In-line recording and off-axis viewing technique for holographic particle velocimetry

Prior approaches (e.g., off-axis holography) to overcoming the limitations of in-line holography for particle fields, namely, intrinsic speckle noise and depth resolution, involved an increased complexity of the optical system. The in-line recording and off-axis viewing (IROV) technique employs a single laser beam to record an in-line hologram, which is then viewed off axis during reconstruction. The signal-to-noise ratio and depth resolution of IROV are higher than conventional in-line holography by an order of magnitude and are comparable with off-axis holography. IROV is a much simpler approach than off-axis holography and is highly promising for holographic particle velocimetry. Measurements of the three dimensional flow velocity field of a vortex ring obtained by an IROV-based holographic particle velocimetry system are presented.     

Multiphase homodyne interferometry: analysis of some error sources

Some sources of error with multiphase homodyne interferometry are reviewed. A major advantage over the classic two-channel approach is that the inaccuracies that originate from laser-power fluctuations and drifts are shown to be automatically compensated for by proper adjustment of the light beams.     

Development of an algorithm for corneal reshaping with a scanning laser beam

The corneal-ablation rate, the beam-intensity distribution, and the initial and the desired corneal topographies are used to calculate a spatial distribution map of laser pulses. The optimal values of the parameters are determined with a computer model, for a system that produces 213-nm radiation with a Gaussian beam-intensity distribution and a peak radiant exposure of 400 mJ/cm(2). The model shows that with a beam diameter of 0.5 mm, an overlap of 80%, and a 5-mm treatment zone, the roughness is less than 6% of the central ablation depth, the refractive error after correction is less than 0.1 D for corrections of myopia of 1, 3, and 6 D and less than 0.4 D for a correction of myopia of 10 D, and the number of pulses per diopter of correction is 2500 when the beam-intensity distribution is Gaussian and 580 when it is flat.     

Fiber-linked telescope array: description and laboratory tests of a two-channel prototype

We present a complete two-telescope version of a fiber-linked coherent array that is meant to be used for mounting on the dish of a radio telescope. This was built with 20-cm amateur telescopes and includes three different servo subsystems for guiding, nulling of the air path difference, and fiber length control. Laboratory tests of the fully integrated system in front of a star simulator are described.     

Filter-feature-based rotation-invariant joint Fourier transform correlator

Rotation-invariant target detection using a trained filter-feature-based joint Fourier transform (JFT) correlator is investigated. First, a composite reference image is obtained from a training set of targets. An optimum filter formulation is then applied on this composite image to come up with a new feature that we refer to as a filter feature. This feature is then used in a JFT correlator, which results in a simple and robust rotation-invariant target recognition system.     

Simple laser velocimeter that uses photoconductive semiconductors to measure optical frequency differences

The dc photocurrents generated by steady-state moving space-charge fields inside photoconductive semiconductors containing deep level donors and traps can be used to determine the relative frequency differences between the two interfering optical fields that establish the space-charge fields. A simple laser velocimeter that uses a semi-insulating GaAs:Cr sample to detect the Doppler frequency shift between two laser beams is demonstrated.     

Performances of endoscopic holography with a multicore optical fiber

A holographic setup that involves the use of a multicore optical fiber as an in situ recording medium has been developed. The hologram is transmitted to a CCD camera for electronic processing, and the image is reconstructed numerically, providing more flexibility to the holographic process. The performances of this imaging system have been evaluated in terms of the resolution limit and robustness relative to noise. The experimental cutoff frequency has been measured experimentally over a range of observation distances (4-10 mm) and presents a very good agreement with the predictions made by simulation. The system features a resolution of 5-μm objects for a 4-mm observation distance. The different sources of noise have been analyzed, and their influence on resolution has been proved to be nonrelevant.     

Angle-dependent diffraction efficiency in a thick photorefractive hologram

The diffraction from a thick photorefractive hologram is shown to be angular dependent, which originates mainly from the angle-dependent effective electro-optic coefficient of a photorefractive crystal. The angle dependency of the diffraction causes a nonuniform diffraction over the pixel positions or the spatial frequency contents of a hologram image in a page-oriented holographic system, resulting in a deteriorated reconstructed image. In addition, owing to the angular variations in diffraction, the wavelength-multiplexing scheme should be a better choice than angular one.     

Extreme-ultraviolet interferometry at 15.5 nm using multilayer optics

The development of multilayer mirror technology capable of operating in the range of 3-30 nm and the construction of thin membranes with excellent uniformity and strength have made it possible to design and implement a Mach-Zehnder interferometer operating at 15.5 nm. We have tested this interferometer by using a soft x-ray laser as a source, and we show its use in probing high-density plasmas.     

Fiducial markers for increasing the versatility of optical correlation in the measurement of faults on integrated-circuit wafers

A microscope-coherent optical processor is used for the measurement of the registration errors on integrated-circuit wafers. The measurements are obtained from the optical correlation of wafers with reference wafer patterns by use of matched spatial filters. Previously, the intricate pattern of the active circuit area of wafers has been used in the correlation process, and a new matched spatial filter had to be created for each different integrated circuit. Here, the results of using comparatively plain fiducial markers on a wafer for the registration-error measurement are presented, and these show that the measurements can be made independent of the design of the integrated circuit while maintaining the advantages and accuracy of the optical correlation technique.