In this paper an electron-optic current transformer (CT) is described which is used in measuring the super-voltage station's line current. The measurement system uses a conventional CT as the sensing probe whose electronics is optically powered. The paper describes the principle of measurement system, including the system design, optically powered sensing probe, the data link, measurement signal processing and result showing current measurement by means of CT.

Electric current sensor employing twisted fiber with a new compensation method for temperature and polarization perturbation is presented and demonstrated for ac current detection. The problem of sensor bias drifting caused by the change of twist induced circular birefringence of the fiber versus the ambient temperature, or by the mechanical perturbations in the sensor leads, is addressed and solved by a novel signal processing method. With the single Wollaston prism scheme described in the paper, the compensation is valid only for limited bias phase drift. However, when the modified compensation scheme is introduced, the current signal can be obtained for arbitrary bias phase drift. The compensation method is demonstrated effective and which provides a new approach for the practical application of twisted fiber in current sensor.

The optical current transformer (OCT) reported in the past decades is mainly based on the traditional principle of Faraday rotation effect. Presented is a new type of OCT based on a new physical effect, the thermal lens coupled magneto-optical effect in ferrofluid. The use of optical array in the measuring system made the instrument complicated and expensive. This paper proposes applying double fibers to detect the current-corresponding variation of light intensity of the diffraction rings to simplify the instrumental structure. The fluctuations of the laser beam were eliminated by differential optical paths. Results obtained showed a DC measurement accuracy of 1 percent with a dynamic range of 0-500 angstrom, extendible to 2000 angstrom. All experiments were computerized. The set-up can also be applied to measure AC currents with similar qualities to the DC case.

This paper describes the action of voltage-induced birefringence and natural birefringences, and presents the design of a fiber optic voltage transformer with double light channels. Due to the special double-channel optical and electric signal processing, the influence caused by nature factors such as temperature, pressure etc. is eliminated, and the stability of the instrument is largely improved. The design idea, sensor structure and circuit processing are also introduced in this paper.

This paper presents a new practical optical fiber voltage transformer (OVT). It is different from the existing OVT. It has the high voltage directly applied across the electro- optical crystal of the OVT sensor and the other OVT uses capacitors to divide the high voltage. New types of silicon rubber insulators in which full SF6 gas is employed for HV insulation. It offers so many advantages like small volume, light weight, high accuracy, immunity form electromagnetic interference, no loss, wide bandwidth etc. The transformer could achieve +/- 0.2 percent accuracy. Compared with the conventional 110KV voltage transformer, its weight is only 1/3 of the former, and its cost is less than 1/2 of the conventional SF6 voltage transformer.

A new multi-function optical fiber sensing system is presented to measure voltage and current simultaneously. This sensing system includes Faraday cell and Pockels cell, Faraday effect is used for current sensing and Pockels effect is used for voltage sensing. To detect sensing light signal, we developed a pulse-controlled polarization converter based on Pockels effect in LiNbO₃ crystal. Four polarization components of polarized light can be obtained through one optical channel by the polarization converter. Some sensing experiments are carried on, and the result of experiments are given.

In this paper, a new type of optical fiber voltage transformer is introduced, which utilizes new porcelain capacitor divider. The optical; fiber voltage sensor is based on the BGO crystal Pockels effect modulated horizontally, sealed in a glass box with low expansion rate. Both the divider and the sensor are sealed in the compound silicon rubber insulator filled with SF6 gas. It has many advantages such as small volume, light weight, low price and high precision.

This paper introduces a multipath optic fiber sensing system for CH₄ gas concentration detection. The system includes three parts: the optic transmitters, the optic receivers and the sensing probes. The computer is used for intelligent management of gas concentration.

We put forward a new balanced optic bridge reference technique in an optical fiber gas sensor. In the system, a loss-compensated signal carrying gas concentration is independent on the variation of optic source intensity, fiber losses and the response of detectors. Also, considering the practical situation such as splitter states, the temperature features of the system etc., we modify the detected concentration signal equations by introducing correction parameters to improve system properties. A software filter program is also introduced by a microcomputer far from the spot.

Pipeline leak detection is a difficult problem to solve up to now. Some traditional leak detection methods have such problems as high rate of false alarm or missing detection, low location estimate capability. For the problems given above, a method for oil pipeline leak detection based on distributed optical fiber sensor with special coating is presented. The fiber's coating interacts with hydrocarbon molecules in oil, which alters the refractive indexed of the coating. Therefore the light-guiding properties of the fiber are modified. Thus pipeline leak location can be determined by OTDR. Oil pipeline lead detection system is designed based on the principle. The system has some features like real time, multi-point detection at the same time and high location accuracy. In the end, some factors that probably influence detection are analyzed and primary improving actions are given.

In this paper, pure and Ag-doped zinc oxide (ZnO) optical sensing thin films were deposited by reactive magnetron sputtering (rf) for detection of nitrogen oxide (NO_x) gas. The optical sensitivity was tested in a vacuum system by introducing NO_x. The transmission spectra of these films were measured at first, then the variation of sensitivity was derived form the transmission spectra. It was found that the sensitivity to NO_x of Ag-doped ZnO films was higher than that of pure ZnO films. The changes of sensitivity with NO_x pressure and concentration of dopant Ag were also studied in the paper. The concentration of dopant Ag varies form 1 percent to 18 percent, which was examined by Auger Electron Spectroscopy. The best sensitivity to NO_x was achieved with ZnO optical sensing thin film which was doped by 5 percent Ag. It may be developed by considering the Langmuir-type adsorption equilibrium relation and it shows that the theoretic analyses coincide excellently with the experimental results.

A novel oil tanks integrative parameters measuring system with optically powered are presented. To realize optical powered and micro-power consumption multiple channels and parameters detection, the system has taken the PWM/PPM modulation, ratio measurement, time division multiplexing and pulse width division multiplexing techniques. Moreover, the system also used special pulse width discriminator and single-chip microcomputer to accomplish signal pulse separation, PPM/PWM signal demodulation, the error correction of overlapping pulse and data processing. This new transducer has provided with high characteristics: experimental transmitting distance is 500m; total consumption of the probes is less than 150 (mu) W; measurement error: +/- 0.5 degrees C and +/- 0.2 percent FS. The measurement accuracy of the liquid level and reserves is mainly determined by the pressure accuracy. Finally, some points of the experiment are given.

A novel wavelength scanning fiber optic interferometer for absolute distance measurement has been proposed. In this paper, the source properties, which affect the measurement accuracy, resolution and dynamic range etc., are comprehensively studied. A tunable external-cavity semiconductor laser is taken as the wavelength scanning source, which can satisfy the demands on the spectrum bandwidth, scanning range and output power. Using the frequency-spectrum analysis, it is found that the random wavelength drift of the scanning can decrease the signal-to- noise ratio SNR of the output interferometric signal. The reduction of SNR limits the measurement accuracy. We can optimize the system with the analysis results. The preliminary experiment results correspond the theoretical analysis well.

By using composite self-focus rod, LD-pumped frequency doubling Nd:YVO₄ laser and 3D fine regulation stand, a new Fizeau-type interferometer is developed. It can apply to measuring the micro-vibration and deformation of micro- surface at the same time. The theoretical analysis and the experimental results for the interferometer of self-focus sensor are given.

This paper puts forward a new method to overcome the polarization-induced fading (PIF) in conventional low- birefringence optical fiber constructed interferometric sensors arrays. By inducing high frequency modulation to the state of polarization at the output end of the array, the maximum PIF variation of each sensor's signal is 4.7dB or 6.3dB according to the selection of modulation signal. Combined with an electronic automatic gain control circuit or dividing each sensor's signal by its visibility, each sensor's signal fading is recovered at the expense of a relevant signal/noise degradation of 4.7dB or 6.3dB.

A four-segmented piezoelectric tube performs XYZ spatial scanning in a SPM system. The measurement of characteristics including scanning range, accuracy, linearity and the lowest resonant frequency are essential to obtain a reliable image and get information about the scanning velocity. An optical fiber laser interferometer capable of vibration measurement is used to characterize the scanning tube with the inner diameter of 6mm, thickness 1mm and length 25mm is measured. We find that the lowest resonant frequency is much lower than the calculated result. The low resonant frequency will limit the scanning velocity and influence the quality of the obtained image greatly. In addition, the non-linearity curve can be obtained by driving the scanning tube with a triangle wave slowly. The measurement is easy, quick and continuous. In addition, we measure the non-linearity statically by inductive micrometer as well. The two results are compared. The result shows that there is difference between them.

As we know, 2 X 2 fused taper Hi-Bi optical fiber couplers are used in many interferometric optical fiber sensor such as FOG, Michelson optical fiber interferometer and Mach-Zehnder optical fiber interferometer. In this paper, the fabrication of 3 X 3 fused taper Hi-Bi optical fiber couplers and their properties and application are discussed. The method for identifying and aligning the birefringent axes of the fibers forming coupler are described. The insertion los of couplers we fabricated is about 1.0-1.5dB. The splitting is about 1:0:1-1:1:1. The extinction ratio is about 15-20dB. We have tested some interferometer system and give some results. The wavelength of light is 1.3micrometers .

In the 6km DOFRPS system, the 1550nm LD is excited photon source. The spontaneous Raman scattering photon are carriers of temperature signal and the Rayleigh scattering photon are carriers of strain and pressure signal. On the 6km optical fiber, the 3000 point temperature, strain and pressure are measured on time and the position of measured local domain have been determined by OTDR technique. The optimum design of 6km LD DOFRPS system and the configuration of the system are discussed in the paper.

In this paper we present the analysis of multimode (MM) interference induced by MM fiber interferometers and report the development of a white light scanning fiber Michelson interferometer with a sapphire fiber sensing head for the measurement of position-distance at high temperatures. The 'mode fading' effect in standard graded 50/125 micrometers fiber and independent 'inter-mode interference' in 100 micrometers step index profile fiber are discussed. By means of the 'mode selecting' technique, proposed and developed in this work, we demonstrated white light fringes with signal to noise ratios of more than 12 with a sensing head composed of a 400 mm long lead sapphire fiber and an uncoated sapphire target fiber.

A new designed sapphire fiber-optic sensor, aiming to improve the performance of the traditional sapphire fiber- optic sensors for high temperature measurement, is provided in this paper, in this system, an additional U-shaped sapphire fiber is used together with a modulated LED reference signal. It has advantages of both high sensibility of the single-band type and the high stability of the dual- band system. It is a good method to improve the stability of the traditional radiation based sapphire fiber-optic sensor without sacrificing high sensibility.

Using numerical calculation methods, we have studied the relations among the key system parameters - the center wavelength, spectral width, crystal thickness, and their influence on the measurement accuracy, resolution, dynamic range, linearity. The optical parameters of the source and crystal are obtained, in the compensation configuration which can provide enough accuracy, resolution and stability. The theoretical and experimental methods proposed in this paper have important roles in other polarization-modulated fiber optic senors.

With the rising cost of building and replacing civil infrastructure systems, as well as increased demands placed on such system, has come an elevated concern about monitoring their health. This requires the development of smart sensors suitable for real-time and in-situ measurement of concrete-based structures. Many smart sensor have been investigated to meet this requirement in the past several years. This paper describes some of the recent research activity undertaken at the Center for Intelligent Structures at Chongqing University regarding both optical sensor and monitoring strategies aimed at civil infrastructure damage assessment and integrity evaluation. Several experiments and sensor applications are also briefly described.

Etched optical fiber is made of multimode optical fiber, whose cladding is single-surface etched or double-surface asymmetrically etched. Because the mode-volume mismatch and the lateral bending can cause a reduction of the transmitted light power, it can be used as a sensor to detect stress and strain in the structure of material. Throughout our experiments, two kinds of fiber are used. It is shown by the static response testing that the power loss increases linearly with the change of strain. In experiments of dynamic response, the maximum frequency of distortionless optical modulation can get up to 21 kHz. This simple method of optical modulation is useful in research of sensor and spatial light modulation.

In this paper, a practical optically powered hydrostatic tank gauging system with optical fiber is presented. It combines the advantages of optical fibers with the micro power consumption sensor. Two multimode fibers are connected between the readout and the sensor, so it has external optical attributes. Optical energy transmitted from the readout is converted to electricity at the sensor. Digital data from the sensor is transmitted over the fiber to the readout. The parameters measured include the liquid level. the interface between water and oil, the tank storage.The paper gives results measured above parameters by means of the semiconductor strain gauges.

The use of optical bridge compensating structure in fiber optic sensor is studied thoroughly in this article. The stability of the compensating structure is essential to achieve good results. A fiber optic pressure sensor based on this compensating structure has been put into practical operation for oil storage measurement in tanks, and the long term stability of the sensor is better than 0.2 percent. This compensating technique has also been used in a LADAR fiber optic liquid level sensor system which combine discrete frequency IMCW LADAR and fiber optic sensor technique together, realized liquid level measurement effectively. Our studies proved that optical bridge compensating technique must be used with appropriate structure in different kinds of sensors and are very important to sure of the stability of fiber optic sensor.

The monitoring of concrete cracks with optical fiber sensors is very attractive for large concrete structures, e.g. dams and bridges. Its distributed measurement capability is particular advantageous for application to detect the crack in concrete dams since it aries at random spatially. In this paper presented are the test result on a new type of the distributed optical-fiber sensor using common communication optical fibers with simple treatment. Sensing fiber is laid in concrete crossing obliquely with the crack expected. Cracking induces tow highly localized microbends and cause attenuation of returning Rayleigh scattering. Its distributed sensing is based on optical time domain reflectometry. Concrete model test show it can not only detect crack width and shear displacement with high resolution, large dynamic range but also localized them with high precision of localization. A calculation model of attenuation is presented. Also, given are typical proposal schemes of application of the optic-fiber sensing system to the Three Gorges Dam, China. The result afford a sound technique-bias for developing the practical distributed optic-fiber crack sensing system.

Signal optical fiber is embedded in reinforced concrete structure to measure internal strain and vibration of structure using Mach-Zehnder interferometer. Calibration test is designed to identify material parameter of optical fiber, so internal strain of concrete structure is measured. In vibration test, amplitude and period of structure are measured, and vibration response property of structure is obtained. The experiment is verified using typical specimen. The research has important application prospect for the measurement of internal strain, vibration monitoring and damage assessment of structure.

A new kind of distributed fiber optic sensor is described in the paper. On the basis of mode coupling between two orthogonal fundamental modes of HiBi polarization- maintaining fiber, a new method is developed to locate the mode coupling point caused by external pressure applied on the fiber.

An automatic method of measuring fiber geometry parameters is described. The parameters include core diameter, cladding diameter, core non-circularity, cladding non-circularity and core/cladding concentricity. The image of the fiber under test is received by a CCD camera and transferred to an image grabber plugged in a microcomputer for further analysis. Several digital image-processing methods are used to remove the noise in fiber image. The image segment algorithm is used to extract data points. The data is analyzed using different algorithms to give the geometry parameters of the fiber. Sophisticated software is programmed. An instrument is developed based on the hardware and software described above. All of the parameters and graphics can be obtained within 30 seconds. The repeatability of the instrument is as follows: core diameter, cladding diameter, core non- circularity, cladding non-circularity, core/cladding concentricity.

This report describes the closed-loop fiber optic gyroscope for aircraft strap-down navigation systems. The closed-loop fiber optic gyroscope consists of a super luminescent diode, a fiber coupler, an integrated optical circuit, a polarizing fiber optic coil, a photoelectric detector and a signal processing circuit. The report describes several technical ways to improve the precision of the closed-loop fiber optic gyro: by improving properties and stability of the integrated optical circuit, utilizing a polarizing fiber optic coil using four grade symmetry twining round technology, improving the properties and stability of the light source, and by using a digital signal processing circuit.

We present a new class of fiber ring resonator gyro with a two-coupler ring as sensing element which can be used as monitoring and simulative setup of integrated optical gyro (MOG) for the way of design is in agreement with MOG. According to the experimental results and further analysis, resolution of approximately 10 deg/hr can be reached as the two-coupler ring structure is used as a gyro system with the couplers of splitting ratio approximately 9:1, insertion losses < 0.1dB, length of 10m of the ring and closed-loop operation satisfied. THis setup lessen the losses and simplified by reducing the number of couplers in traditional ring system.

This paper reports the experimental results of PM-fiber ring resonator in resonator fiber optic gyroscope by our group in Tsinghua University. Adopting the PANDA-type and tied-type PM-fiber, the ring resonators about 12 meters length were fabricated according to the optimized parameters. Using a PZT modulator, the resonance curves and characteristics were recorded and analyzed. A Nd-YAG laser and a laser diode with fiber grating joined the system separately. Both of the lasers' linewidth is less than 200KHz and output power from fiber pigtail is more than 4 mW. We used two miniaturized bulked TiO₂ crystal acousto-optical modulators as the frequency shifters of the clockwise wave and counterclockwise wave light. The experimental results were also presented.

Based on piezoelectric effect of quartz, a design of tuning- fork gyroscopes made of quartz was presented in this paper. The gyroscope is a kind of micro-machined quartz angular rate sensor. Its structure is similar to a tuning fork in quartz watch. In the gyroscope, the piezoelectric effect in quartz is used both to excite a reference vibration in the plane of tuning fork and to detect a vibration normal to this plane due to an externally applied rotation. The amplitude of the second vibration is directly proportional to the angular velocity of the applied rotation. In contrast to ordinary types of tuning fork gyroscopes, this gyroscope uses a single piece of quartz, the sensor element is of a design in which the only vibrationally, active areas are the tines of the tuning fork.

The architecture and performance of single axis digital closed-loop FOG are first described. To achieve high precision and large dynamic range, the optic sensor of single unit uses a multifunction integrated optic chip (MIOC), and fiber used in FOG are polarization-preserving fiber. The signal detection scheme of single axis FOG is also analyzed. Based on single axis unit, three-axis FOG scheme is presented. The optic sensor of three-axis unit includes three MIOC, and the signal detection part is optimal designed. To meet the requirement of real-time processing, high speed digital signal processor is used as center processor. Finally, the test result of single-axis digital closed-loop FOG are presented. As the result shows, the bias drift of 0.1 is achieved.

The engine video system with a fiber optical image guide is an optical equipment for the investigation of all kinds of periodic event sin production engines, such as injection or combustion process in reciprocating engines, etc. It is a reliable and easy-to-handle tool for research and development work avoiding significant alterations to the engine. This is possible by obtaining optical access to most parts of the engine via a fiber optical image guide. On-line observation of various phenomena is realized by connecting a miniature video CCD camera to the fiber optical image guide. System features are suitable for diesel and gasoline engine, on-line observation and recording, system resolution better than 10pl/mm. This system can be used to observe non- luminous phenomena such as fuel injection of cyclic movement of mechanical parts by the use of a high power xenon lamp that has the power 350W guided by an optical fiber bundle to the observed target.

In this paper, the methods and properties of growing the reflective mirror on the ends of the optical fiber used in optical fiber sensor is proposed. There are many kinds of methods used for growing the mirror such as plasma-enhanced chemical vapor deposition, aluminum-coating in vacuum and sol-gel coating. The reflectance and transmittance ratios can be changed from 0.1 to 0.9 with different coating method and parameters. Here, taking the sol-gel coating and vacuum vapor-deposited coating for example, the method and results from the coating test are stated.

On the ground of research of dynamic laser fixed-distance detection system carry on board a protective, this paper discusses and analyzes the factors that affect the ranging accuracy of the system. On the basis of correlation detection theory and according to the system requirements on ranging accuracy and sensitivity, the effects of the values of main parameters, such as lasers' cycle, pulse width and ascending fringe, on ranging accuracy are described; a comprehensive analysis is given on the power requirements due to the reflectance of the target, ranging distance length, the sensitivity of detectors, and the coupling efficiency of the relative coupling devices.

We have studied the effect of the measurement system color temperature change on MTF both in theory and by experiment. Firstly, we simulated sorts of white light sources by computer and constructed the source spectrum characterizations of different color temperature. The simulated calculation results showed that in white light illumination condition, the value of MTF changed little even if color temperature change scope extend to 700K. In experiment we used 'the digital optical transfer-function measuring equipment', an instrument we invested recently. We can see from the experimental data that the measurement results remained quite stable even if the voltage changed in wide range.

We can use laser Doppler effect to measure velocity. But the volume of laser Doppler signal is small and the signal-to- noise is small. If we use an analog filter to precess laser Doppler signal. It is difficult to change the frequency response of the analog filter. So it is difficult to get a good signal. In this paper, we use a digital filter to process laser Doppler signal. When we measure the velocity, we can get the laser Doppler signal volume by an analogy- digital converter. We can process the signal by a Chebyshev digital filter and save the signal in RAM. The digital filter's frequency response can be changed easy until we get a good Doppler signal. In this paper, we show how to design the chebyshev digital filter and how to use the filter to process the laser Doppler signal.

The unmodulation alarm system consists of a receptive device, sensor and an information processing unit, etc. The output noise of the system obeys generally Gauss- distribution. This paper derives the relation between the detection probability and the parameters of the system, designed correlative program using basic language, it is operated by the computer, and we discuss the relation curve between them.

This paper states the kinds of methods to reduce the low- frequency noise in fiber-optic sensor with frequency- spectrum analysis. The author thinks most of the noise frequency will be mixed with the signal frequency in optic- isolated optical measuring system and the result is the low- frequency noise is shifted to the signal frequency band. So, the low-frequency noise can not simply be removed by band- passed filter in theory. The author discusses kinds of methods treating the low-frequency noise and puts forward a new one, the method of A/D conversion. It is an easy realizable, good effective and low-cost method for most application.

A basic concept of an advanced safety vehicle, which will emerge in the next century, is introduced in this paper, with the emphasis on the active obstacle avoidance system. A prototype system is put forward, in which a scanning laser radar is used for detecting ahead objects, the surrounding information can be understood from the radar data, and therefore once the serious situation emerges, the drive will be warned, or the auto-control system will operate according to the analysis of the surrounding condition.

The photographic measuring camera is an instrument for precisely measuring the traces of flying targets and flying parameters. In this paper, the necessary of its periodical correction was described. Especially, through use for a long time, the variations of the numerical value of its focal length will be caused by its carry, vibration and temperature variation; it directly affects the measuring accuracy. Based on theoretical analyses and practices, the datum image plane and datum mark coordinates etc were precisely corrected by using general instruments and equipment, such as T4 theodolite and collimators and so on. The net plate measured precisely was installed on its image plane; the focal length and distortion were measured by angulation method. But the real value of the focal length was defined by means of the minimum value of distortion. The accuracy of measured and corrected results was analyzed, and the result proved that the correction was reliable.

An IR flare is used as a source in the half-active IR night- viewer system, and the non-scanning IR micro-photoelectric camera as a sensor. The clear image had been obtained at night. Some IR flares and the automatic testing equipment had been developed. Then the integral radiation character of the IR flares had been measured. Some samples with more kind of the fuel components are manufactured, the wavelength of the IR flare for maximum radiant flux have matched with the responded wavelength of the sensor. The clearly reflective image of the targets had been obtained at 400m and 700m in the field tests. In this paper, the we develop a sample of IR flare as radiant point and characters, measuring system and method and test in our field are introduced.

We point out that a long-period fiber grating with (pi) phase shift at the center has a transmission window in the center of the stopband. Basing on the transfer matrix treatment we calculate the output spectra of the phase-shifted long period fiber gratings. When the grating total sustains temperature or strain variation, the bright peak will move with the absorbing band and its wavelength shift can be detected to show the variation. The transmission window is a bright signal and easy to be detected.

In the paper, an applying system based on the technique of modulated optical fiber grating model filter is presented. It adopts WDM/TDM method to realize multiple mixture gases monitoring in transformer oil due to fault. A selective- absorption detection, which is based on absorption spectrum of gas molecules in the near IR, is used in our system. For monitoring trace fault-gas, an advanced over-width sine-wave wavelength modulation technique is invented, because of its benefit of stability and applicability. We also point out how to apply a reflection fiber grating optical filter to the harmonic detection. Through using those techniques, light source made up of imperfect components attains quality and level required by the ppm gas monitory. As the result of theory and experiment, the principles of choosing LD/LED and modulating PZT are given too.

Based on the theoretical analysis, the transmission spectrum of a broadband fiber Bragg grating (FBG) have been measured by applying a passive wavelength demodulation system, in which a fiber Bragg grating wavelength scanning laser source is used. The FWHM and the peak reflectivity of the FBG to be measured is 0.48nm and 92 percent, respectively. The experiment result is in good agreement with that measured by an optical spectrum analyzer, with experimental error less than 5 percent.

The coupled-mode equations of chirped fiber gratings are derived using Maxwell's equations, the principle of superposition, and considering the expansion of the perturbative vector fields in the eigen modes which perform orthogonality relation mutually. The boundary conditions of the equations are introduced. Applying these equations to chirped fiber gratings, the phenomenon of a broadband reflection spectrum is qualitatively explained.

It is a commonly encountered statement that shot-noise is the most fundamental and ultimately limiting noise source in any photonic device, because all other noise sources can in principle be reduced to insignificant values. However, this perspective is often of little value, since in practice it is impossible to eliminate other, more trivial, noise sources. It is useful to classify noise sources according to their dependence on the optical power on the detector. Thermal noise and other noise in the electronic circuitry are independent of the optical power. Shot noise current is proportional to the square root of optical power, whereas source intensity noise and thermal noise in the fiber and sensor generate noise current proportional to optical power. Only the noise sources proportional to optical power are of importance if the optical source power can be increased without limit. Often, however, sufficient source power is not available, and shot noise and/or electronic noise become the dominating noise sources. The paper discusses the various noise sources in the light of the above classification. Extra noise sources introduced by fiber- optic amplifiers are included in the discussion.

High voltage transformers use paper to insulate the electrical windings present in the core which is then subsequently immersed in oil. In service, the temperature of the windings will increase to typically 80 degrees C. If the transformer is loaded to beyond its design ratings, the temperature can exceed 100 degrees C causing the cellulose chains in the paper to cleave at an accelerated rate, resulting in the degradation of mechanical strength and performance of the insulation. If unchecked, this can lead to catastrophic failure of the transformed and accompanying disruption to electricity supply and large economic losses to the operating utility. Furfuraldehyde (FFA) is a chemical by-product which is released into the oil by the thermal degradation of the paper winding. The concentration of FFA within the oil has been directly related to the condition of the paper insulation. We have developed absorbance and fluorescence optical techniques incorporating a novel FFA- sensitive material which we have invested at our laboratories. This material has been incorporated into a prototype portable optoelectronic instrument for the measurement of FFA at the site of the transformer. Results of experiments will be presented, and the implication of these results for condition monitoring of HV transformer will be discussed.

A novel humidity optical fiber sensor have been developed using an hygroscopic material, deposited on the thinner zone of a biconically tapered single mode optical fiber. Relative humidity variations form 40 percent to 90 percent have been detected producing up to 5dB optical power change.

This paper describes a simple, low cost technique for producing a micro-oxygen sensor based on indicator chemistry, and results obtained using this device to monitor dissolved oxygen concentration are reported. The technology is based on the use of submicron optical fiber tips which have a fluorescent chemical reagent immobilized on their tip surface. Fiber tips were formed by drawing single-mode optical fiber in a fusion splicer. Sol-gel films doped with a fluorescent, oxygen sensitive chemical indicator were then deposited on the surface of the fiber tip using dip-coating techniques. The sensors that we have developed using these coating technologies are reversible and have response times of a few tenths of a second.

We describe in this paper a sensor system dedicated to the measurement of concentration of organic gases. This system is based on the sue of coherence modulation and allows a remote interferometric measurement. Two different integrated optics technologies are required in a complementary way. Integrated optics in glass is used to realize the sensor itself, whereas lithium niobate technology is used for the demodulation process which performs an active phase read- out. The sensor uses a polymer material whose refractive index varies with the gas concentration, and thus modifies the characteristics of the evanescent part of the wave propagating in the glass waveguide. Experimentally, a sensitivity of 0.8rad/Vol percent is obtained with the vapor of ethanol, and 0.2rad/vol percent for butane.

A theoretical and experimental study of the self-referencing resonant fiber optic intensity sensors based respectively on Michelson and Mach-Zehnder configurations is conducted. Sensor linearity and sensitivity are addressed, being also considered sensors design and optimization.

In fiber Bragg gratings are currently being evaluated as sensors for monitoring various parameters including the structural health of bridges, strain in advanced composites, and high frequency acoustic fields. These devices are attractive because they offer the benefits of ease of multiplexing, the simultaneous measurement of temperature, and a potentially low cost.In this paper, the core of our scheme comprises a fiber-Fabry-Perot sensor which is defined by Bragg-grating mirrors. Dynamic strains applied to the sensor change the optical path-length between the grating- mirrors and are encoded as a change in signal-phase. A low- coherent Mach-Zehnder processing interferometer, with a high-frequency phase-carrier in one arm, is used to interrogate the sensor. Using the fact that the two outputs of the interferometer are in anti-phase, we extend the approach by applying balanced detection to the Mach-Zehnder outputs in order to eliminate intensity-based optical noise. A noise-rejection better than 40dBV is demonstrated.

We investigate theoretically and experimentally the principle of a micromachined pressure sensor integrated in silicon. The optical part of the sensor consists of an imbalanced Mach-Zehnder waveguide interferometer. The waveguide is formed by a silicon nitride layer of high refractive index set between two cladding layers of silicon dioxide of lower refractive index. The sensing part of the device consists in a set of membranes obtained by anisotropic etching of the back face of the substrate under the reference arm. The pressure variations applied to the membrane induce a geometrical deformation of the waveguide arm and modify the optical path difference between the arms. The non-linear variation of the phase versus the applied pressure is obtained from the spectral analysis of the channeled spectrum transmitted by the interferometer. The static optical path-delay introduced between the arms allows the sensor to be introduce din a coherence modulation scheme, which can result in a remote sensor system, involving an active demodulation.

In many concrete bridges, deformations due to static and dynamic loading are the most relevant parameters to be monitored in both the short and long term. Strain monitoring give only local information about the material behavior and too many of such sensor are therefore necessary to gain a complete understanding of the bridge evolution. Recent advances in measurement technology have demonstrated that optical fiber sensors are suitable for monitoring full-scale structures. A network of such sensors installed inside a bridges enables the measurement of parameters such as internal deformation and temperature. In the past four years, our laboratory has installed hundreds of fiber optic deformation sensors of varying seizes in concrete and composite steel-concrete bridges. These sensors give useful information during the construction phases and about the long-term geometrical deformations of a bridge under static load. Recently it has been found that these sensors can also be used to measure the quasi-static part of the dynamic deformation of a bridge under traffic load. The measurement technique relies on low-coherent interferometry and it guarantees high resolution, high precision ad long-term stability. Nevertheless each measurement takes a few seconds, thus it is inadequate for monitoring the dynamic behavior of bridges, where measurement frequencies of up to 1 kHz are necessary. This paper also presents a new technique to demodulate at a high frequency of up to 1 kHz are necessary. This paper also presents a new technique to demodulate at a high frequency the signal for the same sensors used for long-term monitoring. With its large dynamic range of several mm it allows for the monitoring of bridges under dynamic loads while maintaining the configuration of the original fiber optic sensor that has been proven to be reliable.

We review the principle of operation of several reflective fiber optic interferometers and combinations of interferometers that produce an output signal proportional to the derivative, or the product of the position and the derivative of an external phase perturbing signal. The unbalanced Mach-Zehnder or modified Sagnac interferometer, the reflective-ring and the differentiating-ring interferometers, and two other sets of interferometer combinations are discussed. The use of these interferometers is illustrated as sensors for the waveforms of multi-phase alternating currents, an as impact or disturbance position sensors.

In distributed optical fiber temperature measuring system, temperature information is involved in Raman scattering light. In order to separate weak Raman scattering light from other stronger, adjacent scattering light, interference filter is commonly used and must be specially designed and fabricated. A group of fine tunable optical interference filters is presented to avoid the special procedure of filter. The principle of the filter group is discussed. The experimental results is presented. Results show that the accuracy of central wavelength is upgraded from +/- 0.8nm to +/- 0.2nm and the bandwidth is narrowed to the half of original filter's.

Distributed temperature sensors based on Brillouin scattering are attractive because they offer very large sensing length. The intensity of spontaneous Brillouin scattering is directly proportional to the temperature of the fiber, and permits a measurement independent of the strain applied to it. We report on a novel system to detect this signal, incorporating a mode-locked Brillouin fiber ring laser.

The need to achieve higher temperature performance from fluorescence-based optical thermometers has prompted the investigation of a range of rare-earth doped materials. In this work the characteristics of Nd³⁺, Er³⁺ and Tm³⁺-doped intrinsic fiber optic temperature probes are presented and evaluated. It shows that the Tm- doped fiber probe has out-performed in both the temperature sensitivity and the potential operational upper temperature limit.

The effects of a (Delta) T equals 300 K thermal perturbation is modelized theoretically in the case of a telecom singlemode optic fiber that we use as a transducer in an angular displacement polarimetric sensor. This model allows to optimize the fiber's spatial path and to select the light input polarization and the wavelength (lambda) in order to minimize the thermal modulation of the sensor signal. Therefore, the angular calibration error with (Delta) T equals 300 K will stay below 0.1 degrees with a 90 degree measurement range. We present two series of experiments where the wavelength is respectively (lambda) ₁ equals 1300 nm and (lambda) ₂ equals 780 nm. In the first case, we apply a thermal perturbation (Delta) T equals (293 K; 453 K). We observe a sensor signal fitting the one of the simulation, showing that experimental conditions are similar to the model ones. In the second case, (Delta) T equals (293 K; 213 K), we measure a greater thermal modulation of the sensors signal than the simulation one. This linear birefringence is due to the fiber's gluing on its substrate and to the fiber's mechanical homogeneity in the bent parts. These stresses will be minimized by adapted gluing and substrate materials and optic fiber's design selection.

The paper covers an issue of optical polarimetric method for temperature measurement based on the use of a laser autocollimation polarimeter and a sensitive birefringent plate. The sensor works in the following manner. The linear polarized probe beam of the polarimeter illuminates the sensitive element which reflects the radiation backward. The sensitive element result in a phase shift between the beam polarization components which value is proportional to the element temperature. The autocollimation polarimeter determines this phase shift and enables to calculate the temperature value. The distance between the polarimeter an the sensitive element may change from 0 to 100m. The use precisely of the polarization properties of light makes it possible to decrease substantially the effect of beam attenuation on the measurement errors in case where the radiation passes through smoked or dusty space. Small size of the sensitive crystal plate provides rather fast reaction of the sensor to temperature variation in the medium. The basic expression that describes the operation of the system are presented. It is shown that with the use of a quartz plate 1.912mm thick the device makes possible to determine values of temperature in the range of 0 to + 180 degrees C.

A method for detecting interstory drift and chord drift in buildings is proposed and demonstrated based on the motion of a laser generated cross hair in free space. The beam is projected form ceiling to floor onto an arrangement of four position sensitive detectors, each with two separate outputs. Every lateral displacement and relative angle between ceiling and floor gives a unique set of detector voltages, so that as floors shift with respect to one another due to wind and earthquake loads, voltages vary, the new beam axis location is determined, and the 3D interstory drift, velocity, and acceleration can be obtained. Experimental results show excellent linearity and sensitivity of roughly 10 microns for lateral displacement and 0.02 degrees for relative rotation between ceiling and floor.

Integrating fiber optic sensors offer the potential to monitor large spatial extends 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. In this paper we analyze the application of very long gauge length integrating fiber optic sensors to high sensitivity/highly directional seismic sensing. The ability to focus a seismic sensor on particular faults or locations such as underground nuclear test sties offers the potential of enhanced warning of impending earthquakes or detection of the occurrence of nuclear device testing. We provide an analysis of the application of very long gauge sensing to seismic detection and presents some experimental results demonstrating antenna gain.

New developments are reported regarding the fiberoptic curvature gauge proposed recently. Advantages in measuring curvature, rather than strain, are presented. In contrast to direct sensor of strain where reliable structural strain transmission onto the sensor must be assured, the curvature gauge is unaffected by mechanical properties of intermediate objects such as the cushioning effect of the fiber protective coating, adhesives, resin, etc. Furthermore, curvatures can be measured anywhere in the structure's cross section, including along the neutral axis where there is no strain in bending. This simplifies the process of embedding sensor and the interpretation of the signals obtained. Curvature measurements are particularly suitable for thinner structures where extreme resolution in measuring strain would be required.

An original idea of hydrostatic pressure monitoring is presented. The paper reports latest result of liquid crystal-based fiber optic devices for hydrostatic pressure sensing in both low pressure and high pressure regions. In the low-pressure region, the fiber optic transducer based on properties of colorimetry employs pressure-induced deformations occurring in liquid crystalline cells, configured in a homogenous Frederiks geometry. However, in the high pressure region we propose another approach based on a liquid crystal-core anisotropic fiber acting as a sensing medium due to pressure-induced changes in its birefringence. These fibers unite the unique advantages of liquid crystals and quality of fiber technology that can be used for instant pressure monitoring.

The construction of the fiber-optic sensor for the recognition of perturbations and result of its studies are presented. As a sensor head the sample of two-mode optical fiber is used.Changes of intermodal interference condition, caused by external perturbation, generated changes of output speckle pattern. This output has been concerned as an intensity image and diffraction method, known from an automatic image recognition, has been applied for its recognition. For this reason the ring-wedge photodetector has been placed in Fourier plane of the image studied. The digital signal generated by this detector has been processed by software neural network. As the effect of suitable process of network training, a possibility of the perturbation recognition has been obtained without a necessity of troublesome analysis of intermode interactions. An additional advantage of this solution is the possibility to train the network to eliminate slow environmental perturbations.

Refractometers are of interest for measuring concentrations of solutions, since the refractive index of a solution is proportional to its concentration. On-line continuous refractometry is important in many industrial processes, as well as for checking the concentration stability of solutions in stock. An optical fiber long-period grating is presented, which was tested for refractometry in the 1.34- 1.42 range.

We demonstrate that short in-fiber Bragg gratings coupled with an appropriate desensitization of the fiber, may be successfully used to measure MHz ultrasonic fields.

An optochemical sensor network has been developed which performs on-line measurements of multi-point distributions of various chemicals. The side-polished optical fiber elements with wavelength multiplexed fiber optic Bragg gratings are sensitive to refractive index changes in thin film overlays as well as in the more extended superstrate material. A multi-layered slab waveguide model has been used for theoretical investigation of the efficiency of evanescent field interaction with fiber gratings. From these results, the structural parameters of the sensor can be optimized. The sensitivity to small refractive index changes is high for materials with a refractive index near to that of the fiber core. However, the deposition of a thin buffer layer with adapted high refractive index can enhance the sensor efficiency also for detecting materials with lower refractive index. The sensor network can be applied for process control in chemical and biochemical technology, and for environmental and geotechnical monitoring in hazardous or corrosive surrounding.

In-fiber Bragg grating (FBG) sensor technology has become one of the most progressing sensing topics of this decade in the field of optical fiber sensor technologies as FBG sensors are capable of measuring a wide range of parameters and have potential for simultaneous measurement of multi- parameters. In recent years, rapid progress has been made, in particular, in sensor system developments and applications. The fundamental aspects of the FBG sensor technology have been reviewed recently by the author. As a complementary article, this review paper concentrates on the recent progress in applications of FBG sensors for large composite structures, electrical power industry and medicine. Also, recent progress in sensor system developments associated with these applications are addressed.

The optical fiber intrusion sensor is investigated in the configuration of an optical time domain reflectometer using coherent interference of Rayleigh backscattering in a single mode fiber. When the modulation pulse width was 4 usec, the spatial resolution of this intrusion sensor was about 400m, the signal to noise ratio was about 3.3 and the bandwidth was about 30.5Hz.

The fiber-optic refractometer made by the prism-taped fibers is presented. The prism tips of fibers had been making by polishing method and the endface of the load-in fiber was coating by the reflective thin film. The size of the fiber sensor head is of 250-260 microns. The fiber-optic sensor using visible laser diodes of 650-670 nm was performance for the directly distinguish of sugar content in the sugarcanes with accuracy range of 5.10^-4. These refractometers are already to use for the sugarcane plantations.

Modulation characteristics of interferometric noise in fiber optic gas sensors base don wavelength modulation of DFB laser have been studied. Sensor resolution limited by the interferometric noise is calculated and anti-measures are proposed.