Table of contents

The development of optical fibre sensors for chemical species has attracted much interest. This review summarises the body of principles employed in this type of sensor and discusses the implementation of these principles in the design of such sensors and in the associated instrumentation. The use of this type of sensor in various fields of application is illustrated through a description of some representative examples. The trends that are observed, as well as the problems that are encountered, in the development of this type of sensor are also presented.

The basis of NMR imaging is examined in terms of water and mobile proton content in biological and other systems. Relaxation time effects are also discussed and their importance as additional spin parameters examined. The generalised principles of NMR imaging are discussed using the reciprocal space approach. The timing details of the common imaging methods, together with a number of image examples are presented. A discussion of image quality and imaging time is given. Other matters of practical importance in obtaining good quality images are also considered. Some general comments concerning present day imaging and future perspectives are presented.

A method of calculating field functions of circular coils by direct numerical integration is explained, and formulae for the integrands required are given. The method is not only more straightforward mathematically than classical methods, but is also faster. A known principle has been extended, enabling a comprehensive treatment to be presented. The method can be used for calculating both magnetic fields of current-carrying coils, and induced EMFS of sense coils: a complete set of the formulae involved is given. Ways of extending the method to the cases of sets of coils and finite coil volumes and finite field volumes are explained.

An abrasion machine using wiper blade action with a slurry of fine silica particles in water is described and the optimum conditions for producing uniform and reproducible damage over the rubbed area are discussed. The machine has been applied to assessing the durability of a range of optical coatings, examining the abraded films by a combination of techniques, including interference microscopy, profilometry, transmittance and scatter measurements. The method has been shown to provide information both on the adhesion of coatings and on their resistance to scratching and roughening.

The measurement of very small surface features is commonly and most conveniently undertaken by using stylus instruments. The authors bring together various strands of work relevant to their continued use and development. The limits of their performance are investigated and methods are described for calibrating them. The trend in 'nanotechnology' towards the direct exploitation of material properties at the atomic level is illustrated. It is demonstrated that controlled calibration of surface height into the sub-nanometre region can be achieved and that new techniques are rapidly improving the ability to measure surface angle.

A simple method was developed for making subminiature thermocouples for use as dewpoint psychrometers. The apparatus was constructed for entwining pairs of wires as thin as 12.5 mu m and welding them together with an electric spark in an inert gas. Dewpoint psychrometers consisting of chromel/constantan thermocouples, made by this means, were used for continuously monitoring water stress in growing plants.

The 'current integration' method is often used to detect small changes in the intensity of radiation when photon rates are too high for the use of conventional nucleonic counting equipment. When the measurement is made repeatedly in a cyclic fashion, as for example in Mossbauer spectroscopy, some simplifications can be made. A circuit using only a few integrated circuit components which successfully implements the method, and the conditions under which the simplifications are valid, are presented.

The authors describe a DC network based on the Warshawsky generalisation of a Wheatstone bridge to four-terminal resistance measurement which can automatically compare the values of a number of four-terminal resistors of the same nominal value between 1 Omega and 10⁴ Omega with an accuracy of better than 1 in 10⁸ when 1 mW is dissipated in them.

A method for improving the sensitivity of a bridge for the inductive detection of the superconducting critical temperature of thin films based on the screening properties of superconductors is described. In this method, the increase in the sensitivity of the bridge is not caused by the amplification of the unbalanced bridge signal, but by the physical phenomenon itself.

A method is proposed to implement an optical fibre sensor for the measurement of the refractive index of liquids whose optical attenuation may not be considered negligible. The fibres can be immersed directly in the liquid, or placed outside a transparent cell. Three mathematical models are developed for these two configurations. It is shown that the second configuration is more advantageous. Experimental measurements tend to confirm the validity of one of the three models.

An analysis has been carried out on both the degree of synchronisation and the effect of the air gap in Mertz's first field-compensation method. A new stigmatic field-compensating scanning method is proposed based on the result of the analysis. In this method, mirrors are plated on the back of the compensating wedges, the optics remain simple and stigmatic, and both the spherical and the chromatic aberrations are the same as those in Mertz's first method. In comparison with Mertz's first method, the new scanning method allows larger tolerances in the synchronised motions and hence these motions can be achieved without great difficulties. This new method is being applied to a scanning wide-angle Michelson interferometer.

A dynamic alignment system has been developed for a scanning wide-angle Michelson interferometer. The system is controlled by a computer in real time. It dynamically corrects the tilt of the moving mirror in the interferometer and keeps the two mirror surfaces parallel within 1.1*10^-6 rad, while the mirror moves 80 mm obliquely to the optical axis. This relatively simple system is very effective and can be applied to other conventional or wide-angle scanning Michelson interferometers.

A bolometer technique is described for measuring simultaneously the absolute intensity and the average molecular energy of molecular beams with an accuracy of the order of 5%. This is accomplished by varying the operation temperature of the bolometer only. Nozzle beams of He, Ar, N₂ and CH₄ are used for demonstrating the absence of perturbing deviations from complete molecular energy accommodation. This procedure is found to be very sensitive to small (1%) deviations from complete surface accommodation and could also be suitable for accurate measurements of the energy accommodation at other surfaces.

A design of inexpensive PVDF pyroelectric radiation detectors of large aperture and dynamic range is described. The spectral response ranges from the visible into the FIR region. Constructional and performance details are given.

The authors describe the construction of a simple spectrometer for photoconductive frequency-resolved spectroscopy (PCFRS) of semiconductors. The complexity and cost of conventional acousto-optic modulated laser systems is avoided by the novel use of inexpensive light-emitting diodes which can be modulated directly. Sample data of lifetime distributions for several semiconductors obtained with this spectrometer are presented.

The chromatic aberration of optical columns used to produce microfocused ion beams is examined in detail. Numerical solution of Poisson's equation and optimisation algorithms are used to ascertain the best possible design. The results show that, if the ultimate resolution of the column is limited by chromatic aberration, then microfocused spots of less than 20 nm diameter should be achievable with acceptable current densities for a number of technological applications.

A sophisticated flying spot scanner in which 2 kHz AC surface photovoltages induced in a Si wafer due to the chopped irradiation of a photon beam 0.4 mm in diameter are non-destructively measured through a 25 mu m air gap between the wafer surface and a transparent electrode is developed. Photovoltage distributions on a wafer 50 to 150 mm in diameter are measured in 5 min, and displayed as a 512*480 pixel black and white image. With the use of near infrared (peaking at 896 and 1076 nm) and blue (peaking at 448 nm) photon sources (cathode ray tubes), it is possible to measure crystal defect distribution in the bulk and interface trap distribution at the wafer surface separately. The apparatus is successfully applied to observations of swirl patterns and radiation damage induced by an electron beam in thermally oxidised p-type Si wafers.

Two methods are described which allow precise measurements of the ultrasonic absorption coefficient of liquids between about 5 and 400 MHz and 1 and 50 MHz, respectively. In both cases RF substitution techniques are applied to determine the attenuation of pulsed signals transmitted through the sample cell. In the former method a cell with variable spacing of the piezoelectric transmitter and receiver crystal is used and the measurements are automatically driven by a process control computer. The latter method is based on a cell of fixed pathlength, using a simple electronic set-up and a manual mode of operation. The construction of the cells is given in detail and the relative merits and limitations of the two methods are briefly discussed. Examples of acoustic absorption spectra are presented to demonstrate the accuracy of measurements.

A two-dimensional fibre-based laser Doppler velocimeter has been constructed where the orthogonal velocity distributions can be recovered either sequentially or simultaneously using a novel fibre-optic polarisation state control scheme. The scheme is described and the benefits of the two approaches are discussed. The instrument is demonstrated in a simple fluid flow experiment.

The design of an electronically switched flowmeter and temperature sensor employing a single thermistor probe is described. Digital temperature compensation is incorporated in the design. The concept is applicable to geophysical flows; in this case, design criteria for a tidal channel were used. The flowmeter is designed for use in the 0.05 to 0.5 m s⁴-¹ range with a fluid temperature variation of 2 degrees C. A frequency response of 0.25 Hz is obtained.

An opto-isolator using a fibre-optic cable is described designed for operation at voltages in excess of 30 kV DC used in CO₂ lasers where neither side of the supply may be earthed. The use of a fibre-optic isolator enables infinite isolation between the high voltage supply and the output which is monitored by a computer-based data logging system.

The authors report the development of a new opto-thermal radiometry technique, using wavelength-selective detection. The measurement principle is similar to that of opto-thermal transient emission radiometry (OTTER), but includes a grating monochromator as part of the signal detection system, to enable opto-thermal transients at selected infrared wavelengths to be studied. The technique offers the ability to separate the effects associated with the absorption of exciting light from those of infrared emission. Changes of opto-thermal decay time with emission wavelength can therefore be related to the infrared absorption spectrum of the sample. First observations on TiO₂ excited by UV laser light are presented.