The solder glass sealing technology is an alternative to the direct sealing method of the socalled hboptocansu. Using solder glass for the junction of glass and the metal can the temperature at about 500 °C does not destroy the optical quality of the precision glass components. The glass can also be coated with an antireflective layer and even the sealing of filterglass is possible. In cases where coupling losses can't be tolerated, the fiber has to be fed directly through the wall into the housing. Fiber feedthroughs, using solder glass for the sealing of the fiber into a metal tube, are commonly metal soldered or welded into the wall and the fiber surface is directly leading to the semiconductor.

The incorporation of laser diodes and monomode fibre optics into an Electronic Speckle Pattern Interferometer (ESPI) has led to the development of a novel holographic surface contouring system. Height contours are obtained by switching between two laser wavelengths. Contour intervals from 0.5-5 mm have been generated by modulating the injection current of a single laser diode source and are demonstrated on an automotive disc brake hub.

We describe a new configuration for an optical fiber acousto-optic frequency shifter using a section of two-mode fiber where acousto-optic interaction causes mode-convertion accompanied by single sideband frequency translation. A fiexural acoustic wave travels in the stripped section of the two-mode fiber. Because both optical and acoustic waves are confined in the fiber, high efficiencies (close to 100 %) can be achieved with relatively low RF power (< 100mW) applied to the transducer. The acoustic wave is excited in the fiber via an acoustic horn, which focuses the acoustic energy generated by a 5 x 5 mm2 transducer into a fiber-size tip. The new configuration proposed here employs the fiber and the acoustic horn in a collinear assembly, which can reduce excitation of backward acoustic waves in the fiber, contributing to a better image sideband suppression. Moreover, the collinear configuration improves ruggedness of the fiber-to-horn bond as well as device compactness. An improvement of the acoustic bandwidth was also observed. A study of several possible variations of the main configuration was performed and an optimization regarding power requirements, convertion efficiency and acoustic bandwidth is discussed.

A simple technique is presented for azimuthal alignment of the birefringent axes of a polarization maintaining fiber at a splicing point. Alignment accuracy better than ±0.01 is consistently obtained and is fairly independent of polarization cross coupling occurring in the fiber sections being spliced. The technique is particularly suitable for splicing birefringent fiber pigtails to laser diodes or integrated optics devices.

While current sensors based on the Faraday effect in bulk materials have shown good success in field tests, the use of single mode fiber as the sensing element has both technical and economic advantages. In this paper we describe some of the practical problems that have inhibited the development of fiber current sensors. Recent research suggests that most of these problems, including especially the problem of linear birefringence in the sensing coils, can be solved. Instruments providing a measurement quality approaching that set by fundamental material parameters can now be achieved.

By means of modified multicrucible technology multimode and near monomode original fibers of two to nine separated cores have been manufactured. Special topology of crucibles have been used.

The coupling between two spatial optical modes of a birefringent elliptical core fiber due to a travelling acoustic wave is experimentally investigated. Acoustic horns are used to excite acoustic flexural waves on a fiber. The distributed interaction of the LP01 and the LP11 modes along a strip section of fiber is determined by RF pulsing the acoustic wave. Particular attention has been given to studying the local orientation and twisting of the fiber along the acousto-optic coupling region. High coupling efficiency is also demonstrated.

The Rayleigh scattering method for beat-length measurement of polarization-maintaining highly birefringent bow-tie optical fibers under high hydrostatic pressure in the range up to 100 MPa and under temperature up to 7OC is presented. The results indicate that the beat length increases with temperature and decreases with hydrostatic pressure, and are in a perfect agreement with a semi-phenomenological formula we introduced recently.

The problems associated with the fibres used in pulsed time-of-flight rangefinders were studied, and particularly errors due to the transit time disturbances of step and graded index fibres as a function of fibre length, input numerical aperture and temperature. The cladding modes and leaky modes ofa fibre can affect the transit time oflight pulses under suitable conditions, so that fibres become sensitive to environmental effects. The effect of temperature is smaller and more linear for acryl-coated fibres than for nylon-coated ones. The main reason for the non-linear effect of temperature on a nylon fibre is the non-linear Young's modulus of nylon as a function of temperature. The increasing transit time of hard clad silica (HCS) fibres at lower temperatures (below +20°C), contrary to glass fibres, can be explained by the different thermal coefficient of the core and cladding, leading to increased non-homogenities on the core-cladding interface.

In recent years we have been witnessing implementation of automatic handling machines in various manufacturing environments. This, however, has only been applying to large factories. Whilst for automation and robotics to become a matter of routine in our industries we need a wide spread of robots and associated hardware and software in all parts of industry whether they be large multinational companies or small local companies. This will partly be possible if cheap, reliable and easily implementable sensors are introduced to the field. This paper describes an intensity-modulated optical tactile sensor. Light transmission in an optical fibre is through total internal reflection at the core-cladding interface. If the core-cladding interfaces are parallel, an incident meridional ray will continue to be reflected and will be transmitted through the fibre. By applying force to the fibre one can cause attenuation or transmission loss. The attenuation in intensity of light transmitted is due mainly to changes in the angle of reflection at the border of core and cladding.Any pressure on the fibre will result in refraction rather than total internal reflection with the subsequent loss of the light ray into the cladding.

The origins of fiberoptic thermometry, its strengths and limitations and the general characteristics of its commercial applications are discussed. Three classes of measurement systems are considered as commercial: (1) those employing point sensors for measurement of low to intermediate temperatures; (2) those employing point sensors for measurement of intermediate to high temperatures; and (3) those employing distributed sensors. The principal technologies in each class are reviewed and specific applications are then examined. Finally, future developments are suggested.

Evanescent wave spectroscopy at 3.3gm on the surface of the core of an IR transmitting fluoride fibre is used to detect the concentration of flammable gases such as propane or methane in the environment of the fibre. Three probe designs are discussed. In one the gas diffuses through the teflon cladding of a multimode fibre causing an attenuation of the evanescent field and thereby a reduction in transmittance of the fibre. In the second a short section of cladding is removed from multimode fibre to access the evanescent field region. In the third singlemode fibre is used with its cladding reduced locally by polishing. Referencing is carried out at non absorbing wavelengths in the 3.2 to 3.6pm region.

A fiberoptic Michelson interferometer with electrooptic feedback via a piezoelectric fiber stretcher exhibits a stepwise increase of the output signal and PZT position under continuously increasing input power. Depending on the characteristic time constants the system becomes unstable and starts self oscillations at a certain critical input intensity. A linear stability analysis of the 3rd order differential equation used to describe the system reveals the number of stable states increases with the ratio -cIT of the system time constant N) and the feedback delay time (T). In laboratory investigations the maximum number of umax 113 stable states was achieved with i 0.10 and -nT > 45. The ambiguity of the input/output characteristic in the stable region which is due to the multistable hysteresis may be eliminated by chopping the input of the feedback amplifier. The power spectral density of the interferometer signal exhibits a characteristic change after transition into the instable region.

The OPTONET system monitors the position of up to 30 ON/OFF fiber optic limit switches. It is based on Optical Time Domain Reflectometry : an optical pulse emitted by a pulsed laser diode is splitted between the sensors by an optical star coupler. Delay lines are inserted on the optical lines in order to separate the pulses backreflected by the switches. This paper establishes the main characteristics of the OPTONET system from an user point of view. These characteristics depend on the optical fiber properties and the time domain multiplexing method. The well known optical fiber properties lead to high EMI immunity, lightweight, galvanic isolation, in particular while the time domain multiplexing leads to saving in cable, weight and cost, and it requires only the use of a single source and detector. Moreover, various limit fiber optic switches may be connected to the network since the OPTONET system operates properly with any reflective switches. The number of sensors which may be monitored by the system will be examined. The OPTONET system is presently used on several industrial plants and these applications will be presented. Finally, the future developments of the system will be presented and discussed.

Absolute displacement sensing by white-light interferometry is demonstrated without mechanically moving parts in the receiving interferoineter. Its characteristics are: Michelson interferomneter with tilted mirrors, photodiode array of 1024 elements, lead-insensitivity up to 9 dB loss, displacement range 75 /.m, resolution 0.02 pm, read- out time < 7 ins, and multiplexing of several transducers.

A new interferometric pressure sensor providing temperature correction is presented. The temperature correction is obtained by solving a two equations system whose unknown quantities are the pressure and temperature differences applied on a double optical fiber interferometer. The performances and the limitations of the sensor are discussed. Some applications are described, especi.lly the possibility to use it for flowrate measurements.

Experimental and theoretical investigations of the use of multi-mode laser diodes in a coupled Michelson: Fabry-Perot interferometer and a novel Michelson: Bragg interferometer for fibre-linked interferometry are reported. These results show that the coherence regions of such diodes can be varied over a large range of optical path imbalance of the interferometers allowing the interference region in fibre optic sensors to be tailored to specific sensor applications.

Fibre optic interferometers have been extensively studied for sensing applications because of the very high sensitivities with which many physical parameters can be measured using these devices. A limiting feature of fibre interferometers is the random phase changes which occur at the interferometer output because of general environmental fluctuations, for example, due to changes of ambient temperature. These fluctuations can be compensated by active phase tracking techniques which keep the interferometer biased at quadrature. Passive demodulation techniques are of greater practical interest and the J1....J4 technique has been recently proposed as a method for the linear measurement of dynamic phase changes in a fibre interferometer. In this technique the interferometer requires no phase bias. The measurement is unaffected by random phase fluctuations in the interferometer. However, the J1....J4 technique is limited by the fact that only the magnitude of the J1....J4 Bessel components can be measured on a spectrum analyser so that no information about the sign of the Bessel function is available thereby generating some ambiguity in the readout. In this paper, a modified technique which overcomes this limitation is demonstrated.

Optical triangulation is a very well-known classical technique which can be advantageously performed by optical fibers, taking profit from their geometrical versatility, intrinsic safety and good transmission properties. The exploitation of different optical architectures provides spatial information over single or multiple sensing zones, so that a wide class of intensity-modulated optical fiber sensors can be achieved.

A pigtailed DFB source operating at 1520 NM feeds an all-fiber interferometric scheme to obtain absolute measurements. The frequency modulation of the source is in-line controlled by means of an all-fiber ring resonator. The sensor was realized by all-fiber components and it was designed with a multisensing capability. The extremely narrow emitting-line of the DFB enphasizes the resolution features of the scheme. The precision figure is also reported on the basis of the first experiniental data.

A system employing optical fibres for electronic speckle pattern interferometry is demonstrated. Single mode optical fibres are used to provide a reference beam and illumination of the object. The Michelson interferometer formed by the reference fibre and the fibre illuminating the target is used to compensate for environmentally induced phase noise. This compensation is achieved by using an electronic servo and a piezoelectric phase modulator to lock the Michelson at its quadrature point.

A fibre optic interferometric sensor for the analysis of surface texture is presented, which incorporates optical fibre links to achieve a remote and passive sensing head. A synthetic heterodyne signal recovery scheme based upon modulating the frequency of the laser diode source was used to generate a high frequency optical carrier ideal for in-process surface profiling. It is demonstrated that the system is capable of both differential and absolute measurements, a resolution of .O.1 nm/ IHz was achieved.

Imperfections of a phase modulator driven below the proper modulation frequency of a fiber gyroscope can cause offset of the output signal, limiting long term stability. Bias drift induced by non-linear effects in the phase modulator was studied and altenative ways of stabilizing a short loop length gyro were implemented. Long term drift figures below 0.2°/h rms were achieved in a highly birefringent fiber gyro with a 160 m-long sensing spool.

Fabry and Perot first discussed their interferometry concepts in 1898. Over the years, use of the concept has found a wide variety of applications. The availability of solid state sources and optical fiber has further broadened the potential number of applications for interferometry of all types. When considering optical fiber interferometry, one naturally tends to consider single mode operation. Coherent light sources tend to make for an easier conceptual design, and can offer advantages in sensitivity and resolution. Some examples of single mode fiber Fabry Perot interferometers are the work of Matsumoto who described an acoustic sensing diaphragm, and by Lee and Taylor who utilize an in-line fiber interferometer to measure temperature. The work carried out at NetriCor, however, utilizes a multimode Fabry-Perot resonator. This has a number of advantages which makes for a very cost effective trade-off. Features of a multimode interferometer include: * Efficient coupling from long lived LED sources. The mean time to failure for a common LED is approximately an order of magnitude higher for a laser. * Adequate sensitivity for most applications. For most industrial applications, the ultimate in sensitivity is not the predominant issue. * Readily available and inexpensive components. * Easy fiber termination. Although great strides have been made in both single mode and multimode fiber termination technology, the requirements on a multimode termination tend to be less stringent. * Inexpensive sensing element. The sensing element, as will be described later, can be made by 194 / SPIE Vol. 1267 Fiber Optic Sensors IV(1990) semiconductor techniques and produce inexpensively in large quantities. * Common readout. With the technique described, a large variety of sensors can be made to utilize the same and often interchangeable instrumentation.

A time division multiplexing technique is described in which the frequency of a laser source is modulated with a sampled ramp. The concept is demonstrated with two Mach-Zehnder interferometers in a parallel configuration, separated by a fibre delay line whose length is determined by the characteristics of the sampling waveform. Pseudo-heterodyne processing was used to recover the phase information from each interferometer. The expansion of the technique to a combined frequency and time division multiplexed fibre-optic sensing system is considered.

Intensity modulation, induced by a modified microbending pressure sensor is considered as a possible transduction mechanism for detecting environmental changes. Implementations of these kind of sensors into multiple sensor systems is based on simple and reliable technology with multimode fiber, couplers, a reference and a multiplexing technique. A frequency domain technique is presented to interrogate unambiguously time division multiplexed sensors. Line neutrality is obtained by a referencing method. Torsion experiments with multimode fibers in a microbend pressure transducer application, to improve the sensor attenuation, are reported.

A simple optical fibre sensor system based upon a two wavelength source is presented. The system may be operated in two modes: 1) a normal heterodyned interferometric sensor yielding nanometre resolution, and 2) a subcarrier system with much enhanced unambiguous range, but reduced resolution. Results indicate a displacement resolution of greater than 1mm, and vibration amplitude resolution of greater than 1Oim over a 22cm unambiguous range are achieveable. A major advantage of this system is that heterodyne signal processing has been achieved without the use of a frequency or phase shifter.

This paper discusses the potential use of polarisation selective phase modulators, PSPM, in interferometric fiber optical sensors. The principle of the PSPM is shortly reviewed. Two examples of the usefulness ofpolarisation selective phase modulation will be given. The first showing that PSPM introduces a possibility to monitor two measurands, simultaneously, with the same interferometer. In the second, a more detailed look at the consequences of using this concept in fiber optical gyros wiibe presented. A formula is derived describing the suppression ofnoise due to polarisation coupling in the gyro fiber loop. Using a special modulation and detection scheme, the PSPM shows to reduce the noise with the same order of magnitude as a polarizer.

A Fiber Optic BasedSmart Structure wiipossess a structurally integrated optical microsensor system for determining its state. This built-in sensor system should, in real-time, be able to: evaluate the strain or deformation of a structure, monitor if its vibrating or subject to excessive loads, check its temperature and warn of the appearance of any hot spots. In addition a Smart Structure should maintain a vigilant survelliance over its structural integrity. The successful development of Smart StructureTechnolgy could lead to: aircraft that are safer, lighter, more efficient, easier to maintain and to service; pipelines, pressure vessels and storage tanks that constantly monitor their structuralintegrity and immediately issue an alert ifany problem is detected; space platforms that check forpressure leaks, unwanted vibration, excess thermal buildup, and deviation from some preassigned shape.This technology is particularly appropriate for composite materials where internal damage generated by: impacts, manufacturing flaws, excessive loading or fatigue could be detected and assessed. In service monitoring of structural loads, especially in regions like wing roots of aircraft, could be ofconsiderable benefit in helping to avoid structural overdesign and reduce weight. Structurally imbedded optical fibers sensors might also serve to monitor the cure state of composite thermosets during their fabrication and thereby contribute to improved quality control of these products.

Optical probing and interferometric sensing techniques have great advantages of non-loading, high resolution and wide dynamic range for motion measurement of a solid object. For this purpose, laser Doppler velocimeter (LDV) is widely utilized. However, optical measurement of two dimensional (2D) motion is a hard problem for a conventional LDV, because the instrument has a big frame size and heavy wight, and requires stable measurement environment such as an optical bench or a rigid base. In this paper, we discuss a newly proposed 2D LDV with a compact remote sensing probe. Laser light is bidirectionally transmitted via two optical fibers from a base unit to the probe head. The system is designed to measure a specific solid object that moves in 2D plane such as an optical pickup head equipped in an optical data storage system. As testing of the system, 1D displacement, velocity and acceleration of a moving membrane of a loudspeaker and 2D motion of a moving optical pickup actuator were successfully measured.

Bi-Optical Probes have been developed by the French Atomic Energy Commission to perform measurements in liquid/gas or liquid/vapor mixtures encountered in the mock-up of a nuclear Pressurized Water Reactor's steam generator. The purpose of those measurements is to establish both void fraction and gas velocity maps used for the qualification of thermaihydraulic and flow-induced vibration computer codes.

Silicon micromechanical resonators have attracted a great deal of research interest because of their high sensitivity and the feature that the measurement of the parameter of interest is in the form of a frequency. The characteristics of optically excited and interrogated micromechanical resonators have been reported by a number of research groups but little information is available on the behaviour of these devices as sensors. In this paper, an all fibre optical system for the measurement of pressure is reported. Metal coated silicon microresonators have been excited by intensity modulated laser light delivered through an optical fibre. The vibration of the resonator is detected by an optical fibre interferometer. Optimum detection can be achieved by adjusting the distance between the end of the optical fibre and the surface of the silicon bridge. When used as a pressure sensor, the resonant frequency of the device was observed to change from 62kHz to 130kHz as the pressure varied from -0.6 bar to 1 bar (gauge).

The basic electro-optic and magneto-optic effects in monomode fibres are well known. The geometrical flexibility of the optical fibre allows these effects to be used in some novel configurations, for special practical applications.

This paper is devoted to the description of a prototype Emission/Detection Unit, the ACCORD® Module, developed for the accurate demodulation of fiber optic sensors using spectral modulation encoding techniques. This prototype contains a static polarimetric interferometer with 4 ports to provide the four phase signals needed by the passive homodyne scheme, and uses a dual-wavelength arrangement to extend the measurement range by removing the 2ic ambiguity in the phase determination. Resolution as high as A/40000 has been achieved with 1 Hz bandwith and Optical Path Difference mismatch in a 20 microns range.

The pressure sensor described in this paper is an optical adaptation of a high performance commercial pressure sensor that measures the resonant frequency of a double-ended quartz tuning fork as modified by the force applied. The shift in resonant frequency is used to determine the pressure. In this device the tines of the crystal are forced into transverse oscillation by an electrical microcircuit powered by a photodiode array illuminated by an LED via an optical fiber. The crystal oscillation frequency is determined using a second light beam from a separate LED also transmitted by a fiber. (The light beam is interrupted by the two oscillating tines, generating an AC signal at the crystal oscillation frequency.) Since the measured frequency is independent of intensity variations, the sensor is unaffected by light source degradation or by fiber bending. In a recently-developed prototype device, force is applied to the crystal via a balanced Bourdon tube assembly. Increasing the pressure inside the tubes increases the force exerted on the crystal. The prototype exhibits an accuracy, linearity, and stability of better than 0.1 psi over a 100 psi range when operated in air and without any temperature compensation. The sensor also shows good immunity to shock and vibration.