With increasing competition in the manufacturing industries product quality is becoming even more important. The shortcomings of human inspectors in many applications are well know, however, the eye/brain combination is very powerful and difficult to replace. At best, any system only simulates a small subset of the human's operations. The economic justification for installing automatic inspection is often difficult without previous applications experience. It therefore calls for confidence and long-term vision by those making the decisions. Over the last ten years the use of such systems has increased as the technology involved has matured and the risks have diminished. There is now a complete spectrum of industrial applications from simple, low-cost systems using standard sensors and computer hardware to the higher cost, custom-designed systems using novel sensors and processing hardware. The underlying growth in enabling technology has been in many areas; sensors and sensing techniques, signal processing and data processing have all moved forward rapidly. This paper will examine the currrent state of automatic inspection and look to the future. The use of expert systems is an obvious candidate. Parallel processing, giving massive increases in the speed of data reduction, is also likely to play a major role in future systems.
Distributed optical-fibre measurement sensors offer many advantages for industrial use. Most systems which have been studied so far rely on OTDR techniques, where the spatial information on the measurand distribution is obtained via the time resolution of the light backscattered from a pulse propagating in the fibre. An alternative approach is to extract the spatial information from forward-scattered light by allowing counter-propagating light waves to interact non-linearly in the fibre. This might occur through pulse/CW or pulse/pulse interactions. This approach offers possibilities for significant improvements in spatial resolution and sensitivity for these systems.
We describe a number of multiplexing techniques which may be used for fibre optic sensors in which the sensing element is a ring resonator. The basic optical unit comprises a sensing ring resonator connected to a second interrogating ring resonator of nearly equal length, and illuminated by a source of short coherence length. Using such an arrangement, the high finesse of the ring resonator may be exploited to yield a sensor of increased sensitivity over that obtainable with a two-beam interferometer. A number of such units may be combined to form an array, using coherence multiplexing techniques. When a high coherence source is used, then multiplexing may be accomplished using frequency division techniques. However, in this case, the resonators must have low finesse, in order to avoid cross-talk. We also show that when a source of moderate coherence length is used, it is possible to use both frequency division and coherence multiplexing simultaneously, with the potential of realising a large sensor array.
A simple system for multiplexing frequency modulated sensors has been demonstrated and its performance evaluated. The multiplexing scheme is based on spread spectrum modulation of the optical source exciting the sensor network. A pseudo-random binary sequence generated from a digital shift register, is used to modulate the source. This sequence has the feature that its normalized autocorrelation has a value of 1 at zero delay and a constant value of -1/N (where N is the code length) at other delays. A demonstration system consisting of a single mode fibre network of three sensors and the associated electronics has been built. The network is illuminated by a laser diode intensity modulated by 255 bit PN sequence. An analog correlator is used to correlate the signal received from the network with a reference sequence. By altering the delay of the reference sequence it is possible to tune to any specific sensor in the network, and suppress the signals from unwanted sensors. Experimental measurements have shown that the responses from the sensors can be suppressed by up to 30 dB.
A Fiber-Optic Interferometric Sensor with multiplexing capability and noise free behaviour of the leading fiber is presented. The FOIS is based on a differential scheme, where both the reference and the sensing beam are guided by the same polarization maintaining fiber. A microprobe realized with-stacked optics technology permits the measurements in harsh environment. A full comparison of the noise-rejection capability of the realized FOIS versus a conventional ones is made. The experimental results confirm the high-noise rejection features of the developed FOIS.
The application of optical sensor systems in a harsh industrial environment calls for small and rugged optical components. The emerging availability of integrated. optical circuits for distribution, modulation or switching of the sensing light beam may lead to the necessary miniaturization and mechanical stability of the system. The integrated optis technology in combination with fiber optic sensors increases the possibility of advanCed architectures for potentially low cost devices when mass produced. An overview is given with concern to some design criteria, materials and fabrication principles for waveguiding structures for integrated optical sensors.
Integrated-optic wavelength division multiplexing (WDM) is commonly used in communication systems. These WDM-devices are also well suited to build up optical fiber networks for both intensity and interferometric sensor types. The operation principle of our wavelength division multiplexing devise is based on the wavelength dependent two-mode interference in a two-moded waveguide, which is coupled adiabatically to the single-mode input and output strip waveguides. The single-mode input and output waveguides are connected via two Y-branches ( "'kJ- 1° branching angle ) with a two-moded intersection region. The ratio of the light powers in the single-mode output waveguides depends on wavelength . The two-mode interference within the two-moded center waveguide leads to an almost wavelength periodic transmission caracteristic . Dual-channel multiplexers/demultiplexers were fabricated by a charge controlled field assisted pottasium exchange in B-270 glass (Desag). The devices have a typical channel separation of 30 - 40 nm and a far-end crosstalk attenuation of better than 16 dB. The operation wavelength regions of the fabricated devices are 0.6 - 0.8 µm and 1.3 - 1.6 µm, respectively.
Pigtailed Ti:LiNbO3 multifunctional integrated optical circuits for use in fiber gyros have been developed and tested under environmental conditions. Emphasis has been placed on reliability of the fiber-to-wave-guide attachment.
Glass seals may be essential parts for fibre optic sensors, influencing their efficien cy, handling or reliabdlity. Housings for light emitting or light detecting elements with sealed in windows or lenses determine the light coupling to the fibres. The solder glass sealing technique is of increasing importance. Hermetic seals of fibres to metal tubes are also realised with solder glass. The technique is described and possibilities are mentioned to vary geometries and fibre end conditions.
A method for fabricating special fibre, which offers considerable flexibility in both fibre structure and material, is described. The technique consists essentially of selecting borosilicate glasses from a wide range of available material, cutting and polishing rectangular blocks, arranging the blocks to provide any desired combination and configuration of guides and then heating and pulling down the stack to fibre dimensions. A number of possible fibre configurations are described, including couplers, polarization holding and the possibility of developing composite structures. One particular fibre structure, the single fibre Mach-Zehnder interferometer, is described in more detail, as an illustration of the potential of the technique. Some results are given of the operation of the interferometer as a sensor, using a laser source. Some of the difficulties of interpreting the output signal from this form of operation, and the advantages of incorporating a broad band source are considered. It is shown that a spectral analysis of the broad band output can provide an unambiguous interpretation of the state of the interferometer, and hence the measurand.
A fiber consisting of a quartz capillary with a single mode waveguide buried in the wall of the tube is made and some of its optical properties are measured. The outer diameter of the capillary is 110 µm and the inner diameter is 50 µm. The measured changes of the phase difference between the two polarization modes in two fabricated fibers were 3.1 and 3.2 rad/bar.m versus pressure and 0.17 and 0.13 rad/°C.m versus temperature. The beatlengths were 62 and 81 mm at 0.63 µm.
In the paper we are looking for these properties of optical fibers which cannot be obtained easily with classical CVD, VAD, PSL, DC and RiT fabrication methods. These properties are: - fibers with minimized environmental sensitivity, - fibers appropriate for embedding, - specially coated fibers and sensitized, - fibers suitable for smart structures and skins, - fibers with angular dependence of refractive properties against long axis, i.e. fibers with no refractive axial symmetry, - fibers with helical cores and with noncircular cores, - multicore optical fibers. We have developed a completely new manufacturing method to obtain comparatively easy most of these abovementioned fibers. The method consists of two main stages: preform preparation and fiber pulling. The preform preparation process consists of two substages: cold and hot. Cold process involves preparing of mo-dular glass rods and whole preform assembling. Hot process involves (optional) computer controlled high temperature diffusion process and consolidation of the preform in an isostatic computer controlled press. We are going to present in the paper some detailes concerning the newely developed mosaic-assembling--defibering (MAD) technology of sensor-oriented optical fibers and sensing imageguides. We will also show some examples of manufactured unique multimode and singlemode optical fibers for sensor applications. The mosaic-assembling-defibering technology of sensing fiberoptic imageguides is subject to patent claim.
Linear fiber optic long stroke displacement transducers would find many applications in process control, especially because of their immunity to electromagnetic interference, but so far no practical solutions have come up. In order to develop a better understanding of the underlying problems, a top down evaluation of published sensor ideas was carried out, followed by an experimental study of some key aspects of three different concepts. The well known problems in qualifying fiber optic sensors for the field were encountered once again.
A reflective encoder disc is illuminated by the light from a continuously forward biased light emitting diode (LED). As the disk rotates so the level of light coupled back into the LED itself is modulated. By monitoring the LED terminal voltage, the modulation, and hence the motion of the disc itself may be detected. Measurements of optical detection sensitivity in forward biased LEDs and also in silicon photodiodes are presented to illustrate the competing photo-and thermal-detection effects. At low bias currents incident light and voltage change are in phase. at high currents in antiphase. A fiber-connected self-detecting opto-coupler and tachometer based on these effects is described with a signal to noise ratio of 56dB in 10kHz bandwidth.
In view of the application of micromechanical resonators as sensor elements investigations concerning their excitation and interrogation using optical fibers have been performed. It has been shown that excitation by broad-band or white-noise optical power spectra provides systems response that can be well evaluated in the frequency domain. This opens up new possibilities for fiber-optic sensor networking.
The long-term stability of intensity modulated fiber optic sensors under changing environmental conditions is limited by many factors. Introducing the spectral symmetric referencing method it is shown that even linear spectral variations of down-lead damping can be compensated. In combination with a simple modulator consisting of a spectral filter with locally varying density a novel position sensor has been realized. This design reduces the error caused by down-lead sensitivity to 2 %.
A high dynamic range, short response time distributed fiber sensor for control and surveyance applications is presented. It uses the differential mode delay between the LP01 and LP11 modes of a conventional singlemode communication fiber used at 800 nm wavelength. Baseband readout of the relative phase and amplitude delivers information on the location of an event and on the magnitude of the measurand activating a microbending transducer which couples the originally excited dominant mode to the LP11 mode. The spatial separation and focalisation of the two modes can be performed with a 40 dB isolation by means of a monolithic microoptic component acting on the evanescent fields. A dedicated vector voltmeter with 0.1° phase accuracy at 10 MHz provides a transitometry spatial resolution of the order of 10 meters.
An optical fiber vibration sensor designed for monitoring a fixed vibration frequency (in particular 100 Hz) is presented. The device utilizes multimode PCS optical fibers and graded-index microlenses in order to detect cantilever displacements induced by vibrations. A suitable signal processing provides a down-lead insensitive response. Design details of the optoelectronic system and experimental results are presented.
A number of fiber optic methods has been developed for the inspection of critical components of mechanical structures. For inspection from a remote location various methods have been developed for the detection of cracks and strain. Some of these monitoring methods use a fiber mesh or OTDR techniques for distributed measurement of strain or to locate cracks. Fiber optic methods for non-contact ultrasonic inspection require other techniques, e.g. a pulsed laser with fiber optic delay lines and velocity sensitive interferometers.
The accuracy of photoelastic modulated optical fibre sensors suffer from temperature-dependency. Two designs have been developed, aimed at eliminating this deficiency. The evolution of the designs is explained and preliminary results reported.
We describe the development of a commercially available fibre optic disposable pressure transducer. The principle of operation of the device is described, and results presented which show an accuracy of a few percent. A comparison is made with conventional silicon diaphragm disposable pressure transducers.
Two different types of fiber optic sensors for the detection of human heart beats in the harsh electro magnetic environment of a Magnetic Resonance (MR) scanner have been developed. The first type is a photo elastic strain gauge belt sensor, which originally has been developed for the detection of respiratory motion. This sensor has been applied around the thigh in order to detect the pulse of the thigh artery. The second type is a fiberoptic version of a photo plethysmograph detecting the optical transmission of human tissue. A transmissive and a reflective type of this sensor have been tested by application to ear lobes, finger and toe tips respectively.
The use, on the one hand, of the property that light power attenuation occurs in a micro-deformed optical fibre, on the other hand, an other fibre or the fibre themselves happensit, had introduced some new surface pressure sensors (1). In first, we examine different configurations, and then the sensor behaviour is expanded and used for the load sail's sensor conception. Then we consider the opportunerress to use this sensor to the measure of strain in composite material. At last the future development are envisaged.
A simple, low cost optical fiber sensor has been developed in order to compare the temperature value of a medium with a well-defined temperature. The thermosensitive element is a paraffin because of its lack of toxicity in medical applications. The basic physical phenomenon involved in this device is the great variation of the scattering coefficient of paraffins when the temperature is going through the solid-liquid transition. Dielectric configuration and low cost due to plastic fiber are great advantages for disposable devices. So, radio-frequency heater can be used without troubles and blood contamination disappears because of single use.
A number of distributed optical fiber temperature sensors are requested to be only activated where and when the temperature field gets near to, and exceeds a threshold value set to be the alarm value. The proposed system uses the microbending loss effect in a graded index multimode fiber with OTDR readout. The transducing mechanism is based on a continuous bimorphous metal coating that is periodically and partially weakend so that temperature increase induces a microbending at the prescribed modal beat period. The pair of metals, as well as the coating process must be selected in order to place the maximum sensitivity in the neighbourhood of the alarm temperature. The sensitivity can be adjusted to the space-time fea-tures of the events to be detected, eg, semi-local hot spots or a heat source diffusing in an unbounded volume.
This short review presents an outline of recent work on optical-fibre-based sensors for the detection of gases. Emphasis will be placed on aspects related to the detection of flammable gases in general and of methane in particular, in view of the importance of explosion hazards in many industrial workplaces, particularly from naturally occuring methane gas.
The use of diode lasers in fibre optic gas sensors have been demonstrated and it is shown that instrumental drifts can be much reduced or totally eliminated to provide noise limited accuracy. The sensitivities obtained with these diode laser - fibre optic gas sensors are much higher than the sensitivity of the fibre-optic gas sensors utilising broadband radiation sources. The sensors are capable of detecting absorption directly or the photoacoustic signal produced in a resonant cell using a Mach-Zehnder fibre interferometer. For gases with strong absorptions in the near infrared, like HF, ppb level sensitivities have been obtained. Finally the ability of the instrument to make multicomponent gas analysis has been demonstrated.
We have developped a new kind of all-optical device for measuring electrics fields, based on electric field induced second optical harmonic generation in optically nonlinear polymers (polydia cetylenes). Owing to their centrosymmetry these polymers do not generate spontaneously second optical harmonic. When an external electric field Eo is applied, the centrosymmetry is broken, resulting in second harmonic generation proportional to Et Ieb) . This second harmonic is used as a measure of the field E0. The choice of the polydiacetylene (4BCMU blue form) and the wavelength (1.32 em) is dictated by the large value of x(3) (24);(*),u),0) at this wavelength and low absorption of the second harmonic. The beam of a multimode Q-switch Nd:YAG laser (pulse energy 0.3 mJ duration 1 kis frequency 1 kHz) is focused and guided in a multimode optical fibre (50/125). The film, having a thickness close to the coherence length (C21 em), is deposited under vacuum on a thin silica slab (0.1 mm). This slab is illuminated at normal incidence by the laser radiation emerging from the fibre. Another 50/125 fibre, aligned with the previous one, guides the second harmonic generated in the film. This harmonic is detected by a photomultiplier. As all the elements of the sensor are insulator, there is only a slight distorsion of the electric field. In addition precise field maps are available, owing to the small volume sensor (ne1x1x1 mm3). Measurements performed at 50 Hz show that Iwvaries exactly as E 2 0 . It is possible to measure fields down to 0.5 kV/cm.
Optically energised microresonant transducers offer a potentially simple and precise means for implementing fibre optic measurements. In this paper, the principles of such transducers are briefly reviewed. This is followed by a summary of the experimental data and theoretical insights gained over the past few years in applying such transducers to the measurement of pressure and temperature. Factors such as the condition for self-excitation, the transient responses, materials characteristics and systems implications are briefly discussed, in addition to the direct characterisation of experimental sensors. The paper demonstrates that this approach to fibre optic measurements is compatible with simple remotely energised, relatively high precision measurement applications and, furthermore, that the technique can be readily applied to multiple sensor arrays and systems.
The problem of supplying power to optical and in particular fiber-optical sensors may be solved by generating electrical energy locally. This idea is illustrated here with several optical sensors, taken primarily from mechanical measurement applications. Incremental rotary encoders for transducing shaft position in rotating machinery are described which have a mechanical shaft input and 50/125μm fiber-optical signal outputs. They combine a DC generator or AC alternator with an internal fiber coupled optical emitter, allowing self-contained operation, independant of dedicated optical or electrical supplies. Incremental pulse generation is effected either by an encoder disc, or directly from the voltage periodicity of the AC alternator output. Applications include machine control, flow measurement, and limit detection in electrically noisy environments. A second class of sensors is demonstrated, transient optical sensors, which generate an optical output only for a short duration on activation of a particular transducer. With appropriate local intelligence in the form of low-power electronics, and suitable codes and transmission protocols, several sensor heads may be multiplexed onto a common optical bus.
A fibre-optic-system ist described, which ist readily adaptable within 12 Bit closed-loop-controlled machines. The system consists of a standard module which accepts optical as well as electrical sensing heads. The module autoranges and autocalibrates itself as a sensor is plegged-in, reducing installation time of the system to an absolute minimum. Preliminary results as gained during the verification phase of the system are presented.
Two of the main practical difficulties met by fibre point sensors where the measurand acts on the optical signal by means of a membrane or a cantilever are on one hand the thermal and mechanical stability as well as the vulnerability, and, on the other hand, the optical power referencing. Both problems have been solved here by designing a monolithic low order reflection Fabry-Perot microstructure using fibre tip dielectric and metallic mirrors. The suitably designed optical transfer function of the Fabry-Perot can easily be separated from the transfer function of the multimode fibre lead within the optical bandwidth of a cheap light emitting diode. This approach lends itself to batch fabrication and assembling with sensitivity and dynamic range adjustable by controlling the geometrical and technological parameters of planar photolithographic techniques and etching processes. A pressure sensor with 128 resolvable points within a 30 nm spectral width centered at 830 nm is described.
A system of multimode fiber optic sensors is described for the remote measurement of position, angle, force, pressure and other measurands that can be converted into a rotation of polarization. A birefringent filter encodes the polarization angle into the power ratio of two interleaved comb spectra or, in a modified implementation, into the absolute spectral position of a comb spectrum. By using identical filters in all transducers and in the evaluation unit, transducers for the same or different measurands become interchange-able. All sensors are of the incremental type, with accuracies reaching 0.5 % of one period of the measurand, independent of variations in the attenuation of the fiber link of up to 20dB.
Optical fibre resonant rings based on polarisation sensitive couplers give rise to a set of new resonant configurations. Two of them exhibit interesting sensor applications since small changes of the polarisation state produce large effects on the output, due to the resonant characteristics of the system. The sensor arrangements proposed have the advantage of requiring no drift compensation of the optical fibre interferometer against environmental fluctuations, since the parameters used to determine the measurand are the finesse and the visibility but not a phase change. Thus, the system can be used to sense low frequency measurands. Among the particular sensor applications that can be suggested, we shall point out the detection of magnetic fields through small Faraday effects.
The recent developments in the field of high-power semiconductor lasers open a broad area of applications, medical, measurement developments, pumping of solidstate lasers and furthermore. For almost all of these applications it is necessary to collimate and handle the laser beam power by optical elements. On the other side high power laser diodes are mostly gain-guided multi-stripe-arrays exhibiting wavefront aberrations which are a function of drive current, temperature, optical window in the package and even might differ from sample to sample. In order to correct for these aberrations and get an optimized optical system not only the beam intensity distribution has to be known, but also the relative phase of the wavefront in comparison to an ideal spherical wave. A fiberoptic interferometer is described which measures the relative phase of the radiation field locally with high resolution. Summing up the data and assuming a rectangular symmetry the complete wavefront can be reconstructed.