Table of contents

A review of recent progress made in a range of selected fibre-optic sensors for physical measurands is presented, with particular attention paid to new developments, which have the potential to be exploited in a variety of wide applications.

A thin film measuring system for the online monitoring of semiconductor films during processing with a radio frequency plasma is described. The method is based upon the use of chromatic processing of polychromatic interference signals produced by the thin semiconductor film when illuminated with white light. A description is given of the interpretation of the polychromatic signals produced by interference effects and some typical experimental results are presented to illustrate the capabilities of the technique. The approach utilizes cost effective instrumentation with acceptable time response for real time process control via optical fibre transmission. It is currently being incorporated as part of an intelligent knowledge based system for such process control.

Two novel phase extraction algorithms, in which step sizes need not be known or equal, are described. One algorithm requires a minimum of five interferograms from which intensity offset and intensity modulation are calculated at each pixel and the relative phase is calculated for all pixels with respect to a reference pixel. The other algorithm requires two phase stepping mechanisms and a minimum of ten interferograms. The intensity characteristics and reference phase step are calculated at each pixel and a previously published algorithm is used for the final phase calculation.

A mathematical and physical description of a confocal Fabry-Perot interferometer used for the measurement of ultrasound is presented. It analyses the interferometer performance in terms of ultrasonic transfer functions derived for rays of types 1-4. Instrument performance is modelled for two different schemes of operation, namely transmission and reflection. The frequency response is derived from a combination of ray types. In the case of the reflection scheme, a new beam separation method is suggested, which makes full use of the instrument etendue. Additionally, this latter scheme offers a significant increase in sensitivity over a wide frequency range extending towards the free spectral range of the interferometer, typically exceeding 100 MHz.

Scanning near-field optical microscopy (SNOM) is a new technique which can resolve objects with sub-wavelength resolution. A convenient technique is to use a sharp fibre tip which detects an evanescent light field, the intensity of which depends exponentially on distance. The sharp tip is made by heating and stretching an optical fibre to draw it out into a fine tip, and subsequently etching the tip in hydrofluoric acid to reduce its radius of curvature to about 100 nm. Features as small as 5 nm in height and less than 100 nm in width have been observed with the fibre mounted in a novel head adapted for use with Digital Instruments Nanoscope II.

An optical vitamin C sensing probe has been studied based on immobilized 2,6-dichloroindophenol. A kinetic method is employed in the analytical measurement. This optical sensor is shown to respond linearly to vitamin C concentrations in the range 1.6-10 mM at pH 3.0. This linear range varies with pH and temperature, which should be controlled when using the sensor for analysis of vitamin C. A good analytical correlation has been demonstrated for the analysis of vitamin C in samples of orange juice.

Examination of various research efforts has revealed that optical techniques best satisfy the requirements of on-line, non-contact inspection; other methods are usually deficient with regard to speed, flexibility and ability to perform non-contact measurement. Of the optical methods considered, techniques based on statistical analysis of scattered light from the sample are best suited for the purpose of surface inspection. A general scheme for an on-line, non-contact, optically based inspection machine has been developed. The system has the important attributes of fast measurement, simplicity and flexibility. The system's roughness measurement capability was successfully tested on ceramic components of varying surface finish.

The results of an investigation into the use of polarized light for turbidity measurement are presented in this paper. Preliminary experiments were carried out using an ophthalmic slit lamp. Following this, a wider range of turbidity measurements were made using an optical bench-mounted ellipsometer employing a laser diode source. Results have shown that for Rayleigh particles, in addition to changes in scattering angle, different polarization states can be used to obtain a wide range, linear turbidity response. For Mie particles the dependence on polarization state is much less marked.

A new interferometer has been developed for absolute length measurement of length bars between 100 and 1500 mm in length. The interferometer uses a combination of three-wavelength interferometry and phase-stepping interferometry to measure the length, flatness and mutual parallelism of the measuring faces, and thermal expansion of the bars, with direct traceability to realizations of the metre and the kelvin. The interferometer is operated inside a sealed chamber to minimize the effects of refractive index variations. The design and operation of the interferometer are presented, with representative results of length, flatness and parallelism and thermal expansivity measurements. The instrument has an estimated uncertainty of length measurement (at 95% confidence level) of +or-30 nm+or-6.2*10^-8 L, where L is the length of the bar.

Modulation of the emission wavelength of a laser diode is demonstrated as a heterodyning technique for measuring the phase and amplitude of a vibrating object using continuous wave (cw) illumination in a time-averaged electronic speckle pattern interferometer. The technique is demonstrated with an interferometer constructed from single mode optical fibre.

A microprocessor-based electronic system has been designed to detect the position of a black/white transition line viewed by a conventional dosed circuit television monochrome camera. The system examines the video signal for sharp changes in level, corresponding to sudden changes in brightness, and generates a digital output. The detection circuitry works at video rates. The system has been used with the principle of optical sectioning to profile a variety of surfaces, and some preliminary results are presented.

Optical metrology of holographic images, otherwise known as 'hologrammetry', offers the ability to produce in real space a full-scale, three-dimensional image of a recorded structure, which displays sub-millimetre accuracy in both detail and spatial proportions. In an effort to develop this potential for underwater applications, we have been studying the optical aspects of underwater holographic imaging, in particular those aberrations associated with reconstruction of the conjugate real image in air. A similar problem is encountered in conventional underwater optical systems, where a variety of measures have been adopted to limit the effects, particularly on off-axis image fidelity, of refraction and dispersion at the air/water interface. These include use of compensated lens elements in recording or calibration factors in image processing, or some combination of both. If the advantages of underwater holography are to be fully realized these difficulties must be addressed and, if possible, resolved in a context appropriate to holography. In this paper, we have employed both geometrical optics and analytic approaches to assess the extent of the problem and to evaluate a possible solution. By employing third-order numerical methods, we obtain simple analytic expressions for the aberrations encountered when the conjugate real image is replayed in air, and illustrate that a substantial restoration of image fidelity can be realized by introducing a prescribed wavelength change between recording and replay. Finally, we review practical implementation of the technique and outline the conditions that must be satisfied before residual aberrations can be reduced to tolerable levels.

Step index, all-silica UV fibres may have an important role to play in the evolution of new spectroscopic surface techniques such as resonant laser ablation. The intrinsic and extrinsic properties of silica fibres are initially discussed in this paper since the question of their acceptability requires a knowledge of the relative advantages and disadvantages of this type of fibre within high-power laser-based systems. The main characteristics can be clearly established within the required operating ranges of wavelength and pulse energy by measuring the transmission properties of 2 m lengths of fibre using several types of pulsed laser. The results of pulsed UV laser beam transmission measurements are reported as a function of the pulse energy and wavelength. The corresponding attenuation values (dB m^-1) were modelled using equations based on UV absorption and Rayleigh scattering mechanisms. The beam attenuation was found to be sufficiently low in short lengths of silica fibre to satisfy the threshold energy requirements of resonant ablation spectroscopy experiments. As a result, these fibres will now be incorporated into a prototype laser ablation system.

The non-contact excitation of resonant flexural oscillations in structures of significant size, using repetitive pulses from a low-power diode laser, is reported. These vibrations are produced by photothermally induced bending moments which are of sufficient amplitude to produce deflections which are readily measured using a simple optical fibre interferometer. The interferometer uses the reflected signal from the natural surface of the structure and requires only that the surface be aligned approximately perpendicular to the interferometer beam. Applications of this technique for non-contact, non-destructive testing for changes in properties such as size, shape and structural integrity are discussed.

A multivibrating system whose oscillatory frequency is dependent on temperature has been developed. The system utilizes a segment of a liquid crystal display (LCD) in a novel closed loop configuration which produces spontaneous oscillation. The probe is addressed and energized completely optically, and a very low-power budget is required for operation. The system is under full computer control, which leads to a greater degree of control, and the ability to test a wider range of models which describe the behaviour of the liquid crystals in the system.

A prototype single-mode fibre-based Fizeau interferometric medical pressure and temperature sensor system using coherence reading signal recovery is demonstrated. These sensors have been designed in such a way that they have similar optical path differences and hence problems associated with multiplexing are greatly reduced. For the pressure sensor, a range to resolution ratio of about 6.7*10³:1 and an overall measurement accuracy of better than +or-1% over a pressure range of 0-360 mmHg have been achieved. For the temperature sensor, a measurement resolution of 0.006 degrees C and a 1% span linearity over a temperature range of 27.3-62.5 degrees C have been achieved. The performance of both sensors meets or exceeds established medical requirements, and the dimensions of the probes are compatible with commercial electric probes. Hence this new sensor system is expected to be a practical approach for many medical applications.

An instrument has been developed for measuring integrated scattering (TIS) of low-scatter optical surfaces. In the case of TIS measurement using an Ulbricht sphere, Rayleigh scattering on air was found to be the limiting factor in improving the resolution limit of the instrument. Measures for the suppression of Rayleigh scattering are described. The achieved resolution limit for the integrated scattering in the reflection hemisphere is 0.1 ppm (1 ppm=10^-6) of the incident light intensity. This improved resolution offers the capability of measuring surface roughness induced scatter losses of high quality multilayer-coated optics and even uncoated transparent 'superpolished' fused quartz substrates. The result is given of a developed theoretical model describing the straylight emission of the two boundaries of a transparent substrate. Scatter measurement results from uncoated 'superpolished' substrates are presented, showing scatter losses as small as 0.6 ppm corresponding to 0.1 nm rms surface roughness.

An optical sensor has been developed and used to measure ranges of up to 20 m with an accuracy of a few centimetres. The frequency modulated continuous wave (FMCW) technique was employed in conjunction with an intensity modulated laser diode and optical fibres.