Analyzing the progress worldwide made in optical fiber sensing technology and application during the 1990s, current technical as well as commercial trends in this innovative and dynamic field of applied photonics will be discussed. The potentials and chances of industry in this growing markets are demonstrated by selected examples of application and corresponding market forecasts.

The contribution of the UK Optical Sensors Collaborative Association to the first wave of R&D on the application of fiber optics in sensing application is summarized. Safety related R&D has continued and the most recent results are presented. OSCA identified that a suitable component infrastructure for sensing had not yet been realized, and this is probably a significant part of the explanation for the relatively modest commercial impact of this technology at the present time. OSCA also identified the significance of white light interferometry, and realized the potential of silicon as a substrate for integrated optics. Alas, OSCA acted primarily as an onlooker to the important discovery of fiber Bragg gratings. Subsequent global efforts at commercialization have been admirable, but are not yet major in business terms. Distributed sensing has rather plodded along, through some progress is being made. It is argued that we are now on the leading edge of a second wave of development in fiber optic sensing. The emphasis in this wave is not sensing principles but the adaptation and employment of the components and signal processing methods being facilitated by the huge wave of commercialization in optoelectronics which is currently occurring, primarily stimulated by the internet explosion. For sensing systems it is judged that silicon ridge waveguide technology has the potential to create a standardized approach to optical sensing, in concept not dissimilar from the familiar standardized analogue electronic signal and transmission means (e.g. 4 - 20 mA).

The paper describes a micro mechanical Bragg grating acceleration sensor with enhanced sensitivity by force amplification. A new multiplexable and affordable interrogation scheme that uses two overlapping Bragg gratings within the sensor will be demonstrated.

The results of an investigation of the performance of a time-division-multiplexed fiber Bragg grating (FBG) sensor array using a tunable laser source are reported. The system performance is found to be limited by the extinction ratio of the optical pulse modulator used for pulse amplitude modulation. Formulas that relate the crosstalk to the extinction ratio of the optical pulse modulator, the modulation parameters of the tunable laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 FBG sensors with 3 (mu) (epsilon) resolution can be realized with a commercially available single Mach-Zehnder type optical pulse modulator of -35 dB extinction ratio.

Real-time, in situ monitoring for quality control of the polymer cure process is of high interest, since thermoset polymer-matrix composite are widely used in large industrial areas: aeronautical, aerospace, automotive and civil due to their low cost/low weight features. However, their final properties are strongly dependence on the processing parameters, such as temperature and pressure sequence. The key-point for advanced composite materials is the possibility to have distributed and simultaneous monitoring of chemoreological and physical properties during the cure process. To this aim, we have developed and tested an optoelectronic fiber optic sensor based on the Fresnel principle able to monitor the variations of the refractive index due to the cure process of an epoxy based resin. Experimental results have been obtained on sensor capability to monitor the cure kinetics by assuming the refractive index as reaction co-ordinate. The integration with in-fiber Bragg grating in order to measure the local temperature has been discussed and tested.

In this paper a method is proposed for data acquisition and elaboration from a Fiber Bragg Grating strain interrogation system for automotive applications. The results of measurements carried out both under static and dynamic conditions have been compared with those performed with a strain gauge.

In-shoe shear stress sensors are a required tool for the investigation of plantar ulcer development after the onset of diabetes. Recently, several transducers have been developed for measuring in-shoe shear stress using magneto- resistive technology, light intensity modulation, and copolymer piezoelectric materials. Common drawbacks in the previous methods are the relatively large size of the sensors and the difficulty in interrogating many sensors simultaneously in order to achieve distributed sensing. In this paper we demonstrate for the first time a shear stress sensor using Fiber Bragg gratings (FBGs). The small size and the multiplexing capability of FBGs enables quasi- distributed sensing of shear stress on the plantar surface by interrogating a large number of identical sensors. The sensor design is based on the theory of elastic bending of columns. The sensor consists of two FBGs fitted inside a metallic structure which is able to deform elastically under shear stress. This elastic deformation produces strain on the FBGs, which can be detected by measuring the Bragg wavelength shift of the reflected light of each FBG using a CCD spectrometer. Preliminary results on an enlarged version of the sensor have shown the applicability of FBGs for the implementation of the in-shoe sensor.

A multi-point fiber optic sensor array for methane detector (`OMEGA') has been developed and tested under semi- quantitative field conditions. The new system employed wavelength modulation spectroscopy using a DFB laser source scanned across the Q6 methane line at 1.665 micrometers . A branched fiber network connected the single source to up to 64 sensor heads. Controlled releases of natural gas were provided for test purposes within an array of four optical sensors and four pellistor reference sensors. An automated system delivered standard gases to each sensor, to enable routine calibration checks to be carried out. Agreement between the conventional and optical systems was excellent in the range 0 - 100% LEL (lower explosion limit). The optical system offers a simple, intrinsically safe design with a low cost of ownership per sensor head.

A dimension reduction technique is described that allows chemical images to be obtained using an image-guide sensor simultaneously at many different wavelengths with very high spectral resolution. Dimension reduction is accomplished using a custom 2D to 1D square fiber-optic array that is made by using approximately 600 individual 25 micron square fibers. The technique allows the concurrent detection of multiple analytes and is demonstrated using a combination CO₂/O₂ sensor, in two different sensor configurations. The technique is being extended for lifetime-based image-guide sensors.

We present two configurations of new fiber optic Bragg grating sensor developed for petroleum hydrocarbon detection. The sensor includes FBG attached to the special polymer swelling in the hydrocarbon presence. This polymer reversibly strains the fiber section with the Bragg grating inside. The first sensor configuration employs a single grating and standard demodulation technique to determine a strain induced shift of the Bragg wavelength. The second sensor configuration make a use of double grating interferometric sensor consisting of a reference grating free from axial strain and a sensitive one, attached to the polymer. A new demodulation technique is based on monitoring of energy of the interference term in the reflection spectrum. We demonstrated experimentally that such configuration is free from temperature influence. The paper presents the results of the swelling-behavior test of the sensitive polymer material and the results of the experimental investigation of the hydrocarbon sensor performance.

Working with calcium ions in the blood, platelets produce thromboplastin which transforms prothrombin into thrombin. Removing peptides, thrombin changes fibrinogen into fibrin. Cross-linked insoluble fibrin polymers are solubilized by enzyme plasmin found in blood plasma. Resulting D-dimers are elevated in patients with intravascular coagulation, deep venous thrombosis, pulmonary embolism, myocardial infarction, multiple trauma, cancer, impaired renal and liver functions, and sepsis. Consisting principally of a NIR 780 nm GaAlAs laser diode and a 800 nm avalanche photodiode (APD), the fiber-optic immunosensor can determined D-dimer concentration to levels <0.1 ng/ml. A capture monoclonal antibody to the antigen soluble cross-linked fibrin is employed. Immobilized at the tip of an optical fiber by avidin-biotin, the captured antigen is detected by a second antibody which is labeled with NN 382 fluorescent dye. An evanescent wave traveling on an excitation optical fiber excites the antibody-antigen fluorophore complex. Concentration of cross-linked fibrin is directly proportional to the APD measured intensity of fluorescence. NIR fluorescence has advantages of low background interference, short fluorescence lifetime, and large difference between excitation and emission peaks. Competitive ELISA test for D-dimer concentration requires trained personnel performing a time consuming operation.

A new design for a fiberized laser ultrasonic source for process monitoring and bio-medical applications is proposed. The laser ultrasonic source consists of a pulsed laser, a fiber-optic cable, and a generation head. The generation head is a miniature hermetically sealed chamber, which can be embedded into solid structures or immersed in liquid media. The face of the chamber acts as a target for the laser irradiation. Bulk ultrasonic waves generated inside of the target are transmitted into the surrounding liquid media or solid structure. It is shown that ultrasonic pulses of 1 microsecond(s) to 30 ns duration can be generated. Sources with different radiation patterns with respect to the optical axis of the fiber, such as normal, angular, and focused, have been devised. An example use of these sources combined with a fiber optic ultrasonic sensor for inspection of small tubes is presented.

Based on a non-sequential raytrace simulation, a previously developed tubular optical waveguide probe for magnetic- focusing immunosensors has been redesigned and tested. The design features a tubular waveguide tapered at both ends with a tapered magnet embedded inside of the waveguide. For reducing background, the excitation light is directly projected into the sample solution via a fiber against the cuvette wall adjacent to the waveguide. The fluorescent signal is collected by the probe and is coupled into a transmission fiber at the distal end of probe. Simulated antigen samples (paramagnetic microspheres of 5-micron diameter with immobilized antibodies that are labeled with Cy5 dye) have been prepared for testing magnetic signal enhancement and fluorescent signal collection. Typical results show that the sensitivity of the tubular probe is one order of magnitude higher than that of a similar dual- fiber probe. With the improved excitation and transmission scheme, the new tubular probe reduces background noise to one-third that of a previously reported tubular probe and is less sensitive to the interference of ambient light.

This paper presents the operation of an optical current transducer (OCT). The OCT operates by measuring changes in the polarization state of light traveling through a magneto- optic sensor head exposed to the magnetic field of a current carrying conductor. The changes in polarization state are directly related to the magnitude of the current flowing within the bus bar but ia also influenced by external environmental parameters such as temperature and mechanical disturbance. Engineering solutions to these influences have been implemented. Thermally induced errors of greater than 25% are shown to be corrected to less than 0.2% over a range of temperatures from -45 degree(s)C to 140 degree(s)C. In addition to this, signal waveform distortion resulting from mechanical vibration is automatically reduced by more than 30 dB over a range of frequencies from 0 Hz to 5 kHz. This paper reports on the design of this OCT and on detailed investigations carried out at the British Short-Circuit Testing Station to establish the operational characteristics of the OCT when subjected to transient current signals often with a significant dc component.

This paper presents an optical voltage transformer based on the Pockels-effect with a polarimetric signal evaluation. The electrically passive sensor head uses a number of bulk optical sensor crystals which are spatially distributed along the primary voltage drop to minimize the electrical field strength inside the sensor head and reduce electrical insulation expenses. The adaptation of the sensor head's sensitivity is managed by adjusting the absolute crystal length and by cascading similar sensor elements.

The optical fiber flow sensors for automatic measurement in oil industry are considered excellent sensing components owing to the advantages of the immunity to electromagnetic interference and intrinsic safety telemetry. But there are not many commercially fiber optic flow sensors because of the high cost and immature measuring technology. Based on the advanced technology of optical fiber magneto-optic sensor and the matured technology of turbine flow sensor, a new kind of optical fiber mass flowmeter is studied to meet a fast growing demand for measuring flow in mass units instead of volumetric units. It not only keeps on the advantages of the turbine flowmeter, such as high accuracy, large measuring range, but also reduces the effect of electromagnetic noise from the environment, improves the response characteristics in the low frequency. In this paper, the basic principles of the optical fiber mass flowmeter is presented. The design of the optical fiber magneto-optic sensor is studied in detail and the effective method for signal processing is also discussed. Experimental results show that the optical fiber magneto-optic sensor can respond to high frequency of up to 1 KHz and the measurement accuracy of rotational velocity is about 0.1%.

An optical fiber sensor based on In-BaCaVIG crystal for the measurement of high power microwave field has been built. The sensor has a sensitivity of 10^-8T/(root)Hz. The powers of HPM measured by the optical fiber sensor agree well with the results measured by microwave horn coupler.

Sulphur hexafluoride (SF₆) is used as an insulator in high voltage systems. Electrical breakdown in such a system dissociates SF₆ into sulphur fluorides, sulphur and fluorine. Some of these products react with contaminants such as water vapor and oxygen, and with metal surfaces and electrodes, to produce by-products including gaseous sulphur oxyfluorides, hydrogen fluoride and solid by-products. A reduction in SF₆ concentration occurs, and the insulating efficiency of the system is degraded. If the SF₆ is not renewed, failure of the system can occur. A monitoring system for SF₆ loss is required, and since SF₆ insulating systems involve high voltages and electromagnetic interference, optical monitoring provides the best approach. This paper describes the development of an optical sensing technique for continuous monitoring of SF₆ degradation in high-voltage switchgear. Dissociation products of the SF₆ discharge include HF and atomic fluorine, both of which react readily with silica. This reactivity is utilized in our sensor, where etching of a glass fiber surface modifies the transmission characteristics of the fiber. By monitoring the transmitted signal, information on SF₆ dissociation is obtained. The sensor is economical to produce, easy to use, and can be readily retrofitted.

We have developed an optical temperature sensor aimed at early detecting faults in Medium Voltage substations. The advantage of the sensor lies in its full suitability for the target application, which allows it to reach a cost compatible with the service rendered. Heavily doped ruby has shown to be attractive (fluorescence efficiency, cost) but also revealed a multi-fluorescence lifetime behavior. Accuracy of +/- 2 degree(s)C has been achieved over the -20 degree(s)C/+120 degree(s)C range for a 12 point sensor. We hope this system opens up interesting prospects in the field of thermal diagnosis, beyond its application in electric power industry.

In the area of high-voltage apparatus only the use of optical sensors offers the possibility to watch important measurands, e.g. current, voltage, partial discharge and temperature, in regions of high electric or magnetic fields. Among other concepts optical point sensors are able to detect the temperature. The new temperature point sensor is based on the measurement of a varying linear birefringence in optical fibers, produced by the thermal influence of epoxy adhesives. On the basis of the different thermal expansion coefficients of the fiber and the adhesive, a temperature change results in mechanical pressure. Because of this pressure a linear birefringence is induced into the fiber and the polarization of the light is influenced. Experimental investigations show a significant and repeatable temperature dependence of the ellipticity of the light in the range of measurement between 10 degree(s)C and +120 degree(s)C.

Optical Fiber Thermometry (OFT) using single-crystalline, flexible sapphire fibers combines the benefits of a pyrometric temperature measurement concerning accuracy and resolution with a high local sensitivity and low invasiveness as given for a thermocouple. In the first part of the work flexible OFT sensors have been manufactured from bare fibers and characterized with respect to optical properties like the effective acceptance angle and parasitic light entering the fiber sideways and to the minimum bending radius. The tests have yielded that the optical quality of the fibers with a diameter of 325 micrometers , 150 micrometers , or 100 micrometers is sufficient for the application as control sensors in a high-temperature furnace. However, bending of these fibers at temperatures above 1000 degree(s)C has turned out to be very critical. Finally, an active temperature control loop for a multi-zone furnace using rigid and flexible fibers as sensing elements for a temperature of 1500 degree(s)C has been developed. The control algorithm which takes the thermal coupling between neighboring zones into account could establish a short-term stability of (Delta) T equals 0.015 degree(s)C over 25 min for all four zones. This achieved control stability is improved by one order of magnitude compared to a thermocouple based control system.

This paper reports on the first realization of embedding fiber optical Bragg grating temperature sensors inside the stator bars of a 120 MVA air cooled power generator. Furthermore first measurements of the conductor temperature inside the bushings have been carried out during a generator prototype test run.

The effect of strain on the fluorescence intensity ratio technique has been investigated using Nd3-doped fibre as the sensing material. Measurements of the fluorescence intensity ratio for applied strain values up to approximately 2000 show a relatively small sensitivity of (1.5 1.4) x lO %/is while the temperature sensitivity was found to be 1.56 %/°C. This implies a strain-temperature cross-sensitivity of(1.O 0.9) x 1O °C/p., ie. an applied strain of 1000 would induce errors ofonly 0.10 0.09 °C.

In this paper, we report a novel technique to increase the temperature sensitivity of a LPG by nearly two hundred times by exploiting the extremely large dependence of the spectral shift of an LPG on its ambient refractive index. The temperature coefficient of the packaged LPG sensor was measured to be 8.8 nm/ degree(s)C from 16 degree(s)C to 20 degree(s)C which is about 180 times higher than an unpackaged LPG. Correspondingly, it is about 800 times more sensitive than a fiber Bragg grating sensor which has a typical temperature coefficient of 0.011 nm/ degree(s)C.

Certain distributed sensing applications require sub-meter spatial resolution accuracy, and there is interest as to whether the Brillouin linewidth ultimately limits the spatial resolution that can be achieved. We present a single-ended, spontaneous Brillouin-based distributed temperature fiber sensor with measurements operating with a spatial accuracy of 35 cm. The sensor consists of two main components, a laser source to generate the Brillouin backscattered signal and a low cost filtering system which comprises an all-fiber Mach-Zehnder interferometer connected to a InGaAs detector and computer based averaging system. The sensing fiber was 1 km in total, consisting of three sections of conventional single-mode fiber spliced together with lengths of 600 m, 200 m and 200 m respectively. The second drum was at 67 degree(s)C, an increase of 44 degree(s)C from the two other drums at room temperature of 23 degree(s)C. The results show a rise in the Brillouin signal in the heated section. To take absolute measurements independent of fiber attenuation and localized splice/bend losses, the Brillouin signal has to be referenced to the Rayleigh backscattered signal that is independent of temperature fluctuations. The ratio of the Brillouin and Rayleigh signals (Landau-Placzek ratio) provides a temperature dependent signal which is corrected for splice/bend losses and fiber attenuation.

This presentation examines some examples of aerospace demonstration of structural monitoring sensors from one `end-user' company perspective. Some of the key hurdles yet to be surmounted are also identified.

The paper presents the theoretical and experimental results of the strain- and temperature gradient measurement with differential twin Bragg grating sensor. It is shown that this technique could also be used for temperature- insensitive strain measurements.

Long range simultaneous distributed strain and temperature sensors have many applications for measurements in the power and oil industries and also for structural monitoring. We present an efficient technique to measure both the intensity and frequency shift at every point along the sensitive fiber with a low loss filtering device utilizing two in-fiber Mach-Zehnder interferometers. From these two measurements, it is possible to compute accurately the strain and temperature profile.

It is becoming increasingly vital to monitor telecommunication links during operation and installation process. By using a high peak power source and the optical time domain reflectometry technique operating at the wavelength region of 1.6 micrometers , it is possible to monitor conventional C-band Erbium-doped fiber amplifier (EDFA) systems whilst transmitting data, and to characterize losses at the higher wavelengths of extended bandwidth systems designed around the L-band EDFA systems. We describe a compact design based on Raman shifting the output of an Erbium-doped Q-switched fiber laser operating at 1.5 micrometers for obtaining a pulsed source at 1.6 micrometers .

We investigate the measurement of vibrations at the core and windings of the transformer, as well as the differential temperature between zones. We present first results of vibrations measurements at the magnetic core of an oil- filled power transformers. The sensor is based on an intrinsic transducer of the dynamic strain at the surface into changes of the optical path of a segment of fiber. The sensing length is bonded to the core, immersing in oil and inside the transformer environment. Interferometric sensing is accomplished for high sensitivity because the vibrations are due to dynamic strain up to a few (mu) (epsilon) . It allows short probe lengths to be used, though an increase of the length could be used for enhancing the sensitivity. A common accessing of the fibers of the interferometer is used for reducing the common disturbance. The spectral decomposition of the output signals is shown and it is related to the electric reference and the behavior of the transformer.

Rock bolts replace timber sets and steel arcs in securing the structural stability of underground cavities in order to reduce the volume of excavation. Rock bolts used in the mining industry are made of steel allowing up to 20% relative elongation of the bolt. Strain measurements using Fiber-Bragg-Grating (FBG) are limited to relative strains in the order of 1%. In order to allow measurements of up to 20% relative strain, special arrangements of the FBG sensors had to be found to extend their measuring range.

Long-period fiber gratings (LPGs) written into commercially available boron co-doped fibers operating at wavelengths < 1.1 micrometers are shown to exhibit high temperature- and bending-sensitivities. Each resonant attenuation band of such a LPG was observed to split into two when the LPG was bent. The split attenuation bands' separation increased significantly with increasing bend curvature, while the central wavelengths of the split bands provided a measure of temperature. This effect is exploited to allow simultaneous measurement of temperature and bending in smart structure applications. The demonstrated novel sensor system is simple and low cost.

Integrating fiber optic sensors offer the potential to monitor large spatial extents due to their geometric flexibility. To date, this potential has not been widely exploited. By increasing the length of an integrating fiber optic sensor, its gain and/or gauge length can be increased. In addition, by configuring the sensor to a spatial distribution of some parameter field of interest, the antenna gain of the sensor will provide selectivity for that particular distribution and in effect, act as a preprocessor. In this paper we will discuss the theoretical aspects of antenna gain using spatially distributed long gauge length integrating fiber optic sensors. We will then review potential applications including vibration mode filtering, vehicle identification and seismic/nuclear test detection which have been reported. Finally, we will discuss the future directions that the technology might take.

The new Polarization-OTDR technique theoretically and numerically described in this paper is based on the fact that the polarization state evolution of the backscattered signal gives information about the distribution of the fiber birefringence along its length. The knowledge of the full polarization properties of a single-mode optical fiber can be very useful in optical fiber sensing. The distributed measurement of the polarization properties can indeed allow to map external fields like strain and temperature and therefore makes possible the realization of full-distributed optical fiber sensors. There are also implications of this new method in the measurement of polarization mode dispersion in optical-fiber telecommunications systems. Simulations have been done and gave promising results.

Fiber optic TR-EFPI (total reflected extrinsic Fabry-Perot interferometric) sensor is developed to measure the strain of structures, such as building, bridge, aircraft, etc. It has been difficult to distinguish the increase and decrease of the strain from the conventional fiber optic EFPI sensor because their signals only have a sinusoidal wave pattern related to the change of strain. In this study, in order to measure the magnitude and the direction of strain, the fiber optic sensor is simply constructed with the total reflected EFPI sensor probe and the digital signal processing.

Optical frequency modulated fiber optic interferometric sensor is developed to sense the mechanical quantities, such as displacement, strain, force etc. It has been difficult to distinguish the increase and decrease of the mechanical quantities measured by the conventional fiber optic interferometric sensors because their signals only have a sinusoidal wave pattern related to the change of the quantities. In this study, in order to measure the mechanical quantities with the distinction of the changing direction of the quantities, the fiber optic Michelson interferometric sensor is simply constructed by the laser light modulated with saw tooth wave pattern.

Optical fibers are more and more used as chemical sensors. This is, mainly due to their low cost, and their high efficiency to work in harsh and remote environments. Many devices are based on thin film plasmon excitation where a metal coating is evaporated onto the core of an etched optical fiber. In this paper, a new sensor configuration is presented. Instead of exciting surface plasmon waves on a thin film, surface plasma waves are excited on metal islands. The fiber is coated with 3 layers of gold. Each layer is annealed before the next layer is evaporated onto it. this is done to avoid any light leakage, fact which was found on a prior version with only one gold coating. Different sets of fibers were tested and sensitive and reproducible results for liquid with refraction indices varying from 1.3 to 1.7 were obtained.

The optical fiber sensor system and its applications are described in this paper. This sensor system includes two parts: data acquisition part and data transportation part. The data acquisition part is used for measuring the parameters of oil tanks: level, temperature, pressure, flowrate, etc. The specifications of the system are given in this paper. The advantages of the sensor heads are with high stability, because the output signals from it are digital and can be propagated for long distance and long term use with high stability. The sensor system worked continually in laboratory for one year and three systems are installed in the field for field test more than two years in South China and North China now. The constructs and experiment results of the system in the field are described in detail.

The overall aim of this project is to investigate the use of optical fiber sensors for on-line monitoring of particles and droplets having low concentration being conveyed by a fluid. In this project, the system employs lensed optical fiber sensors developed using a low cost approach. A general mathematical model for the lens constructed at each end of the fiber optic was derived which takes into consideration that a large divergent beam is undesirable because it reduces the energy centered in the beam and can cause overlapping of adjacent transmitted beams at the receivers. Initially, the optical fibers are polished. Then, the lenses are formed using a specially-made heating platform. The receiver fiber is coupled to a photodiode, enabling the received light level to be measured. Optical fiber sensors are suitable for monitoring flowing materials where the conveyed component ratio is less than 10% vol./vol. The use of optical fibers provides an opportunity to design sensors with a very wide bandwidth, thus enabling the measurement of high speed flowing particles or droplets. The light extinction method used in this project is suitable for measurement of particles or droplets equal and greater than 100 micrometers .

A fiber acousto-optic device is analyzed experimentally, which based on a single mode fiber coupler. The cutoff wavelength and splitting ratio in 630 nm are about 600 nm and 3 dB. When a strain wave acts on the packaged fiber coupler, the splitting ratio of the coupler will be changed which cause the modulation of output in the fiber coupler. At some frequency of strain wave, the modulation will get at Maximum.

The conventional measurement systems for low frequency (50 - 60 Hz) high electric fields measurement currently lay on active metallic probes which can disturb the measured electric field. The present study concerns an optical device using a LiNbO₃ crystal, which is known to be electrooptic, as sensing medium. The experimental set-up uses two electrodes to create the electric field to be measured. The principle of measurement implements the electrooptical effect in the crystal without any contacting electrode and is based on the classical static method in which the polarization of light is analyzed after going through the crystal. In order to minimize thermo-optical effects, the light beam propagates along the optical axis and the electric field is applied perpendicularly to the beam. Thanks to an excellent agreement between simulation with Finite Element Method and experiment, the shape of the crystal and its protection ring are optimized. The crystal used here has a regular octagonal section (radius #1.5 mm) and a length of 8 mm.

This work deals with the exploitation of optical fiber technology for the recognition of pipeline content within the oil industry, by means of absorption measurements in the 400 divided by 750 nm spectral range.

Chromium is frequently used in industrial processes. However, its residue is rarely discharged directly into the sewers, because of the high pollution level. There are several companies that deal with sewage collection and recycling processes. Since absorption spectroscopy during the recycling process is supposed, high concentration solutions in the 2.6 - 520 mg/l range have been considered. Trivalent chromium (Cr(III) solutions in distilled water and in buffers at pH values 2, 3, 4 and 6 were analyzed by means of a dual beam spectrophotometer in the 250 - 850 nm spectral range, using a 100 mm path length. The spectra showed two absorption peaks, around 400 nm and 580 nm, respectively. The measurements indicated that only Cr(III) concentrations higher than 26 mg/l can be detected, and that the spectral position of the absorption peaks and the absorbance relative to each peak are different for solutions in distilled water or in buffers, and are pH-dependent. The behavior of absorbance as a function of pH for all the concentrations considered was investigated for the two peaks. The value of absorbance increased with the pH. All the measurements carried out demonstrated the possibility of performing on-line absorption spectroscopy of water samples using current optical fiber technology.

Self-stabilizing modulation scheme is described for fiber optical gyroscope, which can be used in car navigation system. Resonance properties of cylinder piezo-electric modulator were tested, which was used as a resonant element for sine generator. Proper modulation index was calculated in case of second harmonic feedback loop for minimum nonlinearity.

The influence of photoconductivity upon operation of a Bi₁₂TiO₂₀-based optical voltage sensor is reported. We have demonstrated, that the presence of photoconductivity defines non-uniform sensitivity of the sensor in the cross section of an infrared light beam. The sensitivity of the sensor is shown to depend on the intensity of light as a consequence of photoconductivity of a crystal. An agreement between our theoretical model and experimental results has been demonstrated.

An interferometric method for non-contact measurement of very small amplitude vibration in confined space, in situation of relatively large and slow vibrating object movement, is presented. The method is based on a fully passive stabilization scheme, with 3 X 3 fused fiber- optic directional coupler. The simple measuring probe head is consisted of two single mode optical fibers of nearly same length, directed towards the vibrating part of target and a stationary part of target. Low coherence source is used to prevent unwanted interferences with beams reflected from fiber ends. If the whole target is slowly moving towards the probe head, the condition for interference is not disturbed and a stable vibrating signal is obtained. The method is experimentally verified with a calibrated piezoelectric transducer as a target, driven an acoustical frequencies. The minimum detectable amplitude of 10 pm/(root)Hz is obtained.

Measurement system based on a time-of-flight technique and Fabry-Perot optical fiber sensors have been used to determine the strain state in the composite structure. In theory the optical fiber causes a local disturbation to the structure such that measured strain slightly differs from the farfield strain. However, the sensors used in this study were insensitive to the disturbance.

The main lack of application of an optical system with Fiber Bragg Grating (FBG), as countermeasure feeler of a management system is a thermal drift, as a result of which depends undesirable oscillations of control parameters of an optical system. Depending on a thermal drift, the wavelength can change by attitude 0.01 nm/ degree(s)N, that is caused by change of a factor of an interception of a fiber and minor effect of heat expansion of a fiber. These changes are unwanted to those narrow intervals of a channel used in fast-response systems, which one today can achieve up to 50 GHz. The suggested technique is based upon a synthesis of two independent concepts of control, the sliding mode and optimal rating. The currently accepted title of this technique is DISCONTINUOUS CONTROL and SETTING ADJUSTMENT. The sliding mode is a technique used for discontinuous control and setting adjustment for optical system with FGS-s optimization. Control within sliding mode enables us to decrease the sensitivity to variations of optical system with FBG-s characteristics making them independent upon the environment. On the basis of offered technique by us is built testing software for synthesis new optical sensors of temperature.

We will present an overview of the different polymer/liquid crystal micro-composites and their uses for the control of the optical flux. The preparation method is the key element of this generic family of materials. There is a great number of pertinent parameters (relative concentration of constituents, irradiation conditions, ...) Acting on the morphology and therefore on the final properties. Materials with very different electro-optical properties can be obtained. We will discuss the case where the spatial repartition of heterogeneity is uniform in the sample: light occlusion films (with direct or reverse mode, selective reflection films, films with optical bistability). The cholesteric films will be particularly emphasized. We will focus on the relation between the preparation, the electro-optical properties and the colorimetric properties of the films. Next, the effect of introducing a gradient in some parameters (as irradiation, ...) influencing the material will be discussed. The use of these electrically controllable materials in adaptive optical materials for some applications such as optical components, smart windows, laser protection... will be reviewed.

The spectacular growth in MOEMS interest is highlighted by the involvement of R&D centres and industrial companies. A lot of application fields offer large opportunities for Micro-Opto-Electro- Mechanical Systems (MOEMS): optical communications (switches, cross- connect matrix, DWDM systems ...), digital image processing, adaptive optics... but also industrial maintenance, environment, medicine,... After general ideas on MEMS and MOEMS, this paper presents the main application fields for MOEMS and a few outstanding devices are presented to illustrate the recent developments. Then the work of LETI in MOEMS is presented. Some devices are fabrication with MEMS technologies such as tunable Fabry-Perot interferometers for DWDM telecommunications or 2D micro-scanners for obstacle detection. But a more specific technology has also been developed by LETI, resulting in devices made of silica such as 1D micro-scanners for obstacle detection. Moreover some devices are constituted of micro-mechanical structures combined with Integrated Optics: micro-switches for protection applications and network reconfiguration in optical communications, micro-vibration sensor for surveillance of rotating machines in electrical generators.