Holographic Plane Research Articles

Photorefractive beam-steering system that uses energy transfer in a BaTiO_3 crystal for a fiber-array interconnect

A beam-control system to write gratings on a holographic plane is studied. The arrangement is designed to interconnect two 1024 monomode fiber arrays. The beam-control system is composed of two subsystems: a beam steerer, which deflects one incident beam toward 1024 positions, and a collimating system, which adapts the shape of the deflected beam to the holographic plane. The collimating system was studied only after the beam steerer had been fully built and tested. It is based on the photorefractive amplification of a beamlet selected by a shutter array. The deflection efficiency is enhanced by a factor 1500 with the photorefractive crystal, and the signal-to-noise ratio is larger than 5500. The influence of the optical aberrations on the coupling losses of the infrared beams in the monomode fibers are evaluated.

Time-multiplexed real-time one-way image compensation for high-spatial-frequency aberration correction

A new self-aligning geometry for real-time holographic image reconstruction for one-way imaging through a phase aberrator is demonstrated. The input beams are time multiplexed to isolate the diffracted image from the reference beams after the image beams propagate through the hologram. This geometry permits the image-bearing beam and the reference beams to copropagate through the holographic plane.

Comparison of diffraction efficiencies for single-exposure and unity-contrast multiple-exposure holograms

The diffraction efficiency for a single-exposure holographic recording is compared with that of a multiple-exposure hologram recorded with maximum signal-to-bias ratio. For the multiple-exposure recording the object is illuminated point by point, and the reference wave amplitude for each subexposure is adjusted to produce a unity-contrast interferogram in the holographic recording plane. It is shown that, for the same M-point object and the same average exposure, the ratio of the single-exposure diffraction efficiency to the mulitple-exposure diffraction efficiency is proportional to M/R, where R is the reference-to-object beam ratio in the single-exposure case. This result is compared with that obtained in the multiple-exposure case when the reference wave amplitude assumes a constant value. Application areas include optical interconnects, holographic data storage, and volumetric multiplexed holograms for display.

Reconstructed double-pulsed holograms: a system for efficient automated analysis

Double-pulsed holograms form the basis of holographic particle image velocimetry. This technique is becoming widely used for measuring three-dimensional velocity fields. The major deficiency of holographic particle image velocimetry is that the double-pulsed holograms must be painstakingly surveyed by a human observer for determination of the location of objects within the holographic volume. A system that automatically and efficiently analyzes double-pulsed holograms of microbubbles in water is developed that eliminates the need for a human surveyor. The system employs a three-part algorithm: (1) Holograms of microbubbles are scanned in the holographic (x-y) plane. Microbubbles within the scanned region are detected by a CCD-based fast object-detection system. (2) Bubble images thus detected are focused (in the z-direction) by an autofocusing technique. (3) In-focus bubble images are analyzed by a simple set of image-analysis techniques, including edge detection and patch correlation. The output of this alg rithm is the size, and the displacement between the two laser pulses, of microbubbles in the water.

Diffractive–reflective optical interconnects

We have demonstrated the feasibility of diffractive-reflective optical interconnects. These interconnects consist of a sandwich of a holographic plane and a reflective plane. Various possibilities like beam relaying, connection switching, and broadcasting are discussed.

Nonlinear effects in holography due to caustics

Nonlinear effects, resulting in the appearance of higher order images on reconstruction, which arise because of the enhanced irradiance due to caustics at the holographic plane during recording are described and analyzed. The possible connection of the fringe patterns seen with catastrophe optics and wavefront dislocations is pointed out.

A new undulator grazing incidence monochromator

We describe a new grazing angle monochromator specially designed for undulator sources. A holographic plane grating is used in conjunction with a refocusing mirror, at a constant include angle of 174°. Astigmatism is eliminated and the odd aberration is reduced to second order terms in the scan angle. Spherical aberration is the main aberration left, and the small emittance of the undulator is used to keep this term small. Scanning is achieved simply by the rotation of the grating. The mount shows a line resolving power of 13 000 at 20 Å, as confirmed by exact ray-tracing. An application of the monochromator to an undulator installed in Aladdin is discussed.

Nearfield holography for wideband sources

Nearfield acoustical holography (NAH), a process developed at the Pennsylvania State University, has proved itself to be a powerful research tool in the study of single‐frequency sound sources. A new high‐speed data aquisition system has been developed which permits extension of the nearfield acoustical holography technique to the study of wideband noise sources. Using a partially parallel, partially sequential sampling network, near simultaneous sampling is achieved for 256 microphones in the holographic measurement plane, for acoustic frequencies up to 1500 Hz. Special problems encountered during the development of the high‐speed data aquisition system are discussed. [Work supported by ONR and NASA.]

Blazed Ion-etched Holographic Gratings

An ion-etching technique has been successfully applied to the production of blazed holographic plane gratings having various blaze angles ranging from 7 to 26 degrees. Efficiencies of the gratings thus produced were measured in the Littrow configuration as functions of an ion-etching condition, wavelength, and angle of incidence. The maximum efficiency for p -polarized light was found to be more than 64 per cent at about 3250 + for gratings etched at the beam angle of 80 degrees. Experimental results show that ion-etched gratings have the blaze property equivalent to that of holographic gratings made by means of Sheridon's method.

Sampling in holography of diffused objects or with diffused illumination

Differentiation between a ‘diffused’ hologram and a ‘specular’ hologram is determined by criteria of the wavelength and the ‘relief’ of the object. In a hologram where the object beam is diffused (due to diffused object illumination, or because the object surface is reflecting diffusively), every part in the holographic plane contains information about the whole object, but from a different ‘angle of view’. In a ‘specular’ hologram, the recording is somewhere between the ‘image recording’ and the ‘diffused holography’. The latter part is contributed mainly by the edges of the object. This paper describes properties of sampled diffused holograms in terms of resolution,S/N (contrast), angle of view, power spectrum and the ability of determination of the object. Different sampling functions are compared and experimentally examined, while the total diffracting areas are preserved constant. It was found that when the sampling areas were distributed over the entire plane and the observation distance was preserved, the resolution, the angle of view and the ability of determination of the object was close to the unsampled hologram. At the same conditions theS/N and the power spectrum were reduced. It seems that these techniques of sampling may be applied for multiplexing, subtracting and correlating between two adjacent recordings.

Ray tracing through holographic gratings

A method of tracing rays through holographic gratings, both plane and concave, is developed. The method is applicable not only to holographic gratings, but also to mechanically ruled gratings with variable spacing and curved grooves. Some examples are given in the form of spot diagrams for certain specific cases. A modification of the method is also described to apply to aspheric gratings.

Measurements on Images Retrieved from Holograms

This paper deals with measurements on images retrieved from holograms when no reference points are on the object and the distance from the holographic recording plane to the object is unknown. For this case, object coordinates formulation was developed and experimentally tested for the cases of stereoscopic and direct measurements in the image plane. Comparing these methods, the direct one was found to be simpler and faster without reducing the accuracy of the results.

Applications of a Holographic Interference Microscope

The holographic microscope has extended the usefulness of holographic interferometry. Without the use of an eyepiece, one is basically concerned with image holography. This is herein considered to include the cases where an object is imaged near, or on, the holographic film plane. The application of image holography for interferometry is described beyond that previously given. Its usefulness for studying small vibrating objects and for real-time interferometry is demonstrated.