Showing papers in "Optical Engineering in 2004"

Fog attenuation prediction for optical and infrared waves

Abstract: The principal disadvantage of using free space optics (FSO) telecommunication systems is the disturbing role played by the atmosphere on light propagation and thus on the channel capacity, availability, and link reliability. The wavelength choice is currently a subject of disagreement among designers and users of FSO equipments. Generally this equipment operates in the visible and the near IR at 690, 780, 850, and 1550 nm. Several authors affirm that equipment working at 1550 nm presents less atmospheric attenuation in the presence of fog and thus better link availability. Others consider that for dense fogs (visibility<500 m), all wavelengths are attenuated in the same way (wavelength independence). Fog attenuation in the visible and IR regions is reviewed from an empirical and theoretical point of view. Laser system performance in the presence of fog (advection and convection) in the 0.4- to 15-µm spectral zone is investigated using FASCOD computation. A transmission gain of 42% for a lasercom system working at 780 nm is observed compared to the same system working at 1550 nm. This gain reaches 48% if the same system works at 690 nm. Finally, we propose a fast transmission relationship based on an exact Mie theory calculation valid in the 0.69- to 1.55-µm spectral bands. It enables us to predict fog attenuation according to visibility without using heavy computer codes.

New Hyperspectral Discrimination Measure for Spectral Characterization

Abstract: The spectral angle mapper (SAM) has been widely used in multispectral and hyperspectral image analysis to measure spectral simi- larity between substance signatures for material identification. It has been shown that the SAM is essentially the Euclidean distance when the spectral angle is small. Most recently, a stochastic measure, called the spectral information divergence (SID), has been suggested to model the spectrum of a hyperspectral image pixel as a probability distribution, so that spectral variations among spectral bands can be captured more effectively in a stochastic manner. This paper develops a new hyper- spectral spectral discrimination measure, which combines the SID and the SAM into a mixed measure. More specifically, letr and r8 denote two hyperspectral image pixel vectors with their corresponding spectra speci- fied bys and s8. Then SAM(s,s8) measures the spectral angle between s and s8. Similarly, SID(s,s8) measures the information divergence be- tween the probability distributions generated by s and s8. The proposed new measure, referred to as the SID-SAM mixed measure, can be imple- mented in two versions, given by SID(s,s8)3tan(SAM(s,s8)) and SID(s,s8)3sin(SAM(s,s8)), where tan and sin are the usual trigonomet- ric functions. The spectral discriminability of such a mixed measure is greatly enhanced by multiplying the spectral abilities of the two mea- sures. In order to demonstrate its utility, a comparative study is con- ducted among the SID-SAM mixed measure, the SID, and the SAM. Our experimental results have shown that the discriminatory ability of the (SID,SAM) mixed measure can be a significant improvement over the SID and SAM. © 2004 Society of Photo-Optical Instrumentation Engineers.

The National Ignition Facility

Abstract: The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is a stadium-sized facility that, when completed in 2008, will contain a 192-beam, 1.8-megajoule, 500-terawatt, ultraviolet laser system together with a 10-m-diam target chamber and room for 100 diagnostics. NIF is the world's largest and most energetic laser ex- perimental system and will provide a scientific center to study inertial confinement fusion and matter at extreme energy densities and pres- sures. NIF's energetic laser beams will compress fusion targets to con- ditions required for thermonuclear burn, liberating more energy than re- quired to initiate the fusion reactions. Other NIF experiments will study physical processes at temperatures approaching 10 8 K and 10 11 bar, conditions that exist naturally only in the interior of stars and planets. NIF has completed the first phases of its laser commissioning program. The first four beams of NIF have generated 106 kJ in 23-ns pulses of infrared light and over 16 kJ in 3.5-ns pulses at the third harmonic (351 nm). NIF's target experimental systems are being commissioned and experi- ments have begun. This work provides a detailed look at the NIF laser systems, laser and optical performance, and results from recent laser commissioning shots. We follow this with a discussion of NIF's high- energy-density and inertial fusion experimental capabilities, the first ex- periments on NIF, and plans for future capabilities of this unique facility. © 2004 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1814767) Subject terms: high-energy-density physics; inertial confinement fusion; labora- tory astrophysics; solid-state lasers.

Simultaneous multiple surface optical design method in three dimensions

Abstract: The simultaneous multiple surface (SMS) method in 3-D geometry is presented. Given two orthotomic input ray bundles and another two orthotomic output ray bundles, the method provides an optical system with two free-form surfaces that deflects the rays of the input bundles into the rays of the corresponding output bundles and vice versa. In nonimaging applications, the method enables controlling the light emitted by an extended light source much better than single free-form-surface designs, and also enables the optics contour to be shaped without efficiency losses. The method is also expected to find applications in imaging optics.

Model for a partially coherent Gaussian beam in atmospheric turbulence with application in lasercom

Abstract: Analytic expressions for the mutual coherence function (MCF) and the scintillation index of a partially coherent lowest order Gaussian beam wave propagating through the atmosphere (based on Kolmogorov spectrum model) are developed for the pupil plane of a receiving system. Partial coherence of the beam is modeled as a thin (complex) phase screen with Gaussian spectrum (Rytov theory and ABCD ray matrices are applied). The relation between the second- and fourth-order statistics for a beam with any degree of coherence in the atmosphere is introduced with the help of ''effective'' beam parameters, deduced from the free-space MCF. In particular, the scintillation (in weak and strong atmospheric conditions), based on these parameters, is stud- ied as a function of the diffuser's strength and that of the atmosphere. The model is applied for the calculation of the SNR and bit error rates (OOK modulation) of the communication link with diffuser at the trans- mitter and slow detection system. The improvement of bit error rates is observed in weak and strong atmospheric turbulence. In the weak re- gime, the optimal diffuser can be found. © 2004 Society of Photo-Optical Instru-

Accurate procedure for the calibration of a structured light system

Abstract: A procedure is proposed to calibrate a generic structured light system, consisting of one camera and one projector. The proposed pro- cedure is based on defining a unique coordinate system for both devices in the structured light system, and thus, a rigidity constraint is introduced into the transformation process. This constraint is used to derivate a simple function for the simultaneous estimation of the parameters, result- ing in parameters that are more reliable. The performance of the pro- posed procedure is shown on examples of the calibration of two different structured light systems. © 2004 Society of Photo-Optical Instrumentation Engi- neers. (DOI: 10.1117/1.1635373)

Fiber Bragg grating sensing system for simultaneous measurement of salinity and temperature

Abstract: A fiber optic sensing system for simultaneous measurement of temperature and salinity based on fiber Bragg grating technology is presented. The fabrication process, which relies on chemical etching of the fibre, is described and its performance is evaluated. Theoretical and experimental results are given, which are summarized on the obtained resolutions of 60.06°C/AHz and 60.2‰/ AHz for temperature and salin- ity, respectively. © 2004 Society of Photo-Optical Instrumentation Engineers.

Review of iterative Fourier-transform algorithms for beam shaping applications

Abstract: We present a comparison of some of the most used iterative Fourier transform algorithms (IFTA) for the design of continuous and multilevel diffractive optical elements (DOE). Our aim is to provide optical engineers with advice for choosing the most suited algorithm with re- spect to the task. We tackle mainly the beam-shaping and the beam- splitting problems, where the desired light distributions are almost binary. We compare four recent algorithms, together with the historical error- reduction and input-output methods. We conclude that three of these algorithms are interesting for continuous-phase kinoforms, and two, namely the three-step method proposed by Wyrowski and the over- compensation of Prongue ´, still perform well with multilevel- and binary- phase DOE. © 2004 Society of Photo-Optical Instrumentation Engineers.

New metric for predicting target acquisition performance

Abstract: The Johnson criteria have been shown to be fundamentally flawed due to their insensitivity to effects below the limiting frequency. This flaw makes predictions for many modern imaging systems inaccurate. A new target acquisition metric, the targeting task performance (TTP) metric, has been developed that provides better accuracy than the Johnson criteria. Further, unlike the Johnson criteria, the TTP metric can be applied directly to sampled imagers and to imagers that exhibit colored (spectrally weighted) noise due to frequency boost. Experimental data using both target recognition and target identification tasks show the problems with the Johnson criteria and illustrate the robust performance of the TTP metric. The simplicity of implementing a range performance model with the Johnson criteria is retained by the TTP metric while extending applicability of the model to sampled imagers and digital image enhancement.

Compression of encrypted three-dimensional objects using digital holography

Abstract: We present the results of applying data compression techniques to encrypted three-dimensional objects. The objects are captured using phase-shift digital holography and encrypted using a random phase mask in the Fresnel domain. Lossy quantization is combined with lossless coding techniques to quantify compression ratios. Lossless compression alone applied to the encrypted holographic data achieves compression ratios lower than 1.05. When combined with quantization and an integer encoding scheme, this rises to between 12 and 65 (depending on the hologram chosen and the method of measuring compression ratio), with good decryption and reconstruction quality. Our techniques are suitable for a range of secure three-dimensional object storage and transmission applications.

National Ignition Facility wavefront requirements and optical architecture

Abstract: With the first four of its eventual 192 beams now executing shots, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is already the world's largest and most energetic laser. The optical system performance requirements that are in place for NIF are derived from the goals of the missions it is designed to serve. These missions include inertial confinement fusion (ICF) research and the study of matter at extreme energy densities and pressures. These mission requirements have led to a design strategy for achieving high quality focusable energy and power from the laser and to specifications on optics that are important for an ICF laser. The design of NIF utilizes a multipass architecture with a single large amplifier type that provides high gain, high extraction efficiency and high packing density. We have taken a systems engineering approach to the practical implementation of this design that specifies the wavefront parameters of individual optics in order to achieve the desired cumulative performance of the laser beamline. This presentation provides a detailed look at the causes and effects of performance degradation in large laser systems and how NIF has been designed to overcome these effects. We will also present results of spot size performance measurements that have validated many of the early design decisions that have been incorporated in the NIF laser architecture.

Alignment and wavefront control systems of the National Ignition Facility

Abstract: The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized facility containing a 192-beam Nd glass laser. Its 1.053-µm output is frequency converted to produce 1.8-MJ, 500-TW pulses in the ultraviolet. Refer to the companion overview articles in this issue for more information. High-energy-density and inertial confinement fusion physics experiments require the ability to precisely align and focus pulses with single-beam energy up to 20 KJ and durations of a few nanoseconds onto millimeter-sized targets. NIF's alignment control system now regularly provides automatic alignment of the four commissioned beams prior to every NIF shot in approximately 45 min, and speed improvements are being implemented. NIF utilizes adaptive optics for wavefront control, which significantly improves the ability to tightly focus each laser beam onto a target. Multiple sources of both static and dynamic aberration are corrected. This article provides an overview of the NIF automatic alignment and wavefront control systems, and provides data to show that the facility is expected to meet its primary requirements to position beams on the target with an accuracy of 50-µm rms over the 192 beams and to focus the pulses into a 600-µm spot.

Holographic photonic crystals

Abstract: Following the realization of photonic crystals in the late 1980s and fueled by the vision of photonic "highways" and all-optical switching, the field has grown into one of the most exciting and fertile areas of cross-disciplinary research spanning the scientific and engineering communities. We focus on the state of the art in holographic fabrication of photonic crystals by providing a unified framework of design principles for the creation of two- and three-dimensional optical lattices, with an emphasis on how all five two-dimensional and all fourteen three-dimensional Bravais lattices may be formed with three and four coherent laser beams, respectively. Additionally, we discuss quasiperiodic crystal interference profiles and aspects of the relevant holographic materials. After firmly establishing the general relationship between the holographic parameters and the resulting optical interference profiles, we offer a comprehensive literature review and set forth the most pressing research issues in holographic photonic crystals.

Correlation-based watermarking method for image authentication applications

Abstract: We propose a correlation-based digital watermarking technique for robust image pattern authentication. We hide a phase-based signature of the image back into its Fourier magnitude spectrum in the embedding stage. The detector computes the Fourier transform of the watermarked image and extracts the embedded signature. Authentication performance is measured by a correlation test of the extracted signature and the signature computed from the watermarked image. The quality of the watermarked image is obtained from the peak signal-to-noise ratio metric. We also furnish simulation results to show the robustness of our approach to typical image processing as found in JPEG compression

Computer-generated holograms in interferometric testing

Abstract: With surface-relief structures, optical functions that are required for radiation power management such as antireflection, light trapping, or light distribution and redirection can be obtained for new applications in solar energy systems and in displays. There, structures with submicrometer features must be distributed over large areas homogeneously. We address the design and the whole experimental process chain from the microstructure origination on large areas to the replication and the system integration in the specific application. Topics are antireflective surfaces for solar systems and displays, light trapping in polymer solar cells, sun protection systems for facades, and diffusers for projection displays and in glazing. For the microstructure origination we investigate the suitability of holographic recording in photoresist using a large-scale interferometer. We use an argon ion laser as a coherent light source at a wavelength of 364 nm. Periodic and stochastic interference patterns are recorded in positive photoresist with the interferometer setup. In the case of periodic structures, grating periods between 200 nm and 20 µm are realized. By carefully modeling the resulting resist profiles it is possible to originate even prismatic surface-relief profiles. Structures with good homogeneity are originated on areas of up to 4800 cm2 by optimizing the interferometer setup and the photoresist processing.

Some application cases and related manufacturing techniques for optically functional microstructures on large areas

Abstract: With surface-relief structures, optical functions that are required for radiation power management such as antireflection, light trapping, or light distribution and redirection can be obtained for new applications in solar energy systems and in displays. There, structures with submicrometer features must be distributed over large areas homogeneously. We address the design and the whole experimental process chain from the microstructure origination on large areas to the replication and the system integration in the specific application. Topics are antireflective surfaces for solar systems and displays, light trapping in polymer solar cells, sun protection systems for facades, and diffusers for projection displays and in glazing. For the microstructure origination we investigate the suitability of holographic recording in photoresist using a large-scale interferometer. We use an argon ion laser as a coherent light source at a wavelength of 364 nm. Periodic and stochastic interference patterns are recorded in positive photoresist with the interferometer setup. In the case of periodic structures, grating periods between 200 nm and 20 µm are realized. By carefully modeling the resulting resist profiles it is possible to originate even prismatic surface-relief profiles. Structures with good homogeneity are originated on areas of up to 4800 cm2 by optimizing the interferometer setup and the photoresist processing.

ARGOS testbed: study of multidisciplinary challenges of future spaceborne interferometric arrays

Abstract: Future spaceborne interferometric arrays must meet stringent optical performance and tolerance requirements while exhibiting modu- larity and acceptable manufacture and integration cost levels. The Mas- sachusetts Institute of Technology (MIT) Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) is a wide-angle Fizeau interferometer spacecraft testbed designed to address these research challenges. De- signing a space-based stellar interferometer, which requires tight toler- ances on pointing and alignment for its apertures, presents unique mul- tidisciplinary challenges in the areas of structural dynamics, controls, and multiaperture phasing active optics. In meeting these challenges, emphasis is placed on modularity in spacecraft subsystems and optics as a means of enabling expandability and upgradeability. A rigorous theory of beam-combining errors for sparse optical arrays is derived and flown down to the design of various subsystems. A detailed elaboration on the optics system and control system is presented based on the per- formance requirements and beam-combining error tolerances. The space environment is simulated by floating ARGOS on a frictionless air- bearing that enables it to track both fast and slow moving targets. © 2004

Volume phase holographic gratings: large size and high diffraction efficiency

Abstract: Volume phase holographic gratings (VPHGs) possess unique properties that make them attractive for numerous applications. After reviewing major VPHG characteristics through theory, we discuss some aspects of the dichromated gelatin recording material and the holo- graphic recording process. The large-scale VPHG research facility set up at the Center Spatial de Liege enables production of VPHGs up to 380 mm in diameter, with fringe frequencies from 315 to 3300 lp/mm. We describe the work that has been undertaken in our laboratory to remove the last limitations inherent in VPHGs. © 2004 Society of Photo-Optical Instru- mentation Engineers. (DOI: 10.1117/1.1803557)

Random phase and jigsaw encryption in the Fresnel domain

Abstract: A number of methods have been recently proposed in the literature for the encryption of 2-D information using optical systems based on linear transforms, i.e., the Fourier (FT), the fractional Fourier (FRT), the Fresnel (FST), and the linear canonical transform (LCT). We propose and examine two encryption schemes using the Fresnel transform (FST) based on the use of random phase screens and random jigsaw transforms (JT). The strength and robustness of the level of encryption of the various techniques are examined with respect to blind decryption. These systems are compared with similar FRT- and LCT-based methods. Optical implementations are also discussed and sampling conditions are investigated in the context of the space-bandwidth product.

Measurement of surface roughness by speckle correlation

Abstract: Measurement of the roughness of solid surfaces by digital speckle correlation of video signals is reported. Speckle patterns appearing in the diffraction field of a laser-illuminated sample are taken by a CCD before and after the change of the incident angle or the laser wavelength, and their cross-correlation is calculated from which surface roughness can be evaluated. The theoretical cross-correlation function is first derived that describes speckle displacement and decorrelation. Its peak height is then provided as a function of the change of the incident angle or the wavelength. The decorrelation curve against speckle displacement is shown to depend on the surface roughness. The derived analytical relationships were verified by numerical simulations that used two-dimensional arrays of random numbers with various maximum values representing the maximum surface roughness. The relationship between the normal correlation calculated by the theory and the phase-only correlation adopted in the experiment was also evaluated by the simulation. In experiments various roughness standards for metal were measured by using a quick processor employing the phase-only correlation algorithm. The results were compared with those obtained from a mechanical stylus, and good agreement was observed between them for root-mean-square roughnesses between several micrometers and several tens of micrometers.

Analysis of practical sampling and reconstruction from Fresnel fields

Abstract: We present an analysis of the different aspects involved with the sampling and reconstruction of Fresnel field distribution. Fresnel fields, describing a propagating optical wave, are digitally recorded in many optical applications. The recording process involves discretization of the continuous Fresnel field using a sampling sensor. Typical nonideal sensors induce degradation of the optical information due to finite spatial sampling rate, finite aperture size, and finite detector element size (finite fill factor). In this work, we investigate the condition and limitation of the reconstruction of Fresnel fields sampled with nonideal sampling sensors. We also analyze the propagation of measurement and reconstruction noise through the reconstruction process. In our analysis, we distinguish between continuous (optical) and numerical (computational) reconstructions. We focus on the different reconstruction conditions and limitations, depending on whether the reconstruction is performed in a continuous space or numerically.

Analyses on physical mechanism of holographic recording in phenanthrenequinone-doped poly(methyl methacrylate) hybrid materials

Abstract: The photoproducts in phenanthrenequinone (PQ)-dissolved methyl methacrylate (MMA) liquid samples and PQ-doped poly(methyl methacrylate) (PQ/PMMA) solid photopolymer samples have been ana- lyzed by various chemical measurements. A mechanism for holographic recording in our PQ/PMMA photopolymer is proposed. By UV-VIS trans- mission and photoluminescence spectral measurements, we find that under light exposure the molecular structure of PQ is transformed to be less conjugated. The measured results of mass spectra, Fourier trans- form infrared spectra, NMR spectra, and gel permeation chromatograph analyses provide some evidence for recognizing the molecular structure of the photoproducts in our PQ/PMMA photopolymers. The results show that under light exposure the PQ and MMA form new molecules, mainly an adduct of one PQ molecule with one MMA molecule. In addition, PQ also reacts as a photoinitiator to form PMMA oligomers in our samples. The structure change of the PQ molecule induces a strong change of the refractive index in the material. It provides a mechanism to record a phase hologram in our PQ/PMMA photopolymer. Holographic recordings in the samples are demonstrated, and the dynamic range of the sample is investigated. © 2004 Society of Photo-Optical Instrumentation Engineers.

Metrology of thin transparent optics using Shack-Hartmann wavefront sensing

Abstract: The surface topography of thin, transparent materials is of interest in many areas. Some examples include glass substrates for computer hard disks, photomasks in the semiconductor industry, flat panel displays, and x-ray telescope optics. Some of these applications require individual foils to be manufactured with figure errors that are a small fraction of a micron over 10- to 200-mm lengths. Accurate surface metrology is essential to confirm the efficacy of manufacturing and sub- strate flattening processes. Assembly of these floppy optics is also facili- tated by such a metrology tool. We report on the design and perfor- mance of a novel deep-ultraviolet (deep-UV) Shack-Hartmann surface metrology tool developed for this purpose. The use of deep-UV wave- lengths is particularly advantageous for studying transparent substrates such as glass, which are virtually opaque to wavelengths below 260 nm. The system has a 1433143-mm 2 field of view at the object plane. Per- formance specifications include 350-mrad angular dynamic range and 0.5-mrad angular sensitivity. Surface maps over a 100 mm diam are accurate to ,17-nm rms and repeatable to 5 nm rms. © 2004 Society of

Remote sensing using passive infrared Stokes parameters

Abstract: We address the problem of detecting vehicle targets on the ground by means of an autonomous polarimetric sensor on board a high-altitude air- or spaceborne platform. We studied a sensor system that incorporates a micropolarizer array, a microscanner, and digital signal processors that host algorithms for detecting small targets. These algorithms use statistical techniques to fuse a target's Stokes-vector infrared signature measurements. Fusion is achieved by constructing the joint statistical measures for the target's polarization states. Those states are expressed in terms of the intensity, the percentage of linear polarization, and the angle of the polarization plane. The performance of the sensor system on synthetic polarimetric infrared imagery was evaluated. Relatively good detection and low false-alarm probabilities were achieved even when the number of pixels on targets is as small as four. This is the first report of the development and evaluation of automatic detection in polarimetric imagery containing targets subtending only a small number of pixels.

Quantitative evaluation of mura in liquid crystal displays

Abstract: The visual performance of liquid crystal displays (LCDs) has usually been evaluated by visual inspection during the manufacturing process. One of the visual problems hardest to recognize are regions of low contrast and nonuniform brightness called mura. The accurate and consistent detection of the mura is extremely difficult because there are various shapes and sizes of mura and the inspection results tend to depend on the operators. We conducted a study on the quantitative evaluation of mura based on visual analysis, intending to clarify the de- tection method and create an automated mura inspection process. We developed an algorithm and a hardware system based on a commer- cially available charge-coupled-device camera and a personal computer system with an image processor board. This system can successfully identify and evaluate mura. The algorithm was developed from research on visual analysis and human perception. We converted the front-of- screen images from the LCDs into distributions of luminance information, and the mura regions were distinguished from the background area us- ing our novel algorithm. Our identification method can also distinguish between the muras caused by flaws in the LCD cells and the intentionally designed nonuniform luminance distribution of the backlight. © 2004 Soci-

Adaptive optical zoom

Abstract: In order to optically vary the magnification of an imaging system, continuous mechanical zoom lenses require multiple optical elements and use fine mechanical motion to precisely adjust the separations between individual or groups of lenses. By incorporating active elements, such as liquid crystal spatial light modulators or deformable mirrors, into the optical design, we can eliminate the need to change the spacing between lenses and create an imaging system with variable optical magnification that has no macroscopic moving parts.

Unsupervised approach to color video thresholding

Abstract: Thresholding of video images is a great challenge because of their low spatial resolution and complex background. We investigate the issue of thresholding these images by reducing the number of colors to improve automated text detection and recognition. We develop an unsupervised approach to video images, which can be considered as an RGB color thresholding method. It applies a gray-level thresholding method to a video image in the (R, G, B) color space to produce a single threshold value for each domain. The three (R, G, B)-generated values will be subsequently processed by an effective unsupervised clustering algorithm that is based on a between-class/within-class criterion suggested by Otsu's method. Since thresholding methods designed for document images may not work effectively for video images in many applications, our proposed RGB color thresholding method has shown to be particularly effective in improvement on text detection and recognition, because it can reduce the background complexity while retaining the important text character pixels. Experiments also show that thresholding video images is far more difficult than thresholding document images, and the RGB color thresholding presented performs significantly better than simple histogram-based methods, which generally do not produce satisfactory results.

Performance evaluation of a liquid-crystal-on-silicon spatial light modulator

Abstract: D optical phased array antennas formed by a liquid crystal on silicon (LCOS) spatial light modulator are described for free-space laser communication and high-resolution wavefront control. The device consists of an 2-D array of 10243768 phase modulator elements, each with controlled voltage, and can induce a phase shift from 0 to 2p for wavelengths up to the near IR. When the device is used as a wavefront corrector, 18.7 waves peak-valley (at 632.8 nm) of aberration in the op- tical system is corrected to a residual of 1/9 wave peak-valley, or 1/30 wave rms. The Strehl ratio improved from 0.006 to 0.83 after correction. An additional linear phase ramp was added to the correction phase ramp to simultaneously correct and steer the laser beam. Continuous steering over 64 mrad in the X-Y plane with a steering accuracy higher than 10 mrad has been obtained. The 1-D beam-steering efficiency is 80% at the maximum steering angle of 4 mrad. These results suggest that an LCOS device can be used to achieve very high-resolution wavefront control at very high efficiency. © 2004 Society of Photo-Optical Instrumentation Engineers.

Volume holographic grating-based continuously tunable optical filter

Abstract: We propose and demonstrate a widely tunable optical filter, realized by angle tuning a volume holographic grating. The volume holographic grating selectively drops a narrow portion of the signal bandwidth into a fiber while passing through the rest of the signals. The demonstrated 1510- to 1590-nm tuning range covers the entire erbium-doped fiber amplifier (EDFA) C band, with small bandwidth variation and low insertion loss (<1 dB). Group delay, polarization-dependent loss, and polarization mode dispersion are measured and investigated for optimizing the filter characteristics.

Performance evaluation of quadratic correlation filters for target detection and discrimination in infrared imagery

Abstract: The detection and discrimination of targets in infrared imagery has been a challenging problem due to the variability of target and clutter (background) signatures. We discuss the application of a novel quadratic filtering method using missile seeker infrared closing sequences. Image filtering techniques are well suited for target detection applications, since they avoid the disadvantages of typical pixel-based detection schemes (such as segmentation and edge extraction). Another advantage is that the throughput complexity of the filtering approach, in the detection process, also does not vary with scene content. The performance of the proposed approach is assessed on several datasets, and the results are compared with that of previous linear filtering techniques. Since we can obtain the signature of some of the clutter "in the field" or during operation, we examine the impact of updating the filters to adapt to the clutter.