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The Editor

Prof. Guillermo H. Kaufmann

Instituto de Fisica Rosario Universidad Nacional de Rosario Facultad de Ciencias Exactas e Ingeniería, Department of Physics and Centro Internacional Franco Argentino de Ciencias de la Información y de Sistemas Rosario, Argentina kaufmann@ifir-conicet.gov.ar

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Print ISBN: 978-3-527-40957-0

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To Carolina

Preface

The development of laser in the early 1960s generated many new research lines in the field of physical optics. The laser is a source that produces a beam of light that is intense, collimated, and coherent. However, the greatly increased coherency of laser light also leads to the appearance of other side effects such as the speckle phenomenon. People working with lasers quickly realized that the grainy appearance of rough objects when illuminated with coherent light was caused by an interference phenomenon. During the first few years after the invention of the laser, while much effort was carried out to minimize the effects of laser speckle, little was devoted to take the more positive path of putting it to good use. Perhaps the turning point came when it was realized that the light forming each individual speckle was fully coherent and it possessed a definite phase. This ability was recognized in 1968 and was quickly applied to the development of new laser-based techniques to measure displacements, deformations, and vibrations produced by rough objects.

The first multiauthor book on the subject (Speckle Metrology, edited by R.K. Erf) appeared in 1978. To fill the need for a book covering new aspects of speckles and novel topics such as fringe analysis and particle image velocimetry that had been mainly developed during the next decade, a new volume appeared in 1993 also entitled Speckle Metrology, edited by R. S. Sirohi. In the following years, the development of video cameras with higher resolution and high-speed data acquisition systems gave great impetus to speckle interferometry and related techniques such as digital speckle photography and digital holographic interferometry. These application-oriented techniques contributed to the publication of a new volume in 2001 edited by P.K. Rastogi, entitled Digital Speckle Pattern Interferometry and Related Techniques.

From the year 2000, new branches in speckle metrology have appeared, several new schemes have been proposed, and also the known approaches and techniques have been revisited and improved upon. The amount and the scope of these developments are reflected in very rich material, published in the specialized journals and presented each year at various international conferences. Therefore, time has come to review and sum up the most significant of these advances, and this book is the result of such efforts.

This book provides an up-to-date collection of the new material published in the field of speckle metrology and related techniques since 2000. Although there were several topics that could be included in such a book, we had to select only a few to keep its length reasonable. This means that other topics such as speckle techniques outside the visible part of the spectrum could not be treated. It is important to note that most of the selection of topics was carried out by taking into account that the? book should be useful for engineers, scientists, and graduate students who are engaged in the application of speckle techniques mainly to solve specific measurement problems in optical metrology, mechanical engineering, experimental mechanics, material science, nondestructive testing, and related fields or who are contemplating its use.

The book is organized into six chapters. The measurement of radial deformations in cylinders and the evaluation of residual stresses are important engineering problems that are better understood when they are analyzed in polar coordinates. Chapter describes latest digital speckle pattern interferometry (DSPI) systems, which are sensitive to polar coordinates. First, the authors present several configurations based on the use of conical mirrors mounted on piezoelectric transducers to allow the application of phase shifting algorithms for evaluating the phase distribution. A more recently developed interferometer based on the use of a diffractive optical element, which overcomes some limitations of earlier configurations, is also described. Finally, these systems are applied to the measurement of translations, mechanical stresses, and residual stresses. Application examples spread all over the chapter show the extent to which radial speckle interferometry has been developed into a powerful tool for industrial measurements.

Chapter 2 analyzes different approaches to measure internal displacement and strain fields within a weakly scattering material. These techniques have many potential applications that range from the development of failure mechanisms in different media to the detection of retinal disease. First, these authors analyze low coherent interferometry (LCI) that involves illumination of semitransparent scattering test objects with a broadband source and scanning the sample through the required depth range. After describing some limitations of the latter approach, the authors present two recently developed techniques called wavelength scanning interferometry (WSI) and tilt scanning interferometry (TSI), which have some practical advantages over LCI, the most important being an improved signal-to-noise ratio. In WSI, temporal sequences of speckle interferograms are recorded while the wavelength of the laser is tuned at a constant rate, so this technique needs a special light source. TSI is based on tilting an illuminating single wavelength beam during the acquisition of an image sequence, a procedure that provides the necessary depth-dependent phase shifts that allow the reconstruction of the object structure and its internal displacements. A theoretical framework is presented to allow the visualization of the spatial resolution and displacement component measured by any of the techniques presented in this chapter. The chapter concludes with a section on recent developments on phase unwrapping in three dimensions.

Chapter 3 presents different spatial methods to evaluate phase distributions from single-image interferograms. This chapter analyzes not only the different approaches that can be used when a spatial carrier is introduced in the interferometric data but also the more difficult task of automatic demodulation of a single interferogram containing no carrier. First, the authors review the well-known Fourier transform method to recover the phase of a single interferogram with a spatial carrier frequency and also different spatial phase shifting algorithms. They also describe various asynchronous algorithms that do not require the knowledge of the carrier frequency. In the rest of the chapter, the authors present several techniques to recover the modulating phase from a single-image interferogram without a carrier. Among the various approaches that are described, we can mention the regularized phase tracking technique and also local adaptable robust quadrature filters that do not require previous fringe normalization. They also describe single-interferogram demodulation methods based on the determination of fringe orientation, on the vortex transform, and on a general n-dimensional quadrature transform.

Chapter 4 reviews temporal speckle pattern interferometry (TSPI) and discusses several approaches that can be used to analyze the recorded data. TSPI was mainly developed during the past decade to measure the temporal evolution of low-speed dynamic deformation fields. It is based on the analysis of a time series of speckle interferograms, which codes the temporal and spatial phase changes produced during the dynamic deformation of the object. In TSPI, the optical phase distribution is extracted from the speckle intensity at each pixel independent of all the other pixels in the image, so that phase unwrapping is also performed as a function of time. As temporal phase unwrapping involves only 1D signals, this procedure is generally much easier to carry out than spatial 2D unwrapping. Until recently, the most common phase recovery technique used in TSPI was the Fourier transform due to its simplicity and short computational time. However, TSPI signals frequently present nonmodulated pixels, modulation loss and noise that affect the bias, and the modulation intensity terms of the signals to be analyzed. This chapter also describes more robust phase recovery methods that were mainly applied in the past decade to analyze TSPI signals. The numerical algorithms described in this chapter include 1D approaches based on the Fourier transform, the windowed Fourier transform, wavelet transforms, the S-transform, quadratic time–frequency distributions, and the empirical mode decomposition and the Hilbert transform. Two-dimensional and 3D approaches based on the windowed Fourier and directional wavelet transforms are also described.

One of the features that characterize a speckle field is the presence of points where both the real and the imaginary parts of the complex amplitude are equal to zero. Therefore, the intensity at this discrete number of points is also zero and their phase is not defined. Phase singularities or optical vortices can be associated with a sign or charge depending on whether the phase rotates clockwise around them or not. Chapter 5 presents some latest work on the application of optical vortices in optical metrology. These techniques are based on the fact that phase singularities are well-defined geometrical points with unique core structures and spatial configuration, which serve as unique fingerprints and present valuable information as identifiable markers. The authors also present various applications of the so-called optical vortex metrology, such as the measurement of nanometric displacements that use the information on the locations of phase singularities before and after the introduction of a displacement. Other applications presented in this chapter include the determination of rotational displacements and the use of optical vortices both for fluid mechanical investigations and for tracking the dynamics of a biological specimen.

Finally, due to the growing importance of security applications, Chapter 6 deals with the application of speckles for coding optical and digital data. In an encryption system, the information is encoded in such a way that the original information is revealed only by applying the correct key. To realize this aspect of security need, most of the optical architectures use random phase masks for coding and encryption. The phase masks made using speckle patterns work well as random phase masks, and have been used in optical and digital encoding of information. This chapter presents a broad review of various coding techniques used for optical and digital data security applications. Various speckle coding techniques are then discussed with an emphasis on the work carried out by the authors' group. This includes methods for preparation of speckle masks and techniques for their easy alignment, the use of elongated speckle patterns and various multiplexing techniques. The chapter also includes a large number of references that will be very useful for the reader interested in the implementation of these approaches.

To conclude, I would like to thank the authors of the different chapters for their contributions and cooperation. I also wish to thank Valerie Molière of Wiley-VCH Verlag GmbH for inviting me to edit this book and to Anja Tschörtner from the same editorial department for her help and support.

Last but not least, I am grateful to my wife Carolina who has tolerated me with patience over the past difficult year while I was recovering from various health problems.

Guillermo H. Kaufmann

Rosario, Argentina

July 2010

List of Contributors