Common-path optical heterodyne profilometer: a configuration

A novel configuration that combines a linearly polarized He-Ne laser and a birefringent lens to produce a common-path polarized optical heterodyne profilometer with respect to the heterodyned P and S waves has been set up. In this profilometer a linear polarized frequency-stabilized He-Ne laser was used with an acousto-optical modulator to replace the Zeeman laser as the light source that had two polarization eigenstates in different temporal frequencies. The proposed interferometer shows a more symmetric and ideal common-path structure than the conventional optical heterodyne profilometers with the Zeeman laser. The phase error aroused by the elliptical polarization and the nonorthogonality of the two eigenpolarization modes of the Zeeman laser can be reduced. The system's resolution in the vertical direction reaches 2 A, and in a 27-mum scanning range the repeatability of the surface profile measurements is shown to be 5 A.     

Relational color constancy in achromatic and isoluminant images

Relational color constancy, which refers to the constancy of perceived relations between surface colors under changes in illuminant, may be based on the computation of spatial ratios of cone excitations. As this activity need occur only within rather than between cone pathways, relational color constancy might be assumed to be based on relative luminance processing. This hypothesis was tested in a psychophysical experiment in which observers viewed simulated images of Mondrian patterns undergoing colorimetric changes that could be attributed either to an illuminant change or to a nonilluminant change; the images were isoluminant, achromatic, or unmodified. Observers reliably discriminated the two types of changes in all three conditions, implying that relational color constancy is not based on luminance cues alone. A computer simulation showed that in these isoluminant and achromatic images spatial ratios of cone excitations and of combinations of cone excitations were almost invariant under illuminant changes and that discrimination performance could be predicted from deviations in these ratios.     

Experimental investigations of a breadboard model of Hall thruster

Various breadboard models of Hall thrusters are analyzed with a view to optimizing the output parameters which characterize the degree of processing of the working medium and the energy efficiency of the device. It is demonstrated that the channel geometry with the magnetic field uniform along the channel affects only slightly the efficiency of acceleration of ion flow. In a number of working modes, the formation of accelerated ion flow is accompanied by the emergence of pulsations of discharge current, this leading to destructive processes of acceleration and formation of ion flow. The development of a breadboard model with a nonuniform magnetic field along the channel significantly decreasing in the direction of anode with cylindrical space makes possible the elimination of discharge current pulsations in a wide range of parameters. The increase in the density of the flow of working medium by a factor of 1.7 results in a 30% increase in the energy efficiency and provides for the working medium utilization factor of the order of 1.2.     

Optical design of a 1024-channel free-space sorting demonstrator

We describe the optical design of a free-space interconnect system to be used for a 32 x 32 sorting system. The system uses 4f imaging relays to connect two hybrid optoelectronic-complementary metal-oxide semiconductor chips. These relays are used to implement a perfect-shuffle interconnect necessary for the sorting algorithm to be implemented. The relay lenses are also used with patterned mirrors and polarizing elements to combine read beams necessary for the optoelectronic chips. Issues relating to the basic system design and the detailed design of the lenses and optomechanics are given.     

Performances and potentialities of a very simple self-compensatedpressure sensor demonstrator

A pressure sensor demonstrator has been designed and mounted using a simple analog-digital BiCMOS converter and a capacitive silicon-Pyrex sensing cell. The measurements as a function of pressure and temperature have enabled the authors to evaluate the efficiency of a ratiometric scheme to self-compensate thermal drifts and nonlinearities. The best demonstrator is characterized by a thermal coefficient of offset smaller than 20 ppm/K and a nonlinearity lower than or equal to ±1.2% FS. The essential features of the sensor have been modeled by a simple analytical expression. The model specifies the potential gains in pressure sensitivity and nonlinearity. The best design is based on a quasisymmetrical architecture in which stray capacitors are minimized. This study demonstrates the feasibility of self-compensated capacitive sensors characterized by an overall accuracy of the order of a few percent of the measurement scale over a temperature range of 100 K at very low cost     

A demonstrator for a low-cost, cordless, multi-carrier spread-spectrum system

Because of the need for spectrally efficient systems for wireless communication, many research activities have been carried out in the area of spread-spectrum techniques. Multi-carrier spread-spectrum (MC-SS) is a new modulation technique with better spectral properties than direct-sequence spread-spectrum (DS-SS). In this paper, a new MC-SS system is introduced. A customized surface acoustic wave (SAW) filter has been designed as a fast analog correlator. A demonstrator testbed has been developed for the 2.4-GHz industrial, scientific, and medical (ISM) band. Experimental measurements of the intermediate frequency (IF) and baseband correlation are presented.     

Aberration measurement from specific photolithographic images: a different approach

Techniques for measurement of higher-order aberrations of a projection optical system in photolithographic exposure tools have been established. Even-type and odd-type aberrations are independently obtained from printed grating patterns on a wafer by three-beam interference under highly coherent illumination. Even-type aberrations, i.e., spherical aberration and astigmatism, are derived from the best focus positions of vertical, horizontal, and oblique grating patterns by an optical microscope. Odd-type aberrations, i.e., coma and three-foil, are obtained by detection of relative shifts of a fine grating pattern to a large pattern by an overlay inspection tool. Quantitative diagnosis of lens aberrations with a krypton fluoride (KrF) excimer laser scanner is demonstrated.     

Predicting color from gray: the relationship between achromatic adjustment and asymmetric matching.

Achromatic adjustment has been used widely to study color context effects. In the achromatic adjustment procedure, an observer adjusts a test stimulus until it appears black, gray, or white. By its nature, achromatic adjustment directly measures the effect of context only for stimuli that appear gray. We present achromatic loci measured in two contexts and asymmetric color matches measured across the same two contexts. The results indicate that achromatic adjustments, together with a gain-control model, may be used to make accurate predictions of the chromaticity of asymmetric matches. Thus measurements of the effect of context for test stimuli that appear gray may be used to predict the effect of context for stimuli that appear colored. The experiments also indicate that accurate prediction depends on ensuring that observers use similar fixational strategies for the two judgments.     

Design of achromatic doublets: evaluation of the double-graph technique

An example of doublet design using the double-graph technique as described by Kingslake is given. The potential of the double-graph technique is evaluated by applying it to three different cases of doublet design. The technique in its present form is found to give a solution only in one case. In the other two cases the solution can be obtained only by modifying the technique.     

Thin-film achromatic phase shifters for nulling interferometry: design approach

Nulling interferometry in the thermal IR is the most promising technique for direct detection of Earth-like exoplanets. This technique requires a pi phase shifter for the parent star of the planet to be completely extinguished by destructive interference. We investigate how thin films can be used to design pi achromatic phase shifters. The design approach that we propose works on reflection and can be carried out by two steps, namely, the design of a mirror and an antireflection structure with no constraint on the phase properties of the thin film stacks. Phase-shift accuracy is derived analytically, and a numerical example illustrates this concept.     

Apodizing of an achromatic lens by a gray-glass negative lens element

The apodizing performance of a plano-concave lens made of gray glass was analyzed numerically. It was found that apodizing performance depends solely on the edge transmittance of the lens (a zero center thickness is assumed here) for any reasonable ratio of radius of curvature of the concave surface and the lens radius. If a modest increase in the size of the central diffraction peak and light losses introduced by such a lens can be accepted, it is possible to reduce energy diffracted into diffraction rings approximately tenfold.     

Investigation of the influence of electron-velocity dispersion on the output characteristics of a laboratory breadboard of a low-voltage vircator

Some ways of forming multivelocity electron beams are considered, and experimental dependences (obtained on a laboratory breadboard of a low-voltage vircator) of the integral output power and generation band on the electron-velocity dispersion in a beam are shown.     

Full-field optical coherence tomography by achromatic phase shifting with a rotating polarizer

We demonstrate two-dimensional detection optical coherence tomography (OCT) using achromatic phase shifting with a rotating polarizer. This phase shifting, which experiences a light beam with a cyclic change in its polarization state, is, in principle, independent of wavelength. We simulated the wavelength dependence of an achromatic phase shifter using Jones calculus and found that the achromatic region exceeded 145 nm when the deviation of the phase retardation was less than +/- 0.5 degrees. Using the achromatic phase shifter and a conventional phase-shift calculation method, we obtained en face OCT images of an onion at different depths. This method is effective to enhance the quality of OCT with an ultrabroad-spectrum light source.     

Methods for multidimensional event classification: a case study using images from a Cherenkov gamma-ray telescope

We present results from a case study comparing different multivariate classification methods. The input is a set of Monte Carlo data, generated and approximately triggered and pre-processed for an imaging gamma-ray Cherenkov telescope. Such data belong to two classes, originating either from incident gamma rays or caused by hadronic showers. There is only a weak discrimination between signal (gamma) and background (hadrons), making the data an excellent proving ground for classification techniques.The data and methods are described, and a comparison of the results is made. Several methods give results comparable in quality within small fluctuations, suggesting that they perform at or close to the Bayesian limit of achievable separation. Other methods give clearly inferior or inconclusive results. Some problems that this study can not address are also discussed.     

Deriving wavelength spectra from fringe images from a fixed-gap single-etalon Fabry-Perot spectrometer

A new method is described for inferring wavelength spectra from two-dimensional images of Fabry-Perot interference fringes. This new method addresses the practical difficulties that have been previously encountered in determining the fringe image's center, magnification, and distortions accurately enough to fully exploit the spectral resolution provided by the etalon. The method proceeds in two steps. First, the instrument's mapping of image position to interference order is characterized by use of images of a scene illuminated uniformly by a highly monochromatic laser. Then this information is applied to resample two-dimensional images of unknown radiation sources down to sets of one or more one-dimensional wavelength spectra. Discrete cross-correlation techniques are used at both stages of anal-     

Improved restoration from multiple images of a single object: application to fluorescence microscopy

We present an approach for the combined restoration of multiple different images of a single object. A linear Tikhonov filter adapted for this purpose is derived in detail. Nonlinear constrained algorithms can also be adapted, and we illustrate this possibility for an iterative constrained Tikhonov algorithm. Both the linear and the iterative constrained Tikhonov algorithms were used to analyze performance in fluorescence confocal imaging by use of simulated and experimental data. One can improve the quality of restored confocal images significantly if the signal that normally is rejected by the detection pinhole of a confocal laser scanning microscope is also recorded on a separate detector such that the two recorded signals are used together for image restoration according to the proposed algorithms.     

Phase retrieval from speckle images

In ground-based astronomy, the inverse problem of phase retrieval from speckle images is a means to calibrate static aberrations for correction by active optics. It can also be used to sense turbulent wavefronts. However, the number of local minima drastically increases with the turbulence strength, mainly because of phase wrapping ambiguities. Multifocal phase diversity has been considered to overcome some ambiguities of the phase retrieval problem. We propose an effective algorithm for phase retrieval from a single focused image. Our algorithm makes use of a global optimization strategy and an automatically tuned smoothness prior to overcome local minima and phase degeneracies. We push the limit of D/r(0)=4 achieved by Irwan and Lane [J. Opt. Soc. Am. A.15, 2302 (1998)] up to D/r(0)=11, which is a major improvement owing to the drastic increase in the problem complexity. We estimate the performances of our approach from consistent simulations for different turbulence strengths and noise levels (down to 1500 photons per image). We also investigate the benefit of temporal correlation.     

Design, implementation, and characterization of a hybrid optical interconnect for a four-stage free-space optical backplane demonstrator

A four-stage unidirectional ring free-space optical interconnect system was designed, analyzed, implemented, and characterized. The optical system was used within a complementary metal-oxide semiconductor-self-electro-optic-effect-device-based optical backplane demonstrator that was designed to fit into a standard VME chassis. This optical interconnect was a hybrid microlens-macrolens system, in which the microlens relays were arranged in a maximum lens-to-waist configuration to route the optical beams from the optical power supply to the transceiver arrays, while the macrolens optical relays were arranged in a telecentric configuration to route optical signal beams from stage to stage. The following aspects of the optical system design are discussed: the optical parameters for the hybrid optical system, the image mapping of the two-dimensional array of optical beams from stage to stage, the alignment tolerance of the hybrid relay system, and the power budget of the overall optical interconnect. The implementation of the optical system, including the characterization of optical components, subsystem prealignment, and final system assembly, is presented. The two-dimensional array of beams for the stage-to-stage interconnect was adjusted with a rotational error of <0.05 degrees and a lateral offset error of <3.5 mum. The measured throughput is in good agreement with the lower-bound predictions obtained in the theoretical results, with an optical power throughput of -20.2 dB from the fiber input of the optical power supply to the modulator array and -25.5 dB from the fiber input to the detector plane.