The state of fiber optic sensor technology is reviewed from the point of view of the potential user. Industrial opportunities and applications are emphasized. An attempt is made to understand why the technology is not seeing widespread acceptance. Issues of general discussion include technical, economic, and other disadvantages (and advantages) of fiber optic sensors, such as maintenance, reliability, and user training. Comparisons with the existing competition of electronic sensors are examined. Present shortcomings of fiber sensor technology are emphasized along with user requirements that must be addressed if fiber optic sensors are to see wide application.

Operation of advanced aircraft in the subsonic, supersonic and hypersonic regimes will require high degrees of propulsion and flight control integration. More sensors will be required for control and monitoring of the complex aircraft as well as more actuators to provide safe and efficient operation while at the same time providing for a high degree of maneuverability. Improved thrust to weight of these vehicles will be achieved through the use of high strength, low weight composites for the aircraft, propulsion system and control accessories (i.e., pumps, valves, actuators, etc.). The increased use of composites makes the digital control more susceptible to electromagnetic effects. In order to provide the protection to the digital control additional shielding will be required as well as protective circuitry for the electronics. This results in increased weight and reduced reliability. NASA and DOD have recognized the advantages that fiber optic technology pro-vides for advanced aircraft applications. The use of optical signals to carry information between the aircraft and the control module provides immunity from contamination by electromagnetic sources as well as other important benefits such as reduced weight and volume resulting from the elimination of the shielding and the replacement of metal conductors with low weight glass fibers. In 1975 NASA began work to develop passive optical sensors for use with fiber optics in aircraft control systems. Passive optical sensors require no electrical energy and operate with very low power optical signals. These passive sensors will result in less maintenance and improved reliability. Many interesting and innovative concepts for using optical signals to measure pressure, temperature, position, flow, speed, vibration, acceleration, etc. have been proposed and demonstrated in the laboratory. The problem now is to choose the best optical sensor concepts and evaluate them for use in the adverse environment of aircraft systems. In 1985 NASA and DOD entered into a joint program, called FOCSI (Fiber Optic Control System Integration), to look at optical technology specifically for use in advanced aircraft systems. The results of this program will be discussed. The objectives of Phase I of the FOCSI program were to conceptualize a fiber optic control system, define the environment in which the optical components must operate and define the specifications for the sensors. The environment issue is highly dependent on the location of the optical components (i.e, turbine region versus compressor inlet) and the aircraft flight envelope. Near term applications of fiber optics will be at relatively low ambient temperatures. Advanced aircraft such as the supersonic/hypersonic aircraft will present more of a challenge due to the higher temperature environment. The conclusion of the study indicated that the use of fiber optic technology in advanced aircraft systems is feasible and desirable. The study pointed to a lack of available sensors from vendors capable of operating in the adverse environments of advanced aircraft. In Phase II of the FOCSI program issues such as the electro-optic architecture required to service the sensors will be considered. Overall system analysis of the optical sensor and the electro-optic architecture will result in selection of the best systems for incorporation into advanced aircraft.

Allison Gas Turbine Division of General Motors Corporation is developing fiber optic based turbine engine control systems and has identified key applications to determine fiber optic sensor requirements. Measured parameters, operating ranges, accuracy requirements, environmental constraints, and speed of response are identified and presented for 3 categories of engine/applications, both for today and tomorrow. A review is presented of fiber optic temperature, speed, pressure, position, and flow sensor types and their potential to meet these critical requirements on turbine engines. Key Allison development and test efforts are highlighted.

Fiber optic sensors have been investigated internationally for the past 15 years or so. Countless papers and articles have been published expounding on their many virtues, and everyone seems to agree with the general claims: (1) being unaffected by environmental electromagnetic fields, neither the sensors nor their interconnections require any form of shielding; (2) they are completely safe in hazardous environments because there is no possibility that a short circuit might cause sparks or heating; (3) true digital sensing is possible in some applications; and (4) the sensors themselves can be very small and light. Clearly, these features should make fiber optic sensors extremely attractive for use on aircraft and in many industrial applications. With this impressive list of attributes, why have they not yet found acceptance in military and commercial aircraft? This paper reviews several of the more common classes of fiber optic sensors and some of the solved and unsolved problems which have probably inhibited the immediate acceptance of this new technology by the aircraft industries.

A review of the performance of various geophysical sensors is given. Included in the discussion are acoustic towed arrays, several types of phase-modulated and intensity-modulated seismometers, and a fiber-optic magnetometer. The presentation is in the form of a brief overview stressing concepts and recent progress. Theoretical derivations and engineering design are left to the references.

The development of the technology for generic fiber optic interferometric sensors is outlined. Attention will be paid to recent developments of remote interrogation/demodulation and to the avoidance of polarization fading. The second part of the article describes some of the applications which have received most attention.

Linearization of the temperature response for temperature sensors based on absorptive cladding techniques has been achieved. Results using both multimode glass and plastic fiber are presented in the range 0 - 150 and 0 -70 degrees Centigrade respectively.

Among the simplest fiber optic sensors, are those which operate in a binary fashion; they were the first sensor types to be developed. Early experiments with fiber bundles and shutters produced demonstrations of, for example, displacement sensors. Typical applications range from position sensing for aircraft landing gear to counting objects on a production line. Because they frequently replace electrical snap action switches, binary sensors are generally called optical switches. Optical switch applications account for a much larger market than the more complex analog measurements discussed in the balance of this volume. This paper presents an optical switch concept that uses a single fiber and is tolerant of back reflections. The sensor element is a low finesse Fabry-Perot pressure sensor which replaces the electrical contact in a conventional snap action switch.

We have developed a prototype system of a fiberoptical distributed temperature sensor with high spatial resolution and have measured the temperature profile of the winding tempera-tures of a transformer model coil in operation. The coil is fabricated from a combined optical and electrical conductor with the optical fiber integrated into the electrical insulation. The results reveal strong variations between outer and inner conductors as well as outer and inner coil surfaces. A strong temperature gradient from the top of the coil to the bottom due to thermal convection can also be seen.

Based on light intensity modification in optical fibers and optical time-domain reflectometry techniques, a proprietary and patented structural monitoring system has been developed. Featuring continuously-distributed spatial coupling of a proprietary optical fiber sensor with a structure, by means of attachment or embeddment, the system can monitor both the magnitude and location of structurally-related changes in strain, pressure or temperature. All measurements are made at a terminal connected to one end of the distributed sensor and located on or near the structure, or remotely operated. Four categories of systems, by length, are outlined. The descriptive term "FIBERTRONICtm System" has been selected for the various groupings of sensor and terminal designs. Initially directed toward structural integrity monitoring for strain, a prototype system has been demonstrated by sensor attachment to a 16-foot "I" beam in uniform bending. Another prototype, involving pressure sensing, was embedded in a pultruded graphite-epoxy composite to determine survivability under high pressure and temperature, as well as to locate a simulated void. Test data are included. Current developments include both sensor and terminal refinements for medium-length, short-length and ultra-short systems. These include pressure-sensor-diameter reduction, pressure tests over short sensor lengths and insertion-loss tests on a Fiber Optic Rotary Joint (FORJ). References are included for more detailed referral, including a companion paper (SPIE 986-14) which reported on a "Smart" aerospace structure utilizing a FIBERTRONICtm pressure sensor.

We present a Michelson-type fiber-optic interferometric displacement sensor that is insensitive to temperature variations. A fused biconical tapered coupler was built and cleaved immediately after the coupled length. The cleaved coupler was fixed with epoxy in a cylindrical glass tube and polished until two cores, very close to each other, could be observed. The end of one core was selectively coated using an aluminium deposition process, and was used as part of the reference arm; the other core was used as the sensing arm. Displacements of a vibrating reflective surface were measured for frequencies ranging from dc to 100 kIIz and extremely stable output signals, devoid of thermal drifts, were observed over sustained periods of time.

A fiber-optic temperature sensor is described that uses a small silicon beamsplitter whose splitting ratio varies as a function of temperature. A four-beam technique is used to measure the sensor's temperature-indicating splitting ratio. This referencing method provides a measurement that is largely independent of the transmission properties of the sensor's optical fiber link. A significant advantage of this sensor, relative to other fiber-optic sensors, is its high stability, which permits the fiber-optic components to be readily substituted, thereby simplifying the sensor's installation and maintenance.

Multisensor networks can be used in a variety of application areas where a number of measurand fields need to be monitored, or where profiling of the measurand is of interest. This paper gives an overview of current developments in the area of distributed and multiplexed fiber optic sensor systems.

We report on the construction and performance of an eight-element time-division multiplexed fiber-optic sensor array. The array, which is based on a transmissive tapped-serial architecture recently developed at our laboratory, utilizes low-coupling-ratio fiber couplers (-1%) to couple light from each sensor element to an output fiber 'bus'. The network exhibits intrinsic optical crosstalk effects which have been fully characterized and compared to theoretical models. Crosstalk figures in the range -25 to -45 dB were experimentally observed, in good agreement with predicted levels of ~-27 to -40 dB.

Recently, we reported a new interferometric fiber sensor array architecture based on a transmissive tapped serial sensor network which, like the reflectometric Fabry-Perot array configuration, exhibits intrinsic optical crosstalk between the sensor elements in the array. In this paper we discuss the origin of these crosstalk effects in both array topologies, and present theoretical models which predict the level of crosstalk between individual sensor elements in each type of array.

A fiber optic thermometric device, which allows to control 19 measuring points, is presented. It utilizes a single electro-optical unit, and an electro-mechanical optical scanner to address the sensor array. Each sensor is intensity modulated type, and utilizes as transducer a differential absorption thermochromic solution, working in the 5°-75°C temperature range. A self-focusing probe is adopted, which presents low intrinsic loss so to achieve high signal-to-noise-ratio.

A method of wavelength division multiplexing (WDM) for fiber optic sensors using a broadband light source and narrow bandpass thin film optical filter coatings on cylindrical graded index lenses has been developed. The WDM system is characterized by parallel information from all channels in real time. The system is being applied to a digital, rotary, absolute position encoder.

The Faraday effect is becoming widely used as an optical method of measuring electric cur-rent or magnetic field. It is particularly advantageous where the measurements must be made at high voltage or in the presence of electromagnetic interference, and where, speed or stability are considerations. In this paper we review the development of the technology over the last twenty years, with an emphasis on the basic principles, design considerations, and performance capabilities of sensors that represent the latest achievements. Faraday effect current sensors are now used routinely in the measurement of large current pulses, and are starting to become available for ac current measurements in the power industry. Recent developments include their extension to the measurement of currents in the milliampere range and substantial reductions in size. Similar devices, in slightly different configurations, can be used for magnetic field measurements. Further improvements, based on new fiber types and new materials, are projected.

It is presented two detections schemes to be used in Faraday Effect current sensors. They enlarge the dynamic range of current measurement preserving a good accuracy with relatively simple optics. These technics overcome the ambiguity problems normally present in this class of sensors. One is able to measure the ampli.tude of AC currents while the other follows it, giving a real time digital output.

Low frequency fiber optic magnetic sensors make use of a magnetic carrier at ω and quadratic magnetostrictive response to upconvert low frequency magnetic signals to be detected. Since the magnetostrictive response is not homogeneous over the transducer length and exhibits hysteresis, one cannot obtain complete nulling of the signal at co, which is typically set at the mechanical resonance of the transducer. This residual signal provides a source of upconversion of low frequency, nonmagnetic noise into the sidebands of the signal at co and, hence, limits the sensitivity of the device. We analyze the sources of the residual signal and determine the critical parameters which define the sensor's minimum detectable field. Experimental data shows how the parameters of the model correlate with the observed residual signal, carrier and sideband signal. We also show how the model parameters vary and change the behavior of the residual signal as one varies the operating regime (dither frequency, dither amplitude, and external dc field) of the transducer.

In this paper details will be presented of fiber optic pressure sensors that utilize the deflection of a thin diaphragm as the sensing mechanism. The magnitude of the deflection is monitored optically by either: (a) Measurement of the change in intensity distribution of light reflected from the diaphragm due to the changing shape of the reflecting surface, or (b) Interferometric measurement of the optical cavity thickness formed by the fiber end and the reflecting diaphragm surface.

A microphone consisting of a hollow cylinder whose flexible, circular endplates are bonded to pairs of flat spiral wound coils of optical fiber is described. When the endplate/disk is deformed due to a pressure difference, the outer and inner fiber coils experience opposite strains resulting in a "push-pull" optical path length difference which is detected in an all-fiber Michelson interferometer. The close proximity of the interferometric fiber coils, separated by the thin thermally conducting end plate, rejects thermal gradient induced signals. The addition of a second identical endplate and fiber coil pair at the opposite end of the cylinder doubles the acoustic sensitivity while canceling acceleration induced signals. The calculated and measured optical strain of a single plate, single coil sensor using static pressure, acoustic pressure, and acceleration are in good agreement and yield a sensitivity of 21 milliradians per Pascal per meter of optical fiber for an 8.0 cm diameter, 3.0 mm thick plate below its resonance frequency of 3 KHz.

Dual purpose fiber optic microbend loss sensors have been developed for measurement of underwater acoustic wave amplitudes and for detection of the direction of wave propagation. Cylindrical sensing elements with external threads have fiber wound around them. Axial slots, cut along the length of the cylinder and deeper than the threads, provide the microbends. Two different construction schemes for cylindrical sensing elements are considered. The dual purpose hydrophones have been characterized for frequencies ranging from 15 kHz to 75 kHz. Long fibers with different coatings and sensing elements with different variations in mechanical wave-lengths can be shown to improve the sensitivity of the sensing system.

A unique design for a polarimetric fiber-optic pressure sensor, its performance, and some preliminary experimental results are presented. Methods for reducing temperature sensitivity, which often limits the sensor's resolution, are also described.

The fiber optic Sagnac interferometer has a series of advantages that make it particularly well suited to many rotation sensing applications when compared to conventional rotation sensors. In addition, the fiber optic Sagnac interferometer may be used to support the sensing of acoustics, strain, temperature, and pressure in modes of operation that are optimized for low frequency or high frequency signatures. This type of interferometer may also be used to support instrumentation for spectrometers to characterize light sources and for fiber diagnostics. This paper provides an overview of these applications.

Fiber-optic gyros are being developed as attractive devices for many navigation and guidance applications. These all solid-state devices promise to have many advantages such as light weight, long life, no moving arts, and low voltage power. One class of fiber-optic gyros, the resonator fiber-optic gyro (RFOG), has been investigated primarily as a means of realizing these advantages while significantly reducing the length of the fiber coil. One of the key developments for interferometer fiber gyros was the use of all-guided-wave single-mode components to achieve a rugged, stable device.5 More recently the incorporation of integrated optics has allowed the use of closed loop schemes to achieve good scale factor performance over high dynamic range and has made low cost practical devices possible.6 On the other hand, while significant progress has been achieved towards the incorporation of semiconductor sources in RFOGs,7 all-guided-wave RFOGs have not yet been reported. In particular, little data yet exists on the use of integrated optics in RFOGs to achieve the necessary frequency shifting and loop closure. In this paper, we present data on a nearly all guided-wave RFOG experimental system, employing a 20m fiber resonator, lithium niobate (LiNbO₃) integrated optics, and a 1.5-μm helium-neon (HeNe) laser. Frequency shifting is achieved by the use of serrodyne phase ramp techniques.8, The gyro is packaged, with the exception of the source and its associated beamsplitter.

Single mode fiber optic couplers have found wide ranging applications in the areas of communications and interferometric sensors. In communications applications, the polarization state of the light in the fiber fluctuates due to environmentally induced changes in the fiber birefringence. Consequently, the major concern of both system designers and users is that the operating specifications of the passive components in the system are insensitive to the light's polarization state. The exception to this is in the application of coherent communications, in which two laser signals are coherently mixed prior to being detected by the system receiver.

Several fiber optic Sagnac interferometers employing multimode fiber of both high and ffedimiNrrumbers and simple LED light sources, have been designed and built by the authors over the past two years. New results showing improved performance fran that reported at the August '87 SPIE are given in this paper. The ratios of maximum unambiguous rate signal to random 3a drift signal are now in the range 50-150 a performance enhancement of between 4 and 10. We have found that a step index ring rather than a grajled Index one is necess for good driftperformance and that best results are obtained when all the other ring elements (PZT coary il and I/O slitter are also fabricated fram step index fiber. The 3a drifts in our 200 meter 10 cm diameter breadboards, in particular, are around 1°/sec. Using high V number fiber (100 pm/0.29 NA) no static mode mixers are required to desensitize this relatively short sense coil fram environmental pertubations. With unambiguous maxi rum rates on the order of ±200°/sec using simple detection of the MT fundamental signal the performance of these breadboard systems is now as good or better than many law cost "Coriolis" type rate sensors on the market.

This paper discusses some potential sources of scale factor error in an interferometric dynamically biased FOG. Scale factor formulae are derived for a serrodyne approach utilizing direct slope measurement, and an open loop direct phase reading (DPR) approach. The source wavelength, phase modulator characteristics and optical fiber parameters are addressed as sources of scale factor error for both moderate and tactical grade FOG applications. A look at the dependence of the predominant error sources on temperature, and how this dependence will affect the magnitude of the scale factor error, will be given. Methods of reducing or compensating for the errors are discussed. The resultant impact on FOG design is examined. Scale factor limitations are examined for each of the above approaches, and comparisons are made between the two methods.

Recent advances in fiber and integrated optical components have made it possible to demonstrate fiber optic rotation sensors which have performance levels adequate for use in tactical missile applications. Fiber gyros are now in the engineering phase and flight worthy prototypes may be ready within the next two to three years. Requirements for advanced projectiles and fiber gyro designs capable of meeting these requirements are discussed below.

The test results of a prototype serrodyne closed-loop interferometric fiber optic gyroscope are reported. The rotation sensor used a low-coherence superradiant diode at 0.85 microns wavelength with a polarization-maintaining sensing coil, polarizer and loop coupler. A wide bandwidth Ti:LiNb0₃ phase modulator was used to provide bias modulation and serrodyne frequency shifting. Performance of the sensor before and after loop closure is discussed. No degradation of the random walk or bias stability was noted with loop closure. The scale factor and dynamic range were improved. Various contributors to scale factor error are discussed.

Two major error sources associated with the scale-factor linearity of serrodyne closed-loop1,2 fiber-optic gyroscope are the stability of light source wavelength, and the perfection of loop-closure compo-nents and the associated driving electronics. LiNbO₃ phase modulators3,4,5 are currently the most promising devices as the loop-closure component due to their wide bandwidth, large modulation index, and baseband operation. However, the non-ideal serrodyne driv-ing waveform, and the undesired amplitude modulation effects of LiNbO3 phase modulator, result in multiple sideband generations which limit the scale-factor linearity of serrodyne closed-loop fiber-optic gyroscopes.

Several criteria were used as a basis for comparing the two types of passive fiber optic gyroscopes: shot noise, non-linear optical effects, radiation induced loss, and applicability of digital technology. The comparisons were performed using theoretically imposed performance constraints that are independent of the specific manner in which the two types of gyros are implemented.

A novel optical fibre rotation sensor which incorporates a multimode fibre storage loop inside a conventional Sagnac interferometer and utilizes a low coherence source has been described . The technique increases the number of transits the light makes around the sensing loop, thus enhancing the rotational sensitivity. The signal sensitivity has been compared with that of an optimised Sagnac interferometer and experimental results for the 10m and 20m multimode fibre loops are presented.

Fiber optic transceivers, consisting of opto-electronic components (LED's, lasers, photodiodes) and passive components (couplers, multiplexers and connectors), have been developed. In this paper details of the design criteria, fabrication techniques and device performance specifications will be presented for a range of fiber, source and detector types.

We describe a low cost optical fibre sensor devoted to the continuous characterization of the density of drilling muds used in oil prospecting. It is constituted by two parallel edge to edge fibres. flee of them permits to light the particles spread in the mud while the other acts at the same time to collect a part of the back-scattered light and to carry back it to a photodetector. With such a device, for the muds used in this field and containing a very high number of particles by unit of volume, the response is always a decreasing function of the density.

The paper describes a Coriolis mass flowmeter employing fiber optic sensing and optical powering. An experimental version was constructed and tested using water flow in the range 0 to 16.5 ml/s.

A fiber optic rotary displacement sensor has been developed in which an optical retardation plate is used to encode angular displacement on a broadband optical signal as a modulated notch minimum. As the waveplate is rotated, the optical beam experiences a variable linear retardation. The signal wavelength at which the retardation is exactly one-half wave exhibits a minimum intensity transmission. The wavelength of the intensity minimum is then a function of the rotation of the retardation plate. Determination of the wavelength of these intensity minima is based on measurements acquired with a grating-CCD configuration. Source variations (wavelength and power) are monitored by illuminating a portion of the grating-CCD. Normalization is obtained by referencing the modulated signal, obtained from the remaining CCD pixels, to the source emission. A theoretical prediction of the sensor's performance is developed and compared with experiments performed in the near IR spectral region using large core multimode optical fiber. Temperature compensation of the system will also be discussed.

A simple but original high-pressure fiber-optic system was developed for transmitting optical data via fiber lightguides to and from the inside of a high-pressure vessel at up to 200 MPa. This multifiber system is fully demountable and can carry up to 20 independent optical fibers of different types and different diameters. The paper discusses the principles and the details of leadthrough system design and describes its performance using single-mode and polarization-maintaining fibers. Preliminary applications include new high pressure fiber-optic sensors as well as measurement of the transmission and/or reflection properties of solid or liquid materials under high hydrostatic pressure. The idea of a new high pressure sensor using highly birefringent bowtie type optical fiber is briefly out-lined.

Thermal conductivities of liquids have been determined by using a variation of the transient hot-wire technique. A short segment of one arm of a fiber-optic Mach-Zehnder interferometer is coated with a thin layer of gold. This gold layer is heated resistively with a 1 msec current pulse producing a temperature change of only tenths of a degree. The magnitude of this temperature change (determined by the phase change in the interferometer) can be used to calculate the thermal conductivity of the medium in which the fiber is immersed. The conventional transient hot-wire thermal conductivity technique employs a much larger temperature rise and requires numerous correction terms. Results obtained in aqueous ethylene glycol solutions are reported.

Under optimized conditions the microbending sensitivity is comparable in metal coated multimode and single mode fibers. Microbending losses in step index fibers may show significant wavelength dependence and this dependence is quite similar for single and multimode fibers. The spectral characteristics of the losses depend not only on the fiber parameters but also on the periodicity of microbending. An inexpensive cladding-free fiber exhibits two-region displacement sensitivity. The maximum sensitivity of the single region fiber can exceed the maximum sensitivities of conventional fibers.

This paper describes the concept of large scale planar polymer light guide networks for automotive applications. These networks are cost effective, easy to fabricate, and easy to install in vehicles. They may incorporate data links, multiplexers, and couplers for communication purposes, as well as sensors, switches, and displays. Fabrication is achieved by cutting a thin sheet of polymer, or compression molding optical grade resin into the desired network topology. Examples of integratable parts are given, including a novel, reflective-type position sensor. The particular sensor we tested exhibited linear response over its designed range. Through normalization of the differential output, the response of the sensor was unaffected by light source intensity fluctuations, and by variations of the distance between the sensor extremities and the moving part. This simple position sensor demonstrates many of the advantages of planar polymer networks, including that of extending the one dimensional geometry of fiber optics into two dimensions.

Low loss, polarisation-maintaining, all-fibre ring resonators can exhibit non-linear propagation effects occurring at relatively low input powers. Observations have been made of Stimulated Brillouin Scattering (SBS) with input powers of 65μW leading to significant non-linear variations in the measured value of finesse, due to depletion of the power via the SBS process. Variations of measured finesse from 210 to 320 have been observed for a variation in input power of 155 p.W. The nature of this variation in resonator finesse with input power has been studied and is shown to depend upon the level of the input power when the resonator response is optimised.

The physical phenomenon behind the sensor described in this paper is related to the modulation of the speckle pattern, at the output end of a multimode fiber illuminated by coherent light, when the fiber is mechanically or acoustically disturbed. This effect is observed either when highly multimode fibers or two-mode fibers are used. In the latter case a demodulation technique is proposed to restore a signal proportional to the acoustical excitation