Phase-only optical decryption of a fixed mask

We present a system for the implementation of phase-only optical decryption of an encrypted fixed phase mask. We achieve decryption by superimposing a decrypting phase key, implemented on a phase-only spatial light modulator with an encrypted phase mask in two equivalent image planes in an optical system. The decrypted phase information is retrieved by the generalized phase-contrast technique. We have constructed a compact experimental system that uses a 635-nm diode laser in which a fixed encrypted 0/pi binary phase mask is decrypted by a binary phase key produced electronically on the spatial light modulator. The key is aligned by electronic scrolling of its position with respect to the mask.     

Fractional derivatives-analysis and experimental implementation

The fractional derivative spatial-filtering operator is useful for image-processing applications, particularly for examination of phase objects. Experimental implementation is difficult because the mask function combines both amplitude and phase. We present a simple one-dimensional analysis of the fractional derivative operation and note similarities with the fractional Hilbert transform. We demonstrate how to encode these amplitude and phase masks using a phase-only liquid-crystal spatial light modulator and present experimental results. Finally, we introduce a radially symmetric extension of this operation that is more useful for objects having an arbitrary shape.     

Generation of Bessel beam arrays through Dammann gratings

In this work we apply the Dammann grating concept to generate an equal-intensity square array of Bessel quasi-free diffraction beams that diverge from a common center. We generate a binary phase mask that combines the axicon phase with the phase of a Dammann grating. The procedure can be extended to include vortex spiral phases that generate an array of optical pipes. Experimental results are provided by means of a twisted nematic liquid crystal display operating as a binary π phase spatial light modulator.     

Multireference binary phase-only filter optimized by regions of support

A new formulation of a multiplex filter for filtering-based optical processors, based on the VanderLugt architecture, is presented. The multireference binary phase-only filter (MBPOF), optimized by regions of support (ROS), constitutes a formal rewriting of some multiplex or composite filters including optimization functions, such as the distribution function and the selection function. The first function optimizes the multiplexing of references into the multireference filter. The second function defines the ROS of an object's Fourier spectrum and can be independently used to optimize the conventional binary phase-only filter. Both functions result from a segmentation of the Fourier plane. The MBPOF with ROS can be optically implemented in a filtering-based optical processor owing to a binary-phase spatial light modulator. Simulation and optical results are given for different examples of the BPOF and the MBPOF, both with ROS optimizing different criteria of performance, such as peak-to-correlation energy, discrimination capability, and distortion sensitivity.     

Reverse phase contrast: an experimental demonstration

We present experimental results showing that the reverse phase contrast (RPC) technique is a viable method for the generation of a binary phase distribution from a spatially varying amplitude pattern using Fourier plane filtering techniques. Experimental results are shown for the generation of a binary 0-pi phase only distribution using either an amplitude mask or a spatial light modulator to provide the input and the results are shown to be in agreement with theoretical predictions for the RPC technique.     

Scale-invariant binary phase-only matched filter using a ferroelectric-liquid-crystal spatial light modulator

We describe the implementation of a scale-invariant binary phase-only matched filter using a 128 × 128 ferroelectric spatial light modulator. A 5-dB discrimination was obtained between a 10-image training set of highly correlated characters (E's and F's) over a scale range of 1.0 to 1.5.     

Low-coherence interferometry with synthesis of coherence function

Synthesis of a coherence function by manipulation of the spectral phase of low-coherent light with a segmented liquid-crystal phase modulator and its application in a low-coherence interferometry are described. Effects of space-time coupling caused at diffractive gratings and second-order dispersion at the spatial light modulator on the coherence function synthesis are theoretically and experimentally verified. Various coherence functions can be shaped with phase-only masks designed by simulated annealing optimization algorithm. We utilized this technique for a novel optical low-coherence reflectometry without any mechanical movement for scanning optical delay.     

视网膜机理启发的尺度空间卷积模板构造算法

针对多尺度模板构造,论述形成人眼视觉尺度现象的两个因素并探讨了感受器与节细胞间的空间信息组合模型。基于人眼视觉机理启发,提出了一种新的多尺度滤波模板构造方法。该方法根据标准高斯曲线的采样率变化生成尺度空间模板族。经过时域、频域分析和滤波图例的实验,验证了该算法的普适性、规范性和有效性。与传统高斯尺度空间模板构造方法相比,具有简单、规范等特点。     

Novel approaches to the design of halftone masks for analog lithography

We report novel approaches to the design of halftone masks for analog lithography. The approaches are derived from interferometric phase contrast. In a first step we show that the interferometric phase-contrast method with detour holograms can be reduced into a single binary mask. In a second step we introduce the interferometric phase-contrast method by interference of the object wavefront with the conjugate object wavefront. This method also allows for a design of a halftone mask. To use kinoform holograms as halftone phase masks, we show in a third step the combination of the zeroth-order phase-contrast technique with the interferometric phase-contrast method.     

Intensity and phase measurements of nondiffracting beams generated with a magneto-optic spatial light modulator

Nondiffracting beams are of interest for optical metrology applications because the size of the beam does not change as the beam propagates. However, accuracy can be increased if the diameter of the beam is smaller. One technique for accomplishing this is to use the dark axial intensity profile associated with a higher-order nondiffracting Bessel function beam. We generate these higher-order Bessel function beams with a programmable spatial light modulator. We study the intensity patterns and the phase dependence of these nondiffracting beams. In addition, we examine interference effects caused by recording these patterns onto a binary spatial light modulator.     

Comment on "Tunable three-dimensional intensity distribution by a pure phase-shifting apodizer"

In the past few years, many radially symmetric pupil masks have been designed for the control of the light intensity distribution near the geometrical focus of optical systems. A recent paper by Gao et al. [Appl. Opt. 44, 4870 (2005)] analyzed the performance of binary 0-pi phase-only masks for an incident beam with Gaussian amplitude. Some inaccuracies of the analysis concerning the evolution of the Strehl ratio with the mask parameters are corrected.     

Optically computing the hit-miss transform for an automated cervical smear screening system

The hit-miss transform serves as a region-of-interest locator for cells from cervical smear images that show abnormal changes, which are indicative of malignancy, in their nuclei. An optical implementation of the hit-miss transform algorithm uses an analog spatial light modulator for gray-scale modulation at the filter plane of a 4f optical correlator. Gray-scale modulation at the filter plane improves correlator performance in comparison with a binary phase-only filter (BPOF) by reduction of the edge enhancement of kernels used in morphological detection of cancerous cervical cells. The hit-miss transform with a gray-scale amplitude and binary phase optical filter (GABPOF) for the hit filter and a BPOF for the miss filter shows a 47% reduction in total error versus the use of only BPOF filters to locate abnormal cells.     

Annular Pupil Arrays

Any modification of the uniform amplitude distribution in the pupil of an optical system by a non-uniformly absorbing filter leads to changes in its spread function. Radially symmetrical pupil masks with a continuously varying transmittance have been used for this purpose. These can be made by vacuum deposition or photographic integration, but the manufacturing process is difficult and involves the use of rotating masks. An alternative is to use a binary mask consisting of a concentric array of annuli which approximates to the continuously varying absorption. The potential of such annular pupils for modifying the point spread of a confocally scanned imaging system is discussed, and some experimental results are presented.     

Application of position encoding to a complex joint transform correlator

Because a joint transform correlator can be used as a general optical signal processor, complex-impulse-response implementations in the spatial domain are often requested. We introduce a position-encoding technique with which complex-valued references for the joint transform correlator can be obtained with an amplitude-modulated spatial light modulator. A proof-of-concept experiment is also provided.     

Self-adjusting dynamic binary phase holograms

An optical system constructed around a dynamic diffractive optic element, a ferroelectric liquid-crystal spatial light modulator in binary phase-only modulation mode, is investigated. The spatial light modulator is successively adjusted according to the direct binary search technique to diffract an incoming laser light beam into a predecided intensity distribution by use of feed back from the diffracted light. It was found that the feedback signal was noisy and that vibrations and limited bistability in the spatial light modulator's pixels were the main noise sources. The final diffraction efficiency depends on the degree of noise in the feedback signal, but even under fairly noisy conditions the iterations were found to converge properly.     

Particle image velocimetry analysis using an optically addressed spatial light modulator: effects of nonlinear transfer function

Optical processors for generating a two-dimensional squared autocorrelation function have been presented for postprocessing particle image velocimetry photographs of fluid flows. The incoherent-tocoherent conversion can be performed by an optically addressed spatial light modulator. The transfer function of these devices is far from linear and will influence the performance of the optical processor.

Two different transfer functions, characterizing the two main types of commercial optically addressed spatial light modulators as an analog and a binary transfer function, have been simulated digitally.

Results of numerical simulations on the influence of introducing these nonlinear transfer functions to the correlation function for particle image velocimetry analysis are presented.

     

Bessel function output from an optical correlator

We introduce a new filter for an optical correlator that produces a Bessel function correlation output. The advantage of this type of output is that the width of the central dark spot at the center of the correlator output is narrower than the width of the usual correlation spot. In addition, the dark spot is insensitive to changes in illumination intensity. We present experimental results with which we write the new filter onto a programmable magneto-optic spatial light modulator.     

Double-exposure phase calculation method in electronic speckle pattern interferometry based on holographic object illumination

Multiple-exposure phase calculation procedures are widely used in electronic speckle pattern interferometry to calculate phase maps of displacements. We developed a double-exposure process based on holographic illumination of the object and the idea of the spatial carrier phase-shifting method to examine transient displacements. In our work, computer-generated holograms and a spatial light modulator were used to generate proper coherent illuminating masks. In this adjustment all phase-shifted states were at our disposal from one recorded speckle image for phase calculation. This technique can be used in the large scale of transient measurements. In this paper we illustrate the principle through several examples.     

Improvement in holographic storage capacity by use of double-random phase encryption

We show that a double-random encryption technique can improve the storage capacity of an angular-multiplexed holographic memory system. In the holographic memory system, input binary images are encrypted into white-noise-like images by use of two random phase masks located at the input and the Fourier planes. These encrypted images are stored as holograms in a photorefractive medium by use of angular multiplexing. All the images are encrypted by different sets of random phase masks. Even when the angle separation between adjacent images is small enough to cause cross talk between adjacent images, original binary data can be recovered with the correct phase mask; the other reconstructed images remain white-noise-like images because incorrect masks are used. Therefore the capacity of the proposed system can be larger than that of a conventional holographic memory system without the random phase encryption technique. Numerical evaluation and experimental results are presented to confirm that the capacity of the system with random phase masks is larger than that of the conventional memory system.