The number of commercial industrial products containing fiber-optic intensity sensors has grown significantly during the past two years. Suppliers of some of the products based on fiber-optic intensity are identified along with some discussion of the corresponding applications or measurement concepts. A summary of technical problem areas as well as future trends is also presented.

A multiple-point fiber optic sensor for the detection of weak and quasi-steady magnetic fields is described. The optical processing involves a single optical fiber sensing element configured as a two beam interferometer employing magnetostrictive sensing elements. Each sensing element is addressed by a different modulation frequency distinct in the frequency spectrum. The principle of operation is based on narrow band phase sensitive detection technique such that the multiplexing of a number of sensing elements is accomplished in the frequency domain. The fiber sensing elements can be remotely deployed, linked to the processing unit by an arbitrary length of down lead fiber. The investigated optical configuration facilitates the realization of a practical multiplexed magnetic field sensor.

A closed-loop, fiber-optic magnetometer/gradiometer, utilizing digital signal proces-sing, has been designed, built and tested. Several novel features of this gradiometer are discussed. In particular, the device is simultaneously a magnetometer and gradiometer. The sensor employs metallic glass transducers in a fiber-optic Mach-Zehnder interferometer. A unique signal processing method utilizing magnetic feedback nulling allows for a stable gradiometer and minimal sensor hysteresis. Directional response of the magnetometer and gradiometer, as well as stability of the combined configuration are presented. Currently, the sensor is being characterized in the laboratory. Preliminary results indicate sub-gamma field sensitivity, and results from a packaged sensor are presented.

Three fiber optic sensor systems designed to remotely determine angular position are described. Two of the systems use a sheet polarizer affixed to a "codewheel" and the third system uses a two-channel digital shaft encoder style "codewheel". The first polarization scheme uses four utical channels, two of which are analog and two digital. The ratio of the two analog channel intensities yeilds tan 6. The four-fold quadrant ambiguity is resolved by the two digital channels which are transected by two semicircular masks on the polarizer codewheel [Ref. U.S. Patent #4,577,414, 25 Mar 1986]. The second polarization scheme again uses quadrant ambiguity masks but only one analog channel which simulates a polarization vector which oscillates through 90°. The oscillating vector is produced by superimposing two sine wave modulated beams at the polarizer codewheel. The modulations of the two beams have a phase difference which is created by time delaying one of the beams in a fiber delay loop. The phase difference between the generated composite signal and electronic reference signal then determines the angle of the codewheel. We have demonstrated experimentally that this type of split analog/digital scheme has a resolution equivalent to a 10-bit digital system (ie ±0.35°). The digital shaft encoder scheme uses only two digital channels and a codewheel which has two concentric masks with 48 equally spaced windows offset with respect to each other by one-half window width. At 0° there is a unique mask which initializes an up/down decoder chip [Hewlett-Packard HCTL-2000]. This system has a resolution better than 7 bits. The supporting electro-optical systems including sources, fibers,lenses, mirrors, couplers, WDM's, polarizers, detectors, and signal processing for all schemes are described and the relative merits of each are compared.

The thermal instability of the working point of integrated devices may arise from photoelastic effect. MACH-ZEHNDER interferometer with special electrode structure, previously designed to have a high voltage sensitivity, leads also to a good thermal stability.

Absolute and differential fiber optic pressure sensors are described in this paper that have .1% resolution and are produced by integrated circuit techniques with yields of thousands per 3" wafer. Sensor performance can be tailored to application with full scale responses ranging from 5" of water (9 mmHg) to greater than 1000 psi (700KPa).

This paper describes a fiber optic pressure transducer intended for use in biomedical applications. The sensing element, or "optrode," is an interference cavity or etalon (essentially a "Hadley - Dennison " filter) formed mainly of glass, which is mounted at the end of a single optical fiber. This construction offers several advantages over earlier transducer designs based on similar principles.

We report the use of bare optical fibers to detect shock arrival times and shock pressures in the collision of a moving flyer plate with a stationary plate. Glass fibers were either laid on the surface of the stationary plate or put through closely fitting holes so that they were perpendicular to the plate, with their cleaved ends flush with the surface being struck by the flyer. The flyer, moving at 5 km/s, struck the stationary plate at an angle of 6.7°. The phase velocity of the line of Intersection, measured from the times of light onset in fibers at different positions, moved at over 40 km/s along the direction of the surface fibers. All of the fibers were shock heated to about 3600K, producing enough light to allow detection by optical receivers viewing the other ends of the fibers. By being reasonably careful about calibrating the absolute transmission of the optical system, we were able to obtain good estimates of the quartz temperature and pressure as a function of time and position along the surface fibers. (We assume that only the light from near the shock front emerges.) By fusing a short piece of smaller-core-fiber to the end of a surface fiber, we were able to get two signals from one fiber and deduce an instantaneous phase velocity. The perpendicular fibers gave understandable signals only during the time the shock ran through the stationary plate.

The attenuation and dispersion of intense stress waves in solids can be measured by fiber optic techniques. This paper describes an in-situ optical fiber strain sensor that can be used to measure transient subsurface strains with a bandwidth of 1 GHz. The sensor consist of a length of polarization maintaining fiber which is illuminated by a coherent polarized light source. The presence of a stress wave is detected and measured by monitoring the state of polarization and phase of the transmitted light, in a manner closely analogous to a classical photoelastic diagnostic-except on a sub-miniature scale. By using two wave-lengths, both components of transverse stress in an incident disturbance can be determined. Experimental results are shown and the sensor principles and measurement techniques are discussed.

One of the main potentiel markets for fiber optic sensors is the absolute static pressure measurement field. However, in most transducing principles that have been proposed a pressure variation causes a variation in attenuation of the propagating light; this leads to intensity modulated sensors which are characterised by low dynamic range because of sensitivity to external sources of attenuation such as bending, variable connector loss, source instability ... Consequently, high dynamic range pressure sensor will be obtained using optical principles unsensitive to uncontroled attenuation variations of the optical path. Spectral encoding is one of the most promising solutions to this problem. In the device we present, the light of a wide spectrum LED is fed into a multimode fiber, at the end of a Fizeau interferometer creates fringes in the spectrum of the light. The two reflectors of this interferometer are the tip of the fiber, and a mobile membrane placed close to the fiber; the spectrum of the light returning from the interferometer is analysed by a spectrophotometer in order to compute fringe number and position in a given spectral interval. This gives an absolute measurement of the path difference of the interferometer, which is directly related in our case to the pressure applied onto the membrane. As the measured quantity is not the intensity of the returning light, this method leads to high precision measurements and preliminary measurements have demonstrated precisions of 10-3 of the pressure range. Theoretical possibilities and experimental results will be presented.

After recalling the principle and the well-admitted operating conditions of fiber-optic gyroscopes, we comment two remaining subjects of concern: polarization and birefringence "non-reciprocities" and scale factor accuracy. Future domains of application are finally outlined.

Fiber Optic Gyro performance has progressed sufficiently to allow productization for sp8ce applications. Data is presented showing a stable bias (<0.1 deg/hr) over 0 to 40^oC environment with low random noise (0.006 deg/J) and modest scale factor stability (0.2%). Its low power consumption, light weight, and potential for low cost and high reliability make it ideal for the space environment.

A fiber optic Sagnac interferometer employing multimode fiber and a simple LED light source was designed and built by the authors. Results from both a computer simulation and actual laboratory tests of such an inherently low cost rotation rate sensor are given in this paper. These are compared to recent theoretical and experimental articles on multimode fiber optic rate sensors appearing mainly in the Soviet literature.

The National Aeronautics and Space Administration and the Jet Propulsion Laboratory are exploring the Fiberoptic Rotation Sensor (FORS) as an alternative to spinning mass gyros for space applications. In particular, FORS is being developed for use in the inertial reference units of the Mariner Mark II series of planetary exploration spacecraft. Other potential space applications of FORS include pointing and attitudinal control of large space structures such as the Large Deployable Reflector, Land Mobile Satellite Service spacecraft, and Space Station; FORS may also find application in the navigation and control systems of the National Aerospace Plane and in the Mars Rover mission.

A formalism is presented that unifies coherence, phase modulation, and dispersive propagation in Sagnac interferometers and is valid for large amplitude phase modulation. Several effects have been found as a result. For the fiber optic gyroscope, parasitic amplitude modulation effects that are maximum at the loop frequency have been found, in contrast to several effects that vanish when operating at the loop frequency. A correction to the gyroscope scale factor that is dependent on modulation amplitude and source bandwidth has also been found. In addition, there exists a maximum rotation rate limited by coherence effects, or maximum sensed phase shift for the Sagnac acoustic sensor. Phase noise effects caused by environmental or other disturbances can cause signal fade out and loss of information for the Sagnac data ring, and the magnitude of these effects are calculated. The Fm/Am conversion effect and its relevance for Sagnac interferometers is also treated. Finally, the use of a frequency shifting approach or various types of modulation to reduce these effects is discussed.

The impact of modulation parameters on signal performance of fiber gyroscopes has been investigated. Amplitude modulation (a.m.) causing bias drift and instabilities of phase modulation index causing scaling problems will be discussed and compared to experimental test results.

A scalar phase conjugator is used to correct for modal scrambling in a fiber-optic gyroscope using multimode fibers. A proof-of-principle demonstration of rotation sensing is reported.

Nonreciprocal phase errors have been identified in Sagnac fiber-optic sensors which use acousto-optic modulators for frequency shifting. These errors have been observed in a system which uses the acousto-optic modulator as both a frequency shifter and the central beamsplitter which generates the counterpropagating light beams in the Sagnac interferometer. The amount of phase error generated is dependent on the acousto-optic modulator drive power as well as other acousto-optic parameters. Data is presented from an inter-ferometer which uses the acousto-optic modulator as a beamsplitter. The data shows good correlation with the predicted phase errors.

Theoretical and experimental performance of guided wave serrodyne phase modulators formed through indiffusion of Ti into LiNb0₃ substrates is discussed. Sideband suppres-sion limitations due to the phase modulator, its driving electronics, and the light source spectral characteristics are presented with experimentally demonstrated suppressions of greater than 51 dB showing good agreement with theory.

Advanced aircraft and spacecraft will require fiber-optic sensor systems to monitor the environment around the platform as well as the structural integrity of the vehicle itself. These sensors when embedded in composite or metal matrix material can also be used to monitor the manufacturing process. Thus, this technology provides a means to sense key environmental parameters from the creation of parts, through assembly, test and flight for the life of the platform.

A fibre optic sensor system is described which employs wavelength filtering, and is capable of being adapted to fulfil a wide variety of aerospace applications. Two sensor heads are described in the paper, one for shaft angle measurement using a diffraction grating, and the other to measure linear displacement using a zone plate encoding element.

This paper describes recent developments for aerospace-structure application of a patented electro-optic monitoring system originally proposed for the integrity determination of long, lineal structures such as pipelines. Background information is presented, including references to the patent and prototype-system test data, followed by a discussion of future systems and aerospace requirements, plus the provision of initial test data involving integration with a relatively-short aerospace-type composite structure.

This paper will present an overview of multiplexing techniques for interferometric fiber-optic sensors, concentrating on the operational details of the various schemes proposed in recent years, and briefly comparing reported experimental performance data such as phase detection sensitivities and crosstalk levels . The limits to the number of sensors which can be multiplexed using a particular approach imposed by the power-budget, sampling and bandwidth considerations, and intrinsic crosstalk are also discussed.

The performance of experimental equipment built to evaluate a fibre optic method for reading instruments in remote locations is described. A retroreflecting encoder and a wavelength division multiplexing technique have been combined giving passive operation at the instrument and the need for only one fibre to read and transmit the data. A signal processing scheme for decoding a five digit decimal number using a microcomputer and means for compensating variations in source output and optical attenuations are discussed.

An RF optical modulation technique for multiplexing and self-referencing a number of fiber optic intensity sensors is described. The optical transducers are incorporated into recirculating optical fiber loops connected in parallel between transmit and receive optical fibers. A linear RF ramped optical signal is coupled into the system and the detected optical signal is electronically mixed with the input. Beat signals are produced in the frequency domain in the form of pulse trains that characterize the output of each sensor module. The relative magnitudes of the frequency components are insensitive to varying optical loss characteristics between the sensor modules and the signal processing location. The theoretical basis of the technique is presented and experimental results are given.

Distributed fiber-optic sensing systems require a spatial resolution of less than one meter, which cannot easily by reached by conventional optical-time-domain-reflectometry (OTDR), because of present electronics response times limited to about 10 ns. Correlation-Optical-Time-Domain-Reflectometry (COTDR) is a powerful technique for high resolution test of distributed single-mode fiber sensors in the long wavelength range 1.3-1.6μm. This paper describes the realization of a 1.3μm COTDR using a 200 Mb/s pseudo-random sequence which allows a spatial resolution of 50 cm, and gives results obtained on a short single-mode fiber length with localized defects simulating a distributed line of sensors.

This paper discusses the application of pulse time division multiplexing and wavelength division multiplexing to reflective digital optical code plates for sensing position on aircraft. Particular attention is given to temperature drifts and component tolerances in the optical power budgets. Both wavelength and time division multiplexing have been proposed for referencing analog ratiometric sensors and for multiplexing digital optical signals such as from a reflective code plate or an array of discrete sensors. The use of either technique in these applications is limited by the aircraft environment, particularly by the temperature induced drift of the source and optical component properties. The paper concludes that each approach is constrained by environmental effects and small power budget. In addition, the performance of the reflective WDM system is strongly dependent on the number of connectors used in the interconnect fiber. Development of either of these sensor concepts requires careful design and evaluation to resolve these problems.

The results of an investigation into the temporal response of fibre optic interfero-meters to heat pulses is presented. A rise time of < 2.5 x 10^-4 s was determined where an electric arc was used to heat the fibre cladding. A response time of < 10-4 s was observed when an optical technique was used to directly heat the fibre core. These measurements were performed with a new remote optical configuration which could be used as a miniaturised sensing element. The feasibility of multiple measurement with this new fibre Fabry-Perot interferometer was studied, and two multiplexed sensing systems based on frequency division and coherence multiplexing are described.

A simple fiberoptic differential temperature probe that utilizes the change of absorption of light in undoped GaAs in the vicinity of its bandgap as a temperature sensing mechanism is described. The probe is similar to that described by Kyuma et al.¹ but uses a different scheme of modulation, measures differential temperature, and is an improvement in accuracy by a factor of two. Prior to reporting on details of the probe that was constructed, the paper briefly discusses temperature sensing based on semiconductor bandgap behavior. This is followed by a report on initial experiments, which demonstrated the desired temperature response but also revealed the disturbing problem of changes in probe output caused by temperature induced mechanical misalignments. To attempt to remedy this problem, frequency division multiplexing was used, in which the output of one LED, of wavelength 0.88 pm was used as the actual temperature sensing mechanism, while changes in the transmitted radiation from another LED, of wavelength 1.27 μm, were used to correct the measured output for the mechanical misalignment. The 0.88 PM LED was amplitude modulated at 100 Hz; the 1.27 μm LED at 3 kHz. Although predicted minimum temperature difference for this system is 1.8 x 10-3°C, our margin of error with the signal generators used to modulate the LEDs that were available to us was ± 0.5°C. Indications are that this figure would be reduced considerably with signal generators of greater frequency stability.

An all fiber interferometric temperature sensor capable of remote operation is described. The fiber sensing element is configured as a Fabry-Perot cavity of typically less than 5 mm situated at the end of a lead fiber of arbitrary length (typically 2 m). The sensing element is addressed via a fiber coupler. Such configuration enables both remote detection and multiplexing of a number of sensing elements for multiple point measurements.

This paper describes a fiber-optic temperature sensor that uses a spectrum-modulating SiC etalon. The spectral output of this type of sensor may be analyzed to obtain a temperature measurement which is largely independent of the transmission properties of the sensor's fiber-optic link. A highly precise laboratory-type spectrometer is described in detail, and this instrument is used to study the properties of this type of sensor. Also described are a number of different spectrum analyzers that are suitable for use in a practical thermometer.

In this work the feasibility of utilizing hydrogenated amorphous silicon (a-Si:H) films in optical fiber temperature sensors is demonstrated. In this sensor, a-Si:H films are deposited from the glow discharge decomposition of silane gas directly on fiber ends, followed by an aluminum layer obtained by vacuum evaporation. In this manner, the a-Si:H acts like a low-pass filter which cut-off frequency is temperature dependent, modulating the light intensity wich is reflected back into the fiber. This arrangement provides excelent mechanical rigidity and minimum dimensions, limited only by the fiber diameter. Preliminary results, using films deposited at 250°C and a HeNe laser as light source, showed a linear 50% variation in the detected signal, for a 50° C to 200°C temperature variation. In order to obtain similar effects for a 0.8μm LED, thermal annealed films are used.

A fiberoptic temperature probe based upon spectrophotometric detection of the change in wavelength of the optical absorption edge of a semiconductor sensor has been fabricated and tested. Broadband light is transmitted through the sensor--a small prism of GaAs--attached to the end of a two-fiber probe. The returned light is analyzed by a fast-scanning spectrometer consisting of a diffraction grating and a silicon photodiode array. For handling multiple probes, a rectangular detector array may be employed. The system has been operated over a temperature range from 20°C to 175°C.

The refractive index of a material such as silicon changes rapidly with temperature at wavelengths near the band edge. This phenomenon can be used to sense temperature if the silicon or other material is used as a Fabry-Perot etalon. As temperature changes and the index varies, the etalon's spectral reflection characteristics will be affected. Using this effect and a spectral distribution readout technique it is possible to construct an optical temperature sensor with many desirable properties. These include a wide temperature range, typically -100 to + 400°C, long-term stability, insensitivity to connector and fiber losses, tolerance of fiber bending effects and rapid response. These sensors are fabricated using techniques similar to those used to manufacture semiconductor devices. This results in low production costs and the ability to produce sensors with reproducible performance.

A novel technique to analyze analog data in fiber optic sensing systems with temporal separation of channels is proposed. A theoretical explanation of the process is presented and an experimental setup that was used to obtain the data is described.

A compact seismic sensor has been fabricated by supporting a seismic mass between two fiber-wrapped rubber mandrels which form the two arms of a Michelson interferometer. This sensor has a sensitivity of 104 raft below resonance and 3500 rad/gm above resonance, is easy to fabricate, easy to modifiy for other applications, uses less than fifteen meters of sensing fiber, and has a detection threshold limited by its own intrinsic thermal noise (= 2 μrad/√Hz below resonance) rather than the opto-electronic demodulator noise.

A fiber optic accelerometer exhibiting high sensitivity has been developed. The transduction element is configured optically as a Michelson interferometer. Mechanically it consists of a pair of liquid-filled mandrels wrapped with equal lengths of single mode fiber. Acceleration sensitivities in excess of 2000 radians/g are readily achieved. Such accelerometers have been employed as sensing elements in gradient hydro-phones that have been tested successfully both in the laboratory and in the field. The practicality of this accelerometer's design and operation is compared with that of other recently proposed fiber optic interferometric vibration sensors.

A novel optical fiber sensor for dust concentration measurement has been developed, which is based on the Lanbert-Beer's Law. The construction of the sensor can limit electrically noises, resists heat and transmission difference and detects the concentration profile. The principle, performance, advantage and limitation are discussed.

The method for measuring non-contactly the position of an object using an optical fiber and a frequency modulated semiconductor laser is proposed. This method uses the coherent FM heterodyne principle of the Michelson interferometer. The optical fiber gives the flexibility to the system. The maximum detection range was limited to several meters by the speckle effects to the diffuse targets and the dependence of the semiconductor laser.

A new technique to utilize a single optical fiber simultaneously for communication and sensing is proposed. The sensing channel's operation is based on the modulation of the interference pattern projected from the output end of a multimode optical fiber when the fiber is subjected to an external disturbance which changes its optical properties. It is demonstrated that the communication quality is not affected by the simultaneous operation of the sensing channel. Potential applications such as the protection of fiberoptic com-munication from information tapping, and the use of a data link simultaneously for intrusion detecting are also proposed.

A design for a Fiber Optic Rotary Position Transducer (FORM') has been developed, and major components tested, at Teledyne Ryan Electronics in San Diego, California. This rotary position transducer exhibits a full angular span of 90 (±45) degrees with an angular resolution of approximately 0.09 degrees (10 bits). The physical dimensions are approximately two inches outside diameter with a thickness of about one inch. The transducer is designed to meet standard military specifications. Its optical telemetry system operates on the principle of Time Division Multiplexing (TDM). An optical signal processor consisting of an optical transmitter and receiver communicates via a single optical fiber to the transducer. A digitally encoded reflective disc coupled mechanically to a shaft and optically to a fiber optic read head constitutes the actual transduction mechanism.

Process control instrumentation is a large potential market for fiber optic sensors and particulary for fiber optic microswitches. Use of such devices brings a lot of advantages such as lighter cables, E.M. immunity, intrinsic security due to optical measurement, no grounding problems and so on. However, commercially available fiber optic microswitches exhibit high insertion losses as well as non optimal mechanical design. In fact, these drawbacks are due to operation principles which are based on a mobile shutter displaced between two fibers. The fiber optic microswitch we present here, has been specially designed for harsh environments (oil industry). The patented operation principle uses only one fiber placed in front of a retroreflecting material by the mean of a fiber optic connector. The use of this retroreflector material allows an important reduction of the position tolerances required in two fibers devices, as well as easier fabrication and potential mass production of the optical microswitch. Moreover, such a configuration yields good performances in term of reflection coefficient leading to large dynamic range and consequently large distances (up to 250 m) between the optical microswitch and its optoelectronic instrument. Optomechanical design of the microswitch as well as electronic design of the optoelectronic instrument will be examined and discussed.

Six self-contained fiber-optic accelerometers have been built based on the microbend fiber-optic sensor. The accelerometer systems feature a simple loss-compensation technique that removes the DC offsets that are developed as a result of spurious signals that arise due to cable bending and vibration. The sensor optical and mechanical systems are described and the results of the sensor system calibrations are reported.

This paper describes a fiber-optic spectrometer for measuring the wavelength of light sources. This device makes use of an acousto-optic modulator to frequency shift counter-propagating light beams in a Sagnac fiber-optic interferometer. The fringe spacing at the output is a function of the wavelength of the light source and the frequency difference of the two counterpropagating light beams. This same device was used to determine the length of the fiber coil ofothe spectrometer. Experiments showed that the resolution of the spectrometer was 0.007 Å and the optical pathlength resolution was 8 microns. Applications of this device include the characterization of optical fibers and light sources under dynamic conditions.

The paper describes a novel, distributed fibre optic sensor capable of detecting and locating a disturbance on a continuous length of fibre. The sensor is based on a Sagnac loop interferometer. Any disturbance that causes a phase shift in the light propagating within the fibre loop, such as a change in the mechanical strain or temperature of the fibre, can be located. The basic principle of operation is described and preliminary results, obtained from a feasibility experiment, are given.

Fiber optic interferometric sensors can be extremely sensitive. A sensor that operates in the 1 to 10K radian regime has been developed at the Naval Postgraduate School. Present demodulators operate over a 10 microradian to .1 radian range. Therefore a different approach to demodulation is required for this sensor. A fringe rate scheme is described here. The output of an optical shaft encoder driven by a pendulum resembles the fringe pattern produced by a fiber optic interferometric sensor in response to a sinusoidal perturbation. The HEDS 6000 shaft encoder used in this project provides both in-phase and quadrature waveforms --both are required for demodulation. This simulates the two waveforms provided by an interferometer with a 3X3 coupler. The well behaved outputs of the shaft encoder were used to test the fringe rate demodulator circuit. The cicuitry that reconstructs the analog signal from the in-phase and quadrature signals must do two things: convert the interferometric fringe rate to a proportional voltage level and determine whether this level should remain positive or be inverted. A frequency-to-voltage converter chip (SK9209) accomplishes the first task. Since the chip is based on a charge pump/RC integrator there are some trade-offs for noise vs. slew-rate. A possible solution is a digital frequency-to-voltage circuit, i.e. a digital counter and a D/A converter. The up/down decision can be made by determining which waveform (in-phase or quadrature) is leading. This is accomplished by a JK flip-flop and edge detector circuitry. The flip-flop output controls a switch which is the converting element in a voltage follower/inverting amplifier. The output of the circit is a waveform which indicates particle velocity.

The effects of longitudinal strains and temperature changes on modal interference patterns from elliptical core two-mode optical fibers are investigated. The results suggest that simultaneous measurements of both temperature and strain are practical.

A new synchronous sampling demodulation scheme capable of providing passive stabilization for nonlinear sensors has been successfully demonstrated in a fiber optic magnetometer.

Investigations of optical losses in silicone-clad fibers immersed into water have shown that water diffusion through the reflecting cladding results in the loss increase, which is of a reversible character. Such fiber can easily be integrated in an optical fiber cable; water penetration can then be localised by the back-scattering method.

A porous optical fiber is developed for use as a sensor for gases and liquids at low concentrations. The porous structure of the glass which remains after selective heat treatment, phase separation and chemical leaching of a borosilicate glass gives a very high surface area to the fiber. The sensitivity of the fiber is very high due to the large surface area. The porous fiber is initially tested for measuring ammonia vapors at very low concentrations. A small region (about 0.5 cm) of the porous fiber is treated with a reversible pH indicator dye. The device was demonstrated to be capable of reversibly sensing ammonia vapors. Ammonia vapor concentration as low as 0.5,4.01, was easily detected. The resulting light absorption in visible region is related to the sample ammonia concentration. The sensor device is calibrated for different ammonia vapor concentrations.

The use of fiber optics in the field of optical holography is described with emphasis on an experimental arrangement whereby several object and or reference beams are implemented simultaneously. This scheme enhances a holographic system's versatility by offering multiple illumination planes of any given object. Using several passive singlemode couplers and a closed loop phase stabilization technique, a multiple beam fiber optic holographic system with stable phase correlation among output fibers is presented. The feedback system incorporates a Michelson interferometer which detects relative phase fluctuations between output optical fibers. A servo controlled piezoelectric phase modulator automatically compensates for environmentally induced phase fluctuations inherent to a fiberized system. The resulting scheme is a very stable and highly versatile system suitable for remote holographic interferometric sensing and other applications where conventional holography techniques are impractical. Futhermore, new results on the ability to vary object and reference beam intensity ratios in a fiber optic holographic system will be discussed.

A technique for the pseudo-depolarization of a long-coherence length source is described, which is based on a simple polarization modulation scheme. Using a length of high birefringence fiber wound on a piezoelectric transducer, a simple all-fiber polarization scrambler can be constructed. Details of an electro-optic scrambler are also reported. The depolarization technique is shown to eliminate the effects of input-lead-birefringence induced polarization fading in interferometric sensors.

A fiberoptic intrusion detecting system bases on intermodal interference has been installed for field test. The line-sensing system demonstrates high sensitivity and capability to provide position information. It is shown to be able to detect an intruder walking at 16 meters from the buried fiber. Except during heavy raining , or when the field is being flooded, detection rate of crossing the sensing line, by walking, running or crawling, is 100%. Improvements on sensor design and method of installation to enhance reliability are discussed.

Recently, greater emphasis has been placed on the need for remote sensing in all fiber-optic interferometric sensor systems. In order to accurately understand system response, one must investigate the optical effects associated with the use of a long lead cable in conjunction with interferometric sensors. This paper describes an empirical study of the responses of a 12 meter sample of armored fiber-optic cable to which static and dynamic stresses were applied. The results include quantitative measurements of variations in light amplitude, polarization state and interferometer output, and the correlation between these effects under stress conditions.

High purity fused silica fiber offers low attenuation and broad spectral transmission. When this fiber is used in imaging sensors, greater information may be transmitted over longer lengths when compared to conventional silicate fibers used in such imaging devices as endoscopes and boroscopes. Heretofore, imaging devices made with fused silica have been limited by small size and poor flexibility. Two imagescopes have been fabricated from two types of high purity glass core/glass clad fused silica fiber providing 8 linepairs per millimeter resolution over 4.6 m length with a 3 mm² viewing area. Bend radius without loss is less than 2 cm since the loose fibers are fixed only at the ends. Transmission range is from 300 nm to 1100 nm, with attenuation increasing exponentially as a function of wavelength. This is attributed to evanescent tunneling loss through the cladding. Transmission, bend loss, and attenuation were determined for the two fused silica imagescopes and a standard multifiber imagescope.

Optical fiber modal domain methods which use mode-mode interference as the sensing mechanism have been used to detect strain, temperature and pressure. Typically, bending-induced strain applications involve mode coupling which changes the mode field distributions. In this paper, we report the use of axial strain with minimal bending on a few-mode optical fiber ( V = 4.47) to study mode pattern modulation. Such modulation due to quasi-statically varying strain in particular has been observed and quantified. More complicated low frequency strain variations about "quiescent DC strain" points associated with different system strain sensitivities have also been analyzed. A heuristic discussion of the modal domain phenomenon responsible for this fiber waveguide modal behavior is presented.