This PDF file contains the editorial “Editorial: The Year 1991 in Review: Part II” for OE Vol. 31 Issue 03

This PDF file contains the editorial “Guest Editorial: Optics in Poland” for OE Vol. 31 Issue 03

A specific interterometric method has been developed. The method uses monochromatic light of continuously variable wavelength through the visible spectrum, and thus is referred to as variable-wavelength interferometry (VAWl). In some special circumstances, the VAWI method takes a specific form, referred to as object-adapted (or adaptive) variable-wavelength interferometry (AVAWI), which enables the optical path difference or phase retardation and derived quantities to be measured very precisely by relatively simple means. In practice, however, the most frequent interlerometric situations at the microscopic level can be qualified as the quasi-object-adapted ones. Therefore, a modified approach to quasi-object-adapted variable-wavelength interferometry (QAVAWI), its features, and performance is presented. In general, the VAWI method is much more accurate than conventional visual two-beam interferometry techniques. In particular, the AVAWI version enables the measuring accuracy of the optical path difference to be improved by two orders of magnitude, while the QAVAWI procedure offers better accuracy by at least one order of magnitude.

Three new and modified wavefront analysis concepts for interferometric optical testing are presented, specifically a moire fringes version of a temporal phase-shifting method and spatial-carrier phase-shifting and Fourier transform methods. All of these techniques require working with a finite fringe observation field in the interferometer, therefore additional analysis of the imaging optics is necessary. The impact of the interferometer construction and the method of analysis on the measurement error is discussed. A detailed experimental and theoretical comparison of the techniques is presented to fulfill a wide range of user requirements.

An implementation ofthe phase-shifting technique for computer-aided processing of interference fringes obtained by the moiré interferometry method for in-plane displacement measurements is described. Polarized light is used for both temporal and spatial (multichannel) approaches. The latter approach enables work at a single-frame rate. The experimental results obtained serve as a comparison of the two approaches.

An approximate analysis of gain saturation in two-mirror and distributed feedback lasers is presented. Using energy theory and threshold field approximation, an expression relating the small signal gain to the output power and system parameters is obtained, including nonlinear dispersion effects in the active medium, spatial hole burning effects, and longitudinal as well as transverse field distribution. The laser characteristics obtained reveal the optimal coupling strength for given system parameters providing maximal output power level for given pumping rate of the active medium, i.e., maximal efficiency of the system.

Changes of output beam parameters due to the pumping power level of a high-power cw YAG laser and the resulting focus shift have been investigated theoretically and experimentally. The effect of thermal focusing on the resonator parameters is compared with measurements of the output multimode beam properties. Theoretical results of focus shift analysis, verified experimentally, make it possible to determine the best coupling conditions of such a laser beam with an optical fiber for a wide range of pump levels.

Systems for optical diagnostics of plasma created in a plasma-focus device are described, showing progress in the development and application of different optical methods used in plasma-focus research. In the experiments carried out on the plasma-focus device at the Institute of Plasma Physics and Laser Microfusion, multiframe Iser interferometry and shadow photography, the Faraday rotation method, and high-speed photography in different spectral ranges were applied. The system used for investigation of the plasma structure and dynamics is fully automatic and is able to acquire images on 10 channels with time resolution of about 1 ns and spatial resolution of about 0.1 mm. Subnanosecond synchronization between different diagnostics was achieved. Some examples of recently obtained results are given.

We present equations describing the basic grids that when superposed with themselves and then mutually translated create moiré patterns in the form of spherical, equilateral hyperbolic, and linear zone plates. The family of basic grids described forms a complete set of solutions. Moiré zone plates can serve as imaging elements with very long, variable focal lengths and have an application in alignment in the three-point technique. The derived solutions are then checked for cases of undesired residual translations and rotations. Evaluations of the focus based on paraxial ray tracing equations are also performed.

We report some characteristic features concerning informative properties of the periodic and quasi-periodic light fields. On the basis of the sampling Fresnel-Dirac transformation, the idea of Fresnel periodicity is defined. Periodic or quasi-periodic light fields revealing this type of symmetry have properties of lensless imaging and self-restoration, such as the elimination of lattice defects-vacancies or inclusions, correction of quasi-periodic structures, and extraction of the common part of elements.

Diffraction pattern representation of images (optical Fourier transform or Wiener spectrum) is very useful for computer image processing. An accurate analysis of spatial frequencies requires very well corrected lenses, and the diffraction pattern must be sampled by means of a moving detector or a stable multiple detector array strictly in the Fourier plane. A theoretical analysis of obtaining and sampling the diffraction pattern is provided. Fourier diffractometry with a directionally changed incident light wave can be realized without the above-mentioned limitations. In this method, a single static detector is placed at the rear focus of a Fourier transform lens while the sampled diffraction pattern is moved over the Fourier plane. Suitable movement of the diffraction pattern can be obtained when the analyzed image is continuously illuminated with a parallel light beam whose inclination is changed with respect to the optical axis of the Fourier transform lens. This idea was tesied experimentally and applied to a new generation of compact diffractometers with unsophisticated transforming lenses.

The imaging quality of holographic lenses depends on parameters that include the shape of a holographic lens surface or an input pupil position. Based on the formulas for third-order aberration coefficients derived for such cases, conditions that ensure the correction of aperture and field aberrations are given. The possibility of joint correction of spherical aberration, coma, and astigmatism is discussed. The formulas presented are illustrated with a number of examples; two types of holo-lenses are taken into account: imaging and focusing. For imaging quality assessment an aberration spot calculation method based on numerical evaluation of an appropriate diffraction integral is used. The results of this method are compared with the results of imaging quality estimation using the geometrical ray tracing method.

Two measuring systems are presented enabling determination of the refractive index profile and its anisotropy as well as principal stress components in optical fibers and preforms, respectively. The system for optical fibers is a scanning-type, differentiating interferometer used to measure directly the wavefront derivative, from which, after the inverse Abel transformation, the index profile is obtained. The required high sensitivity of measurement is achieved by applying the sinusoidal modulation of the input beam ellipticity and the homodyne detection of the first harmonic component of the output intensity. After removing the Wollaston prism, the system can be used to measure the retardation function that is related to the fiber residual birefringence. The dynamic spatial-filtering technique, used until now to measure the ray deflection function, has been modified for testing the preforms. An optionally applied linear modulator of ellipticity of the input beam was added, to enable the measurement of the retardation function also. The system can be easily switched from the measurement of the ray deflection function to the measurement of the retardation function by moving only a single element.

A prototype of a new optical length transducer is presented. The system is based on a laser diode illumination and a holographic grating applied as a length master. The theory of the optical method is formulated using Doppler description. General characteristics of the device construction and metrological feasibilities of the prototype are presented. Resolution of 0.02 μm and nonlinearity error ±0.05 μm in the measuring range of 4.5 mm are obtained.

We present an analysis and accuracy determination of the phenomena of mechanical, optical, and electronic signal transducing that takes place in optoelectronic measuring systems. As an object of theoretical and experimental investigation, length-measuring transducers have been adopted in which incremental masters are applied in the shape of binary amplitude diffraction gratings. In detector systems of those transducers, the moiré phenomenon is utilized. This phenomenon causes averaging of the errors of masters as well as makes possible the creation of optoelectronic structures that compensate for other error components, e.g., clearances in the transducer system. The results of analysis of the moiré pattern kinematics, which take into account parameters connected with the process of detection and structural transducer parameters, make possible the optimization of the transducer design.

A general, nonparaxial differential equation is derived describing the refracting surface of revolution, which is able to focus light into a segment of the optical axis that has an arbitrary length, position, and longitudinal intensity distribution. The approach is based on the energy conservation principle in the geometric-approximation and ray-tracing equations. Also presented are numerical solutions for uniform-intensity axicons because the equation has only analytic solutions in the limiting cases of a stigmatic lens and classical axicon.

CCDs used for low light level applications require a long integration time and/or cooling to have acceptable SNR. Standard commercially available CCD cameras (e.g., for TV applications) are used with a relatively short integration time (≈1/60 s per field) and are operated at room temperature. Consequently, these cameras will have poor SNR if used for low light level applications. The addition of successive frames from a standard CCD camera for the improvement of the SNR is considered here. We show, using a simple model, that a range of signal levels (as low as a few millilux) exists for which frame addition is a viable alternative to long integration times and cooling.

Mathematical expressions are presented to relate object and image plane tilts in nonaxially symmetrical optical systems. Examples that illustrate the use of these expressions are given. Keystone distortion is described and quantified as a function of image plane tilt.

A carrier fringe technique for measuring surface deformation is described and verified by experiments. In contrast to conventional holography and fringe analysis, this holographic system is based on fiber optics and automatic spatial carrier fringe pattern analysis techniques. Single-mode optical fibers are used to transfer both the object and reference beams. Carrier fringes are generated by simply translating the object beam between two exposures. The fast Fourier transform method is used to process the interferograms. The experiment gives an example of the tile-level minimum spanning tree phase unwrapping technique and introduces a new pixel-level noise immune unwrapping strategy also based on minimum spanning trees. The test object is a centrally loaded disk. An excellent correlation between the theoretical deformation profile and that suggested by the technique is given.

A quadratic-phase holographic lens can give a Fourier transform that is multiplied by an undesired parasitic phase, caused by the off-axis configuration. We show that it is possible to significantly reduce the parasitic phase by controlling the recording and readout geometries of the holographic lens so as to allow complete Fourier transformation. As a practical example we incorporate such a holographic lens into a conventional coherent light correlator, and show how to optimize the correlation peaks even when the illumination wavelength of the correlator differs from that used when recording the holographic lens and the filter.

The pulsed laser holographic vibration analysis on high-speed rotating structures presents several difficulties due to the rotational movement of the object between the two laser pulses recording the holographic interferogram. Apart from the vibrational motion, the recorded holographic fringes can be additionally modulated due to the object's rotary motion. Beyond small angles of rotation, a decorrelation of the recorded object waves occurs, preventing the formation of high-visibility interference fringes. To overcome these effects, different holographic recording techniques were used, including an interferometer in which both the reference wave and the holographic recording plate are rotated simultaneously in relation to the object and an interferometer incorporating an image derotator. Conditions for the practical application of these experiments are outlined, and examples of vibrational studies of high-speed rotating components, i.e. , cantilever beams, disks, cooling fans, and radial impellers, are presented. The radial impeller vibration investigation was done on a compressor test facility up to object speeds of more than 13,000 rpm, corresponding to a circumferential speed of approximately 200 m/s.

A computer simulation was performed to obtain an estimate of the infrared radiant intensity and radiative contrast produced by missile noses. The temperature distribution on the nose was computed by taking into account the effects of aerodynamic and solar heating, sky irradiance, and radiative cooling. The corresponding absolute and contrast signatures were obtained by considering the influence of both atmosphere (self-emission and absorption) and target geometry (distance, aspect, and elevation angles) for six target classes, including cruise missiles, tactical ballistic missiles, and air-to-air and surface-to-surface missiles. Both radiant intensity and radiative contrast were computed as a function of missile altitude or missile distance for all targets. A comparison of radiative quantities in the 3- to 5-μm and 8- to 12-μm bands shows that usually the spectral region that gives the strongest signal is not the one that gives the highest contrast.

The Upper Atmosphere Research Satellite (UARS) is dedicated to the study of the earth's atmosphere, composition, and temperature. It carries several instruments. Among them, WINDII is a Michelson interferometer used to observe wind and temperature in the upper mesosphere and thermosphere. The onboard calibration of the interferometer requires a He-Ne laser of frequency stability of 10^-9. To be used in the space environment, particular specifications have been met by a special design described here. Performances of the system are shown and analyzed.

A high-gain, high-energy extraction efficiency, phase-conjugated, multiple-pass amplifier was developed using Nd:YAG. The amplifier was used with a low-energy, self-injection locked, passively Q-switched, highrepetition-rate oscillator to obtain energies of 0.85 J in SLM laser pulses of durations variable between <7 and >17 ns. At high repetition rates (20 Hz), the average power reached 14.5 W. Beam quality was better than 2 x diffraction limited. A two-dimensional multiple-pass model was developed for the purpose of interpretation and design.

Low-frequency mechanical vibrations are a significant problem in robotics, machine vision, and practical reconnaissance where primary image vibrations involve random process blur radii. They cannot be described by an analytical MTF. A method of numerical calculation of MTF, relevant in principle to any type of image motion, is presented. It is demonstrated here for linear, high, and low vibration frequencies. The method yields the expected closed form solutions for linear and high-frequency motion. The low-vibration-frequency situation involves random process blur radii and MTFs that can only be handled statistically since no closed form solution is possible. This is illustrated here. Comparisons are made to a closed form approximate MTF solution suggested previously for low-frequency motion. Agreement between that analytical approximation and exact MTF calculated numerically is generally good, especially for relatively large and linear motion blur radius situations. For nonlinear short exposure motion, MTF levels off at relatively high nonzero values and never approaches zero. Such situations yield a two-fold benefit: (1) larger spatial frequency bandwidth and (2) higher MTF values at all spatial frequencies since MTF does not approach zero.

wideband optical heterodyne spatial tracker for use in diodelaser-based heterodyne optical space communications is described. The tracking error signals are formed by correlating the output of an intensity-noise-canceling communications receiver with the outputs of the tracking photodetectors. This correlation technique removes noise-induced offsets, which may significantly limit performance of other wideband tracking schemes and is an alternative to adding amplitude modulation in the track channel. The technique also enhances the SNR in the track channel relative to square-law-based demodulation of the track channel alone. The correlation concept is experimentally verified and closed-loop tracking performance is demonstrated.

We consider the effects of clutter in infrared and television imagery on the performance of the joint transform correlator operating in the adaptive mode. Computer simulation results show that the energy of the target object must be greater than the energy of the background to have confidence in the system's tracking capability.

A narrowband signal excision system uses spatial light modulators in the spatial frequency domain to notch energy caused by narrowband interferers. We review the signal excision process and discuss the optical system developed to monitor the noise spectrum. A postdetection system searches for narrowband interferers in wideband spectra and provides control signals to implement the required notch; experimental results of the distortion imposed on short pulse signals are given.

Blockwise transform image enhancement techniques are discussed. Previously, transform image enhancement has usually been based on the discrete Fourier transform (DFT) applied to the whole image. Two major drawbacks with the DFT are high complexity of implementation involving complex multiplications and additions, with intermediate results being complex numbers, and the creation of severe block effects if image enhancement is done blockwise. In addition, the quality of enhancement is not very satisfactory. It is shown that the best transforms for transform image coding, namely, the scrambled real discrete Fourier transform, the discrete cosine transform, and the discrete cosine-III transform, are also the best for image enhancement. Three techniques of enhancement discussed in detail are alpha-rooting, modified unsharp masking, and filtering motivated by the human visual system response (HVS). With proper modifications, it is observed that unsharp masking and HVS-motivated filtering without nonlinearities are basically equivalent. Block effects are completely removed by using an overlap-save technique in addition to the best transform.

Results are presented from a horizontal path imaging experiment in which a 0.5-m telescope was focused on targets located at a range of 1 .2 km. The targets varied in complexity from simple binary letters to extended representations of satellites with gray scale and size variations. Imaging at a center wavelength of 0.7 μm, we found an atmospheric degradation factor of D/_r0 = 17, on average. We used a slow read-rate bare CCD detector and thus had to deal effectively with additive noise in the speckle measurements. Our image reconstruction algorithms are based on the use of the complex bispectrum, and we have demonstrated diffraction-limited imaging down to light levels approaching a few photons per speckle per resolution area. We have paid careful attention to the effects of additive noise on the reconstruction process and have shown that they can be adequately overcome. These results support the feasibility of high-resolution speckle imaging of high-earth-orbit satellites using CCDs.

This PDF file contains the editorial “Book Rvw: The Physics of Moire Metrology," by Oded Kafri and Ilana Glatt for OE Vol. 31 Issue 03

This PDF file contains the editorial “Book Rvw: Introduction to Nonlinear Optical Effects in Molecules & Polymers," by Paras N. Prasad and David J. Williams for OE Vol. 31 Issue 03