Fiberoptic sensors of many types are being developed. They can be categorized as amplitude-or phase-modulated devices. Various transduction processes have been employed including: microbend losses, evanescent coupling, polarization shifts, florescence, and elastic strains induced by changes in various physical parameters. The list of physical phenomena which have been detected is quite long including: acoustic, electric, and magnetic fields; angular and linear displacement, velocity, and acceleration; temperature, pressure, stress, strain; radiation dosage and trace gases; etc. A detailed presentation of fiberoptic sensors along with discussions of optical components etc. has been given in ref. 1. Examples of both an amplitude and an intensity modulated device are given below.

Stability and reproducibility problems in single and multimode fiber optic microbend strain sensors are found to be due to interference between the core mode and cladding modes in the bend region. Microbend strain sensors are stabilized by preventing the interference effect. A high sensitivity fiber optic microbend strain sensor for detecting bending of large mechanical structures has been constructed and tested. OTDR technique has been utilized for detecting strains at several structural points simultaneously by placing many sensors in tandem along a single fiber.

A conventional optics experimental investigation of a (frequency domain) Passive Laser Accelerometer (PLA) has been performed. The successful experiments and analysis led the author to suggest other (phase and frequency domain) integrated optical sensors such as pressure, strain or temperature. These optical sensors are particularly attractive for application requiring either high sensitivity, small size or immunity to electrical noise.

The dynamic range of fiber optic signal processors driven by relatively incoherent multimode semiconductor lasers is shown to be severely limited by laser phase-induced noise. It is experimentally demonstrated that while the noise power spectrum of differential length fiber filters is approximately flat, processors with recirculating loops exhibit noise with a periodically structured power spectrum with notches at zero frequency as well as at all other multiples of 1/(loop delay). The experimental results are aug-mented by a theoretical analysis.

Recent developments in an external cavity laser diode sensor are discussed. The sensor consists of an external reflector mounted such that the mirror to laser facet distance is smaller than the laser cavity. This device has micro-radian sensitivity and a large dynamic range. This sensor has been configured to sense acoustic waves, magnetic and electric fields, current, and temperature and acceleration.

The current status of integrated optics components for fiber gyro applications is reviewed. Progress on the reduction of loss in these components including coupling loss, propagation loss, and bending loss is summarized. Preliminary experimental results on the photorefractive effect in Ti-indiffused LiNbO₃ directional couplers at GaAs wavelengths are presented. The photorefractive effect, measured as a drift in the switching state, has been observed for intensities (powers) as low as 32 W/cm² (3.2 at 0.85

It is thought that the future production of fiber optic gyros and other optical sensors depend on the development of integrated optic circuits to replace the various bulk optic components and labor intensive components presently utilized in all waveguide systems. This paper will present recent data for integrated optic chips and components specifically designed and fabricated for fiber optic sensor applications.

Single-mode optical couplers are critical elements in fiber optic sensor systems and communication networks which use single-mode techniques. In 1980 ITT Electro-Optical Prod-ucts Division (EOPD) developed single-mode 2 x 2 fused biconical taper couplers using con-ventional fabrication techniques with a fiber designed especially for the couplers. This paper reports recent efforts, sponsored by a Naval Research Laboratory contract, which have further developed the coupler.

Due to extremely high gain of semiconductor lasers, single pass light amplification can be realized by simply eliminating the Fabry-Perot cavity of a high gain laser diode using antireflection coatings. The resulting diode (Super Radiant Diode) has very low noise, high output power and a very broad spectrum. It also retains the low divergence of the light output of a laser diode. Thus, large amounts of light can be coupled into either multimode or single mode fibers. Applications of this diode in fiber optic sensors and fiber optic communications have shown dramatic performance improvement.

A number of different methods of stabilizing the output of GaAlAs diode lasers are described and discussed. Forms of both optical and electronic feedback are considered.

Stable fundamental transverse and single longitudinal mode CW behaviour is demonstrated using a Gaussian channel substrate planar waveguide laser. Highly reliable performance including low threshold current, extremely low spontaneous background level and high-power CW operation has been achieved. In addition, the longitudinal mode has a locking range of 7-8°C, where the wavelength changes at a rate of 0-6 Â/°C.

Fiber optic sensors are being developed to measure a large variety of physical quantities such as temperature, pressure, rotation, voltage, current, strain, etc. These sensors are compact, lightweight, free from RFI & EMI, and hold the potential for low cost with performance comparable or better than their present day counterpart. Until recently most fiber optic sensors could only be built with both fiber and bulk optic components. With the advent of all-fiber technology it is possible to build sensors solely with fiber optic components greatly simplifying the fabrication of these sensors. The principles and techniques of all-fiber technology are reviewed along with the mechanization and principles of operation of various fiber optic components. The applications of all-fiber technology will be examined. A new class of sensors unique to all-fiber technology will be presented. This class of sensor employs evanescent field interactions to change the amplitude, phase, or polarization of the light propagating in the fiber. The operating principles and several examples of evanescent field sensors will be discussed.

We have previously reported the development of a simple set-point temperature sensor based on the variation in intensity of monochromatic light transmitted by an optical fiber with a short section of modified cladding. In this paper we discuss the development of a wavelength dependent temperature sensor based on the extension of the previous results to include polychromatic light and wavelength dispersive effects.

A technique is described for measuring the displacement of a thin glass diaphragm relative to a fixed glass reference surface. The method employs a single-mode optical fiber to deliver coherent light to the center of the diaphragm and reference surface. The interference fringe pattern caused by partial reflection of light from each surface is focused onto a multimode receive fiber, which conducts the light to a detector. When the diaphragm deflects, the fringe pattern traverses the input end of the multimode fiber causing the detector output signal to follow a cosine-squared time dependence matching the cosine-squared spatial dependence of the fringes. Appropriate up/down counting electronics may be used to count the interference fringe peaks and convert the count to a measure of diaphragm displacement. Results of a preliminary investigation of this diaphragm deflection sensor are presented. Application of the method to pressure measurement is discussed. The technique is inherently insensitive to thermal drift and vibration, and its "all glass" construction makes it applicable to pressure measurements in high-temperature environments.

A novel fiber optic sensor that determines pressure from diaphragm curvature is discussed. This sensing technique exhibits a high degree of sensitivity and linearity and is less susceptible to environmental perturbations than previous intensity modulating techniques. Experimental results are presented for a pressure transducer operational in the 0-60 psi range, and a miniature pressure transducer operational in the 0-5 psi range designed for in vivo medical applications.

Measurements are always subject to errors resulting from the interaction of the measurement device with the quantity being measured. When dealing with electromagnetics, the fields are typically detected with some type of antenna which must be electrically connected to a recording instrument. The antenna as well as its electrical connection can, at times, significantly perturb the field being measured. By using fiber optics to the maximum extent possible in the detection process, microwave fields are perturbed only minimally since most of the probe and associated connections are dielectric. This paper presents the preliminary results obtained by using a commercially available fiber optic thermometer in the measurement of microwave power. In the measurement scheme a small amount of slightly conductive material is placed in contact with the fiber optic sensor. In the presence of a microwave field, currents are induced in the conductive material which in turn produces joule heating. It is shown experimentally that under certain circumstances the probe temperature is linearly related to the power level present. Experimental results will be presented for power measurements at 2.45 GHz and 94 GHz. Probe design criteria as well as limitations are also discussed.

Multimode fiber optic sensor research carried out at the Sperry Research Center will be discussed. Emphasis will be placed upon a description of a generic technique, based upon the photoelastic effect, for the creation of a whole class of multimode fiber optic sensors. A presentation of recent results will be given.

Fiber optic levers have been found useful in the design of intracardiac pressure transducers, low displacement biological measurements, and more generally as noncontact optical proximity detectors. In this paper the utilization of a bifurcated fiber optic lever in the design of hydrophones of the pressure and pressure gradient types has been considered. This approach leads to simplicity, low cost, and relative ease in implementation. Some optical and acoustical considerations are given to establish the validity of the experimental data presented. It is shown that with little effort these devices can provide acoustic sensitivities at least equivalent to those of piezoceramic elements within the same frequency range and can generate acceptable directivity patterns in the pressure and pressure gradient modes.

Two variations of each of three types of digital optical transducers have been developed to provide the technology base for a digital optical flight control system for US Army helicopters. The transducer types are rotary position, linear position, and differential hydraulic pressure and all are electrically passive at the transducer location. The development and test of these six transducers has led to the selection of a reflective, time delay multiplexed concept using a single interconnecting fiber for use on the Advanced Digital Optical Control System Flight Demonstrator Program (ADOCS FDP).

Recently we have proposed and demonstrated two broadband single mode fiber optic sensor systems for detection of ultrasonic waves propagating in fluids. A review of such systems and their theoretical analysis is presented here. One such system is based on polarization modulation and only requires the use of a single fiber in a very simple configuration. This system has a frequency response from 0.7 MHz to 50 MHz, and a flat response between 11 MHz and 25 MHz. The other system is based on phase modulation, in the form of a two fiber Mach-Zehnder interferometer. The ultrasonic wave is detected through the pressure induced changes in the interference between the two optical beams propagating in the "ref-erence" and "sensing" fiber, i.e., the two arms of the interferometer. This system has a frequency response from 50 kHz to 50 MHz, with a flat response between 50 kHz and 1 MHz.

A sensitive single-fiber acoustic sensor has been constructed by securing single-mode fiber to the diaphragm of an acoustic cavity. The acoustic pressure induces a birefrin-gence in the fiber and alters the state of polarization of the light. The sensor is capable of detecting a minimum pressure of 33dB re 111Pa/âœ"Hz and exhibits a sensitivity that is comparable to fiber-optic Mach-Zehnder interferometers.

A passive stabilization scheme using a (3x3) fiber directional coupler in an all fiber Mach-Zehnder interferometer and suitable signal processing has been successfully demonstrated. A stable output with large signal dynamic range and a minimum detectable phase shift in the microradian range has been achieved.

The passive quadrature demodulator eliminates the complexity of the phase stretcher and feedback electronics frequently used in fiber interferometers by passively extracting the desired signal using two distinct interferometers which differ in phase by π/2.

Guidedwave optical modulators have been designed and tested for applications as electro-optic data link for electromagnetic sensing. The ruggedized data link consists of an injection laser diode pigtailed to a stress induced birefringent fiber, an electro-optic coupled channel waveguide modulator, a graded index multimode fiber for return signal, and a laser driver optical receiver electronics box. The electronics box can be physically separated from the laser diode-fiber-modulator-fiber assembly for ease in field installation. This link has demonstrated a frequency range between 10 kHz to 200 MHz, and a dynamic range of 40 dB at 200 MHz bandwidth operation.

An improved spectrophone for measurements of laser absorption in gases has been tested. The device employs a fiber-optic interferometer, wound into a cylindrical shape, as the acoustic sensor. Performance of the device was evaluated by measuring the absorption of 3.39 μm HeNe laser radiation in a calibrated mixture of methane in Argon. Results indicate a minimum detectable absorption capability of 2x10^-7cm^-1 W/√ MHz , although a background signal 30 dBV above the noise limited performance. We report an improvement in sensitivity several orders of magnitude above the first fiber-optic spectrophone.

A passive interferometric demodulation technique is described and experimentally demonstrated. This technique uses a piezo-electrically stretched coiled fiber as a phase shifter to drive the interferometer into two states with a 90° phase difference between them. The method exhibits good linearity, high sensitivity, and the potential for large dynamic range. The demodulation technique eliminates the phase tracker resetting problem encountered in active homodyne detection schemes.

A fiber optic system consisting in its main parts of a GaAlAs laser diode, a monomode launching fiber and a fiber Fabry Perot (FFP) resonator is investigated in view of two main aspects: 1) use of the FFP as fiber optic spectrum analyzer (FOSA) with the laser diode presenting a light source of rather complex spectral behavior, and 2) application of a FFP fabricated with a birefringent fiber as a one fiber double Fabry-Perot (OFDFP) device for temperature sensing. When varying the laser injection current, temperature drift of the laser wavelength as well as instable discontinuities (mode hops) can be identified. The temperature tuned FOSA is suggested as a device for diode laser spectral performance moni-toring. The OFDFP with its double system of transmission maxima offers a scheme to determine parameter change sign and principally provides a calibration means for this type of interferometer.

A single mode optical phase modulator utilizing a piezoelectric plastic jacket is studied theoretically. Due to the short wavelength at high frequency and the axial symmetry of the optical fiber, a radially dependent deformation is assumed. Computational results of the optical phase modulation reveal that the optimal performance may be achieved if the modulator is driven at its resonant frequencies. These resonances are caused by the thickness modes of the piezoelectric plastic jacket at lower frequencies and by the breathing modes of the glass fiber at higher frequencies. Experimental measurements (using a Mach-Zehnder interferometric configuration) up to 5 MHz on a 3 cm long sample show reasonable agreement with the theoretical predictions.

We propose a new optical device for measuring inertial rotation. The device uses the nonreciprocal frequency shifts of the self-oscillations generated in a phase conjugate ring cavity under rotation. We present the analysis of this concept and obtain the scale factor equation of such a gyro. The results of a preliminary experiment are also discussed.

Conventional ring laser gyroscopes (RLGs) are planar; the light beams are constrained to lie in a single plane. This need not be the case. We have found that nonplanar ring resonators offer certain advantages for the multioscillator (four frequency) RLGs developed by Raytheon. An overview is given of those properties of nonplanar rings which are pertinent to RLGs, including the inertial rotation sensitivity (the scale factor and the direction of the sensitive axis), the mode polarization, and the frequency spectrum. Design equations for a class of simple nonplanar rings are given and applications to multioscillator RLGs are discussed.

It is shown that optical resonators may be broadly divided into two classes based on their mirror count and other properties, and that the two classes exhibit very different axis stability properties. When a resonator of one class is slightly misaligned, its optic axis (and hence its input axis for rotation sensing) remains close to its nominal position independently of the focussing action of the resonator optics. In contrast, when a resonator of the other class is slightly misaligned, the displacement and reorientation of its optic axis is restrained only by the focussing optics. Hence resonators of this second class are restricted in the range of mirror radii of curvatures they may employ. This has implications in noise and lockin performance for rotation sensors.

Cluster Sampling Technique (CST) is a powerful method for measuring the output stability of the ring laser gyro or any other oscillator in the time domain. The relationship between the cluster variance and the power spectral density of noise contained in the data is derived. The former, being a directly measurable quantity, can provide information on the types and magnitudes of various noise terms. Analytical expressions for the cluster variance are derived for a number of prominent noise terms that are known to affect the gyro performance. The technique is applied to a long-term drift run of a Honeywell gyro and various noise terms are determined.

Thin-film waveguides are attractive for applications in sensors using a resonant cavity to sense rotations. The relationship between waveguide parameters and gyro sensitivity is discussed and measurements of relevant parameters, particularly losses due to absorption and scattering, are presented.

The square ring laser, unlike the triangular ring, can exhibit a bias due to the combined effect of slight nonplanar deformations (a reciprocal rotation effect) and an applied magnetic field (a nonreciprocal rotator). In this paper, the bias as a function of net tilt out-of-plane, magnetic field and net mirror retardance is calculated and its importance as an error source in the planar square ring laser gyroscope is discussed.

This paper reports our experimental results with the Passive Cavity Gyro (PCG). The angular random walks in our long- and short-term drift runs are within a factor of 3 of the setup performance limit. Optical misalignment tests indicate fairly low sensitivities of the bias and random walk to component misalignments. The PCG concept allows for the development of very high performance gyros. Also, recent results in fabrication of low-loss waveguides make PCG an attractive candidate for developing small, low-cost gyros.

We consider a ring resonator filled with a nonlinear medium which is pumped by a pair of counter-propagating laser beams with the same frequency. As a result of optical four-wave mixing, transmission "gains" are available for the oppositely directed traveling waves sup-ported by the resonator. Thus, oscillation will result provided the coupling is strong enough to overcome the mirror loss. Since the two lasing modes are coherently coupled via four-wave mixing, they are mutually phase conjugated and are invulnerable to frequency pulling due to backscattering. Thus, such a gyro will have a reduced lock-in effect. We will present this new concept as well as the result of our analysis.

This paper summarizes the expected advantages of incorporating integrated optical waveguide components into fiber gyros, describes the status of the basic material and component technologies, and outlines the development work which remains.

During the last few years the sensitivities of fiber optic Sagnac gyroscopes and Mach Zehnder interferometer sensors have improved significantly. Part of this improvement is due to replacement of bulk devices with integrated optic and fiber optic devices.1 Guided wave devices reduce errors resulting from component motion and surface reflections. They allow compactness and offer a well defined optical path. One of the most commonly utilized guided wave devices is the evanescent wave directional coupler. This device can be used to split or combine guided optical waves and thus rgpl aces the bulk optic beam splitter. Veuions of directional couplers have been fabricated from optical fiber 4,;) and from integrated optical waveguides4.

Optical fibre interferometric sensors are described in which the path imbalance of the interferometer is measured by using a frequency modulated source. The technique is analogous to frequency modulated continuous wave (FMCW) radar. Optical FM techniques are described and initial results from a closed loop system presented. The interferometers are totally passive and their state can be read with high accuracy, depending on the total frequency excursion and output signal to noise ratio.

Recent literature has presented characteristics for 1.3 micron optical devices which indicate that there is significant advantage in using this technology for Sagnac passive gyros. Here, we present our measurements on several optical devices and through analysis, evaluate their impact on performance improvement in a generic fiber optic rate sensor. We conclude that although the use of 1.3 micron technology causes a sensor scale factor reduction, this effect is more than offset by the improvement in signal to noise ratio.

We examine the non-reciprocal behavior of Sagnac interferometers exposed to linear perturbations, and develop a geometrical representation for the magnitude of the pointing error in fiber gyroscopes. We show that proper winding techniques may greatly reduce the environmental sensitivity of these devices.

Commercial CW single-mode GaAlAs diode lasers tuned to the 795 nm Rb line were used in the optical pumping and magnetometric detection of the nuclear magnetic resonance of Xe in a Rb-Xe NMR gyroscope. Besides reduced size, weight and power requirement, the diode laser has the advantage over a conventional rf Rb lamp of frequency tunability and high beam collimation. A gain of 20 dB in signal to noise was measured. Frequency-locking of the laser output to the Doppler-broadened Rb absorption line stabilized the drift over 13 hours to within 10 MHz. The diode laser was also used, with spectral shaping by current modulation, to optically pump the Rb-Xe system.

The current status of fiber optic gyroscope development is reviewed and summarized. The companies, agencies and universities involved in this development are listed. Included in the review are these gyro categories: