometers, which is nearing completion of its productionization and has been selected for the new Army Comanche helicopter. Performance data on the LN-200 system will be presented.

FOGs are developed for industrial applications such as car navigation or location systems, self-guided robots, bore-hole route survey systems, geodesic compasses, etc. The FOGs are based on an open loop and minimum reciprocal configuration with all polarization maintaining (PM) fiber components. Because the fiber optic components are almost ready for mass production, FOGs provide an attractive economical aspect.

FOGs are developed for industrial applications such as car navigation or location systems, self-guided robots, bore-hole route survey systems, geodesic compasses, etc. The FOGs are based on an open loop and minimum reciprocal configuration with all polarization maintaining (PM) fiber components. Because the fiber optic components are almost ready for mass production, FOGs provide an attractive economical aspect.

A fiber-optic gyroscope(FOG) has already been selected to fulfill the inertial navigation system(INS) function of the new satellite launching vehicle of ISAS in Japan, and the flight test of our FOG was successfully demonstrated on February 22, 1990. This paper covers the aiming of development, design description, evaluation and productization of our FOG. Finally, reviews of a broader perspective of this new gyro technology are descussed.

We describe our fiber gyro activity that has been directed toward prototypes with a very high dynamic range. Compact gyros ((phi) 50 mm X 28 mm) have been realized with reproducible performances (bias stability of 0.5 deg/h at 1 (sigma) , and scale factor linearity of 5 to 20 ppm at 1 (sigma) over +/- 1300 deg/h).

British Aerospace has a long history in the design and development of optical gyros. The closed loop fiber optic gyro using the interferometric gyro architecture operating at 1.3 micron wavelength has been the principal development in recent years. Parallel digital data is output within a 70 mm cube gyro package. Performance data is presented for these gyros covering a rate range of +/- 1000 deg/sec under various environmental conditions. A resonator fiber optic gyro is also under development. Design principles are reviewed and ranges of application are given.

The transition to manufacturing a single-axis rate sensor as the first IFOG product began in 1987. It included the design and development of semi-automated work stations for production and test. Up to now, 200 units have been produced and delivered. The gyro is based on a modular design that is also part of the qualification strategy. It gives the opportunity to offer an expanded line of single-axis and three-axis devices using the same modules for different package versions.

The present investigation of FOG behavior under vibration conditions gives attention to effective techniques for vibration-compensation. The FOGs tested are noted to yield an intermediate level of performance, of the order of 3 deg/hr at 1-sigma, under commonly encountered vibration conditions and over the temperature range associated with aerospace applications. These FOGs have been deemed adequate for use in the triaxial inertial devices of the guidance and control system aboard the TR-1 rocket.

The LFS-90 FOG system for flight-control applications presently discussed, in which three gyros are combined in a 'triad' configuration, operates on the basis of digital closed-loop electronics and uses a digital signal processor for error compensation. The LFS-90 unit is able to communicate with a flight control system by a variety of commands, and is ideal in cases where volume restraints are a paramount consideration; system volume is only 0.56 liters.

An investigation is underway regarding the usefulness of altazimuth-mounting telescopes' incorporation of laser gyros for pointing and fiber gyros with extremely small random-walk coefficient for telescope inertial stabilization during tracking. A star tracker is expected to help stabilize long-term gyro bias. Gyro and telescope specifications have been derived by means of computer simulations and systems analyses.

A test program has been conducted with a view to defining methods for reducing the drift of optical passive ring-resonator gyros (OPRG). Attention is given to such noise sources and noise-reduction techniques as Rayleigh backscattering, polarization fluctuation, and fiber-resonator optical Kerr effect. A novel, partial digital feedback scheme is defined for preventing the OPRG from lapsing into large drift, due to thermal expansion of the resonator.

A novel fiber-optic resonant ring has been demonstrated that eliminates one of the orthogonal resonances in the cavity. Standard birefringent fiber resonators support two resonances, associated with the fast and slow eigenmodes of the resonator. The addition of an intercavity device increases the loss in one of the eigenmodes, thus squelching that resonance. We have fabricated resonant rings from birefringent fiber polished couplers with a length of polarizing fiber spliced into the cavity, which eliminates the orthogonal resonance, even for moderate amounts of subcomponent polarization cross-coupling. Resonators with deep, symmetric dips and with finesse of about 25 have been obtained, fabricated from adjustable and fixed polarization-maintaining couplers.

The theory of polarization effects in an optical passive ring resonator gyro (OPRG) will be discussed using Jones matrices. The consequences of a mismatch in the states of polarization incident to the optical fiber ring resonator (OFRR) and a misalignment in the principal axes of its directional coupler shall be considered. Particular attention will be paid to ways of improving the gyro's reciprocity by means of an in-loop polarizer.

For the light source of the passive ring resonator fiber-optic gyro (RFOG), we have developed 1.3 micron highly coherent laser diode module (HCLDM) using resonant optical feedback from a compact hemispherical reference cavity. The HCLDM had the 100 KHz linewidth and was frequency-modulated by directly modulating the injection current. A FM spectroscopy technique was employed for rotation-sensing. The resolution of the present RFOG system was estimated to be 0.9 X tau exp -1/2 (deg./hour) (where tau is the integration time of measurement) from the signal-to-noise ratio of the demodulated signal.

A novel fiber optic resonator employing a 90 degree polarization rotation was developed to reduce polarization related bias errors of the resonator fiber optic gyro. For the first time, we have fabricated and tested a nearly all guided-wave gyro having a polarization-rotating fiber resonator. Bias stability better than 0.4 deg/hr, random walk of 0.1 deg/root-hr, and day to day absolute bias stability of 10 deg/hr are reported. The major error mechanism of bias instability and other error sources are discussed.

An account is given of a novel instrument that employs white-light interferometry to determine the spatial distribution of parasitic polarization coupling along as much as 1 km of high-birefringence optical fiber. This device is directly applicable to test-gyro coils in which localized spurious coupling points are able to affect bias stability. The required measurement time is of the order of 1 min for 1 km of fiber.

An inertial-grade interferometric fiber optics gyroscope (IFOG) was developed for potential use in NASA deep-space missions. The closed-loop IFOG system, which incorporates beat detection circuitry exhibits the best measured long-term bias drift of 0.0016 deg/hour. The key component of the system is an advanced 13-component LiNbO3 integrated optical circuit (IOC) fabricated by annealed proton exchange process.

The present digital filter structure and processing scheme employs a minimum of hardware for the efficient computation of FOG rotation rates. The filter system encompasses preamplification, a demultiplexer, a directional comb filter, a complex bandpass filter, an equalizer, and a frequency demultiplexer. This scheme has used the evaluation of four spectral lines to achieve an unequaled level of scale-factor stability.

The present, simplified analog signal-processing technique for an open-loop FOG is based on Kersey and Moeller's (1990) closed-loop phase-shift-nulling concept. This approach emulates the operation of a true closed-loop gyroscope on the basis of low-cost phase tracking, and furnishes a linear output from an open-loop gyro configuration over a +/- 2-pi radian range in Sagnac phase shift. Good linearity and comparatively low noise and drift characteristics are obtained.

The present treatment of FOG system phase-modulation theory proceeds on the basis of the Mach-Zehnder interferometer's theoretical framework. Attention is given to the phase-bias, open-loop, and closed-loop modulation aspects of the theory; a general theoretical characterization of phase-modulated FOGs is obtained and compared with experimental results for a Ti:LiNbO3 electrooptic crystal waveguide phase modulator.

A new topology is introduced for an analog phase detector for open-loop fiber-optic gyroscopes which consists of a linearization function and a control function. It is shown that five different trigonometric functions are suitable to perform the linearization function. It is shown that by introducing a feedforward loop, an exact replica of the input phase will be produced at the detector output. In addition, a technique is also presented which increases the angular range of the phase detector significantly beyond the +/- 3 pi range which is the limitation of the trigonometric generators in the circuit.

Open-loop signal processing techniques for a phase-modulated optical fiber gyroscope are discussed and classified according to sampling or non-sampling, frequency- or time-domain, and real or equivalent time schemes. A duality theory between frequency and time for the gyroscope signal is developed. The rotation rate supplied to the gyroscope is detected from both the frequency and time signals. As examples of equivalent time signal processing, down sampling and pulse modulation techniques are proposed.

A FOG has two electrical signal interfaces: the input u(t) for the modulation signal and the output v(t) from the photodetector. The signal path u(t) to v(t) through the Sagnac-interferometer can be modeled as a nonlinear transmission channel. The characteristics of this transmission channel are dependent on the parameter rotation rate Phi(s). A combination of variable period serrodyne and fixed frequency sine signals is used as the modulation signal u(t). The received spectrum of v(t) depends on the rotation rate Phi(s). It is shown analytically that the Sagnac-shift is exactly compensated, when the amplitudes of two of the spectral components are equal.

Medium to high accuracy gyroscopes will require scale factor corrections in the range of 100 ppm to better than 10 ppm. Recently efforts have been made to attain these accuracies by using stabalized light sources. This paper reviews an alternative method that uses the dispersive properties of the fiber coil to monitor changes in the light source spectral content.

Nonreciprocal phase biases were experimentally investigated in a single-mode fiber-optic Sagnac type gyro with nonsingle-mode spatial filters. A good correlation between the nonreciprocal phase biases in the fiber-optic ring interferometer and the intensities of the higher order core modes and cladding modes in the fibers used as spatial filters was obtained. A new experimental method for determination of the optimum spectral range between the source wavelength in the gyro and the effective cutoff wavelength of the single-mode spatial filter was proposed.

Honeywell's first-generation, high-performance interferometric fiber-optic gyroscope (IFOG) was tested along with sensor electronics over a limited static and dynamic temperature environment. This serrodyne closed-loop IFOG consisting of polarization-maintaining fiber components and a LiNbO3 phase modulator has simultaneously achieved inertial-grade requirements for bias stability, bias repeatability, bias temperature sensitivity, bias temperature-rate sensitivity, random noise, and scale-factor error over the tested temperature range.

We review the Sagnac effect in the application of the fiber optic gyroscope. Non-reciprocal phase shifts resulting from environmentally-induced fluctuations in polarization-maintaining (PM) fiber parameters are investigated theoretically and experimentally. It is pointed out, based on our earlier measurements, that temperature and stress cause random perturbations in fibers designed for gyroscope applications. Analysis for the experimental data are presented. The data at low temperature show large noise that rule out some PM fibers for high accuracy applications.

We evaluated the phase and amplitude characteristics of output noise in fiber optic rotation sensors with a single-mode fiber sensing loop. We found that the major source of the noise was amplitude modulation (AM) noise caused by a phase modulator, and that the AM noise always had an approximate 90 degrees phase difference from the rotation signal. Thus, by adjusting the lock-in reference signal phase to the node of the AM noise, the output drift was markedly reduced to about 2 deg/h.

An account is given of a proprietary method for the production of flawless quadrupole windings between flanges, facilitating the maintenance of the levels of precision required for interferometric FOG (IFOG) coils intended for low manufacturing cost implementation. In addition to presenting IFOG coil-winding requirements, attention is given to factors affecting winding costs, winding equipment options, and results obtained to date with cost reduction efforts.

Miniature semiconductor laser diode (SLD) modules have been developed and tested under conditions representative of aerospace applications; it is established that a 1.0-dB change in output power occurs over the -55 to +85 C temperature range. The SLDs are shown to exceed conventional modules in both compactness and output levels.

Stimulated Brillouin scattering (SBS), a nonlinear process in which an intense beam of pumping light propagates in a single mode fiber in such a way as to generate gain for a counterpropagating, frequency-downshifted beam, is presently used in the design of a fiber ring laser gyroscope. In this gyro configuration, two independent, counterpropagating SBS lasers are generated that share the same cavity and frequency, as well as almost the same spectral width. The readout of such a gyro is inherently digital.

We introduce models and realizations demonstrating the feasibility of multimode fiber optic gyroscopes (MFOG). Some solutions to specific problems of MFOG (contrast instability and zero drifts) are described in terms of coupling effects on nonidentical counter propagating modal distributions. Our first experimental results show good correlations between phase and intensity variations under modal coupling effects. We propose a method of detection adapted to MFOG which uses a zero drift stabilization device based on these correlations.

The present FOG uses a fiber-coupled 1.06-micron isolator between the source output and gyro input to achieve stable operation with forward pumping. The rms optical scale factor stability thus achieved is of the order of 15 ppm, in conjunction with 0.05 deg/hr rms phase-bias stability. Due to the specific doping used for the Nd fiber employed by this FOG, the source optical linewidth is only 6.8 nm wide; this is significantly less than in comparable fibers.

A 2.8-inch diameter, low-cost interferometric gyroscope has been created through the application of a prototype LiNbO3-based integrated optical circuit (IOC) that incorporates polarizing waveguides, phase modulators, a power splitter/mixer, and polarization-maintaining 'fiber pigtails'. Instrument closed-loop performance exceeds 0.1 deg/hr (1-sigma) bias-drift performance at 100-sec integration time.

A comparative evaluation is conducted of the performance obtainable by means of (1) an all-fiber gyro based on a high-extinction polarizer, a Lyot-type depolarizer, and a conventional birefringent fiber coil, and (2) a gyro incorporating a polarizing integrated-optics chip fabricated by means of proton exchange in lithium niobate, as well as a high-birefringence fiber coil. Performance levels are obtained for the cases of both open and closed loop signal processing.

A theoretical and experimental treatment is presented for the feedback-dependent intensity-noise characteristics of a laser diode used in a FOG. The feedback induces an additional, time-varying noise signal in the laser output which has the same waveform as the Sagnac signal, and is amplified by as much as 13 dB. The effect of this intensity noise in the output of a 2D frequency-division multiplexed gyro is to introduce two additional terms.

One of the major error terms in the fiber optic gyro is the polarization crosscoupling in the components. This coupling has been analyzed with a white-light Michelson-interferometer with which position-resolved crosscouplings lower than -60 dB can be measured. We have analyzed sensor coils and pigtailed integrated-optic-chips to optimize winding and pigtailing. Also the extinction ratio of different types of polarizers have been measured.

A development status evaluation is presented for recently developed analytical and experimental methods for the characterization of inline fiber-optic polarizers and polarizing beam splitters that employ surface plasmon polaritons in thin metal films. Both standard and highly birefringent fiber devices are considered. Attention is given to the effect of birefringent-axis misalignment, which requires that devices on a polarization-maintaining fiber must be characterized by one or two coupling matrices for polarizers and beam splitters, respectively.

Recent research has indicated that broadband fiber sources are a viable alternative to superluminescent diodes for fiber-optic gyro applications. The key issues in using such sources are possible source configurations, the effects of feedback, source power output, and the stability of such sources in the gyro system. Both Nd:silica and Er:silica sources are discussed with an emphasis on superfluorescent fiber sources pumped by laser diodes. In both media, 100 mW of pump power can produce more than 10 mW of source power with a spectral width in excess of 10 nm. Furthermore, such sources have consistently produced mean wavelength thermal coefficients of less than 10 ppm/C.

An account is given of the design parameters of the polarization-maintaining single-mode optical fibers currently under development for incorporation in interferometric FOGs (IFOGs). Attention is given to a fiber design that satisfies IFOG requirements; optical performance parameters that encompass coil and splice results are presented. The polymer coating is found to play a substantial role in the maintenance of coil performance over a range of temperatures.

Performance of Multi—Function Chips fabricated in Ti—indiffused LiNbO3 to operate at 1.3jm is presented. Achieved polarization extinction ratios exceeding 60dB represent significant improvements over published data. Measurement techniques which isolate the various chip—performance parameters and interpretations of their influence on gyro performance will be emphasized.

The present low-performance, but very simple and inexpensive FOG contains only a single fused coupler. An account is given of the dependence of gyro bias on light source and fused coupler properties. Coupler elongation was found to result in oscillating gyro bias, for the case of a low-coherence light source and high-birefringence fibers.

Phase modulation is studied with regard to high stability, low distortion coefficient, and minimum parasitic modulation effects for the case of a gyroscope of 1 deg/hour performance based on normal birefringent single-mode fiber. Signal processing involves both a novel modification of the optical feedback to compensate for the rotation-caused Sagnac phase shift, as well as a simulation of the optical interferometer output for periodic zero rotation rate adjustment and scale factor calibration.

This paper presents a passive fiber optic gyroscope without nonreciprocal phase modulation. A fused fiber 3 X 3 directional coupler provides a constant phase shift, thus enabling the detection of rotation rate at the quadrature point. Bias and scale factor errors due to indeterministically changing birefringent coupling centers in the non-polarization preserving monomode fiber coil are eliminated by using a contrast insensitive signal recovery scheme. A simple temperature compensation procedure has been implemented in the gyro software which yields a bias stability < 0.04 deg/sec and scale factor errors < 0.5% in the whole temperature range between -40$DEGC and +70 degree(s)C.

In the past, the interferometric fiber optic gyro (IFOG) has generally been viewed as an angular rate sensor. This may appear true on a coarse time scale. However, a study of its behavior within the transit time of light through the fiber optic coil shows that, in reality, the IFOG is a rate integrating gyro with a very short memory. Making use of fast sampling and accumulation, the rate integrating characteristic can be retained. Mechanized that way Litton IFOG exhibits an output response which is similar to

The high performance optical waveguide components essential to FOGs are multifunction integrated-optics chips, fiber coils, and polarization-maintaining couplers. An account is presently given of the component performance levels thus far obtained by a major proprietary effort toward high-volume/low-cost FOG production.