Table of contents

The special feature in this issue is a response to recent breakthroughs in the technology of sources of ultra-short electromagnetic pulses; such pulses can now be delivered at the limits of coherence over a wide frequency range. Studies of ultra-short electromagnetic pulses and their interaction with matter are attracting increasing interest in many areas of science and technology because of the inherent potential of such pulses to reveal transient features of physical phenomena and probe peculiar electromagnetic and physical properties of all kinds of matter. The laser is widely involved in generating or detecting ultra-short electromagnetic pulses. These developments have consequences for the instrumentation of measurements, as scientists endeavour to implement new methods and analytical approaches in the difficult challenge of modelling the dynamics of ultra-fast physical phenomena.

The aim of this feature is to provide the reader with a comprehensive overview of the current state of research and to stimulate interest in implementing new ultra-short pulse generators and device technologies in order to study more deeply the rich field of ultra-short wideband electromagnetic pulse technology. This issue includes articles exploring a `new frontier in physics' and contributed articles on broadband aspects of the field.

Thus, we report new developments and trends in the following areas:

• ultra-wideband short pulse electromagnetic waves in the microwave range of frequency, developed in Section I;

• ultra-short laser pulse technology and coherent beam interaction with matter, developed in Section II;

• ultra-short high-intensity laser and plasma interactions and transient x-ray laser pulses, developed in Section III;

• ultra-short laser pulses inducing the emission of coherent soft x-ray beams as a result of high order harmonic generation or high contrast energy modulation, developed in Section IV;

• the temporal aspect in synchrotron radiation sources and applications, developed in Section V;

• methods of time-resolved spectroscopy applied to measurements of ultra-short electromagnetic pulses and diagnostics of dynamical phenomena, developed in Section VI.

The techniques for generating, manipulating and detecting picosecond and sub-picosecond electromagnetic pulses have been documented extensively in the literature [1-6]. To measure an ultra-short electromagnetic pulse in the picosecond (or sub-picosecond) time-scale, an electromagnetic source with a high repetition rate is required (such as a laser source or eventually a synchrotron radiation source). When this condition is fulfilled, a pump-probe optoelectronic sampling measurement method can be applied. The relatively fast detectors used for infrared and visible ultra-short pulses of radiation generally have a weak sensitivity for x-ray detection. This is because of the fast response of the photocarrier generation and the weak x-ray absorption of semiconductors. In the VUV and x-ray domain, the main system fordetecting picosecond to femtosecond signals is generally a streak camera [7, 8] photoswitched by a synchronized femtosecond laser pulse (see Section VI).

These researches and experiments on pulsed electromagnetic radiation interacting with matter open new domains to theoretical research and technological development [9], promising previously unsuspected knowledge about the universe. The generation, detection and measurement of transient electromagnetic waves, i.e. short pulses, are of considerable interest to experimental studies of interaction with various media and modern information and communications technologies. Numerous applications are found in the field of wideband radar echoes, measurements of electromagnetic properties of materials and time-resolved spectroscopy (e.g. x-ray diffraction, reflection and absorption, etc). New measuring techniques and data processing methods have been and are being developed and are, in fact, indispensable for analysing the detailed features of the real-time results delivered by high power lasers, wideband impulse radar and x-ray beamlines. The propagation of a transient wave packet through plasmas, for instance the ionosphere, condensed matter (obtained by inertial compression) and biological aerosol streams, is now of widespread interest.

References

[1] Eloy J-F 2001 Progress in Ultra-Short Electromagnetic Pulses Technology. A New Frontier in Physics (London: Hermes-Penton)

[2] Elton R C (ed) 1989 X-Ray Laser (New York: Academic)

[3] Kim K-J, Chattopadhyay S and Shank C V 1994 Generation of femtosecond X-rays by 90° Thomson scatteringNucl. Instrum. Methods Phys. Res. A 341 351-4

[4] Klisnick A et al 2000 Generation of a transient short pulse X-ray laser using a travelling-wave sub-ps pump pulse 7th Int. Conf. on X-ray Lasers, St Malo, France, 18-23 June

[5] Lappas D and L'Huillier A 1998 Generation of attosecond XUV pulses in strong laser-atom interactions Phys. Rev. A 58 4140-6

[6] Smith P R, Auston D H and Nuss M C 1988 Subpicosecond photoconducting dipole antennas IEEE J. Quantum Electron.QE-24 255-60

[7] Knox W, Nordlund T M and Mourou G 1982 Jitter-free streak camera system Picosecond Phenomena III, Proc. 3rd. Int. Conf. on Picosecond Phenomena ed K B Eisenthal \etal (Berlin: Springer) pp 98-100

[8] Watanabe M, Koishi M, Kan H, Kaufmann K J and Tsuchiya Y 1990 Picosecond fluorescence microscopy Picosecond and Femtosecond Spectroscopy from Laboratory to Real World ed K A Nelson Proc. SPIE'90, Los Angeles, 16-17 July vol. 1209, pp 157-64

[9] Svanberg S 1998 High-power lasers and their applications Adv. Quantum Chem.30 209-33

This paper reports on a free-space broadband terahertz (THz) spectroscopy technique with frequency ranging from 30 GHz to over 40 THz. A historical review is given on the development of THz spectroscopy and its unique features. We focus on a time-domain detection technique using free-space electro-optic sampling. This broadband THz spectroscopy technique opens the door to many interesting applications. We present some recent developments in applications of THz spectroscopy, including characterization of the optical properties of materials, study of the dynamics of phonon and photocarriers and THz imaging. This THz time-domain technique continues to attract interest in both fundamental research and real-world applications.

The objective of this paper is to present a methodology for the analysis of electromagnetic systems irradiated by an ultra-short ultra-wideband electromagnetic pulse. This is accomplished through the use of a hybrid method that involves simultaneous generation of early time and low frequency information. These two data sets, namely early time and low frequency, in the original and transform domains are not only easy to generate, but also contain all the necessary mutually complementary information to electromagnetically characterize the system. This assumes that a sufficient record length is available in both domains. A criterion is provided to assess whether the record lengths are sufficiently long. Utilizing orthogonal associate Hermite functions, a time domain signal representing the electromagnetic quantity of interest (be it current or the scattered electromagnetic fields) can be expressed as a weighted sum of these quantities in an efficient way. The associate Hermite functions are orthonormal and are expressed through the Hermite polynomials. The frequency domain response can also be characterized by the same set of functions weighted by the same sets of coefficients, as the Hermite polynomials are the eigenfunctions of the Fourier transform operator. The available data in both domains are then simultaneously used to solve for the unknown weights. Once these coefficients are known the data can be simultaneously extrapolated in both the time and frequency domains. Computational load for the electromagnetic analysis for this method is quite modest compared to solving the electromagnetic analysis problem exclusively in either domain. Numerical examples are presented to illustrate the application of this hybrid methodology.

Since the beginning of the 1990s the generation of high-intensity laser pulses has known an unprecedented evolution thanks to the conjunction of the possibility of the chirped pulse technique and the availability of spectrally broad-band laser media. Lasers capable of producing multi-terawatt pulses can now be built on few optical tables in a small laboratory. We review the generation and the amplification of ultra-short pulses by the chirped pulse amplification technique.

Ultrashort laser pulses are finding increasing applications in biology and medicine. Such pulses can be used directly or after non-linear modification. Direct utilization includes propagation studies in scattering media with applications in optical mammography, dosimetry for photodynamic therapy and species concentration assessment. Intense continua of electromagnetic radiation of very brief duration are formed in the interaction of focused ultrashort and intense laser pulses with matter. Two different kinds of experiment using such radiation are described, employing visible radiation and x-rays, respectively. Focusing into water leads to the generation of a light continuum through self-phase modulation. The propagation of the light through tissue was studied, addressing questions related to specific tissue chromophore absorption. When terawatt laser pulses are focused onto a solid target with high nuclear charge Z, intense x-ray radiation of few ps duration and with energies exceeding hundreds of keV is emitted. Biomedical applications of this radiation are described, including differential absorption and gated-viewing imaging.

Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.

The development of high-energy, high-power ultra-short laser pulses in the 1 TW to 1 PW range has initiated a number of studies on the mechanisms of interaction between a laser and a plasma at very high (relativistic) intensities. In this new regime, a large fraction of the laser energy is transferred to ultra-short bunches of collimated relativistic electrons. Protons or heavier ions dragged by these electron jets can also be accelerated inside as well as outside the target. Moreover, the interaction of these electrons with various types of materials gives rise to powerful sources of x-rays and γ-rays and to a large number of nuclear reactions. These interactions can result in the emission of neutrons and positrons and in the production of nuclear isotopes and other types of particles.

We review the production of high-energy protons and neutrons in the interaction of intense lasers with gases, clusters and solid targets. Some potential applications are briefly discussed.

We summarize the progress achieved at LSAI in the understanding and optimization of the transient collisional pumping of x-ray lasers using an ultra-short picosecond heating pulse. A strong ×300-400 enhancement of the 13.9 nm Ni-like 4d-4p lasing emission is obtained when a travelling-wave short pulse irradiates the preformed plasma. The temporal history of the 13.9 nm laser pulse is measured with a high-resolution streak camera. A very short, 2 ps x-ray laser pulse is directly demonstrated for the first time.

The generation of high-order harmonics of intense laser pulses focused in gases is an intriguing and spectacular process: nonlinear orders as high as 300 have been reported recently. In addition to its fundamental interest, this radiation presents unique properties of coherence and ultrashort pulse duration that makes it a useful XUV source. The measurement of such short durations has required us to develop new characterization techniques in the XUV range, that have evidenced harmonic pulses as short as a few femtoseconds. This property opens the way to studying ultrafast dynamics in atomic and molecular spectroscopy, solid-state and plasma physics. Moreover, the possibility of generating yet shorter pulses, so-called attosecond pulses, is under investigation.

The possibility of producing femtosecond x-rays through Thomson scattering high power laser beams off laser wakefield generated relativistic electron beams is discussed. The electron beams are produced with either a self-modulated laser wakefield accelerator (SM-LWFA) or through a standard laser wakefield accelerator (LWFA) with optical injection. For an SM-LWFA (LWFA) produced electron beam, a broad (narrow) energy distribution is assumed, resulting in x-ray spectra that are broadband (monochromatic). Designs are presented for 3-100 fs x-ray pulses and the expected flux and brightness of these sources are compared.

Recent developments have allowed x-ray diffraction techniques at synchrotron radiation facilities to be employed with a temporal resolution of around 1 ps. These developments are the availability of firstly, high brightness third-generation x-ray sources, secondly, passively mode-locked lasers with 100 fs temporal duration and thirdly, averaging streak cameras with sub-picosecond temporal resolution. In this paper, we discuss how these novel devices are combined in time-resolved x-ray diffraction experiments at synchrotron radiation facilities.

The recent development of ultrafast x-ray sources makes conceivable the analysis of subpicosecond transient structures in physics, biology and chemistry. This paper focuses on the laser-produced plasma technique. Mainly on the basis of the work done at LOA (France), we report the first experiments dedicated to the analysis of subpicosecond atomic motions on this time-scale.

Femtosecond spectroscopy with 250 fs temporal and 150 nm spatial resolution is made possible by combining near-field scanning optical microscopy with ultrafast pump-probe techniques. Femtosecond-resolved near-field microscopy allows transport studies in nanostructured materials as well as the study of carrier dynamics in single nanostructures with unprecedented temporal and spatial resolution. Measurements on single nanostructures are a powerful means for the characterization of the inhomogeneity of nanostructure ensembles. We describe the design and performance of femtosecond near-field scanning optical microscopes and give examples of typical femtosecond-resolved near-field experiments on semiconductor nanostructures.

An x-ray streak camera system uses a laser triggered photo-conductive switch to synchronize the high voltage ramp applied to its sweep plates to the photo-excitation of a sample. This technique allows the jitter between successive sweeps to be kept below 100 fs compared to over 1 ps using classical methods. By accumulating a stable pulsed x-ray signal over many sweeps, a very high dynamic range can be achieved whilst maintaining a sub-picosecond time resolution. The ultimate time resolution is limited by the dispersion of electrons in the streak tube. Whilst triggering with a laser at 900 Hz we have achieved a time resolution (FWHM) of 640 fs at a detection wavelength of 267 nm for a 60 s accumulation period, corresponding to over 50 000 sweeps.

A technique is described for detecting misformed clips in hermetically sealed packages. The technique involves use of a machine vision system incorporating an image processing algorithm, that calculates the directions of contours of intensity in grey-scale images of clips. The algorithm approximately locates the clip by scanning a box over the contour direction transformed image, and finding the position of the box such that the standard deviation of the directions in the box is a minimum. The clip is accurately located by scanning a rectangular template over the contour direction transformed image in the vicinity of the approximate location of the clip, and determining the best-fit position of the template. Accurate location of the clip enables the critical clip dimensions to be calculated and used for assessment of clip integrity. The clip integrity is considered acceptable if all of the critical dimensions lie within an acceptable range, as defined by predetermined upper and lower thresholds.

The design and performance of a new tandem parallel-plate analyser (PPA) are presented. The main advantage of this instrument is its time-focusing; i.e., within its angular acceptance (±10°), particles entering at different angles reach the detector nearly at the same time. In coincidence measurements, this is particularly useful at low energies for increasing signal-to-background levels. In its present realization, its relative energy resolution is 7.5%.

In the past, in order to obtain the stress-strain behaviour of a test piece under a temperature gradient field, first the uniaxial tensile test was carried out under some uniform temperature field, and then the stress-strain data were utilized for computer analysis using the finite element method. However, in this study, a simpler and more realistic testing machine for measuring the mechanical properties under a temperature gradient field has been devised and employed. Stress-strain curves measured with this machine for a 0.105 mass% C carbon steel have shown good agreement with the curves calculated from the thermal elastic-plastic analysis using stress-strain curves obtained from hot tensile tests in uniform temperature fields. Also, the fracture strength obtained with the temperature gradient tensile test is a little lower than that of the conventional hot tensile test corresponding to the average temperature of the two sides of the test piece. The fracture strength measured with the temperature gradient tensile test and a submerged split tensile test agree well with each other.

Knowledge of wing orientation and deformation in free flying insects is necessary for a complete aerodynamic analysis. It is important to determine the body coordinate of an insect because the kinematic features are described based on the body coordinate. A reflection beam method has been developed for measuring the body vector of a free-flying bumblebee with large angle range and high accuracy. The method uses a piece of small cover glass attached to the bumblebee for reflecting the laser beam, and a trapezoid screen for measuring the direction of the reflected beam, allowing us to determine the body vector based on the direction of the incident beam and that of the reflected beam. The method has been successfully used to measure the body vector of a free flight bumblebee.

The advantageous employment of multimode fibres for beam delivery in laser-Doppler anemometers (LDAs) is presented. Multimode fibres allow the transfer of significantly higher power into the LDA measurement volume and need less alignment effort than do the single-mode fibres which are usually employed. Powerful laser diodes can now be applied for LDA set-ups, allowing sensitive velocity measurements of fluid flows. It is demonstrated for the first time that the length of the measurement volume is less than that of the volume of intersection of the two laser beams because of the low spatial coherence of the multimode light. Highly spatially resolved measurements of variations in velocity in flows can be achieved. As a first result, a measurement volume with a diameter of 584 µm, length 40 µm and 280 fringes is presented. This allows precise frequency measurements and the determination of movements of accelerated particles. The speckle pattern of the multimode beam is sufficiently suppressed by choosing high-aperture fibres with high intermodal dispersion and by the use of laser diode arrays with low coherence lengths. A high accuracy in the determination of velocity gradients in laminar or turbulent boundary layers can be achieved with the multimode-fibre LDA.

The application of particle image velocimetry to turbulence measurement is described. An analysis of physically necessary spatial resolution is presented by using a model spectrum function. A comparison is made with hot-wire anemometry. Some aspects of spatial filtering and two-dimensional sampling are presented with a comparison to large-eddy simulation. The estimation of time mean turbulence quantities from the measured vector fields in a laboratory mixer is used as an example. The measurement results for two different image sizes at the same position in the flow field are compared with different interrogation area sizes. The assumed dependence of the velocity field on the interrogation area size could not be confirmed. The image size seems to produce dependence in all estimated quantities. The measurement errors are critical for the achieved results.

Local field correction particle image velocimetry (LFCPIV), which was first presented in 1997, is the only correlation PIV method able to resolve flow structures smaller than the interrogation window. It presents advantages over conventional systems and thus offers an alternative in the field of super-resolution methods. Improvements of the initial version are likely to promote its application even further. The issues defining some of these possible improvements were already indicated in the paper that originally introduced LFCPIV, but not developed. This work presents refinements and also simplifications of the technique, so that it can be applied using current algorithms of advanced correlation PIV systems. Furthermore, these refinements reduce the measurement error and enlarge the range of application of LFCPIV. In particular, the application of the system is no longer constrained to images with mean distances between particles larger than 4 pixels. Besides that, the use of interrogation windows smaller than in its previous version is evaluated. This allows multigrid LFCPIV implementations. The results show how multigrid LFCPIV can obtain better measurements than can the usual multigrid PIV, but still the refined version of the LFCPIV technique performs even better, at the expense of a larger computing time. The performance of these methods is evaluated for synthetic and real images. This includes examples in which the ability to cope with gradients in velocity, gradients in seeding density and the presence of boundaries is highlighted.

In this paper, a technique based on genetic algorithms is proposed for improving the accuracy of solar cell parameters extracted using conventional techniques. The approach is based on formulating the parameter extraction as a search and optimization problem. Current-voltage data used were generated by simulating a two-diode solar cell model of specified parameters. The genetic algorithm search range that simulates the error in the extracted parameters was varied from ±5 to ±100% of the specified parameter values. Results obtained show that for a simulated error of ±5% in the solar cell model values, the deviation of the extracted parameters varied from 0.1 to 1% of the specified values. Even with a simulated error of as high as ±100%, the resulting deviation only varied from 2 to 36%. The performance of this technique is also shown to surpass the quasi-Newton method, a calculus-based search and optimization algorithm.

A systematic study on the uncertainty in spectral responsivity of silicon photodiodes due to the calibration spectral bandwidth is presented. Two of the most common types of silicon photodiodes in the field of optical power measurements have been used in this work: one with thickness of about 100 nm of SiO₂ and the other with thickness about 27 nm of SiO₂. It is shown that the spectral responsivity error will be negligible (⩽5×10^-4) using a spectral bandwidth up to 20 nm in the calibration if the effective wavelength of the spectral transfer function, including the distribution of the radiation source, is calculated and the measured responsivity is assigned to that effective wavelength. In other cases, the error depends not only on the extent of the interval isolated by the spectral analysis system but also on the shape of the spectral isolation function.

A practical method for determining the broadband microwave complex permittivity of particulate materials is described. In this method, particulate materials are dispersed randomly in paraffin wax; thin disc samples are prepared for measurement from the particle-wax mixtures. During measurements, the samples are backed by a conducting plane, and an open-ended coaxial probe is used to determine the permittivity of the samples. A mixture equation is used to calculate the permittivity of the particulate materials from the permittivity of the samples. The validity of six well known mixture equations is examined. The experimental results indicate that only the QCA-CP and Bruggeman mixture equations can accurately describe the microwave permittivity of the particle-wax mixtures over the wide particle concentration range. To validate this described method, the complex permittivities of PbTiO₃ and Pb(Zr_0.53Ti_0.47)O₃ particles are determined over a frequency range of 0.2 to 6 GHz. The determined results are found to be in agreement with the coaxial transmission/reflection measurement results. The advantages and limitations of this method are also discussed in this paper.

The Mueller matrix is the transfer matrix in the Stokes algebra that describes the polarization of natural light. This matrix is very versatile for the task of characterizing the optical properties of liquid-crystal cells, since it can be used for comparison with theoretical calculations, the determination of material parameters and the modelling of the cell as an optical building block for technological use. We have constructed a Mueller-matrix spectrometer, with the ability to perform fast, dynamic measurements of the Mueller matrix of small areas of liquid-crystal cells throughout the visible range. To illustrate the potential of the instrument, dynamic measurements on a ferroelectric-liquid-crystal cell are presented and analysed. The optical measurements indicate that there is an asymmetry between the up and the down state, tilted smectic layers and polarization reversal initiated at the boundaries.

A method suitable for the thermal characterization of gases is described. It is based on thermal wave interference (TWI) in a cavity. A TWI device was constructed and the thermal diffusivity of different binary gas mixtures as a function of their relative concentration was measured. From these values, using a logarithm mixing model for the thermal conductivity of a two-phase gas system, the thermal properties of the corresponding pure gases were calculated. The data obtained for several test gases show good agreement with values reported in the literature.

An ultrasonic clad steel buffer rod sensor embedded in the die has been used to in-line monitor the die casting process using aluminium alloy billets with semi-solid (thixotropic) and dendritic microstructures as feedstocks. This clad steel buffer rod provided better performance than the previously reported clad zirconium buffer rod. A multiple recording technique was used for the fast data acquisition with a rate up to 0.003 s per acquisition. This single ultrasonic probe monitored the completion of the die filling, the release of the pressure, the opening of the die, part detachment and solidification of the part. The change of the ultrasonic travel time in the clad buffer rod and cast part relates to the variation of the average temperature in the die and part, respectively. The measured amplitude profile of the reflected echo at the probe end/cast part interface can be used to monitor the cold shot and complete filling. The variation of the ultrasonic attenuation coefficient in the cast part may also relate to the change of the composition, microstructure and porosity.

Keyhole arc welding (KAW), including the keyhole double-sided arc welding process being developed at the University of Kentucky and keyhole plasma arc welding, can achieve much deeper narrower penetration than all other arc welding processes. If it could be controlled such that the heat input and weld pool are minimized while at the same time the desired full penetration is guaranteed, it could become an effective yet affordable technology to improve productivity in welding thick materials. However, the key in developing such a controlled KAW technology is the sensor which can detect the evolution of the keyhole. Preliminary study shows that the plasma reflection could lead to a practical yet accurate sensor. In this study, the dynamic behaviour of the plasma reflection is described using the reflection arc angle (RAA). It is found that the RAA series can be considered an autoregressive moving-average (ARMA) process. The orders of the ARMA model are determined using auto-correlation and partial auto-correlation functions. The parameters of the ARMA are recursively estimated using the extended least squares algorithm. It is found that the recursive estimates of the model parameters change as the state of the keyhole changes. A discriminator has been proposed to determine the state of the keyhole based on the recursive estimates of the model parameters.

We present a new technique of real-time measurement for the pure axial force distribution exerted by a full size active polishing tool on large aspheric surfaces. The technique uses an array of custom developed flat line load cells incorporating semiconductor strain gauges that can withstand the large shear forces in polishing. Design, manufacture and characterization of the force sensor array and its instrumentation to the active polisher are described. In particular, thermal characterization of the sensors proved that the temperature change by a few degrees required for a polishing tool has negligible effects on sensitivity and zero-point offset. Effects of non-axial force components were compensated with four independent half-bridge signals inside a sensor unit. This was further removed with a gap condition between the sensor array and the structural membrane, of which the latter functions as a point of pushing and pulling the polishing lap. The sensor array shows a potential usage in detecting and controlling the non-axial force components in operation. The real-time graphical map of axial polishing force generated by the sensor array improved the polishing efficiency by at least 30% compared to the traditional polishing technique.

As an alternative to strain measurement, a sensor of structural deflection curvature has been reported by the present authors. The operation of this `curvature gauge' is now explained and computer simulated in order to improve its performance and visualize the effects within its optical fibre, that curves together with the supporting structure under bending loading. The simulation results have been verified against known test cases. They allowed fabrication of `modal curvature gauges' that, through distributed measurements, can resolve deflection mode-shapes of vibrating structures such that each one is only sensitive to a designated vibration mode while virtually insensitive to other dominant modes.

This article presents an original method of B/A measurement with a single plane transducer used in pulse-echo mode. To determine the best conditions of measurement, we use an ideal model with a new solution for the second harmonic. We take into account the phenomena of attenuation and diffraction. Experiments validate simulations in the case of comparative method measurement. Then we present a device to improve the detection of the second harmonic. Thus measurement of the B/A parameter of ethanol and glycerol validates this device and we also show the possibility of its use in nonlinear imaging with a weak excitation.

A novel method is proposed for the measurement of layer parameters using spectroscopic double-beam interference microscopy. The contribution of each partial wave reflected from the layer interfaces to the interferogram is treated separately and the total interferogram is obtained as their coherent sum. When an annulus aperture is used, an analytic expression is derived for the interferogram that allows fast direct comparison between measured and calculated spectra from which refractive indices and thickness of layers can be determined simultaneously. Subnanometre accuracy of the film thickness measurement is shown to be possible.

Although the role of the periodic errors of optical heterodyne interferometers in displacement measurements is fairly well understood, their influence on the calibration of other types of displacement sensor does not seem to be studied as extensively. We have performed a careful analysis of the role of these errors on the calibration of a capacitance displacement sensor, which is integrated into a linear piezoelectric transducer for nanometrology applications over displacement ranges in the sub-micrometre range.

Assuming a linear dependence of the output voltage of the capacitance displacement sensor on the displacement it was found analytically that periodic interferometer errors lead to a decaying oscillation of the sensor sensitivity as the displacement range used in the calibration increases. For use of a double path interferometer with a polarization mixing amplitude of 0.5 nm and a polarization mixing wavelength of 79 nm, deviations of sensor sensitivity of 0.3% for a range of 0.2 µm and 0.029% for a range of 1 µm were estimated. The pronounced oscillations of the sensor's sensitivity over very small displacement ranges as predicted by the theoretical description were verified experimentally.

A new method for non-stationary temperature-field measurement is presented. Optoacoustic generation is used to emit short acoustic transients within the observed field. The transient deflects a probe laser beam, which is folded by two parallel mirrors in such a way that it passes through the observed field several times before it reaches a position-sensitive photo-detector. The timing of the acoustic wavefront transits at several distances from the source is obtained from a single oscilloscope trace of the photo-detector output. By measuring the acoustic wavefront transit times and the distances between the probe-beam segments, the velocity of sound is determined and from this we were able to calculate the local temperature.

Non-relativistic equations of motion have been solved numerically in order to calculate the focusing properties of a cylindrical field between concentric cylinders of radii R₁ and R₂ with two boundaries along the Z-axis, for charged particles emitted by a source that is distant from the analyser and located on the axis of symmetry. It was shown that this system exhibited second-order focusing. For a point focus and a distant source a dispersion value was obtained equal on average to 4.6R₁. The effect of variation of parameters on the focusing has been studied and some regressions for estimating the focusing characteristics were obtained.

As the preface to An Introduction to Space Robotics proclaims, this work is aimed squarely at `students and professionals engaged in the analysis and design of robotic space missions'. As a reviewer I should come clean and state that this does not apply to me - rather, as a space scientist I am very much a user (some would say, abuser) of robotics within the context of space exploration. To me robotics are a means to an end, and often a limitation on my own personal scientific enquiry. And so, having made my position clear, what were my impressions about the book itself? The first thing that hits you when you pick it up is its high mass - this is a heavy book (one that would make your arms ache if not supported on a desk). This characteristic arises because of its overall high quality - the hardback format, the glossy paper and clear print; with 663 pages arranged into 21 chapters you will certainly feel like you have obtained your money's worth if you purchase it yourself. There is no question: the book itself will last a lifetime.

As far as content is concerned, the opening three chapters introduce the subject of space robotics with reference to the environmental considerations under which they need to operate, and with examples from past mission successes. The next two deal with the man-machine interface (in these enlightened times we really should try to think about a human-machine interface) and systems design. These two chapters begin to establish the overall direction of the book, which takes, as two pertinent examples of space robotics, in-orbit servicing and planetary rover exploration. These two concepts are described with regard to telerobotic control processes initiated from a ground station, which introduces the notions of robotic programming languages, voice recognition software, virtual reality systems, and the overall ground station computer hardware. Then a hypothetical freeflying space robot dedicated to satellite servicing is introduced; known as ATLAS, this device is used as an example to describe the complex interrelationships which have to be considered within the overall systems design. As a space scientist the underlying concepts are entirely relevant to, say, a robotic device that would reach out from a spacecraft and grab a particular rock from the surface of an asteroid. Interestingly, in this example, the constraints on ATLAS would become even tighter since the operation may need to be done without imparting any contamination to the sample in question (something else for engineers to worry about). Having said that, space scientists would do well to appreciate the technical limitations attached to robotics (which might then prevent them from asking for the impossible).

The text then changes gear and goes into a chapter-by-chapter account of some of the specific elements of robotic manipulation, i.e. sensors, control systems, dynamics, kinematics etc. This is all pretty standard robotic stuff, but is dealt with in a marvellous degree of detail. After two further chapters on imaging and autonomous control, there follow six which are very much oriented towards spacecraft themselves - orbit and attitude control, avionics, communications, power supplies, structures etc. At every possible opportunity the text deviates from the flow of generic information to consider a real spacecraft example. This constantly emphasizes the very practical applicability of the book. Finally, there are chapters which consider the commercial and legal ramifications of robotic space missions, and a conclusion section that focuses on the future of space exploration, including the International Space Station, and resource utilization on the Moon and planetary bodies.

So, quite apart from being a heavy book, its content is also best described as heavyweight; it is hard to imagine that anything has been missed out. In conclusion this book should be on the shelves of all academic libraries that specialize in engineering, mechatronics or space science. Furthermore, if space robotics is to be your chosen career then clearly you need to have your own copy. As far as this subject goes, Ellery's book should surely be considered as the Bible.

I P Wright

The concept of the electronic nose has attracted attention in many branches of industry for its potential in routine odour analysis. Being first reported in 1982 by Persaud and Dodds, the research in the field rapidly increased, and a number of companies have now been established to commercialize the concept. Basically, an electronic nose has the mammalian olfaction as a model and consists of a sensor array with partially overlapping selectivities and a pattern recognition system. It can be trained to detect and discriminate a large number of both simple and complex odours.

In 1994, the electronic nose manufacturer AlphaMOS in France initiated a symposium to promote the field and to bring together representatives from industries and the research field. The symposium has since been held annually, alternately in Europe and the USA. In 2000, the 7th symposium on the subject, called the International Symposium on Electronic Noses (ISOEN), was held at Brighton, UK, and this book, Electronic Noses and Olfaction 2000, consists of a series of papers presented at this event. The 44 contributions contains a broad mixture of research and state-of-the-art papers, covering aspects such as new sensor concepts, instrumentation, data processing methods and a number of applications.

In the first part, entitled Odours, Taste and Physio-Chemical interactions, an excellent overview of oral malodour is presented, including 60 references. This section also includes the description of a concept of an electronic tongue (yes, you can measure in aqueous solution with it), based on similar strategies as for the electronic nose.

In the next section, Sensors/Instrumentation, various sensor principles are described, of which conducting polymer FET, amperometric devices and impedance spectroscopy can be mentioned.

Sensor performance is affected by ageing and degradation of the sensor surface, causing drift effects. In the third part of the book, devoted to Data Processing, some methods to compensate for this drift are presented. Methods for sensor selection are also presented as well as data processing based on fuzzy logic and artificial neural nets.

In the next two sections, Medical/Microbial and Applications: Food, Agricultural and Environmental, a large number of different applications are described, including microbial growth, quality measurements of olive oils, wheat and raw milk odour, search for truffles by a portable nose, monitoring hot flue gases, odour emissions in the field of agriculture and automotive trim materials.

These five sections of the book cover well the current research and development areas of electronic noses. Especially valuable is the data processing part, dealing with new algorithms for signal processing and the algorithms for drift reduction, which is a new area with increasing importance. Also interesting is the application part dealing with microbial detection. A large section deals with quality estimation of olive oils - is this a trend? Contributions covering electronic noses based on new techniques such as ion mobility spectroscopy, mass spectroscopy or gas chromatography were not given, however, and sample handling procedures were only sparsely reported, techniques that also are important aspects of electronic nose technology. There were also no real industrial applications reported.

The book is aimed at the researcher in the field as well as at instrument makers and people with a general interest in the subject, and it gives a good update on what happens in the area.

Fredrik Winquist

Why do we need a monograph on measurement of mobile antenna systems?

There are two reasons. Firstly, despite the problems that the mobile communications industry are currently having as a result of uncertainties of third generation system uptake, it remains one of the most important technology sectors across the world. Secondly, whilst the topic of antenna measurement techniques is a largely mature one, there are significant new challenges thrown up in measuring antennas on mobile terminals. Most antennas used in fixed systems are intended to operate in a single direction. They are thus measured in an anechoic, or reflectionless, chamber or on a special outdoor range that ensures incident energy arrives from a single direction only. When operating a mobile telephone in a built-up area, possibly without a line of sight, a multipath propagation environment occurs with rays from the base station incident from many directions simultaneously. The measurement of the antenna must in some way simulate this environment. The close proximity of the head is a further factor that leads to novel and unique measurement problems both in terms of the characterization of the radiation properties and the measurement of the absorption of energy into body tissues. For these reasons this book is both welcome and timely.

The book aims to cover, in a comprehensive way, antenna measurement methods for both mobile and base station antennas. To some degree these two topics sit uneasily together. The techniques for the mobile antenna are very different from those for the base station. Mobile antennas are usually small low-gain types and need the new methods noted above. Base station antennas, on the other hand, require conventional, although nonetheless demanding, measurement methods and these have been well described elsewhere. Having said that, the book does form a comprehensive introduction to measurement of all types of mobile system antennas and is thus an excellent reference and primer for workers new to the field.

The chapters on measurement of mobile antennas form the meat of the book and do justice to what is both a difficult problem for antenna designers and manufacturers yet also a fascinating and challenging area for researchers. For example, the use of dielectric phantoms as physical representations of the head and hand is now an acknowledged method to allow the body effect on the antenna radiation properties and the specific absorption rate to be measured. It is well covered. Fading and incident field simulators are also extensively dealt with. Here, the creation of a multipath environment within a metallic enclosure by the use of multiple sources is a demanding and fascinating new development in antenna measurements and the book presents some of the latest results using this method.

Measurement of Mobile Antenna Systems is a book that will be of interest to engineers, both those who are experienced in antenna measurements and those who are just entering the field of mobile antenna design and measurement. It is also sufficiently novel to appeal to researchers looking for new challenges. It is most heartily recommended.

P Hall

Adaptive antenna arrays and their control represent areas of considerable importance in modern radio communications. Much is already written about the subject, a great deal of it embodied in formidable vector and matrix mathematics. In this book, Halim adopts the posture that the inherent complexity of a rigorous mathematical treatment unnecessarily obscures the operating principles, rendering them inaccessible to many who require a practical understanding, and to many college and university students encountering adaptive array techniques for the first time. Thus, he sets out to describe the functioning of an adaptive array, and some of the systems-level principles, using just elementary mathematics. Does he succeed? In my view, he does to a very large extent, and he has produced a text that, whilst some practitioners will find it idiosyncratic, clearly exposes the principal array characteristics with emphasis on their physical interpretation and significance.

In essence, the book sets out to solve just one problem, that of predicting the performance of an active, adaptive array system to eliminate interfering signals. The beamforming and signal processing parameters are derived and interpreted for a range of important operating conditions, including different combinations of the numbers of source elements and output beams. Nowhere does the treatment demand more than basic calculus and algebra: the analysis principles will be familiar to anyone versed in control theory. Of course, the subject matter is such that unwieldy expressions are unavoidable, but the author softens the impact by relegating the more intimidating examples to a series of appendices.

Generally, then, the book will be of interest to practising antenna specialists and to students alike. It is not substitutional for any of the established texts on adaptive arrays, however, and is best viewed as augmenting them by providing a useful, readable account of the underlying theory. Array technology is only briefly addressed, and signal-processing techniques are excluded. It is not generous in its references to literature in the field, it must be said, and the cited works are mostly more than ten years old. As ever, the Artech House production standard is high.

I Bennion

It was a pleasure to read this important book. To understand and predict the development of turbulent flows represents both a continuing scientific challenge and also a serious practical problem in many different fields.

Professor Pope has based his book on graduate level lecture courses on turbulence that he has presented at MIT and at Cornell University. It is intended for students in engineering, applied mathematics, oceanography and atmospheric sciences, as well as researchers and practising engineers. The emphasis is on turbulent flows, rather than on the theory of homogeneous turbulence, and only constant-density, nonreacting flows are considered. The author states that his aim is to explain concepts and develop the necessary mathematical tools, rather than to provide a practical guide to turbulence modelling. The text is divided into two parts. Part I provides an introduction to turbulent flows and the fundamental physical processes involved. Topics discussed in separate chapters include: the equations of fluid motion, the statistical description of turbulent flows, the mean flow equations, free shear flows, scales of turbulent motion and wall flows. The chapter on statistical methods for turbulence is particularly good; together with the related appendices it represents a valuable and accessible introduction to the subject. Several approaches for modelling or simulating turbulent flows are then described inPart II, in which the various chapters describe direct numerical simulation (DNS), turbulent viscosity models such as thek-ε model, Reynolds stress and related second-moment models, probability density function (pdf) models and large-eddy simulation (LES) techniques. Finally, some necessary mathematical tools are summarized in ten Appendices dealing with a wide range of relevant topics including tensors, Dirac delta functions, Fourier transforms, random processes, derivation of pdf equations, characteristic functions and stochastic descriptions of diffusion processes.

I particularly enjoyed the chapters on modelling and simulation of turbulent flows. A unified treatment of the different types of model has enabled the author to make valuable connections between them and to reach clear and logical conclusions about the strengths and weaknesses of each approach. Over many years Professor Pope has made very important contributions to the development and application of pdf methods for the prediction of nonreactive and also reactive turbulent flows. The chapter on this subject is an exceptionally clear description of these powerful and often under-utilized simulation methods. The final chapter, on LES, provides an excellent introduction, with valuable and novel insights into both the promise and the problems of this relatively new and rapidly developing approach. It should be compulsory reading for many LES practitioners.

Each section of the book contains a number of exercises, which typically invite the reader either to derive an expression that is quoted in the text or to generalize an analysis set out in the text. These exercises are often divided into several stages, and contain appropriate instructions, so that a diligent student will find his or her way through them. An advantage of the extensive use of appendices and student exercises is that analytical clarity and physical understanding are not obscured by unnecessary algebraic detail or by the need to review basic mathematical tools. The result is an exceptionally clear presentation, together with an often penetrating critique of both classical methods and recent developments in the theory and modelling of turbulent flows. There is excellent cross-referencing between one section and another, and an extensive and up-to-date bibliography.

I strongly recommend this book to advanced students of fluid mechanics, to their teachers and to all researchers, engineers and others with a professional interest in turbulent flows.

K N C Bray

The conception of this book lies somewhere between a proceedings volume and a monograph. It consists of 16 sections on intermittent hydrodynamic or magnetohydrodynamic flow. Each one is written by a different author or team of authors. Each subsection itself offers a review type of presentation, written for this book. It contains many references to recent original work, emphasizing the authors' expertise and contributions.

While there is coherence of the presentation, the notions, the symbols, etc, within the subsections, there is hardly any cross-referencing between them, except for the common headline intermittency. The reader finds a rich variety of topics, of building blocks, on the one hand, but on the other hand will miss the unifying view as well as various features and authors he knows to work on intermittent fluid flow. But altogether he gets quite a representative overview on recent progress in this very active field. Some of the articles particularly nicely demonstrate why there is increasing interest in turbulent fluctuations and excitement of researchers about the phenomenon called intermittency. The book is less devoted to applied fluid flow; it concentrates on the physics of intermittency, even in the broader context.

A few items treated in this volume should be mentioned in order to provide the reader with appropriate information. These include how to relate the anomalous scaling exponents of the structure functions - the signature of intermittency in fluid flow - with the near-singular flow structures. Are there relations to the often discussed finite time singularities of the Euler equations? The question of universality of intermittent space-time fluctuations in fluid flow in particular and in large classes of nonlinear dynamical systems is dealt with repeatedly. The cascade picture, fractality, multifractality and beyond, are offered. Clearly the mixture of new ideas, notions and conceptions together with discussions and results of many new experiments, also visualization techniques, stimulate the reader's interest and excitement.

Competent authors and new material on very recent approaches are the main reasons to recommend this book for researchers in fluid flow and in nonlinear dynamical systems theory, for students as well as for experts. They can find stimulating information on new ideas, for example on multipoint correlators, on the so-called inverted structure functions, on geometrical statistics, etc, and they find this together with less recent material such as on so-called extended self-similarity, on wavelet use etc, and finally on long-standing items.

Intermittency is a very vivid, active field. A unified physical understanding is still missing, but a whole host of models, Ans\"atze and experimental findings contribute to the developing nature of this unique phenomenon intermittency. This volume contributes in compiling many details and providing an overview which might lead, in the future, to a summarizing explanation. The reader already looking for that here will be disappointed. Still, it is worthwhile to read or at least look into this book, to experience the dynamics in this field and, perhaps, become engaged by it.

Siegfried Grossmann

The second edition of the authors' well-received 1987 text has been thoroughly revised, updated and modestly reorganized within the same basic framework and under the same title as the earlier edition. It contains a selection of new material and improved treatment of other topics in its later chapters.

The book is again divided into three major sections: chapters 1-4 provide an introductory short course on modelling and numerical simulation; chapters 5-10 treat advanced topics in numerical simulation of reactive-flow processes; while chapters 11-13 consider the simulation of complex reactive flows. It is in the choice of topics and their treatment in the last section that significant new material is introduced, including a very useful treatment of recent developments in turbulent combustion, the latter very much welcomed by the reviewer.

Both editions use the same form of words in recommending the technical background requisites of readers: `an advanced undergraduate or beginning graduate school understanding of the basic principles of chemistry and fluid dynamics and their potential interactions. Some familiarity with the rudiments of numerical methods is helpful.'

The purpose and target readership of the book are identified in both the frontispiece and prologue as: `an indispensable guide/manual on how to construct, use, and interpret numerical simulations of reactive flows.' The frontispiece adds: `It will be welcomed by advanced undergraduate and graduate students, as well as a wide range of researchers and practitioners in engineering, physics and chemistry.'

To what extent the new edition succeeds, only time will tell. It ought to do at least as well as the first edition, but the subject area is now so large that a single text, albeit one concentrating on fundamentals, cannot do justice to more than a few applications. As was the case in the first edition, the text is primarily concerned with gas phase flows, with only a small excursion into multiphase systems. Perhaps it is this characteristic that may induce a small degree of disappointment; has an opportunity been lost to provide a little introductory material on condensed phase reacting flows, including a few words on `hydro-codes'?

The second edition offers a significantly higher quality product, in terms of organization, presentation, illustrations and layout, than the original text.

Norman Pratt

The book Turbulent Combustion by Norbert Peters is a concise monograph on single-phase gaseous low Mach number turbulent combustion. It is compiled from the author's review papers on this topic plus some additional material. Norbert Peters characterizes turbulent combustion both by the way fuel and air are mixed and by the ratio of turbulent and chemical time scales. This approach leads naturally to detailed models, which are based on results of turbulence modelling and asymptotic flame theory. In both areas Norbert Peters has contributed significantly over the last two decades.

The book has four sections. In chapter 1 he discusses briefly the state of the art of combustion models as they are used by different authors. Important turbulent and chemical scales are introduced, which are then used to introduce and explain the different combustion models. He distinguishes between premixed and non-premixed combustion and also between infinitely fast and finite rate chemistry. The current turbulent combustion models are described in order of their complexity and physical accuracy. He explains Eddy BreakUp and Eddy Dissipation Models, the fundamentals of the PDF transport equation, and the laminar flamelet concept applied to non-premixed and premixed turbulent combustion. Then, the Conditional Moment Closure, the Linear Eddy Model, and combustion models used in Large Eddy Simulations are described very briefly.

Chapter 2 is devoted to premixed turbulent combustion. After introducing some characteristic dimensionless numbers Peters uses the level set approach and the flamelet concept to formulate a combustion model valid in the thin zone and corrugated reaction zone regimes. He shows parallels between this more fundamental model and standard models like the Bray-Moss-Libby model. He also presents models for the turbulent burning velocity, the Flame Surface Area Ratio, and discusses the effects of gas expansion. Very helpful for the reader's understanding is the presentation of three worked examples of a slot burner, a propagating spherical flame and an oscillating counter flow. Peters' model for premixed turbulent combustion is based on the equations for the mean and the variance of the $G$-equation, some closure relations as well as the flamelet equation for premixed combustion. A numerical example is used to discuss its accuracy.

Non-premixed turbulent combustion is the subject matter of chapter 3. Peters uses the mixture fraction variable and asymptotic flame theory to explain the regimes of non-premixed turbulent combustion. Two worked examples of a counterflow diffusion flame and the one-dimensional unsteady laminar mixing layer help the reader to understand the theory. After discussing turbulent jet diffusion flames and introducing the flamelet equation he develops steady and unsteady flamelet models for non-premixed turbulent combustion. In particular, the Eulerian Particle Flamelet Model and the RIF (Representative Interactive Flamelet) Model are discussed. These models have been used to predict pollutant formation in a gas turbine and a direct injection Diesel engine, respectively.

Finally, partially premixed combustion is discussed in chapter 4. Lifted turbulent diffusion flames are reviewed and the prediction of the lift-off height is identified as a key problem. This leads directly to the introduction of the concept of a triple flame. Different models for partially premixed combustion are then presented and the numerical simulation and the scaling of lift-off heights in turbulent jet flames are studied.

There is no doubt that this book is well written and is an important contribution to combustion literature. Peters uses asymptotic theory and scale separation to develop combustion models from first principles. Also, the book contains a comprehensive review of the current literature on turbulent combustion. It is clearly a `must have' for experienced combustion modellers and experimentalists. The book has not been written for non-experts and beginners; these readers would probably have liked more worked examples and exercises, a list of symbols, some material on the mathematical techniques of asymptotic analysis, and a more detailed discussion of the standard combustion models that are often used in the literature. Also, the numerical aspects of turbulent combustion modelling are not discussed in the book. Nevertheless, the book will be a useful source for advanced courses on combustion.

Markus Kraft

The books provides a good introduction to the field of nonlinear science. In the first part of the book the authors present a well written theoretical description of nonlinear phenomena and modern analysis methods. The examples in the text are chosen from many different phenomena in nature and range from biology and medicine to physics, including the selection and evolution of biological molecules, electronic oscillators, the beating heart, snowflakes, Hamiltonian chaos, the rings of Saturn and others.

Maple codes on the CD-ROM (in the second edition for Maple 6) allow the study of illustrative nonlinear examples. An introduction to the usage of Maple is provided by the authors in the book. In the second part of the book about 30 interesting experimental activities for the better understanding of nonlinear phenomena are suggested. The book contains about 400 problems for the student and instructor.

The book has well met its purpose of giving an introductory survey of the basic concepts and applied mathematical methods of nonlinear science as well as an introduction to some simple related nonlinear experimental activities for students in engineering, physics, chemistry, mathematics, computing science and biology.

Peter Nielaba