This PDF file contains the editorial “Editorial: Here We Go Again” for OE Vol. 34 Issue 01

The design and characterization of a 54-μs, continuously variable, acousto-optic delay line developed for radar testing applications is documented. The operating principles of this new common-path delay line architecture are reviewed, and key component selection issues are discussed. Ultimately, the characteristics of these key components limit the achievable performance of the completed delay line. For example, the laser diode introduces high-frequency noise at the output of the delay line. This noise, which is a direct function of the relative intensity noise of the laser, is the limit to the output noise floor. Bragg cell design is presented with emphasis on the minimization of multiple time-delayed signals caused by acoustic reflections. A computer design and analysis tool is introduced that predicts delay line performance in terms of bandwidth, insertion loss, and acoustic attenuation for this slow shear, tellurium dioxide (TeO₂) based delay line. Experimental results are presented showing 10-MHz operating bandwidth at a 33-MHz center frequency. The 3-dB bandwidth is maintained over delays ranging from 0.75 to 54 μs.

The resolution of a large telescope is limited by various kinds of dynamic wavefront disturbance such as atmospheric turbulence. This has been a problem since the invention of the telescope. An approach for solving the problem is provided by adaptive optical techniques. Compensating for stellar objects in real time is, however, very difficult, because only limited light energy can be used and the temporal-spatial characteristics of dynamic disturbances place very high demands on system specifications. This paper reports the first experiment of a 21-element adaptive optical system for stellar objects on the Yunnan Observatory 1.2-m telescope and presents the improvements of the system for low-light-level stellar objects. The experimental results show that the adaptive optical system can work satisfactorily when the photon counts are as low as 93 per subaperture per integrating time. The experiments show that the effects of adaptive optics in reducing the divergence and wandering of the image spot, in improving resolution, and in increasing image contrast are significant.

The theory and methods of adaptive noise cancelling (ANC) are discussed briefly. By theoretical analysis and experimental studies, the ANC technique is successfully applied in the straightness measurement system based on optical activity. The experimental results of various alignment measurements for long and short distances are processed by the ANC technique, and it shows that the compensation effect after filtering under different conditions is 30% to 80%.

Analytic expressions are obtained for the aperture-averaging factor associated with optical scintillations of unbounded plane waves and spherical waves in the weak fluctuation regime. This analysis is based on a modified spectrum of refractive-index fluctuations that features inner scale and a high wave number bump. Asymptotic results for both small- and large-aperture receivers compared with the first Fresnel radius (L/k)^1/2 are used to construct simple interpolation formulas, which are generally within 5% of exact numerical-integration values in the case of a plane wave and within 12% of exact values for a spherical wave. On comparison with published experimental data on spherical waves, we found similar agreement, with maximum error 8%.

We have designed a multisource frequency-domain spectrometer for the optical study of biological tissues. Eight multiplexed, intensity-modulated LEDs are employed as the light sources. Four of them emit light at a peak wavelength of 715 nm (λ₁); the other four, 850 nm (λ₂). The frequency of intensity modulation is 120 MHz. This instrument measures the frequency-domain parameters phase, dc intensity, and ac amplitude at the two wavelengths λ₁ and λ₂ and for different distances between light source and detector. From these frequency-domain raw data, the absolute values of the absorption and reduced scattering coefficients of tissue at λ₁ and λ₂ are obtained. The oxy- and deoxyhemoglobin concentrations, and hence the hemoglobin saturation, are then analytically derived from the molar extinction coefficients. Acquisition times as short as hundreds of milliseconds provide real-time monitoring of the measured parameters. We performed a systematic test in vitro to quantify the precision and accuracy of the instrument reading. We also report in vivo measurements. This spectrometer can be packaged as a compact portable unit.

Characterization of a Kodak KAF 4200 CCD chip, coated with the UV converter Lumigen, is described. This chip is very similar to a chip described by Ninkov, Backer, and Bretz [Proc. SPIE1987, 14 (1993)], except for the Lumigen coating. The major difference in perlormance is in the existence of significant deviations from linear output for a given input up to 30% at low count rates. The nonlinearity varies across the chip and is largest for pixels that are the most distant from the readout serial register. Tests revealed that the nonlinearity depends on wavelength in a minor way only, and is temperature dependent. Characterization of the nonlinearity allowed the creation of a function that allows for correction of the data. It is unlikely that this nonlinearity can be explained using arguments invoking deferred charge effects or peculiar behavior of the on-chip amplifier. Comparison with similar but uncoated CCD chips makes the Lumigen coating a likely source of the problem.

The output from diffraction gratings is formed using a Fourier transform lens. Because optical detectors are sensitive to intensity, information about the phase of the diffracted beams is lost. The relative phase shift between different diffracted orders can be measured using a multiplexed lens consisting of two offset lenses This lens forms two separate copies of the diffraction pattern. If a diffracted order from one copy overlaps a different order from the second copy, the resulting interference allows analysis of the relative phase between the different diffracted orders. Experimental results are presented using multiplexed lenses written onto a magneto-optic spatial light modulator.

The temperature dependence of polarization-maintaining fibers, wound onto two different diameter (4- and 1-in.) glass cylinders, is measured and compared. The fluctuation of the extinction coefficient for the smaller spool is much greater for the same set of conditions and is roughly proportional to the rate of the temperature change.

Cross-modulation distortion arising from the dynamic nonlinearity of a semiconductor laser is shown to be an important entity in a multicarrier fiber optic system. The interplay between cross-modulation distortion and intermodulation distortion is investigated for lasers operating at 1.3 and 0.835 μm.

An analytical model has been developed to account for coupling losses between two Gaussian beams with misalignments in different planes. The verification of the model is implemented via the coupling of laser diode light into an optical fiber. A nonzero end-face angle on the fiber tip allows for simple adjustment of misalignment planarity and can be interpreted as independent of traditional longitudinal, transverse, and tilt offsets. The model is intended for applications where variations in coupling efficiency must be accurately evaluated and where the potential for out-of-plane misalignments must be considered. Experimental data are presented for the coupling of light between a laser diode module and several single-mode fibers with different end-face angles. The predicted change in coupling efficiency agrees with theory. However, the absolute coupling efficiency is only 0.71 of that predicted by theory, as is common for initial launch. Inasmuch as the theory presented is for any two Gaussian beams, the absolute coupling efficiency is expected to be more accurate for fiber-to-fiber applications.

Utilizing the Herschel condition, a method is presented for null testing most refractor doublet objectives, including the Baker air-spaced design with equal internal radii, and Maksutov corrector plates using auxiliary spherical mirrors. A special Baker air-spaced objective is offered that utilizes a spherical mirror in null testing that is generated by silvering the front face of the doublet's crown lens or immersing it into mercury. The classical Schmidt corrector is found to deny null testing using the technique.

A change of image orientation caused by a small rotation of a reflecting prism about an axis P is discussed. Based on image formation for reflecting prisms, a set of simple and explicit formulas for their adjustment is derived. The rotation theorem for reflecting prisms is proved, and the extreme-valued nature of the rotation axis is examined.

The diffractive phase profile of holographic stereograms is calculated using different types of diffusing functions, which can be categorized in two groups: object-dependent and object-independent diffusers. It is shown that object-independent diffusing profiles lead to considerable computational savings at the expense of some interperspective leakage. Object-dependent diffusers solve that problem, but their computational complexity probably restricts their use to noninteractive displays.

Most of the existing lossy encoding schemes rely on the stochastic image model with a spatially uniform correlation and show decreasing performance as sharp edge features increase in images. To cope with images where sharp edge features convey more important information, a relatively new encoding scheme utilizing a Laplacian edge detector is proposed and experimented with prototype sample images. In this scheme, the Laplacian operator is applied to an image to get a corresponding edge field and, in turn, the edge field is encoded with the Huffman code. A quantization, a fundamental information reduction step, is also carried out in the edge field, and the corresponding error field, the overall intensity variations, is approximated with some discrete sine transform coefficients. A compression ratio up to 5 or 6 can be achieved with the current implementation without a severe degradation of image quality. An implementation with an image divided into tiles and an extension to an hierarchical model will be studied in the future.

The recognition of raised alphanumeric markings on rubber tires for their automatic classification is presented. Raised alphanumeric markings on rubber tires show different characteristics from those of printed characters. In the preprocessing step of the proposed method, we first determine the slope of an arc, along which alphanumerics are marked, using the Hough transform, and align them horizontally. Then we separate each character using vertical and horizontal projections. In the recognition step, to recognize characters hierarchically we use several effective features, such as width of a character, number of cross points, partial projections, and distance features. Computer simulation results show that the proposed system can be successfully applied to automatic classification of rubber tires.

An analytical approach for the three-dimensional recognition of quadric surfaces for range images is presented. Initially, after describing each of the quadrics in terms of their implicit representations, a new iterative technique is utilized to determine and eliminate the rotation parameters of the surfaces. The main characteristic of this technique is that it converges in a very few iterations. Next, based on the intersections of surfaces with planes of different orientation, a unique set of feature vectors of characteristic curves is determined for each of the quadrics considered. Furthermore, within the same analysis, angular bounds for the orientation of the intersecting planes are evaluated, so that the necessary and sufficient number of planes is determined for characterizing a surface. Experiments conducted on several sets of real data on quadric surfaces indicate that the proposed method is very accurate and computationally inexpensive.

Segmentation of an image based on texture can be performed by a set of N Gabor filters that uniformly covers the spatial frequency domain. The filter outputs that characterize the frequency and orientation content of the intensity distribution in the vicinity of a pixel constitute an N-element feature vector. As an alternative to the computationally intensive procedure of segmentation based on the N-element vectors generated at each pixel, we propose an algorithm for selecting a pair of filters that provides maximum discrimination between two textures constituting the object and its surroundings in an image. Images filtered by the selected filters are nonlinearly transformed to produce two feature maps. The feature maps are smoothed by an intercompetitive and intracooperative interaction process between them. These interactions have proven to be much superior to simple Gaussian filtering in reducing the effects of spatial variability of feature maps. A segmented binary image is then generated by a pixel-by-pixel comparison of the two maps. Resuits of experiments involving several texture combinations show that this procedure is capable of producing clean segmentation.

We design, implement, and test an on-board optical joint transform correlator, using a nonlinear optically addressed ferroelectric liquid-crystal spatial light modulator in the Fourier plane. This compact correlator performs real-time and on-board recognition tasks, such as road sign recognition. The influence of the nonlinearity in the Fourier plane is investigated, and scale-invariant filters are tested and implemented. Preliminary results from real-time dynamic scenario experiments are presented. Electronic and optical pre- and postprocessing as well as the extension to silicon backplane devices are discussed.

A system approach is applied to overcome atmospheric degradation of remotely sensed images. A comparison is presented between different filtering techniques for restoration of distorted images for both the visible and thermal IR spectral regions. Restoration methods include spatial and spatial frequency filters. Best results are obtained by using a fractal model to describe the image's power spectral density for scenes in the visible spectral range. Atmospheric effects are best modeled by a noisy spatial frequency filter composed of an average component described by the average atmospheric and hardware modulation transfer function and a noisy component modeled by the atmospheric point spread function's power spectral density. The most impressive restorations in the visible range are achieved by combining the last model and the fractal model for the object's power spectral density. In the thermal range, however, several restoration techniques yielded very good results, with no one technique the most advantageous. The methods presented here are capable of yielding real-time image restoration with the resolution essentially limited by only the hardware in both wavelength regions.

One of the problems associated with nondestructive testing by infrared thermography with heat injection is related to the inspection of nonplanar objects. A method is proposed to determine the shape of an object present in the field of view from the corresponding single, early-recorded thermal image. Such shape reconstruction enables the correction of thermal image distortions due to object shape.

A new share-buffered direct-injection (SBDI) current readout circuit with high injection efficiency, low noise, high dynamic range, and good threshold control is proposed. The circuit is superior to the traditional direct-injection (Dl) current readout circuit. Using the SBDI readout circuit, the same excellent performance of the buffered direct-injection (BDI) current readout can be achieved, but only half the chip area and power consumption are required. Thus the SBDI is more suitable for infrared (IR) readout applications, especially for 2-D focal plane arrays under strict power and area limitations. A dynamic discharge source follower (DDSF) output stage is also proposed and analyzed. It can improve the speed performance of the conventional source-follower output buffer and requires very little power dissipation. Both simulation and experimental results have verified the functions and the advantageous features of the proposed readout structure.

A novel beam tester based on a single wedge plate shearing interference using a two-time rotatable method described by Xu and Rosenbruch in 1991 is developed. It is used primarily to test laser beam collimation and divergence and the defocusing of collimator, beam expander, and telescope systems. It also is used to test laser wavefront aberrations and parameters and to adjust optical systems. We briefly introduce the tester's basic principles, capabilities, features, and present an accuracy analyses of the tester.

Accurate static fringe-pattern analysis is very important for the successful application of a variety of interferometric techniques. In most cases of practical application, especially in aerodynamic flow testing, substantial noise can be introduced due to prevailing adverse environments. A means for efficiently reducing interferometric noise is thus desirable. Conventional noise reduction has mostly depended on ordinary averaging or median filtering in a squared mask to remove high-frequency components. These techniques, however, can induce some side effects of image blurring. If the structural integrity needs to be preserved, the method to be adopted should be able to eliminate noise efficiently without altering local intensity gradients, that is, local contrasts. In this paper, the concept of directional smoothing is introduced and its application to interferometric noise reduction is presented. Interferograms provide locally similar fringe directions, that is, isophase lines contaminated by noise, and thus contain directional information. In essence, the method exploits this valuable fringe directionality by setting up a slender mask of large aspect ratio along a fringe. A new value, that is, the average or median intensity of the mask, is then assigned to each pixel. The mask can be straight or curved. For a straight mask, the average direction of fringes within a processing region is employed. A curved mask is made to conform to a fringe curve. The proposed method is tested by computer simulation of experiments as well as with real interferograms. The results appear to be promising as compared with conventional techniques, especially for high-level noise.

A new numerical phase-measuring algorithm for digital phases-hifting interferometry is presented. In this algorithm, the actual values of phase shifts are taken as unknowns together with the wavefront phases ultimately to be determined, and they are computed through iterative spatial and serial least-squares fittings. The algorithm is more general than existing methods in that any arbitrary number of unequal phase shifts may be adopted with no prescriptive restrictions on their spacings. The algorithm is therefore found inherently free from any errors of phase shifts, allowing accurate measurement even in harsh environmental conditions with vibratory disturbances.

We briefly review the present status of work on all-optical photonic devices and discuss the properties of nonlinear optical materials that control and limit device performance. We then describe some recent work that indicates that As-rich GaAs is a promising material for ultrafast all-optical devices, and conclude with some projections for future progress in this field.

A dual-laser writing scheme is proposed in which an unmodulated short-wavelength read laser augments the writing process effected by a longer-wavelength laser. Apart from increasing the thermal efficiency of the laser marking process itself, the dual-beam writing scheme is shown to decrease the mark width and the recorded mark-length variability. Coupled with the increased resolution of the short-wavelength read spot, these enhancements could improve the performance and packing densities of media.

Servo-actuated limited-rotation motors provide an effective solution in many applications that require laser or infrared beam scanning. Early designs used moving iron rotors. The advent of rare-earth magnet materials has made possible the creation of a new type of scanner in which the rotor is a moving magnet. This paper describes the dynamic behavior, thermal characteristics, and scanning errors of this device. Comparisons made with the prior moving iron technology show moving magnet scanners to have much greater speed and accuracy. The discussion includes the effect of servo design and signal conditioning on scanning accuracy, as well as techniques for the measurement of very small dynamic scanning errors.

A new approach to fiber optic microbending sensors is demonstrated, in which the spatial frequency of the bends, rather than of the amplitude, is varied. This scheme has the potential for greatly increased sensitivity, greater flexibility of design, and longer lifetime. The fiber is prebent and embedded in a compliant material, and the spatial frequency is chosen to bias the sensor on the steep slope of the resonance in the loss curve. Compression in the longitudinal direction causes a dramatic change in the transmission. Sensitivity depends on the resonance slope, which is controlled by bend number, bend shape, and bend amplitude. In conventional microbend sensors, friction and abrasion severely limit the number of bends, and the maximum bend amplitudes and bend shapes cannot be controlled. In the new approach, all three parameters can be independently controlled, resulting in greater design flexibility,high sensitivity, and a long sensor life.

Four wavelengths (266, 355, 428, and 532 nm) generated from a prototype laser fluorosensor system were used to investigate the fluorescence spectral and temporal characteristics of crude and refined Kuwaiti oils in the laboratory. Temporal and spectral measurements were gathered for fresh and naturally weathered oil samples. Furthermore, a principal-components algorithm was developed for the purpose of identifying and classifying unknown oils.

Methods are presented for designing and estimating the performance of artificial apposition compound eye optical sensors. Apposition compound eyes have been investigated for a wide range of applications from robotics to smart weapons. While artificial apposition compound eyes have been constructed and demonstrated, optical design issues and performance prediction for these systems have never been adequately addressed in the literature. Apposition compound eyes are a useful paradigm for applications where wide field of view is critical but high spatial resolution is not required. Natural arthropod compound eyes and their biological models are first scrutinized to give insight into designing and modeling artificial apposition compound eye optical sensors. Such sensors are shown to have comparable sensitivity to traditional single-aperture sensors for extended sources. A method for enhancing resolution over the extended-source case is demonstrated for single point sources. For the first time, the frequency response of an artificial apposition compound eye is addressed while taking into account the differences between artificial and natural ommatidia. Both a numercal integration technique for determining the contrast transfer function and a Fourier-transform modulation-transfer-function method are presented and validated with experimental results.

A vibration sensing technique using the speckle contrast ratio from a multimode fiber is introduced. This sensing technique is based on time integration by a CCD camera, from which the contrast ratio of the speckle patterns can be measured. Important aspects of the proposed sensor are that it can be applied to high-frequency vibration sensing and that its operation is rather simple.

A fiber optic microdisplacement sensor is realized by placing a diffraction grating surface in front of multimode fiber endfaces. This grating serves as a dispersion component. The incident white light from a multimode fiber is dispersed by the grating and a portion of the optical power of the color band is picked up by the same fiber or another multimode fiber placed side by side. The spectrum of the light received by the second fiber is a function of the air gap between the fiber endface and the grating surface. Because diffraction angles are different for different wavelength components, the received power of different wavelengths varies differently as the air gap changes. A standard two-wavelength demodulation technique is employed as a self-referenced scheme to compensate for the signal power shift that occurs as a result of light source power fluctuations, fiber bending, and other disturbances. A resolution of 2 μm has been obtained over the range from 0 to 150 μm. Potential applications of this device are also discussed.

The quality of the optical phase conjugation, realized by means of the four-wave mixing in Kerr-like media, is examined. The fidelity of the generated replica of a signal field is investigated in suitable approximations and related problems are solved applying the Fourier transformation. The presented methods are based on the derivation of integro-differential mode equations valid for general conditions, including interaction of polych romatic nonparaxial waves.

In a series of papers we have developed various mathematical models for the solid angle centered at a scintillation point and subtended by a circular photomultiplier tube (PMT) in a gamma camera. In this third paper, the scintillation crystal, where the gamma ray is converted to visible light, shows a very diffusive plane in the PMT direction. The media on each side of the boundary are homogeneous. We develop a full mathematical model for the PMT response. This model depends on several physical parameters. They can be set up according to the kind of diffusion occurring at the interface. We also show that the direct and refracted solid angles are particular cases of the diffusive model. Then, for computational purposes, we approximate the model to bring its four-level integral representation to a two-level one. Numerical results show the effectiveness of our mathematical model of the solid angle function transmitted through a diffusive plane.

A correction to the definition of the atmospheric coherence diameter is suggested here, based on the existence of a practical instrumentation-based aerosol modulation transfer function (MTF), which is often the dominant ingredient of the atmospheric MTF. As defined classically by Fried about 25 yr ago, atmospheric MTF and coherence diameter were related to turbulence MTF only. Lutomirski considered diftractive aerosols, too, but did not consider effects of instrumentation on scattering angles actually recorded in the image. These are limited in the real world by instrumentation to milliradians, rather than by the broad angular spread of diffraction to radians. In the case of a Gaussian approximation of the practical aerosol MTF, an analytical expression is derived for the practical aerosol-derived coherence diameter. This parameter is related to the practical aerosol MTF's cutoff frequency, and to its asymptotic value at high spatial frequencies. Thus, a more general concept of atmospheric coherence diameter is proposed here, which is relevant to actual real-world imaging systems, whether they are passive or active. Quantitative validation of the theory is presented, based on both simulations and actually measured atmospheric MTFs in both the visible and thermal infrared spectral ranges. Overall atmospheric coherence diameter is determined generally by the smaller of the turbulence and practical aerosol coherence diameters, depending on optical depth. The results here appear applicable particularly to cost-effective thermal imaging system design, although applications are considered, too, for the visible and near infrared. For example, blur deriving from aerosol scatter should have much less effect in coherent detection laser radar (LIDAR) than in direct detection imaging.

We investigate the use of space/spatial-frequency and multiscale analysis methods for texture segmentation, with emphasis on the two-dimensional Wigner-Ville distribution and Morlet wavelet transform. For these two methods, the discrete versions that are necessary for numerical implementations are discussed. Texture segmentation paradigms making use of local spectral measurements from these two types of representations are described. The interest of the proposed spatial-frequency-and scale-based segmentation methods is illustrated with the aid of examples on both synthesized and natural images, and their segmentation performance is analyzed and compared.

The loss of throughput observed at higher energies for traditional grazing-incidence x-ray telescopes coated with high-Z elements can be partly countered by employing multilayers on the outermost reflectors. Using 8-keV reflectivity data from a periodic W/B4C multilayer, the expected performance of intermediate-sized telescopes of (1) the nested Kirkpatrick-Baez geometry and (2) the conical approximation to a nested Wolter-I geometry is computed. Depending on the multilayer design, the throughput was increased by a factor of 3 to 5 in a 1.5-keVwide band, or by 30% to 100% in a 3-keV-wide band. This gain is obtained at the expense of a 20% to 30% loss of throughput over the 2- to 4-keV band. These designs lend themselves well to astrophysics missions, such as spectroscopy of the H- and He-like iron emission lines (6.4 to 7.1 keV). The technology for multilayer coating, mounting, and configuring of the flat reflectors required by the Kirkpatrick-Baez telescope exists, so that an Fe-line multilayer telescope could be built today.

This PDF file contains the letter “Letter: Comment on the paper” for OE Vol. 34 Issue 01

This PDF file contains the letter “Letter: Response to Comment on the paper: New diode laser light source for absolute ranging two-wavelength interferometry [see 34(1)289(Jan1995)]” for OE Vol. 34 Issue 01

This PDF file contains the letter “Letter: November 1994 special section [see 33(11)3504(Nov1994)]” for OE Vol. 34 Issue 01

This PDF file contains the editorial “Book Rvw: Handbook of Lens Design," by Daniel Malacara and Zacarias Malacara for OE Vol. 34 Issue 01

This PDF file contains the editorial “Book Rvw: Fundamentals of Laser Optics," by Kenichi Iga for OE Vol. 34 Issue 01