With successful long term reliability testing completed, fiber gratings are emerging from the laboratory to find use in communications, signal processing, and sensor systems. For sensor applications fiber gratings can act as generic transducer elements to sense many different measurands. Important emerging applications areas are for civil and embedded composite structure sensors and acoustic array sensors. Strength, reliability, and temperature limitations of fiber gratings are important issues in real world industrial applications. Also important for wide acceptance of fiber gratings is their manufacturability. In-line systems are currently being set up that will make low cost fiber gratings a near term reality. Although driven by communications markets, high volume manufacture of low cost, reliable fiber gratings will be important for their expanded use in sensor applications.

We report on recent work on sensing using in-fiber Bragg gratings carried out in our laboratory. First, an alternative method of discriminating between temperature and strain effects using a conventionally written, in-fiber Bragg grating is presented. The technique uses wavelength information from the first and second diffraction orders of the grating element to determine the wavelength dependent strain and temperature coefficients, from which independent temperature and strain measurements can be made. Secondly, we describe an all-fiber, passive scheme for making extended range interferometric measurements based on the dual wavelength technique. A coherence turned interferometer network is illuminated with a single superfluorescent fiber source at 1.55 mm and the two wavelengths are synthesized at the output by means of chirped fiber Bragg gratings.

Recent findings strongly suggest that the glass in the core of a fiber optic Bragg grating has been periodically compacted resulting not only in residual tension but also in a structural transverse isotropy within the core of the fiber. These effects result in a difference of strain- temperature dependence within and outside the Bragg grating that provides a tool for separation of measurands. Hence temperature compensation can be achieved for a Fabry-Perot interferometer with chirped Bragg gratings as end reflectors when used in conjunction with a pulsed light source. The analysis presents in this paper show that if carefully chirped gratings are used as reflectors in a Fabry-Perot interferometer it is possible to control the optical path imbalance of the sensor such that one measurand can be fully compensated for, e.g. strain or temperature. Mainly temperature compensation is treated since it is considered the primary objective, even though the concept applies for any arbitrary measurand. Several relationships are derived for temperature compensation of Fabry-Perot interferometers with chirped Bragg grating reflectors including both free and embedded sensors. The boundary conditions of an embedded sensor are significantly altered since the surrounding host material superimposes temperature induced and transverse strains that will contribute substantially to the strain field inside the sensor. Simulations were carried out according to a geometric grating model assuming continuous chirp and a strict geometrical reflection criterion. These simulations are illustrating the compensation effect for various load cases to both free sensors and sensors embedded in composite laminates.

In this paper we present a fiber-optic sensing system designed to verify the identity of objects or personnel. The spectrum of a fiber-optic Bragg grating can store information, which can then be accessed simply by examining the reflection spectrum of the grating. Typically, fiber- optic Bragg grating sensors measure perturbations in the spectrum of the grating caused by the environment. However, spectral information can also distinguish various Bragg gratings, a principle that is used to wavelength division multiplex limited numbers of Bragg gratings sensors. Going one step further, a very large number of gratings can be distinguished by their spectra. In this paper, we present the experimental validation of a system capable of distinguishing over 1000 sensor heads. The fiber-optic Bragg gratings can be thought of as keys, or ID cards, that can be read by a detector to limit access to secure information or places. The system can readily expand to provide the capability of distinguishing over a million keys. In addition, the keys can also function as sensors.

Fiber Bragg gratings (FBGs) are simple intrinsic sensing elements which can be 'photo-imprinted' into fiber and represent one of the most exciting developments in the area of fiber optic sensing, probably since the inception of the all-fiber interferometer. These devices are based on the photosensitivity of optical fibers: Conventional tele- communications grade germanium-doped (Ge) optical fiber has been shown to exhibit a significant photosensitive response when illuminated with UV light in the region of 248 nm, a wavelength which corresponds to an absorption band, or 'color center,' in the glass associated with the Ge/SiO₂ bonding. Absorption of the UV light in the glass breaks bonds creating changes in color centers which modify the absorption characteristics of the glass. This change in absorption results in a shift in the index of the glass at wavelengths removed from the absorption region through the Krammers-Kronig relationship. As the Ge dopant is usually confined only to the core (light guiding) region of the fiber, the effect is observed only in the core. The required UV light can be readily produced by various sources: KrF excimer lasers, dye lasers, frequency doubled Ar lasers and quadrupled Nd:YAG lasers. The optical power levels produced by these sources vary, and the most common source used is the KrF laser which can produce intense pulses at 10 to 50 Hz repetition rates. Changes in index are generally on the order of 10^-3 or less, although recent work on fibers with enhanced photosensitive response (hydrogen- loaded fiber), index shifts of approximately 10^-2 have been reported. The advent of the holographic side- exposure and phase-mask techniques for writing gratings has made the devices readily available for widespread usage in fiber optic systems. Although the primary application area for Bragg gratings appears to be in the fiber communications field, there has been strong interest in FBG-based sensing, and progress in this area has been rapid with many significant developments over the past 5 years.

Surveillance of facilities and critical structures by monitoring mechanical strength and integrity is necessary for safety in use. Fiber-optic sensors still have poor industrial acceptance due to their lack of demonstrated reliability and long-term stability. Reliability testing for objects with expected life times of 10 to 100 years has to rely on accelerated aging procedures. We report on a series of aging tests of optical fibers and Bragg gratings at elevated temperature, humidity and mechanical stress performed in regard to field applications. Tensile and climate tests were performed with optical fibers embedded in glass fiber reinforced polymers (GFRP) and surface attached to carbon fiber reinforced polymers (CFRP). For surface attached fiber sensors relative humidity was found to be a critical parameter with strong influence on life time. A cable-stayed bridge (Storchenbrucke in Winterthur, CH, still under construction) where for the first time two steel cables are replaced by CFRP cables was equipped with optical-fiber Bragg gratings and standard resistance strain sensors.

We investigate the feasibility of using optical fiber Bragg gratings for the sensing of ultrasonic fields for medical applications. In preliminary experimental investigations, ultrasonic waves with a frequency of 950 kHz have been detected with a noise limited pressure resolution of approximately 10^-2 atmospheres in a 3 KHz measurement bandwidth.

Fiber sensors have found numerous industrial, military and civil applications in the last decade. These sensors possess small size and high sensitivity, are immune to electromagnetic interference and can be easily modified for distributed or multi-parameter measurement. The extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor, though it retains all these advantages, still has a few limitations which have prevented its large scale commercialization as an effective strain gage. We discuss these limitations and report the status of manufacturing techniques. Recent progress in the development of extrinsic Fabry-Perot interferometric sensor is reported. Techniques for absolute measurement are demonstrated and adhesives and endface coatings for sensor fabrication are analyzed. Many different applications are presented along with discussions of packaging and attachment issue problems and solutions. The applications include high temperature, low temperature, and harsh environments such as Mach 6 wind tunnels and rock drills.

A long-distance fiber-optic white-light displacement sensing system, using a synthesized source generated by combining the outputs from two low coherence sources, is demonstrated. As the wavelengths of both sources are compatible with standard 1.3 micrometer single-mode telecommunication fiber, allowing the peak of the central fringe in interferogram to be determined precisely, a measurement range to resolution of better than 10⁴:1 over a remote distance of 3.3 km has been achieved. Optimum synthetic source is also proposed to further reduce the SNR requirement for identification of the peak of the central fringe. It is expected that such a system could find applications in long-distance, remote, and absolute measurements for quasi-static parameters.

The METRICOR 2000 instrument has been developed as a solution to numerous needs for fiber optic sensors in metrology and in process control. It is able to measure over 20 different physical parameters through three different operating principles, corresponding to three different conditioners. A first intensity conditioner is designed to deal with signal generated by probes with a limited dynamic range. It is particularly suited for on/off sensors such as level detection and void fraction probes. A dual wavelength conditioner handles signals generated by probes requiring referenced operation. It can be used with opacimetry and chemical concentration probes as well as fail-safe dual wavelength on/off type sensors. A third conditioner deals with the complex signal generated by interferometric sensors. Those sensors, operating under a white light illumination, return a spectrally encoded information which is not affected by on-line attenuation. This third conditioner can be used with micro-machined interferometric temperature, pressure or index of refraction sensors. It offers the highest performance in resolution and accuracy. The standard mainframe unit includes a display for 4 simultaneous measurements. Each probe is equipped with an electronic personality key which includes the calibration.

This paper describes the use of the first and second optical return paths in a moderate to high finesse Fabry-Perot sensor to measure the absolute phase in extrinsic Fabry- Perot interferometric (EFPI) sensors. Path-matched differential interferometry (PMDI) using high finesse EFPI sensors, a low finesse Fabry-Perot read-out interferometer, and a broadband light source consisting of amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA) is used to illustrate the idea. The first and second multiple paths in the Fabry-Perot read-out sensor are used to provide two distinct path-match conditions from the same scanning Fabry-Perot read-out interferometer. The difference in fringe numbers between the centers of two orders of interference fringe packets formed by the distinct path-match conditions makes possible a simple method of measuring the cavity length of EFPI sensors, which in turn can be used to measure absolute phase and the corresponding strain. Sensor cavity length measurement using the multiple return paths in the high finesse Fabry-Perot sensor is compared with measurements made using the modulation transfer function found using an optical spectrum analyzer. Then the multiple return path technique is then used to make strain measurements on a cantilever beam. Comparisons with resistance strain gate measurements are favorable. Characterization tests indicate that the proposed technique has a cavity length measurement resolution on the order of 1.1. micrometer, which translates to a strain resolution of 28 (mu) (epsilon) for a 4 cm gage length sensor.

A technique using dual-wavelength interferometry which permits the demodulation of quasi-static measurands from very short cavities is introduced. It requires the optical path difference of the sensor to be perturbed and a careful choice of the two wavelengths. Under these conditions comparable resolution and range to conventional dual- wavelength white-light interferometry is obtained. A remote receiving interferometer is not required. Experimental verification of this technique using a fiber Mach-Zehnder under conditions of natural drift are presented. A resolution of 0.2 nm (corresponding to 1.5 mrad at 827 nm) over an effective wavelength of 114.8 micrometer was achieved. Multiplexing using this demodulation technique is also presented. A network of eight sensors is demonstrated with the same resolution and range as the single Mach- Zehnder.

A division-of-aperture in-line fiber-optic photopolarimeter using a 1 by 5 fused fiberoptic star coupler has been demonstrated. All five output beams of the star coupler have nearly identical states of polarization except that they have unknown orientations of optical axes. The photopolarimeter output is taken from one of the star coupler output beams, whose state of polarization is measured from the other four beams. The construction of the in-line photopolarimeter is identical to that of conventional ones except that the linear and circular analyzers are aligned with the respective beam optical axes determined from calibration. Such a system is compact, light weight, efficient, robust and inexpensive. The new photopolarimeter agrees within one percent with a HP 8509A off-line photopolarimeter.

A low finesse fiber Fabry-Perot interferometric sensor is illuminated with five equally spaced axial modes from a multimode semiconductor laser diode source. The resulting intensity output is composed of five equally phase shifted optical frequencies, the intensities of which are used to determine the interferometric phase modulo 2 pi. This technique is shown to reduce the phase errors arising from multiply reflected beams, characteristic of the Fabry-Perot interferometer, by two orders of magnitude compared to techniques using two or three intensity outputs. The use of this technique is also shown to result in a reduction in cavity miscalibration errors when compared to previously reported signal processing techniques for Fabry-Perot interferometric sensors. The signal processing technique is demonstrated for temperature measurement where a 70 degree Celsius range and 0.07 degree Celsius resolution are obtained.

We demonstrate a common-mode rejection scheme for a bulk- optic triangular Faraday current sensor that can eliminate optical noise induced by fiber-link vibration. The noise floor before applying common rejection was about 30 dB for a 100A Faraday signal and transceiver vibration levels of approximately 30 g. This was reduced to about 60 dB for the same vibration levels. The sensor's exploitation of Ampere's circuital law is also demonstrated.

An electric field sensor based on the linear electro-optic effect is described and the performances of available electro-optic cubic crystals are compared. The sensor sensitivity to an electric field is characterized by a new figure of merit evaluated using crystal data available. The intensity of electric field in the crystal is estimated using the approximation of the rectangular crystal shape by an ellipsoid of revolution. Crystal geometries appropriate for sensors with transverse and longitudinal electro-optic effects are suggested. Influence of the crystal conductivity and the photoconductivity on the sensor performance are investigated. These effects are found to influence the sensor performance if it is used for the dc and extra low frequency electric field measurements.

We characterized the performance of a commercial fiber optic extrinsic Fabry-Perot interferometer for use as an ultrasonic sensor, and compared the performance with a standard lead zirconate titanate (PZT) detector. The interferometer was unstabilized. The results showed that the fiber sensor was about 12 times less sensitive than the PZT detector. Ultrasonic frequency response near 100 kHz was demonstrated. We describe the design of the fiber sensor, the details of the tests performed, and potential applications.

Optical fiber sensor technology has not yet penetrated the relatively conservative world of nuclear instrumentation. Although the main effect of ionizing radiation on optical fibers is well known to be an increase of the optical absorption, little is known about the response of optical fiber sensors exposed to radiation fields. Our experiments therefore aimed at gaining insight about the behavior of standard commercially available optical fiber sensors in a representative nuclear environment. As experimental target, we have chosen semiconductor absorption optical fiber temperature sensors for monitoring temperature excursions in-situ, within SCK(DOT)CEN's BR1 gas-cooled graphite- moderated nuclear reactor. The sensors are located in the reactor's graphite, where they are exposed to thermal neutron fluxes on the order of 10¹¹ n.cm^-2.s^-1. Temperature excursions were measured for several reactor cycles, i.e. from reactor start to reactor shut-down, and compared to readings obtained from iron-constantan thermocouples. The sensor performed well until severe embrittlement of the fiber protective jacket caused a premature mechanical failure, after only a few days of reactor operation.

A new sensor for measuring high temperature in harsh environments is presented. The sensor concept is based on the fact that the band gap energy of semiconductors, and accordingly the cut-on optical wavelength, depend strongly on temperature. The optical wavelength at which these semiconductors start transmitting light typically increases as the temperature increases. Durable optical fibers transmit light to and from the semiconductive material. This paper addresses the feasibility of the concept at temperatures up to 800 degrees Celsius, and presents results using different semiconductors including gallium arsenide, silicon, and gallium phosphide.

A temperature sensor is constructed in which light strikes the boundary between a glass (BK7) corner cube reflector and a small volume of liquid (water). The liquid is contained in a metallic housing, which is bonded to a test structure. As the structure changes temperature, the indices of refraction of both the liquid and the glass vary, and the amount of light reflected out of the corner cube changes. A full three-dimensional vector analysis of the corner cube sensor is presented in conjunction with the Fresnel equations and the refractive index temperature dependence of both BK7 and water. Experimental results show the sensor output to be roughly linear with temperature for the range between 30 and 90 degrees Celsius.

Blackbody radiation sensors are well known for measuring high temperature in harsh environments. Conventionally, these sensors require materials with high and stable emissivity. Any dimensional changes or shifts in the emissivity over the lifetime of the device would degrade sensor performance. This paper presents a new temperature sensing concept where thermal radiation is integrated over the length of an optical waveguide. Under some easily attainable conditions, it is shown that the new sensor output is independent of both the emissivity of the material and the length of the integrating waveguide.

An experimental demonstration of fiber optic imaging inside a furnace at 1000 degrees Celsius is described. A sapphire fiber optic video system was designed, fabricated and tested for basic performance in a small tube furnace. The imaging fiber bundle was assembled using 100 high quality sapphire fibers aligned and bonded at each end. Experiments to achieve a high temperature cladding are described. Reference imaging experiments at room temperature were performed with the sapphire fiber bundle and a commercial glass fiber bundle of comparable size. Imaging experiments at 100 degrees Celsius are described and discussed.

The TeX glass optical fibers have been developed for their broad transparency in the 3 to 13 micrometer region and their good thermal, mechanical and chemical properties. The minimum losses of these fibers are approximately 0.5 dB/m in the mid-IR domain of 7 - 9 micrometer. Owing to these properties, these fibers are useful in a wide range of applications, requiring a relatively low power level, such as temperature sensing. Temperature measurements using a TeX glass fiber have been investigated. The set-up was mainly composed of a polymer-coated fiber which transmitted the signal of a black body to a HgCdTe detector. Fibers with different lengths and different diameters have been used for this experiment and the temperature sensing has been performed in the region of minus 30 degrees Celsius up to 400 degrees Celsius. It has been found that the signal transmitted to the detector increases very rapidly when the temperature is higher than 60 degrees Celsius. However, the sensitivity of the used set-up is still high, even at temperatures as low as minus 30 degrees Celsius. The resolution of the sensor is estimated to be better than 1 degree Celsius in the region of room temperature. These fibers provide a possibility of non-contact low temperature sensing.

In the present work, the performance characteristics of a set of six fiber optic temperature sensors based on the fluorescence lifetime of neodymium-doped glass have been investigated. The experiment is aimed at verifying whether a satisfactory probe-to-probe repeatability and sensor accuracy can be obtained with minimum calibration effort. To accomplish this task the fiber-optic probes were calibrated, in the temperature range 0 degrees Celsius to 250 degrees Celsius, by comparison against a platinum resistance thermometer in a metal block furnace. The experimental set- up consisted of an electro-optical unit for fluorescence excitation and detection and an analog-to-digital signal processing circuit for lifetime measurements. The temperature probes were assembled by placing the Nd-doped glass at the distal end of a silica optical fiber and by splicing the other fiber end to a 1 by 2 wavelength- independent coupler. The calibration results and the temperature repeatability of the Nd:glass probes are reported. The reproducibility of the measurement method, with an optimum calibration function, and the resulting accuracy are discussed.

This paper describes a novel birefringent fiber remote strain sensor which is based on the FMCW technique and consists of a single length of single-mode birefringent fiber. The lead-in/lead-out fiber and the strain sensing fiber probe are separated by introducing a twist in the middle of the fiber. The sensor demonstrated is shown to have several advantages including a resolution of 2 microstrain resolution, a dynamic measurement range of 5000 microstrain, an environment-insensitive lead-in/lead-out signal section and a variable length strain sensing probe. The system is ideally suited to the measurement of absolute and relative strain.

A technique for detection of high-frequency strain using an AR-coated laser diode with a fiber Bragg grating to form a short (approximately 1 cm) external cavity is reported. The laser system becomes a single mode, narrow-linewidth (30 MHz) source under the influence of the grating's feedback. An interferometric detection technique was used to demodulate the strain signals generated by a piezoelectric transducer into a steel block. The sensing system was able to detect frequency components up to 140 kHz.

Results of initial studies of polarimetric fiber optic sensing systems for smart structures applications are presented. The smart structures based on fibercore bow-tie fibers embedded in a specially prepared cylindrical epoxy cylinder were subjected to deformation effects such as those induced by hydrostatic pressure, temperature, and loading conditions, whereas polarization properties of the transmitted optical signal have been investigated. The presence of the smart structure modifies the output characteristics of the highly birefringent fiber do to elastic properties of the structure. The applied experimental procedure had an objective to compare the phenomena occurring in both: the embedded fibers and the separated highly birefringent fibers influenced by the same deformation effects. Potential applications in a rotor blade of a helicopter have been tested.

We present a novel optical fiber remote strain sensor which is based on the FMCW technique and consists of only one single piece of single-mode birefringent fiber. The lead-in and lead-out fibers of the sensor, which are simply separated by two polarization mode couplers, are insensitive to environment so that the sensor can remotely detect the strain variation of a distant structure. The advantages of the sensor, such as no fiber junctions, no feedback light, reasonable resolution (4 microstrain), large dynamic measurement range (5000 microstrain) and long length of sensing fiber are demonstrated in this experiment.

Real-time determination of contact forces due to impact on composite plates is necessary for on-line impact damage detection and identification. We demonstrate the use of fiber optic strain sensor data as inputs to a neural network to obtain contact force history. An experimental study is conducted to determine the in-plane strains of a clamped graphite/epoxy composite plate upon low-velocity impacts using surface mounted extrinsic Fabry-Perot interferometric strain sensors. The plate is impacted with a semi-spherical impactor with various impact energies using the drop-weight technique. The impacts did not produce apparent damage in the composite plates. The significant features of the strain and contact force response are contact duration, peak strain, strain rise-time and full-width at half maximum. We have designed and built an instrumented drop-weight impact tower to facilitate the measurement of contact force during an impact event. The impact head assembly incorporates a load cell to measure the contact forces experimentally. The load cell data is used to train a three-layer feedforward neural network which utilizes the back-propagation algorithm. The output of the neural network simulation is the impact contact force history and the inputs are fiber optic sensor data in two different locations and time in 10 microsecond intervals. The efficiency and accuracy of the neural network method is discussed. The neural network scheme recovers the impact contact forces without using any complex signal processing techniques.

Fiber optic interferometry is applied to the measurement of seismic vibrations at temperatures of about 300 degrees Celsius and several km of signal transmission lines. The measuring system consists of three orthogonal seismic receivers which are coupled via free propagating light beams to fiber optic Michelson interferometers. White light interferometry with fiber optic phase modulation in the readout interferometer is used for less than 1 micrometer resolution of absolute position of seismic mass, high- resolution monochromatic interferometry using wavelength modulation of the laser diode yields less than 0.1 nm sensitivity of seismic vibrations.

A new method is proposed for measuring interstory drift, the shifting of floors relative to one another when a building undergoes wind or earthquake loading. A laser crosshair is projected from one story to the next, and onto a set of photodetectors which sense changes in the position of the projected light. This paper reports the theory of operation and a quasi-static verification of the method using micropositioning stages to provide input displacements. Lateral positions, including translational and rotational components are calculated from the photodetector outputs, and show excellent agreement with input displacements. The overall performance of the sensor system is extremely linear and predictable, and appears robust enough for field deployment.

BEAM sensors include treated loops of optical fiber that modulate optical throughput with great sensitivity and linearity, in response to curvature of the loop out of its plane. This paper describes BEAM sensors that have two loops treated in opposed fashion, hermetically sealed in flexible laminations. The sensors include an integrated optoelectronics package that extracts curvature information from the treated portion of the loops while rejecting common mode errors. The laminated structure is used to sense various parameters including displacement, force, pressure, flow, and acceleration.

Single headed 3D laser Doppler velocimetry (LDV) geometries generally rely upon the use of 3 Doppler difference channels, inclined at differing angles with respect to the mechanical axes of the probe. The transformation between the non-orthogonal measurement coordinate system and the Cartesian system can result in large errors in the calculated velocities. A theoretical analysis of the geometrically induced uncertainties in measurements produced by four single headed 3D LDV configurations is presented. These considerations have lead to the development of a single headed LDV probe based around the use of two Doppler difference channels to directly measure the transverse velocity channels, and a reference beam channel to measure the on axis velocity component. The probe may be operated in two regimes using cw radiation and wavelength division multiplexing to distinguish the three channels, or using a pulsed source and time division multiplexing.

In this paper possible ways of increasing the signal to noise ratio of a laser Doppler vibrometer with fiber optic components are discussed, in order to improve its metrological characteristics and to enlarge its field of possible industrial applications. System improvements, related to the frequency shifting block of the vibrometer, to its photodetector stage and to the optical head, which recovers the light scattered by a vibrating target, have been experimentally evaluated. It must be noticed that this activity has been carried out taking into account the requirement of simplicity and handiness of the measurement system and using solutions from both an optical and an electronic point of view, which are inexpensive as much as possible.

The monochannel fiber optic sensors measuring accuracy depends on such destabilizing factors influence, such as light emitter power variations, different research surface reflecting and scattering properties, photoreceiver parameters alterations and light from external emitters. The suggested vibration measuring method and this method realized two-channel fiber optic sensor permits to set and control the rated gap between the research surface and the fiber optic guide roving face plane with 0.1 micrometer accuracy during the vibration measuring process in conditions of light source power variations to 50% relative to its rating value, the surface reflecting coefficient a few times alterations and if there are small misalignments of the fiber optic guides roving face plane. The linear displacement measuring method and the four-channel fiber optic sensor could even keep the form of the transfer function curve, which varies during the destabilized factors' influence. The measuring result doesn't depend on prevented factors influence. The operating range of such devices coincides with the operating range of the monochannel sensors and makes up to 0.5 - 2 mm. The functional test forming method, monochannel fiber optic sensor with three time steps measurements and three-channel fiber optic sensor characterized by not only destabilized factors independece of the outputs signal, but they also widen the sensor operating range from 10 mm to 20 mm which differs from the known monochannel fiber optic sensors operating range in ten times. The three elaborated methods are described. Schematic diagrams of these methods realized devices are shown and the main concerns of this methods and devices correlations are discussed.

An interferometry system for absolute distance measurement is described. In the system, an optical fiber Mach-Zehnder interferometer with a chirped laser diode is employed to position the measured object. Another interferometer with He-Ne laser is used to measure the optical path difference. The coherence-length of the pulsed multimode laser diode is studied. In the experiments, the system measured the absolute distance up to 1 meter with the accuracy of 2 micrometer.

A miniaturized probe comprising a circular array of optical fibers coupled to a graded index (GRIN) microlens has been designed for analyzing colloidal solutions by photon correlation spectroscopy (PCS). The system's suitability for PCS measurements was validated by experimental testing performed in small drops of monodisperse test colloidal solutions.

A detailed theoretical analysis with numerical calculations was conducted to study the frequency spectrum resulting from the application of a sinusoidal phase modulation to a two beam interferometer illuminated by a low coherence source with a Gaussian spectral profile. Results of these calculations shows behavior of the frequency components which was unexpected from earlier derivations of the frequency spectrum. The results of the calculations have been experimentally confirmed using an all-fiber interferometer illuminated by a source with a Gaussian spectral profile. The results are useful for the signal processing of tandem interferometers for 'white light' interferometry.

A new self-calibrating modulation ellipsometer (SCME) has demonstrated outstanding accuracy, utility, reliability, and speed. The ellipsometer is well suited to in-situ monitoring of surface degradation, film growth or etching, and quality control. The design incorporates several novel features including: (1) full self calibration, (2) high speed, (3) high accuracy, (4) high signal-to-noise ratio, (5) compactness, (6) reliability, and (7) no moving parts. The design is portable, can be fully automated, and is suitable for use in remote and harsh environments. A complete prototype instrument incorporates all optical components, mechanical mounts with flexible configuration options, custom electronic components, signal acquisition, computer control, data analysis, and a user interface, all integrated into a self-contained, user-friendly, system. It operates at fixed wavelength and incidence angle, though both can be changed by the operator in a few minutes as desired. Quantitative testing verified the absolute accuracy and suitability for monitoring real-time in-situ film growth and etching.

The objective of this paper is to present a detailed investigation of the frustrated-total-internal-reflection (FTIR) phenomenon in silica-glass-based optical fibers and its application to simple intensity-modulated strain and pressure sensors. Such sensors may be readily fabricated using silica-based fibers and can be easily modified using sapphire fibers for high temperature industrial applications where conventional silica-based fiber sensors are not feasible. In this paper we present the all-fiber FTIR sensor and show good correlation between theory and experiment. We also present results for the design and implementation of a prototype FTIR-based fiber pressure sensor.

A white-light fiber interferometer working in the spatial domain, using two fiber ends in a hollow tube as the sensing head and an electric-magnetic actuator-mirror reflector as the path compensation-measurement element, is presented. Analysis and preliminary experiments have demonstrated repeatability of 0.02 degrees Celsius within 1 degree Celsius temperature range and repeatability of 0.6 degrees Celsius within 100 degrees Celsius temperature range for temperature measurement. Suggestions for further improving the measurement accuracy are also given.

A method for respiratory monitoring, which uses a fiber- optic probe, has been developed and evaluated. The fiber- optic probe makes it possible to monitor from different sites on the patients, and the method is convenient to use. Therefore the method is suitable for the observation of both adults and children, in hospital as well as in other environments.

A new kind of practical fiber optic flow sensor system is studied in this paper. Combining the matured technology of conventional Rhodes flowmeter with advanced technology of fiber optic sensor, we proposed a practical scheme of hybrid fiber optic flow sensor system which remains the advantage of high accuracy of conventional Rhodes flowmeter, improves its characteristic in low speed and has large measuring range well over that the magneto-electric Rhodes flowmeter has. Thanks to the advantages of isolation, nonconduction and immune from electromagnetical interference, fiber optic flow sensor has steady output signal and can work without electricity on worksite. So it is used specifically in hazardous environments such as oil depot for oil and chemicals measuring. In this paper, the working principle of the flow sensor is introduced, the structure of the fiber optic Rhodes wheels rotary displacement probe is designed, and the signal-processing system is described. Finally the in-situ test of the system in the oil terminal of Zhanjiang Harbor in China is introduced.

An optical fiber sensing scheme used for cure monitoring during composite curing cycle is presented. The sensing mechanism is based on the change in refractive index during cure process of the composite. An optical fiber sensor embedded in the composite was used for detecting the variation of the refractive index of cure resin in composite materials during its curing cycle. A carbon fiber/epoxy composite laminate was used for experiments in this paper.

A multiplexed Mach-Zehnder interferometer is proposed for measuring time varying signals. The system was implemented to measure electrical currents. These measurements are facilitated by a current transformer terminated with a resistive load, which derives a piezoelectric phase modulator. Frequency domain demultiplexing allows interrogation of the sensors in each of the conductors of a three-phase electrical system. Different phase modulating carriers are applied to each arm with additional piezoelectric phase modulators. Synchronous phase detection followed by integration yield the current signals. Results of current measurements between 9 A and 200 A on a single- phase system and on two 50 Hz phases of a three-phase system are discussed.

This paper presents a pre-biased fiber optic microbending sensor where perturbation is based on the number of microbends rather than the bending amplitude. The sensor uses a graded index fiber where a portion of the fiber is pre-bent to bias the sensor precisely at the peak of the loss curve. This can be achieved by matching the bending period of the pre-bent fiber to the resonance spatial frequency of the fiber itself. In principle, the sensor operates based on the conventional microbending sensing schemes, however, the emphasis on the bending amplitude is relaxed to prevent the rapid degradation of the fiber and to prolong the sensor lifetime. It also is shown that the proposed sensor shows higher sensitivity than the amplitude based sensor.