The paper summarizes recent developments in interferometric fiber sensors for physical field applications, based on two-beam interferometric configurations, mainly the all-fiber Mach-Zehnder interferometer. Developments in signal processing which are reviewed include those in phase noise reduction, low-phase noise sources, passive homodyne detection, and polarization phenomena, and recently developed multiplexing techniques. Special attention is given to applications of interferometric fiber-optic sensors in the areas of underwater acoustic sensing, magnetic sensing, and dc sensing.

A fiber-optic, interferometric, flexural disk hydrophone cast from an epoxy resin is described. This hydrophone is designed in the shape of an enclosed, hollow cylinder with pairs of flat, spiral wound coils of optical fiber embedded in each sensing plate. An all-fiber Michelson interferometer is used to detect the optical phase shift which results from pressure induced strains in the optical fiber. The sensing coils are positioned in the plates in a manner to enhance the acoustic response and provide cancelation of acceleration induced signals. An epoxy resin was chosen for its relatively high tensile strength, its low Young's modulus, and its ability to cure at room temperature. The acoustic sensitivity of this sensor in both air and water was measured to be 0.277 0.005 rad/Pa (-131.2 dB re rad/μPa) which corresponds to a normalized sensitivity (formula available in paper)re 1 μPa^-1 below 1 .0 kHz. This measured result is in excellent agreement with simple elastic theory and the measured epoxy elastic constants. The normalized acceleration sensitivity is (formula available in paper)dB re g^-1. The acceleration-to-acoustic sensitivity ratio (figure-of-merit) of -134 dB re g4tPa is the largest reported to date for any fiber-optic hydrophone.

Results are presented of long-term testing of the reliability and performance of fiber-optic sensors developed by the Advanced Fiberoptic Technologies Corporation. The performance tests, conducted in accordance with national reference standards, involved various subsystems including optical sources, cables, and sensors. The results show no degradation in the performance of optical sensors over a period of 7-8 years. The optical sensor systems tested exceeded the requirements of Mil-Specs tests. System accuracy of 0.02 percent F.S. over an extended period of time and sensitivity of 0.05 C were exhibited by the optical temperature-sensor systems operating over a wide ambient-temperature range.

Recent progress achieved in the field of fiber-optic sensor applications is discussed with emphasis placed on LiNbO3-based integrated optics (IO). Particular consideration is given to advanced electromagnetic-field sensors, an integrated laser vibrometer system, and a fiber-optic gyroscope system. It is shown that the multifunction IO chips have enabled high perforamance fiber-optic sensors (e.g., fiber-optic gyros), provided advanced and unique signal processing capabilities and advanced architectures, and have a potential of making fiber-optic sensors at low cost.

A simple phase tracking circuit for interferometric fiber sensors is described. The scheme is based on the generation of sine and cosine dependent interferometric outputs using phase generated carrier homodyne detection, and cross-multiplication of these outputs with sine/cosine components of an electronically adjustable phase term. The use of the scheme for providing wide dynamic range sensing with an open loop fiber gyroscope is also reported.

The design principles, the performance, and the problems exhibited by some typical fiber-optic sensors (FOSs) which are being currently developed in China, and some of which are already in practical use, are discussed. Special attention is given to the development of polarization modulation FOSs, intensity modulating FOSs, phase modulating FOSs, a fiber-optic gyroscope, and components and devices for FOSs. Research is being presently conducted on a single-mode fused directional coupler, a single-mode fused wavelength division multiplexer/demultiplexer, and PZT phase modulators.

The paper describes a compact (50-cu cm) fiber-optic gyro (FOG) prototype with a dynamic range of +/- 1500 deg per sec and a bias stability of 1 deg per hour. The FOG uses an all-digital signal processing scheme which is intrinsically free of electronic bias drift, making it possible to work over the whole potential dynamic range of the FOG without degrading the bias performances. With a stability and a linearity of 10 ppm, this FOG is a good candidate for high-performance tactical-grade applications. A configuration scheme of the FOG prototype is presented.

Results are presented of experimental measurements and an analysis of the magnetic field sensitivity of several types of depolarized fiber-optic gyros. It was found that depolarized gyros of the Bohm et al. (1981) and Fredricks and Ulrich (1984) configuration (having one depolarizer in the loop) can be highly sensitive to magnetic fields, while depolarized gyros incorporating Pavlath and Shaw concept (i.e., having unpolarized light enter and exit the loop) are insensitive to magnetic fields. It is shown that a gyro having all the advantages of both designs can be obtained by incorporating two depolarizers, one between the polarizer and the loop coupler and the other within the loop. A scheme of this type of fiber gyro is presented.

The issue of excess14 noise is of interest with respect to the broadband optical sources commonly used in interferometric fiber optic gyroscopes because it can limit the ultimate sensitivity of the device. Excess noise has been studied in superluminescent diodes (SLD's) by Yurek, et .al. 1 and in an Erbium doped fiber by Morkel, et.al.2 . The latter group also presented a simple model for the excess noise and demonstrated a quantitative fit with the linewidth of their source. In this letter we report measurements of excess noise in three potential fiber gyroscope sources, SLD's at 0.83 pm and 1.3 .tm and a superfluorescent Nd doped fiber at 1 .06 pm. These noise measure-ments are shown to be in good agreement with the model of Ref. 2. The model for excess noise is used to calculate the random walk coefficient due to shot and excess noise in a interferometric fiber gyro to demonstrate the impact excess noise in these sources will have on such a gyroscope. We show that the gyros utilizing SLD sources are just barely impacted by excess noise due to their limited output power (1-3 mW in a single mode fiber). The fiber source at 1 .06 tim, with its higher potential output power, will likely be limited by excess noise.

A novel approach in fiber-optic gyro (FOG) technology was developed and incorporated into a strapdown inertial measurement unit (IMU). In this approach, optical switches are used to multiplex a single light source, detector, and electronics among three FOG axes. The multiplexed operation of the FOG triad extends the inherent advantages of the single-axis FOG to attain small size, light weight, and low power dissipation and provides a high-performance low-cost IMUs with improved reliability and reduced random drift. A scheme of the triad system is presented.

It was previosly observed (Aronowitz, 1971; Zarinetchi et al., 1989) that, in a Brillouin fiber-optic laser gyro with identical laser frequencies along the counterpropagating directions, there occur, at low rotation rates, the lock-in phenomena normally encountered in bulk-optic ring laser gyros due to backscattering. This paper presents rotation data that are free from lock-in phenomena, from a laser gyro in which the two Brillouin lasers are separated by a fixed offset at zero rotation rate. Also presented is a measurement of the beat between the two counterpropagating Brillouin lasers, which determines the ultimate performance of this gyro.

The paper examines the current status of Litton's design approach to the attitude-and-heading reference system interferometric fiber-optic gyro (IFOG). Theoretical models used to predict the IFOG bias performance over a temperature range are described. Data are presented for the performance of IFOGs over an extended environment. The combined test data on IFOGs and IFOG-based inertial measurement units (IMUs) obtained to date, along with the environmental performance and survivability data obtained with IFOG components of the type anticipated for production, support the prediction that low-production-cost IFOG-based systems are capable of meeting the performance requirements of medium-accuracy systems for near-future military applications.

Experiments were carried out to quantify the contribution of backscattered light and orthogonal polarization to the bias stability of a tactical-grade resonant fiber-optic gyro. The data were found to agree with theory to within 25 percent. It was found that suppressed carrier phase modulation of the light for each input was effective in reducing the backscatter bias error by shifting the modulation frequencies for the counterpropagating signals. The polarization errors could be reduced by adding fiber polarizers at the ring outputs.

Digital signal processing schemes which provide amplitude and phase tracking of PZT modulation harmonics in a fiber optic rotation rate sensor are presented. The estimation algorithm implementations are designed to be extremely low cost, yet sufficiently accurate and responsive to be compatible with a multimode fiber optic rotation rate sensor (FORS). A digital detector board, featuring a digital signal processor and simple 13-bit data conversion circuitry is utilized, along with an electronic FORS simulator and an external PC to provide the data acquisition and digital signal processing for the detection schemes. It is also demonstrated how false rate signals induced by PZT amplitude modulation instabilities in the multimode FORS may be eliminated in the signal processing.

The requirements of fiber-optic gyros (FOGs) for the most demanding mission profiles of future weapon systems are examined. A concept of a high-performance depolarized FOG was developed that uses a standard single-mode fiber coil with a depolarized optical circuit including a multifunction integrated optic device, making it possible to achieve automated and reproducible production of high-performance tactical FOGs at low cost. The paper describes the optical configuration of this FOG and the gyro's electronics and presents the test data.

Results are presented from a comparison of the performance of various multimode fiber optic rotation sensor designs. The data include optical bias, PZT polarization modulation bias, and photomixing signal contrast data as a function of fiber core diameter, index profile, and LED operating wavelength, for 50-, 100-, and 200-micron core diameters, step and graded index profiles, and 0.85- and 1.3-micron wavelengths. It was found that optimal operation at the shorter wavelength lies between the 50- and 100-micron core size and near 100 microns at the longer wavelength. Step index profiles yielded considerably less bias than did the graded index profiles.

This talk reviews the fifteen-year development of fiber-optic position transducers for aircraft controls (Refs. 1-7). First the design of a modern digital electronic aircraft control system is presented to show that position transducers are the most numerous type of sensor on an aircraft. Performance requirements for position transducers on aircraft are then reviewed. These include accuracy, resolution, range, bandwidth, slew rate, size, reliability, and cost. Then the variety of presently available fiber-optic position transducer technologies are reviewed. These include digital optical code plates, analog optical code plates, chirped diffraction gratings, macrobend loss coils, polarizers, interferometers, and optically powered electronic sensors. The performance of these various technologies are compared and flight test experience is reviewed. In conclusion, prospects for installation of these transducers into aircraft of the 1990's are discussed.

High precision fiber coupled resolvers, which have a monotonically varying density analog track, or multiple binary tracks, have either stringent analog stability requirements, or require the interrogation of at least 10 optical channels. The resolver to be described has 3 variable modulus (6,10,15) analog tracks encoded using a weighted number system. The resolution exceeds that of a 10 bit binary resolver of comparable size with reduced optical complexity, and has moderate (3%) analog stability requirements. Degradation characteristics compare favorably with those of an analog sensor.

A fiber optic speed sensor (FOSS) has been developed, bench tested and rig tested in a real turbine airflow environment. The FOSS employs an innovative design using a pressure tube and fiber optic microbend transducer in order to capture turbine blade pass wake frequency. The blade pass frequency can be converted by a signal processor into turbine rotational speed. The FOSS offers unique potential to meet future requirements for performance (0 to 25 KHz) and environmental tolerance (1200 F temperatures and EMI/EMP threats). Future efforts include development of the signal processor and environmental/durability testing focused on developing reliable, long life operation in the hostile environment of an advanced gas turbine engine.

The paper reports the results of work to demonstrate the feasibility of embedding a metal-buffered optical fiber inside a thin metal diaphragm to create a pressure-sensitive transducer. A method was developed to embed butt-coupled optical fibers inside brass diaphragms. Butt-coupled fibers with two different end spacings were successfully embedded in the diaphragms. The pressure response of the diaphragms was calibrated by measuring the changes in light transmission through the butt coupling as a function of pressure. In addition to embedded fiber pressure sensors, this method may be useful for other applications. The calibration results indicate the method could be used to make connections between signal processors and optical fibers embedded in composites.

Knowledge of angular displacement is important for many system applications. There have been numerous optical techniques applied to this sensing need with the most pervasive at present being the optical code plate sensor. Other optical measurement techniques have not been as widely used due to their limited angular measurement range. An alternative to code plate sensing is described here that uses period modulated diffraction gratings attached to the interior of a rotatable element. Rotation of the element or anything to which it is attached has the result that the bandpass wavelength of the sensor is modulated due to the change in the grating segment being illuminated. The sensor is interrogated via optical fibers together with an LED source and high speed spectrometer and can measure angular displacements approaching 2ir. Sensor performance is quantified and compared with existing sensor technology.

A high-performance fiber optic pressure sensor system is being developed for gas turbine engine applications. Based on the photoelastic effect, the four sensors convert differential pressure into bending stress in transparent plates. The bending stress is then measured by its effect on polarized light transmitted through the plates. Three different pressure ranges, from 0-15 psia to 0-500 psia, are provided to measure compressor inlet (CIP), compressor discharge (CDP), ambient static (ASP), and nozzle total pressure (NTP). The sensors are designed for accurate onengine operation at temperatures from -55 to +800°C, so the optical components, housing, and fiber cables have all been designed for stability over this temperature range. The sensor design employs fused silica and Inconel 718 to achieve these results. The fiber cable materials include metal-coated fiber, ceramics, and stainless steel. The transducer electronics interface unit (EIU) is a multiplexed analog system which gathers eight data readings (four pressures and four temperatures) and processes all eight within 2 ms. The analog intensity-modulated nature of the basic sensing method has been modified to yield highly accurate results by the use of a novel self-referencing technique. At the same time the receiver circuitry measures the temperature of each sensor by two-color pyrometry. This in turn allows continuous temperature compensation of each sensor. The multi-sensor system thus produces four separate temperature-corrected pressure readings in 12-bit digital format every 2 ms. A further benefit of the self-referencing scheme is that it allows the cables to be freely disconnected and reconnected, or reconfigured portto- port, without loss of calibration or accuracy. Each sensor pressure and temperature reading is resolved to within 0.1% of full scale.

A fiber-optic displacement sensor with subnanometer resolution is described. The sensor is based on a highly sensitive detection of power change at a 'zero' working point in light reflected from an object surface to be measured when the surface is subject to a small displacement. The sensor consists of a He-Ne laser source, a directional fiber coupler as a beam splitter, a small-diameter (less than 5 mm) optical-fiber probe, and a photodetector setup. Measurements by this sensor are not sensitive to the power fluctuation of the light source. The sensor has high accuracy, stability, and wide working range, is simple, and can be manufactured at low cost.

A novel fiber-optic sensor for measuring external pressure distribution along a high-birefringent fiber is proposed, based on the polarization mode conversion due to pressure. The sensor exhibits spatial resolution better than 0.15 m, high signal-to-noise ratio, and wide dynamic range.

This paper is devoted to the description of a new type of fiber optic magnetic field sensor based on spectral modulation encoding techniques. This device uses a Faraday magneto-optic element as transducer and converts the rotation of the plane of polarization of the light induced by the magnetic field into a small change in the large optical path difference provided by a birefringent coding plate. The use of a broadband source and of an interferometric detector, with an optical path difference matched to the sensor one, allows to reach high resolution and perfect down-lead insensitivity. A dual-wavelength arrangement is proposed to determine the temperature and magnetic fied values with the same probe.

The class of ferrimagnetic materials known as substituted iron garnets display characteristics which make them suitable for applications of magnetometry requiring high sensitivity, high spatial resolution, or high speed. Diamagnetic substitution, in which specific iron ions are replaced by diamagnetic ions, reduces the saturation magnetization and increases the sensitivity. We find that the sensitivity of a composition of gallium-substituted yttrium iron garnet is six times greater than of pure yttrium iron garnet. The noise-equivalent magnetic field for a sample of this material has been measured as approximately 100 pT/THz.

We review developments over the past five years in fiber optic magnetometry based on magnetostriction. Recent work has demonstrated detection of magnetic fields from dc to over 1 MHz with resolution of 10 pT P1Hz at 1 .0 Hz and 0.07 pTpIHz at35 kHz in laboratory devices. We present the basic device operating principles and discuss factors which limit the resolution of fiber optic interferometric magnetic sensors. Results are presented for a number of configurations including a ruggedized, compact, three-axis magnetometer for dc and low-frequency measurements, a single-axis gradiometer, a single-axis heterodyne configuration for narrowband detection of fields in the frequency range 0.01 Hz to 1 MHz, and a magnetostrictive oscillator which exhibits period doubling bifurcations and chaos.

In this paper one presents the results obtained in the development of a fiber optic temperature sensor based on the principle of the variation of the index of refraction of a liquid with temperature. The sensor is built by barincj the nucleus and replacinci the clad by the liquid which will act as a new clad with a hiqhly temperature dependent index of refraction. This intensity type sensor can work in several ranQes of temperature dependinQ on the particular I iquid used.

The paper proposes polarimetric and two-mode differential interferometric schemes incorporated in an elliptical-core fiber to resolve strain and temperature simultaneously. Initial results indicate accuracies of +/- 10 microns/m and +/- 5 C for strain and temperature measurements, respectively. A technique, based on the evaluation of the condition number of a matrix, is shown to be useful in evaluating comparative merits of multiparameter sensing schemes. Experimental results for four fibers are presented, and cross-sensitivity issues are discussed.

A gated optical multichannel analyzer was used to detect the Rayleigh and Raman shifted backscatter in a UV grade silica fiber using a pump laser at 337 nm. The temperature dependence of the anti-Stokes to Rayleigh ratio, at 337 nm, was demonstrated and used as the basis for a distributed temperature sensor. Temperatures from 293 K to 483 K were measured with a spatial resolution of 0.5 m.

The specular reflection pyrometer has the advantages of decrease the influence of surface ernissivity on temperature measurement, but the short continuous working du ration limits its wider application. By use of an optical fiber to seperae the reflector from the instrument and change the structure of the reflector, the performance and duration can be improved. Analysis and experiments are presented.

A push-pull fiber optic accelerometer using a Michelson interferometer to detect the surface strains produced on a pair of coaxial flexing disks is described. The sensitivity below the disk's fundamental resonance of 2.45 kHz is 50 radians/g. The fractional phase change per unit force A4/AF is 5.9 x 106 W1 . A noteworthy feature of this sensor is that the 5 meter sensing coils comprising the legs of the interferometer are totally internal to the sensor structure and thus are not exposed to platform or fluid under investigation.

A multiwave interferometer was developed for accurate measurements of absolute distance over distances on the order of inches. The principle of the device is based on using multiwave interferometry to create artificial wavelengths between 1 micron and 50 mm. Phase measurements for pairs of optical wavelengths are used to mathematically produce a phase measurement equal to that of a much longer 'equivalent' wavelength; if the pair of wavelengths are close together, a long equivalent wavelength is produced. Each optical wavelength is assigned to a distinct heterodyne frequency and is multiplexed onto the same measurement beam.

Extinction ratio is an important characteristic of polarization-maintaining fibers and their in-line fiber devices. A novel measuring system based on the bidirectional-double-path measurement proves to be of advantages over previous ones. The experimental results show that high precise measurements of extinction ratio (error