Laboratory prototypes of a novel pressure sensor have been produced using a hollow glass microsphere, bonded, in an on-axis position, to the end of a monomode optical fiber. The sphere surfaces form a low finesse Fabry-Perot interferometer. The construction of the probe is simple in concept, yet the sensing element is intrinsically hermetically sealed. Experimental trials, under the influence of hydraulic pressure have been carried out and show a good match with predicted behavior. The observed shift in wavelength with pressure was -0.93 nm/MPa, two orders to magnitude higher than that we have measured with a in-fiber-grating sensor under similar conditions. The ratio of the pressure sensitivity to the temperature sensitivity for our microsphere sensor was more than two orders of magnitude better than the in-fiber-grating type, so therefore less compensation is necessary to correct for temperature changes. This new form of sensing probe has potential for many high-pressure sensing applications.

An experimental study of the performance of an all-fiber polarimetric pressure sensor is carried out. The sensor is in the form of a multifiber assembly composed of a low- or a highly-birefringent (Lo-Bi or Hi-Bi) single-mode lead-in, a Hi-Bi sensing fiber and a Hi-Bi polarizing fiber all spliced in sequence. Theoretically the polarization behavior of the assembly is described by means of the Mueller-Stokes matrix method in the general quasimonochromatic case. The response of the sensor to hydrostatic pressure at different temperatures and with different lengths of sensing fiber is described. The spectral distribution of the semiconductor laser was monitored by changing the injection current so that the effect on the visibility and the stability of operation could be considered.

Progress in developing fiber-optic interferometric sensors for aeroacoustic measurements in wind tunnels, performed under the NASA program, is reported. Preliminary results show that the fiber-optic interferometer sensor array is a powerful instrument for solving complex acoustic measurement problems in wind tunnels, which cannot be resolved with the conventional transducer technique.

This paper describes various reliability and accelerated aging tests that have been performed on LiNbO3 integrated optical FOG circuits fabricated by annealed proton exchange. Fully packaged devices have been temperature cycled 100 times from -65 to 125 C and subjected to 11 Grms random vibration with less than 0.5 dB variation in insertion loss. Potential failure mechanisms of LiNbO3 integrated optical circuits are discussed. Ten devices with passive 3-dB couplers have been aged at 150 C and tested every 1000 hr with little, if any, change in device performance.

Intracavity phase modulation in a fiber-optic ring laser gyro can provide 'optical dithering' to reduce the effects of frequency locking while retaining optical reciprocity in the cavity. We show that the use of two antiphased phase modulators placed symmetrically within the fiber cavity can provide uniformly distributed dithering. A modulation index of 2.4 theoretically eliminates the zero-order lock-in band around zero frequency, while use of a high modulation frequency puts higher-order lock-in bands outside the beat frequency dynamic range. Push-pull modulation allows for high modulation frequency with minimum dynamic perturbation of the cavity resonant behavior. We describe results with an experimental Brillouin fiber optic gyro operating at 1.3-micron wavelength using push-pull modulation together with a novel synthetic heterodyne detection scheme for sensing rotation rate and direction. A ten-fold reduction of the width of the zero-order lock-in band is observed. We also demonstrate that the observed frequency bias at zero rotation rate is caused by the Kerr effect due to the power imbalance between the two oppositely directed circulating lasers.

Polished coupler technology has developed several niches for prototype and breadboard applications, particularly for birefringent fiber. Applications include variable splitting ratio couplers, ultralow-loss devices, resonant rings and cavities, and evanescent-field-based devices. This paper will discuss the processing of polished couplers, including coupler substrate preparation, principal axis alignment, fiber-to-groove bonding, and substrate polishing. Coupler assembly, including adjustable, adhesive-bonded, optical contact bonded (OCB) designs, and resonator assembly will be described. Data from several spliced and spliceless resonators will be presented, including single-eigenstate-of-polarization, polarization rotating, and high-finesse devices.

Recirculating light in a fiber optic gyro can cause significant scale factor distortions and uncertainty when the signal processing makes use of the second harmonic of the bias modulation to sense rotation. Significant reduction of this problem can be realized by operating the gyro at particular modulation depths and/or at one-half the proper frequency.

Fiber optic gyroscopes (FOGs) are developed for industrial and consumer applications. The bias error, which is determined by electrical circuits rather than the optical system, varies between 0.01 and 36 deg/hr depending on the construction of the electrical system. The scale factor error at each input rate is less than 0.5% in the temperature range of -30 to 85 degree(s)C. These FOGs are compact, highly reliable, and need only a +12 V D.C. power source. They are ready for mass production.

We compare the use of lithium tantalate and lithium niobate as a substrate material for integrated optics chips for fiber optic gyroscopes. We also show the performance of a fiber optic gyroscope using a proton exchange lithium tantalate chip. The results indicate that, when normalized for detected power, the gyroscope built with the lithium tantalate chip performs as well as that with the lithium niobate chip.

A novel error compensation technique capable of significantly improving the gyroscope performance by reducing the polarization related bias errors of a polarization-rotating (PR) resonator is presented. It is shown that the PR-resonator has residual errors which depend on the polarization-dependent losses and coupling ratios of the resonator output coupler and on the polarization crosstalk within the ring. A bias error reduction by a factor of 50 or more is achieved by employing the technique which involves periodical switching of the resonance tracking operation between adjacent resonators.

An optical gyrocompass is based on the high-performance fiber optic gyroscope's sensitivity to the earth's rotation rate. The advantage over conventional gyrocompasses is that it starts up quickly and should also have a long operating life. An optical gyrocompass is developed with a north-seeking accuracy of 0.05 degree.

We describe sources of polarization errors in depolarized IFOGs and methods for enhancing their suppression. A test-gyro circuit and associated experimental results showing 0.1 degree/hour (one-sigma) open-loop bias stability over a thermally dynamic environment are presented.

A polarization model is used to experimentally study polarization phase nonreciprocity in all-fiber ring interferometers. The possibility of measuring the extinction ratios of fiber polarizers by means of fiber optic ring interferometers is demonstrated. The maximum extinction ratio values obtained are 84 and 86 dB.

Fabry-Perot interferometers have attractive features for use in fiber optic sensors. However, they suffer from undesirable amplitude modulation introduced as a result of coupling losses due to mirror absorption, mirror tilt, curvature of the fiber end-face, fiber mirror separation and splice losses. In addition, non-linearities of high finesse Fabry-Perot interferometers require special readout techniques. A method using digital signal processing is proposed which is able to solve the problems caused by the non-linearities, the directional ambiguities and the coupling losses. This method is demonstrated by using a fiber optic Fabry-Perot interferometer for the measurement of magnetostriction.

A new method and apparatus are presented by which electric fields (AC and DC) can be measured in high voltage (HV) environments above ground using a variable gap, Fabry-Perot micro-cavity transducer which is mounted on the tip of a multimode optical fiber. By constructing the Fabry-Perot cavity as a conductive Faraday cage, external electric fields can be detected by the electrostatic forces they exert on the top surface of the cavity where a flexible, corrugated, silicon diaphragm is incorporated. Under the action of the electrostatic forces, the diaphragm deflects varying the gap of the cavity which is measured as a change in the backreflected light. To make the device insensitive to bending and transmission losses in the fiber, a dual wavelength referencing technique is employed. Thus, the magnitude of the electric field can be related to a change in the intensity ratio at two different wavelengths. The sensor is characterized by being small, lightweight, unobtrusive, accurate and immune to electromagnetic interference (EMI), temperature or pressure effects. DC electric fields in the range of 0 to 300 KV/m have been successfully measured. The minimum field intensity detected was of the order of 40 KV/m. This relatively low sensitivity is due to the high stiffness of the diaphragm arising from the high boron-diffusion. However, higher sensitivities are possible by thinning the diaphragm, increasing the radius, reducing the boss ratio or decreasing the corrugation depth.

Two very simple electric-optic hybrid schemes which demodulate the output from a conventional current transformer (CT) are presented. The first scheme would be very useful for indicating whether the conductor was carrying current or for making measurements within an accuracy of 10% at extremely high potentials. Whilst the second scheme, with a demonstrated minimum detectable signal of 0.32 Amp/(root)Hz, offers low cost accurate current measurement at high potentials.

A new triangular topology for a bulk optic Faraday current sensor is presented with a demonstrated resolution of 20 mA/(root)Hz over a measurement range from 1 to 3000 A. The sensitivity of the system was 2.35 X 10^-5 rad/A. This sensor is relatively easy to fabricate and overcome problems encountered using current sensors based upon bulk optic 'square' configurations and all fiber systems.

The behaviors of the near- and far-field two-lobe radiation patterns from a circular-core two- mode fiber are studied by using both the mode and Kirchhoff diffraction theories. The electric field outside the output endface of the fiber is derived from the electric field distribution of the LP₀₁ and _LP11 modes at the fiber endface using the Kirchhoff diffraction formula. Different phenomena are studied by solving this expression numerically at near- and far-fields. The Gouy phase shifts are quantitatively evaluated at cross-sections of the near field. The results reveal the distortion and dislocation of the far-field two-lobe pattern induced by an inclination angle of the fiber endface.

A tunable erbium fiber laser which utilizes a broadband mirror and an intracore Bragg grating reflector in a side-pump configuration is designed and developed. This arrangement is used as a laser sensor which greatly enhances the performance of a recently developed wavelength demodulation system by University of Toronto Institute for Aerospace Studies. Sensor design parameters for optimal performance in terms of the fiber laser geometry, doped fiber length, and the required pump energies are presented. Laser tuning with temperature has been achieved and shows much improvement in the SNR with respect to the previous techniques, i.e., interrogation of the Bragg grating sensor with a broadband source.

This paper describes novel bend enhanced fiber (BEF) sensors used to make continuous, linear, real-time measurements of curvatures, which often relate more directly than strains to the control of vibration and position. BEF sensors are made by treating optical fibers to have an optically absorptive zone along a thin axial stripe a few millimeters long. Light transmission through the fiber past this zone then becomes a robust function of curvature, three orders of magnitude more sensitive to bending than in the untreated fiber. Directionality and polarity of curvature are preserved in the transmission function, over a linear range covering five orders of magnitude, centered about zero curvature. Thus, BEF sensors are curvature-measuring optical analogs of elongation-measuring resistance strain gauges, with similar sensitivity. BEF sensors add little or no thickness to the fiber, can be instrumented with simple analog electronics, and have been successfully embedded in composites. Results of dynamic curvature measurements are included, along with characterization data for BEF sensors made with plastic and silica fibers as small as 125 microns.

A fiber-optic strain gauge system for general purpose applications capable of operating with either Fabry-Perot, modal domain, or polarimetric sensors, is presented. The different sensor options enable the user to choose between short gauge length Fabry-Perot sensors for localized measurements, or long gauge length modal domain or polarimetric sensors for integrated measurements. The system is fully connectorized to facilitate interchange of sensors without a fusion splicer, which makes it accessible to a wide range of customers with no knowledge of fiber optics.

We propose a phase delay balanced two-mode fiber interferometer, which consists of two sections of two-mode, elliptical-core (e-core) fibers of the same length. These two fibers are fusion spliced together in such a way that there is a radius offset along their major or minor axes. We refer these two fibers as the sensing and reference fibers, respectively. Each of the LP₀₁ and LP₁₁^even modes of the sensing fiber excites the LP₀₁ and LP₁₁^even modes in the reference fiber. By launching a short coherence light into this sensing-reference fiber system, the original phase delay difference between LP₀₁ and LP₁₁^even modes in the sensing fiber can be balanced. The unique feature of this sensor is that the temperature coefficient is expected to be at least two orders of magnitude smaller than that of a conventional e-core two-mode fiber sensor. Because of the compensation effect, phase noise caused by the frequency fluctuation of the light source may become negligible.

Special, relatively inexpensive communications grade fibers have been developed to minimize the effects of humidity and temperature on lightwave propagation. Two of these fibers, hermetic and dual-coat, were tested in interferometric (Mach-Zehnder type) and microbend sensing configurations. The results are compared both with theory and identical measurements using standard single-coat fiber. Axial strain (interferometer) and transverse displacement (microbend) were measured at 20 degree(s)C intervals from ambient to 80 degree(s)C. In both the interferometric and microbend configurations, only the sensing portion of the fiber was heated. The ambient relative humidity was not controlled but was monitored during the experiments. Measurements were made with displacements at frequencies up to 1 kHz. In the interferometric sensor, the static sensitivity of the hermetic and dual-coat fibers changed very little over the temperature range while the sensitivity of standard single-coat fiber decreased by 20%. Low frequency sensitivity of the microbend sensor with single-coat fiber was 40 dB higher at 80 degree(s)C relative to 20 and 40 degree(s)C, while responses measured using dual- coat and hermetic fibers were less sensitive to changes in temperature. The results indicate that fiber designed to minimize the effects of humidity and temperature on communications may be used in sensing applications to minimize or eliminate unwanted response to fluctuations in ambient temperature.

In a previous paper one described a sensor that worked by replacing a small region of the clad by a material possessing an index of refraction highly sensitive to temperature. The variation of the index of refraction of this material leads to losses that modulate the transmitted optical power bringing information about the temperature. In this paper one presents a theory for the explanation of the behavior of the above mentioned sensor.

A digital fiber optic temperature sensor based on the abrupt change in the absorption coefficient observed when a two constituent mixture undergoes a solid/liquid phase transition is presented. Varying the weight ratio of the two constituent mixture, the phase transition temperature can be tuned over several degrees Celsius.

A single-mode silica fiber placed normal to a polarized 10.6 micrometers CO₂ laser beam absorbs heat and the resultant temperature rise can be determined with an optical fiber interferometer. A theoretical analysis of the heat balance for the fiber provides an expression for fringe shift in terms of fiber parameters and the laser power absorbed by the fiber. An expression for the absorbed laser power is determined assuming a Gaussian distribution for the single-mode laser beam and using Fresnel's reflectance equations. This allows fringe shifts to be calculated for different fiber positions across the laser beam and for different directions of linear polarization of the beam relative to the fiber axis. A series of experimental measurements of interferometer fringe shift as a function of different fiber and beam parameters has been completed, and the results show good agreement with the theoretical predictions. The work reveals that an optical fiber can be used to monitor the details of a CO₂ laser beam, and that a CO₂ laser beam provides a controlled heating source for thermal studies involving optical fiber interferometry.

A probe temperature sensor is constructed by attaching a long single mode fiber to a short (a few millimeters) dual mode elliptical core (e-core) fiber with an offset at the interface. The other endface of the e-core fiber is carefully polished to increase the reflectivity. The LP₀₁ and LP₁₁^even modes are excited in the e-core fiber which establishes a spatial two-lobe intensity pattern within the fiber. A temperature variation on the e-core fiber oscillates these two-lobe intensity pattern which gives rise to an intensity variation of the reflected light in the single mode fiber. A resolution of 0.5 degree(s)C has been achieved over the range of 25 degree(s)C to 122 degree(s)C. By adjusting the length of the dual mode e-core fiber, the output of this sensor can be linearized.

An optical fiber thermometer for electrical energy applications has been developed. The instrument uses the radiation of leaky modes in a short bend of standard singlemode fiber covered with polymeric resin, to sense temperature variations in the external medium. Suitable precision (<EQ +/- 2 degree(s)C) and good stability in the usable range of 5 degree(s)C - 170 degree(s)C are achieved.

Continuous and discrete liquid level measurements made with fiber optic sensors generally depend on refraction of light in the liquid to be measured. Problems, including erratic or incorrect readings, arise when the index of refraction of the liquid is near that of the outer surface of the optical probe. This is often the case for common optical probe materials and many organic compounds including fuels and lubricants. This paper describes a continuous liquid level sensor which operates over a wide range of indices. The lower limit of the range is determined by an intermediate optical material in the probe, rather than the material of the outer surface; there is no upper limit. The liquid being measured may have any index equal to or greater than that of the intermediate material. Water is used as the intermediate material in the sensor to be described; it surrounds a lambertian fiber optic emitter and detector pair. The sensor is linear within +/- 5 percent over a five-inch measurement span, for liquids of any index equal to or greater than 1.33, even though the outer surface of the probe is fused quartz, with an index of 1.46. Since most liquids have an index greater than that of water, and many liquids of interest have an index near that of quartz or glasses, this method greatly extends the range of liquids that can be measured with a given sensor.

A passive fiber optic fire sensor that uses transmission quality silica fiber is described. The fiber optic is used only to direct radiation from a fire down to a detector, as a light conduit. The sensor is characterized by resistance to high temperature environments, dirt, and electromagnetic fields. It is small, compact, lightweight, and senses fires from a wide variety of sources.

In a previous paper(1) the background, organisation and early results of a collaborative project on the safety of optical measurements in hazardous industrial environments involving five European laboratories in France, Germany and the UK were described. It is necessary that safe operating power levels should be established for the peace of mind of both manufacturer and user of optical sensors, and paradoxically, to enable an intensity of radiation to be employed which does not disadvantage light as a measurement medium and thereby act as a technological constraint. In general it can be assumed that the more light that goes into an optical measurement system the more utilitarian will be the result. Technically, determining safe operating radiation levels is a difficult task because of the many variables involved. We have concentrated on a single mechanism: thermal ignition of an atmosphere surrounding a body which is heated by the incident radiation. Influential factors are the nature of the absorber (size, shape, form, chemical reactivity), the nature of the environment (air, dust, level of flammable gas present) and the nature of the incident radiation (wavelength, coherence, beam geometry and temporal variation). Because of the breadth of this problem experimental investigations are being supplemented by theoretical calculations in the hope of realising a predictive capability which can anticipate hazards for situations for which experimental results are not available. It is the objective of this paper to present an interim statement on the results obtained up to the half-way point in this 3-year European project. This paper draws heavily on the work of one of the collaborating laboratories, a fuller account of which will shortly appear in the scientific literature2. More importantly it builds bridges to the other relevant results which have been published over the last several years. Of particular relevance are the publications from the Australian 'school'.

The radiation induced loss of multimode lead glass fibers is measured during and after irradiation by a Co-60 source in the temperature range from 10 degree(s)C to 50 degree(s)C. The measurements were performed at visible and infrared LED wavelengths using time multiplexing of the LED's. The radiation sensitivity and relaxation effects of this fiber depend on temperature and on the wavelength of the read-out light. The wavelength dependence allows estimation of the actual fiber temperature. Thus a method can be developed to compensate temperature and fading effects on radiation induced loss in situations where the fiber temperature is unknown. The application of multiple wavelength measurements of induced loss for radiation dosimetry with optical fibers is discussed.

The presented fiber optical confocal scanning microscope (FOCSM) using multimode fiber offers many advantages with respect to the simplicity and compactness of the sensor configuration and low system cost because only commonly available optical components are used, such as an inexpensive LED source, a GRIN rod lens, a multimode Y-coupler. For diffusely scattering or weakly reflecting industrial objects the sensor provides a high level detector signal owing to higher numerical aperture of the multimode fiber. By considering the optical 3D transfer function the image formation in such a FOCSM has been studied in detail. The small size of the sensing fiber enables us to construct a sensor head with fibers positioned closely side by side. This parallel sensor structure allows us either to accelerate the scanning speed or to perform local real-time filtering operations on objects during scanning which is of importance for many problems in industrial surface inspection.

In this paper a detailed error analysis for photoelastic fiber sensors is presented. The errors introduced by the photoelastic sensing element, the quarter-wave-plate, the polarization beam splitter, the optical source and the signal processing system have been evaluated quantitatively. Practical issues for photoelastic fiber sensors applications are also discussed.

The deflection of a flexible cantilevered optical fiber is used to measured the flow of gases and fluids. For practical applications a combination of an emitting optical fiber with a half-shade edge and a positional sensitive diode (PSD) is used. For a PSD with a length of 3 mm a range of measurement of 1:500 and more is possible. The responsivity of this device is 10 ml/s. The effects of deflection caused by the mass of the fiber are compensated for by using a second fiber of equal length in a separated compartment in the sensor device.

A differential fiber sensor is used as an aiming device in on-line measuring size of the workplace. It can be employed as a tracker in admeasuring micro-displacement of a specimen edge as well. The homodyne detection has been implemented by a closed loop system made up with microcomputer, right-angle prism and PZT. The experimental standard deviation of aiming the edge is confirmed to be within 0.1 micrometers .

A fiber optic pressure sensor is being developed for the Naval Surface Warfare Center- Dahlgren Division for use in measuring the pressure within sonar domes of Naval ships. The sensor comprises a quartz crystal tuning fork and force transduction mechanism such that the frequency of the quartz crystal is proportional to pressure within the sonar dome. An optical fiber interface is used to transport light to power the quartz crystal and measure its frequency. These sensors are compatible with the fiber optic cable plant of Naval ships.

The development of a dual wavelength referenced intensity-modulated optical fiber sensor system is described. The system utilizes a single broadband LED source whose emission is spectrally sliced to provide two separate narrow-band wavelength signals, one as the measurand and a second as the reference. The allocated wavelength bands were selected to minimize differential intensity effects between the two optical signals that may arise from the LED thermal variations. Subsequently, the sensor system provides an output fully referenced for all major common-mode variations arising from the optical propagation effects as well as those due to the LED instability. The complete system comprises of a terminal transceiver unit which incorporates the system electronics together with a specially designed LED coupler and wavelength demultiplexer device which is interfaced to an optical sensor using a single optical fiber. A Graded-Index (GRIN) rod microlens together with an interference filter and a moving mirror element are used in the optical sensor to provide a mechanism for intensity modulation of the measurand signal. The performance characteristics of an advanced prototype sensor system are reported which demonstrate substantial amount of linear displacement. Finally, an initial investigation of the immunity of the system to common-mode variations is also described.

The ability to multiplex sensors with different measurands but with a common sensing scheme is of importance in aircraft and aircraft engine applications; this unification of the sensors into a common interface has major implications for weight, cost, and reliability. A new class of sensors based on a common sensing scheme and their E/O Interface has been developed. The approach detects the location of the centroid of a beam of light; the set of fiber optic sensors with this sensing scheme include linear and rotary position, temperature, pressure, as well as duct Mach number. The sensing scheme provides immunity to intensity variations of the source or due to environmental effects on the fiber. A detector spatially multiplexed common electro-optic interface for the sensors has been demonstrated with a position and a temperature sensor.

The most common problem of interferometric sensors is their inability to measure absolute path imbalance. Presented in this paper is a signal processing system that gives absolute, unambiguous reading of optical path difference for almost any style of interferometric sensor. Key components are a wide band (incoherent) optical source, a polychromator, and FFT electronics. Advantages include no moving parts in the signal processor, no active components at the sensor location, and the use of standard single mode fiber for sensor illumination and signal transmission. Actual absolute path imbalance of the interferometer is determined without using fringe counting or other inferential techniques. The polychromator extracts the interference information that occurs at each discrete wavelength within the spectral band of the optical source. The signal processing consists of analog and digital filtering, Fast Fourier analysis, and a peak detection and interpolation algorithm. This system was originally designed for use in a remote pressure sensing application that employed a totally passive fiber optic interferometer. A performance qualification was made using a Fabry-Perot interferometer and a commercially available laser interferometer to measure the reference displacement.

A self-referencing technique compensating for fiber losses and source fluctuations in air-gap intensity-based optical fiber sensors is described and demonstrated. A resolution of 0.007 micron has been obtained over a measurement range of 0-250 microns for an intensity-based displacement sensor using this referencing technique. The sensor is shown to have minimal sensitivity to fiber bending losses and variations in the LED input power. A theoretical model for evaluation of step-index multimode optical fiber splice is proposed. The performance of the sensor as a displacement sensor agrees well with the theoretical analysis.

In the field of optic sensors, in most applications, the measurement of intensity is of main interest. There are many sensors, which by means of fiber optic evanescent spectroscopy are used to measure the temperature, concentration of methane or carbon dioxide in fiber optic transmission cells and glucose or other important substances. Using these sensors, the compensation of the influence at the data link dependencies is problematic. There are several proposals to compensate these influences. Among them there are the utilization of two or more wavelengths, the earliest possible digital conversion and the application of optical bridges. However, these methods are circumstantial and problems cannot be solved in general. A new method for the compensation of fiber optical connectors is presented.

Mang analysis have been done on the optical fiber position sensor based on intensity modulation technique. In this paper we present an analysis on Optic-Fiber-Array Sensor with a shutter wheel as intensity modulator, which can work on large dimensional measurement such as length or rotation angle with high sensitivity, accuracy and S/N. Different arrangements of optic fiber array versus shutter wheel are analyzed, and so are two signal processing methods, Switch-Digital Processing Method and Analog-Digital Processing Method. The angle sensitivity of the sensor can reach up to 10^-6 (in rad.) and the S/N is higher than that of the Moare Technique. At the final, experiment results are shown.

Electronic speckle pattern interferometry (ESPI) based on incoherent multimode fiber-optic bundles (MMBs) and coherent multimode image bundles is considered. Application of MMBs ESPI to measure displacement, strain, and unbonded faults in the carbon/epoxy honeycomb components is experimentally evaluated. It is concluded that the MMBs ESPI method is an extremely powerful tool for analyzing displacement, strain, and nondestructive testing.

In this paper we propose replacing a widely used but often difficult and cumbersome technique of hydraulic evaluation of stress in concrete materials with a new fiber-optic measurement device, which has all inherent advantages of fiber-optic sensors. The sensing element of the device consists of a highly birefringent (HB) polarization-maintaining optical fiber. The stress inside it induced by external pressure modulates the polarization state of the output light signal at the detection end of the system. The all-fiber instrumentation system of the sensor consists of a semiconductor pigtailed laser, input and output HB optical fibers, an analyzer and a computer-controlled synchronous detection system. A specially designed leadthrough integrated with the sensor head allowed us to insert the sensor inside a pressure pad filled with oil or alternatively with mercury. For calibration purposes, the pressure cell was placed inside a large pressure chamber designed to simulate the real environment. Characterization of the device for hysteresis, selectivity and sensitivity was performed for pressures up to 70 bar and for ambient temperatures. The described sensor is simple, cost-effective, safe in explosive environments and well adapted for stress monitoring in the large-scale structures.

In a previous paper (SPIE Conf. 1584, 1991, 304-307) we proposed an optical fiber interferometric sensor for X-ray dosimetry. Basically, on absorption of a modulated X-ray beam, a temperature rise is produced in a silica fiber, which is detected with a Mach-Zehnder interferometric scheme. In this paper we report on the test measurements performed with our prototype detector using a X-ray rube. A possible application for X-ray synchrotron radiation monitor is discussed.

Signal transformations around an open-loop fiber gyroscope are identified. This shows that the interferometer performs a derivative. An electronic integrator is used to cancel this effect. Control theory shows that a second integrator can then be used to complete the closed-loop architecture. Variations of this architecture are used by both dual-ramp and serrodyne approaches. The method that these two techniques use to manage the second integrator, and avoid saturation of its output, is described.

This paper critically analyzes a new open loop interferometric demodulation technique which utilizes digital processing approaches to achieve a direct optical phase measurement. This approach is compared to traditional FM discrimination approaches which are currently used for interferometric demodulation. The comparisons include intrinsic performance capabilities and measurement errors resulting from imperfect interferometric sensor conditions.