This PDF file contains the editorial “2008 in Review” for OE Vol. 48 Issue 02

We use a paraxial approximating solution to calculate the point spread function of the collinear holographic storage system and show that the point spread function can be dramatically enhanced by the reference pattern with random binary phase modulation or random phase modulation.

The simultaneous modulation of coherent light distributions on several closely spaced planes by one phase-only spatial light modulator is described to create 3-D light distribution. The pure phase distribution on the phase-only spatial light modulator is retrieved by a new multiplane iteration method based on the Gerchberg-Saxton iteration method, which consecutively propagates the coherent light on several parallel planes with amplitude constraints. A comparison between our proposed method and the complex distribution addition method is presented by numerical simulation. The result indicates that a sharp grayscale image is realized on each closely spaced plane by our proposed method, implying potential applications both in 3-D holographic displays and optical tweezers.

We demonstrate a field programmable gate-array-based real-time optical Doppler tomography system. A complex-valued bandpass filter is used for the first time in optical coherence tomography signal processing to create the analytic signal. This method simplifies the filter design, and allows efficient and compact implementation by combining the conversion to an analytic signal with a pulse shaping function without the need for extra resources as compared to the Hilbert transform method. The conversion of the analytic signal to amplitude and phase is done by use of the coordinate rotation digital computer (CORDIC) algorithm, which is an efficient algorithm that maps well to the field programmable gate array. Flow phantom experiments, and the use of this system for in vivo imaging of cardiac dynamics in the chick embryo, are presented. We demonstrate the visualization of blood flow in the early embryonic heart as well as in the aorta, small peripheric vitelline vessels, and coronary arteries of fully formed chick hearts.

The correction of the human eye's higher-order wave aberrations by a phase plate has been studied. The phase plate was made by using a single-mask-moving technique. The imaging quality has been analyzed in terms of several metrics. The results show that the fabrication error is 0.05 μm, which is close to λ/14 at the medium value of the wavelength of white light, and the performance of the eye can be improved by the phase plate, indicating that the single-mask-moving technique has potential applications in clinically correcting ocular aberrations.

Characterization of deformation and surface shape is an important parameter in quality testing of micro-objects in view of the functionality, reliability, and integrity of the components. Single-wavelength TV holography is widely used for deformation analysis. However, the single-wavelength TV holographic configuration suffers from overcrowding of fringes for large deformation that sets a limitation due to speckle decorrelation for quantitative fringe analysis. Furthermore, shape cannot be determined when using single wavelength. In this paper, we describe a multiple-wavelength microscopic TV holographic configuration that uses sequentially recorded phase-shifted frames at three different wavelengths before and after deformation of the specimen for evaluation of relatively large deformation fields at the effective wavelengths. Use of multiple wavelengths for deformation and shape evaluation is discussed. The design of the system along with the experimental results on small-scale rough specimens under static load is presented.

Fusarium head blight is a worldwide fungal disease of small cereal grains such as wheat that affects the yield, quality, and safety of food and feed products. The current study was implemented to develop more efficient methods for optically detecting Fusarium-damaged (scabby) kernels from normal (sound) wheat kernels. Through development of a high-power pulsed LED (green and red) inspection system, it was found that Fusarium-damaged and normal wheat kernels have different reflected energy responses. Two parameters (slope and r²) from a regression analysis of the green and red responses were used as input parameters in linear discriminant analysis models. The examined factors affecting accuracy were the orientation of the optical probe, the color contrast between normal and Fusarium-damaged kernels, and the manner in which one LED's response is time-matched to the other LED. Whereas commercial high-speed optical sorters are, on average, 50% efficient at removing mold-damaged kernels, this efficiency can rise to 95% or better under more carefully controlled, kernel-at-rest conditions in the laboratory. The current research on free-falling kernels has demonstrated accuracies (>90% for wheat samples of high visual contrast) that approach those of controlled conditions, which will lead to improvements in high-speed optical sorters.

Design of an optical pattern in a light-guide panel (LGP) has relied on empirical methods. However, the characteristics of developing liquid-crystal display (LCD) products such as frequent design modifications, various design conditions, and a short development period make it difficult for the empirical design approach to cope with various design requirements for size, shape, and optical performance of the LCD products. The most important tasks for the design of LGPs are improving average illuminance and the uniformity of the backlight unit. To meet these requirements, a design for an incoupling and an outcoupling part of the LGP is presented. These two parts can be designed in two separate phases: the first for the incoupling part and the second for the outcoupling part. The shape of serration in the incoupling part was first determined by design of experiments, and the dot patterns in the outcoupling part were subsequently determined by a density-based approach with progressive quadratic response surface modeling. Using this design approach, the illuminance was increased from 2241 lx in the initial design to 2299 lx in the optimal design, and its uniformity also increased from 38% to 82%.

Development of maskless lithography techniques may solve the problem of photomask cost. Furthermore, it could open a market for small-scale manufacturing applications. Since femtosecond lasers have been found suitable for processing of a wide range of materials with submicrometer resolution, it is attractive to use this technique for maskless lithography. In this paper, we report direct laser writing of lithographic patterns with submicron feature width on thin photoresist film by a femtosecond laser. The patterns were analyzed with a scanning electron microscope. The effects of laser energy and number of pulses on the feature size were investigated. Finally, we present various results on submicron photoresist patterning, which show great potential for future application.

Incipient fault diagnosis is closely related to insulation condition assessment. A great number of methods are available for condition monitoring and diagnosis of power transformer insulation systems, but only few of them can take direct measurements inside the transformer. Fiber optic sensors can be applied to incipient fault diagnosis. In particular, acoustic sensors have been developed for detection and location of partial discharges in oil-filled power transformers. We report the study of extrinsic and intrinsic fiber Fabry-Pérot sensors that can be used to detect the acoustic waves that are generated by a partial discharge inside a power transformer. A comparative analysis is done to determine the best sensing head configuration and some methods to improve the parameter readout sensitivity are proposed. The sensing head behaviour when immersed in different fluids (air, water, and oil) is also investigated.

A fiber Bragg grating (FBG) pressure sensing scheme based on a flat diaphragm and an L-shaped lever is presented. An L-shaped lever transfers the pressure-induced defection of the flat diaphragm to the axial elongation of the FBG. The curve where the L-shaped lever contacts the diaphragm is a segment of an Archimedes spiral, which is used to enhance the responsivity. Because the thermal expansion coefficient of the quartz-glass L-shaped lever and the steel sensor shell is different, the temperature effect is compensated for by optimizing the dimension parameters. Theoretical analysis is presented, and the experimental results show that an ultrahigh pressure responsivity of 244 pm/kPa and a low temperature responsivity of 2.8 pm/°C are achieved.

We prove that the tuning range of a one-pump fiber parametric wavelength converter based on degenerate four-wave mixing can be maximized by selecting fibers with appropriate zero dispersion wavelength and high-order dispersion coefficients. The maximal tuning range is proportional to the cube root of the product of the pump power and fiber nonlinear coefficient. If the effects of fourth-order dispersion are utilized, the tuning range can increase dramatically.

We experimentally demonstrate that a 3×3 port fused fiber grating coupler can be implemented by combining the technique of fabricating fused-fiber couplers as well as that of writing fiber Bragg gratings for application in all-fiber optical filters. A Bragg grating with a reflectivity of 65% is written into the coupling region of a 3×3 fiber coupler with the coupling ratios of 60, 24, and 16%, respectively, for three different output ports. This device can be applied in optical multi/demultiplexing and add/drop filters or in special optical filters.

Based on Sagnac interferometric structure, a simple novel ultrafast scheme for an all-optical wavelength converter is proposed. The operations of this scheme with a 80-Gbits/s return to zero (RZ) pseudorandom bit sequence (PRBS) are simulated correctly with an output extinction ratio of more than 17.2 dB. Through numerical analysis, by comparison of the performance at 40- and 80-Gbits/s operation, the operating characteristics of the scheme are illustrated. Furthermore, the carrier recovery time of the semiconductor amplifier (SOA) is no longer a crucial parameter to restrict the operation speed of this scheme.

We assess the effectiveness of a previously proposed noise reduction technology for hyperspectral imagery to examine whether it can better serve remote sensing applications after noise reduction using the technology. Target detection from hyperspectral imagery using a spectral unmixing approach is selected as an example in the assessment. A hyperspectral datacube acquired using an airborne short-wave-infrared Full Spectrum Image II with man-made targets in the scene of the datacube is tested. Three criteria are proposed and used to evaluate the detectability of the targets derived from the datacube before and after noise reduction. The evaluation results show that the detectability of the targets is significantly improved after noise reduction using the technology. The targets not detected from the original datacube are detected with high confidence after noise reduction using the technology. A noise reduction technique that is based on a smoothing approach is also evaluated for the sake of comparison to the proposed noise reduction technology. It also improves the detectability of the targets, but is less effective than the proposed noise reduction technology.

An optical implementation of the amplitude encoded double random phase encryption/decryption technique is implemented, and both numerical and experimental results are presented. In particular, we examine the effect of quantization in the decryption process due to the discrete values and quantized levels, which a spatial light modulator (SLM) can physically display. To do this, we characterize a transmissive SLM using Jones matrices and then map a complex image to the physically achievable levels of the SLM using the pseudorandom encoding technique. We present both numerical and experimental results that quantify the performance of the system.

We develop an effective algorithm for colorizing a grayscale image. In our approach, a reference color image, an RGB to Lαβ color transform L=luminance, α β=chrominance), and a block-based vector quantization of luminance mapping (VQLM) technique are used to automatically colorize the grayscale image. The VQLM technique compares the grayscale image with the luminance of the color reference image to obtain the information of the αβ planes of the grayscale image. After the chrominance is padded, the inverse color transform, L αβ to RGB, colorizes the grayscale scene is colorized. Meanwhile, we create a mean of VQLM (MVQLM) method to improve the quality of the colorized grayscale image. Experimental results show the MVQLM method is better than the VQLM method. Also, we investigate colorizing the grayscale image working in the Lαβ and YIQ spaces. The simulation results also reveal that working in the Lαβ space is slightly better than working in the YIQ space. Compared to other colorizing schemes, our proposed method has two advantages: 1. the codebook and MVQLM techniques colorize the grayscale images for any size image that can be evenly divided by 2 automatically; and 2. the MVQLM method obtains a smoother colorizing effect and improved quality compared to the VQLM method.

Since algorithms based on Bayesian approaches contain smoothing parameters associated with the mathematical model for the prior probability, the performance of algorithms usually depends crucially on the values of these parameters. We consider an approach to smoothing parameter estimation for Bayesian methods used in the medical imaging application of emission computed tomography (ECT). We address spline models as Gibbs smoothing priors for our own application to ECT reconstruction. The problem of smoothing parameter estimation can be stated as follows. Given a likelihood and prior model, and given a realization of noisy projection data, compute some optimal estimate of the smoothing parameter. We focus on the estimation of the smoothing parameter for mathematical phantom studies. Among the variety of approaches used to attack this problem, we base our maximum-likelihood (ML) estimates of smoothing parameters on observed training data. To validate our ML approach, we first perform closed-loop numerical experiments using the images created by Gibbs sampling from the given prior probability with the smoothing parameter known. We then evaluate the performance of our method using mathematical phantoms and show that the optimal estimates obtained from training data yield good reconstructions in terms of a percentage error metric.

Based on a multiplicative iterative algorithm, a novel algorithm named multiframe multiplicative iterative algorithm (MMIA) has been developed for performing multiframe blind image restoration. By further extending the multiplicative iterative algorithm to multiframe blind deconvolution, the MMIA is derived. Experimental results well demonstrate that the MMIA not only has good performance and efficiency, but also achieves high resolution of the restored image.

Color-based object indexing and matching is attractive because color is an efficient visual cue for characterizing an object. However, different light sources will lead to color deformation of objects, which will inevitably degrade the performance of recognition. In order to circumvent this confounding influence, we present three effective illumination-independent descriptors that are substantially insensitive to the variations of light conditions, object geometric transformation, and image blur level. To this end, we define two new two-dimensional color coordinate systems, the central color coordinate system and the edge-based color coordinate system, based on the diagonal-offset reflectance model. By introducing normalized moment invariants, this paper then provides two color-constant descriptors in each system separately. Either descriptor is a feature vector consisting of several normalized moment invariants of the color pixels' distribution with different orders. Furthermore, the combination thereof can characterize not only color content but also color edges in an image, so it serves as a third descriptor of the image. Experiments on real image databases with object recognition and image retrieval show that our descriptors are robust and perform very well under various circumstances.

The joint transform correlator (JTC) is a classical optical architecture, recently associated with interesting applications in optical security. In addition, multiplexing is a tactic associated with optical encrypting mechanisms that provide security for multiple users. However, experimental constraints arise when we intend to reproduce a diffuser shifting multiplexing option. This is due to the invariance to lateral shifts under a JTC setup. To overcome this problem, we propose a setup modification that allows its use under a multiplexing approach. Instead of placing the encrypting mask in contact with the input JTC plane, we use the actual optical Fourier transform of a diffuser projected over the entrance plane of the JTC. We present a theoretical explanation, along with computer simulations and experimental results, that support our proposal.

Overlapping is a major problem for machine vision applications in agriculture. We present a robust marker-controlled watershed transform algorithm to automatically perform the accurate segmentation of overlapping plant fruits. The marker-controlled watershed algorithm mainly involves image preprocessing, marker extraction, and watershed transform. Marker extraction is the most important and difficult step of the whole process. Using K-means clustering, cut point decision making, spline interpolating, and morphological processing, markers can be detected automatically. Due to the good localization performance of detected markers, the accurate contour of separated fruits can be extracted by the watershed transform based on detected markers. The face validity of the segmentation algorithm is tested with a set of grape images, and segmentation results are overlaid onto original images for visual inspection. The algorithm is further evaluated by comparing segmentation results with a "gold standard" established by professional agronomists. Quantitative comparison shows that the segmentation algorithm can obtain very good spatial segmentation results.

A billiard ball tracking system is designed to combine with a visual guide interface to instruct users for a reliable strike. The integrated system runs on a PC platform. The system makes use of a vision system for cue ball, object ball and cue stick tracking. A least-squares error calibration process correlates the real-world and the virtual-world pool ball coordinates for a precise guidance line calculation. Users are able to adjust the cue stick on the pool table according to a visual guidance line instruction displayed on a PC monitor. The ideal visual guidance line extended from the cue ball is calculated based on a collision motion analysis. In addition to calculating the ideal visual guide, the factors influencing selection of the best shot among different object balls and pockets are explored. It is found that a tolerance angle around the ideal line for the object ball to roll into a pocket determines the difficulty of a strike. This angle depends in turn on the distance from the pocket to the object, the distance from the object to the cue ball, and the angle between these two vectors. Simulation results for tolerance angles as a function of these quantities are given. A selected object ball was tested extensively with respect to various geometrical parameters with and without using our integrated system. Players with different proficiency levels were selected for the experiment. The results indicate that all players benefit from our proposed visual guidance system in enhancing their skills, while low-skill players show the maximum enhancement in skill with the help of our system. All exhibit enhanced maximum and average hit-in rates. Experimental results on hit-in rates have shown a pattern consistent with that of the analysis. The hit-in rate is thus tightly connected with the analyzed tolerance angles for sinking object balls into a target pocket. These results prove the efficiency of our system, and the analysis results can be used to attain an efficient game-playing strategy.

The Richards-Wolf approach to analyze tight focusing by high-numerical-aperture aplanatic optical systems was designed to treat incident waves having a planar (or negligibly curved) wavefront at the entrance pupil. In this paper we extend the approach to incorporate also wavefronts with piecewise quasi-constant phase. The evaluation of the field distribution in the vicinity of the geometrical focus is accomplished by linear superposition of the contributions from all the segments with quasi-constant phase. As an application example, a tightly focused optical field is evaluated for a π-phase-step-modulated incident wavefront.