On-axis Holograms Research Articles

On-axis diffuse screen based on a reference-free volume hologram

A new type of hologram that elaborates upon the simplest (Gabor) on-axis hologram is suggested. The new approach makes it possible to eliminate the projected image distortions typical of the Gabor hologram. Specifically, based on a reference-free thick hologram, an on-axis holographic screen that does not transmit the zeroth order is prepared. In addition, this screen does not produce a halo and the conjugate image. It allows for recording in one spectral range and reconstruction in another, thereby greatly simplifying the choice of a light-sensitive record medium. With this screen, a color image can be projected.

On-Axis Diffuse Directional Screen Based on a Transmission Gabor Hologram

Holographic single-component diffuse screens based on on-axis transmission holograms are studied theoretically and experimentally. The configurations of extra holographically reconstructed images of the diffuse screen (conjugate image, halo, and zeroth-order spot) are analyzed. A so-called centered hologram is suggested in which the conjugate image of the diffuse screen spatially coincides with the main image, thereby eliminating distortions inserted by the conjugate image. Also, hologram recording using a Mach-Zender interferometer is suggested and implemented. Such an approach makes it possible to eliminate the shadow of an object on the hologram. It is noted that, in recording a planar diffuse screen, the presence of the halo does not distort the image of an object projected through the screen: only the light intensity is partially lost. Analysis of the images projected shows that the luminance of the zeroth-order spot, while relatively low, should be diminished in one way or another.

On-Axis Holograms Made with Speckle

On-Axis Holograms Made with Speckle

Optimum on-axis computer-generated hologram encoded into low-resolution phase-modulation devices.

A computer-generated hologram with minimum spatial complexity is proposed for encoding arbitrary complex transmittance with low-resolution phase-modulation devices. The phase modulation of the proposed hologram is determined such that on-axis reconstruction with high signal-to-noise ratio, optimum reconstruction efficiency, and the largest possible signal bandwidth can be obtained.

Characteristics of the reconstructed image upon multiplexing on-axis holograms

A comparative analysis of the experimental data on recording and multiplexing holograms, including speckle holograms, by an axial scheme is presented. The contrast and signal-to-noise ratio in the images reconstructed are compared. It is demonstrated that the thickness of the photosensitive material has an effect on the image quality during the recording of filamentary holograms.

On-axis computer-generated holograms based on centrosymmetric partitioning of the pixel

A novel, to our knowledge, configuration for the design and fabrication of zero-order computer-generated holograms in which each pixel is split into two centrosymmetric equal-sized regions is proposed and tested. In a manner similar to other approaches this configuration also permits the encoding and the reconstruction of a complex function that exhibits phase as well as amplitude variations by use of a phase-only filter. A detailed mathematical analysis is followed by evaluation of the error of the encoding approach, which is calculated and compared with the error exhibited by other approaches. Computer simulations as well as optical experiments demonstrate the capabilities of this novel configuration.

On-axis computer-generated holograms for three-dimensional display

The feasibility of on-axis synthetic near-field amplitude holograms for three-dimensional display applications is demonstrated. An iterative optimization algorithm is used that generates an object-dependent diffuser that utilizes the phase and, to some extent, amplitude freedoms in the reconstruction plane. The discrimination between twin images and undiffracted terms is thus improved. The on-axis approach presents important advantages: a low coherence requirement for the illuminating source, a lower spacebandwidth and higher viewing angle than with the off-axis alternatives. Defocusing and parallax are experimentally attained with an extended white-light source and a lensless setup.

On-axis binary-amplitude computer generated holograms

On-axis holograms are synthesized on positive binary-amplitude recording media. These holograms present unique characteristics: The signal is reconstructed around the optical axis without overlapping orders and it is the brightest order in the reconstruction plane, the coding procedure is fast, and the holograms can be implemented on low information content recording devices. Various examples and applications are discussed and the method is illustrated with an experimentally implemented array illuminator.

Wave-front control using liquid-crystal devices

We report on complete wave-front control with amplitude- and phase-modulation liquid-crystal devices (LCD's). A twisted nematic device is used for amplitude modulation, and an electrically controlled birefringent device is used for phase modulation. Because the LCD's are optically coupled with afocal optics and are driven by individual LCD driver circuits, the amplitude and the phase can be controlled two dimensionally and independently. Complex amplitude data are calculated, and on-axis computergenerated holograms are directly recorded. Furthermore, we discuss LCD performance requirements for high-quality reconstruction.

Computer generated holographic optical elements for optical interconnection of very large scale integrated circuits

The use of computer generated holographic elements for implementation of free-space optical interconnection of very large scale integrated circuits is presented. The design of the holographic optical elements is based on laser source divergence, source to hologram spacing, signal fanout, and resolution of the hologram recorder. The results of a computer simulation of diffraction from the hologram are compared with the analytically predicted results, particularly to confirm the effect of spatial sampling on the performance of an on-axis interferogram hologram. Measurements taken using transmission absorption and transmission phase holograms in photographic emulsion, as well as reflective surface relief holograms on silicon, are compared to predicted results. Performance benefits expected from extension of the design process to holograms fabricated using electron-beam lithography are discussed.

Cell-oriented on-axis computer-generated holograms for use in the Fresnel diffraction mode

A type of computer-generated hologram (CGH) is introduced which works in the Fresnel diffraction mode and generates the desired wave front on-axis. Such holograms rely on the use of the detour-phase method suitably modified for Fresnel diffraction and on the sampling theorem; therefore, they are of the cell-oriented type. The concept of a carrier or reference wave is not needed to introduce them; the Huygens-Fresnel principle provides a complete description of their diffracted waves. Consequently, no orders in the usual sense of grating orders or zone plates or natural holograms can be defined. Only orders in the sense of the sampling theorem exist for these computer-generated holograms, and the useful order is the central one; it is also the brightest and does not overlap with other orders. These questions are discussed and illustrated by experimental examples in connection with the problem of diffraction efficiency. Among the examples, the CGH of a single point on-axis is considered; it is in general not a zone plate.

On-axis holograms for teaching principles of holography

Instructions are given for making a simple on-axis hologram in which a line on the object is displayed as a Fresnel grating on the hologram. The location of the reconstructed image may be calculated from measurements made on the grating and compared with the experimental value.

Image information by means of speckle-pattern processing*

When a laser beam is reflected from a diffuse surface, a granular scattering distribution is commonly observed, called a speckle pattern. We will discuss some of its ramifications: (1) this pattern is the time-independent analog of the signal used in the classic Hanbury Brown–Twiss stellar interferometer of the 1950’s; (2) this pattern can be processed by standard incoherent optical techniques to yield information pertaining to the object radiance distribution; (3) this same signal, when processed by coherent-optical techniques, is equivalent to the Gabor on-axis hologram or the Fourier-transform hologram, depending on the specific source configuration; and (4) signals processed by all of the above techniques are comparatively insensitive to atmospheric turbulence. An experiment is performed to illustrate the procedure of item (2) and then modified to show assertion (4).

New ways to make computer-generated color holograms

Color images can be produced from three computer-generated holograms, one for each of the primary colors. Two basic problems must be solved: scaling the images according to wavelength and avoiding false images. The former can be solved by a computational step, during synthesis of he holograms, or during reconstruction. The latter problem, false images, can be solved by the reconstruction geometry or by using multi-emulsion color film. The wavelentgh-selective absorption of the different layers in color film can be used to selectively absorb light of an undesired color while transmitting light of the desired color at a given point on the film. This effect allows the spatial multiplexing of binary detour-phase holograms. In addition to density variations, there exist corresponding phase variations in color film, particularly Kodachrome II, making onaxis holograms possible. Furthermore, the phase variations can be used to eliminate an undesired color by selectively diffracting it off to the sides. Experimental results are presented.

Recent Approaches To Computer - Generated Holograms

Computer holography is a subject of interest to a small but increasing number of researchers. This paper briefly describes the concept and history of computer holography, how it works, and its possible applications. Also presented are the recent approaches to computer holography developed at Stanford University, and a procedure for synthesizing this new class of computer holograms. These new on-axis holograms are very efficient both in their use of reconstruction light and in their use of display resolution elements during synthesis. They keep the advantages of the kinoform, a previous approach by IBM, but do not have its limitations. One approach makes use of color reversal film, such as Kodachrome II. Another approach only requires phase-transmittance material, but preserves both amplitude and phase information. Experimental results are presented.

Analysis of Multiple Hologram Optical Elements with Low Dispersion and Low Aberrations

When holograms are used as optical elements two of their properties are of interest: dispersion and aberrations. This paper uses a general hologram analysis computer program based on ray tracing to analyze these properties. Numerical examples are given for both single and multiple element systems where the goal is to reduce the lateral dispersion. A significant reduction in this dispersion is accomplished with the introduction of a plane grating adjacent to the hologram. The problem of reducing the longitudinal dispersion is more complex, and it is studied in the paper by minimizing the variation in focus distance as a function of the reconstruction wavelength for two- and three-element on-axis hologram systems. To optimize the reduction in dispersion and design a system with low aberrations, the hologram ray tracing programs were modified to be driven by an optimization program. These techniques realized a three-element on-axis f/3.84 system with aberrations and dispersions less than 0.32lambda(c) and 87 mu, respectively, when operated over a band from 4385 A to 7060 A.

Television Transmission of Holograms Using a Narrow-Band Video Signal

It has been experimentally verified that holograms can be transmitted via television using the synthesized carrier-frequency method. A vidicon camera system was used to transmit three on-axis holograms to the receiver where the video signal was used to amplitude modulate a 15 MHz carrier. This modulated carrier was applied to the control grid of a cathode-ray tube, and the synthesized carrier-frequency hologram was then photographed from the tube screen. Several of the experimental problem areas, such as mechanical stability, camera tube storage effects, and display-recording process optimization, are considered in detail.

Television Transmission of Holograms With Reduced Resolution Requirements on the Camera Tube

This paper proposes a technique for the television transmission of a hologram of a two-dimensional transparency. The spatial resolution required on the camera tube is reduced by a factor of four compared with the transmission of a conventional off-axis reference beam hologram. The resolution required is therefore no higher than that required for the direct transmission of the transparency itself. Implementation of the proposed arrangement should be easy. Three holograms formed with an on-axis reference beam are transmitted. The phase of the reference beam assumes the values 0°, 120°, and 240° for the first, second, and third hologram, respectively. The carrier-frequency hologram is “synthesized” from these three on-axis holograms at the receiver. The technique has the further advantage that the undesirable zero-order terms are eliminated.