Locally linearized synthetic near-field holograms.

An efficient procedure for the generation of synthetic near-field hologram structures for display purposes is presented. The formation process requires virtually no computation effort.

Pulse delay at diffractive structures under resonance conditions.

An optical pulse incident upon a diffractive structure under resonance conditions may undergo a temporal delay. The effect is demonstrated with a pulse of Gaussian profile at two geometries: a waveguide grating and a single-mode waveguide. It is shown that the order of magnitude of the delay can be controlled by the parameters of the structures.

Use of error diffusion with space-variant optimized weights to obtain high-quality quantized images and holograms.

Error diffusion is an important procedure for image and hologram quantization. The spatial distribution of the spectrum of the quantization noise is shaped by a filter function, which depends on the diffusion weights. The customary weights applied during the whole quantization process are optimized to yield the desired filter functions.

Beam displacement at diffractive structures under resonance conditions.

A beam of light incident upon a diffractive structure can under resonance conditions be subject to a rather large lateral displacement. The effect is demonstrated with a Gaussian profile beam and two geometries based on a waveguide grating and a single-mode waveguide.

Confined wave packets in the domain of Rayleigh-Wood anomalies.

It is known that near grating anomalies of the resonance type a beam may undergo a lateral shift of the order of the beam width, and a pulse may be delayed by a time of the order of the pulse duration. These numerical investigations are extended to Rayleigh anomalies that occur when, upon variation of the wavelength or the angle of incidence, an additional propagating diffraction order emerges. It is shown that delays and displacements are 1 order of magnitude smaller than in the resonance case.

Diffractive efficiency improvement of diffractive cylinder lenses by Gaussian-beam illumination.

To maximize the diffraction efficiency of cylinder lenses with high numerical apertures (such as F/0.5 lenses) we use an iterative algorithm to determine the optimum field distribution in the lens plane. The algorithm simulates the free-space propagation between the lens and the focal plane applying the angular spectrum of plane waves.

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.

From evanescent waves to specified diffraction orders.

A grating was designed to convert an evanescent wave into propagating diffraction orders that fulfill a specified optical function.

Gradient-controlled iterative half-toning.

In digital half-toning there exists a general desire to adapt the properties of the half-toning algorithm to local properties of the input image. We show that consideration of the image gradient in an iterative convolution algorithm with a space-variant impulse response can be used to improve reproduction at high frequencies. Experiments demonstrate the properties and control of this algorithm.

Hybrid color holograms.

The familiar optically recorded rainbow hologram is frequently used to produce color holograms. It is simple to compute the corresponding synthetic hologram structures, which would require no recording process and would be capable of displaying virtual three-dimensional objects. It is difficult, however, to materialize the computed structures because of the high space-bandwidth product required.

Synthetic near-field holograms with localized information.

Synthetic near-field holograms with large apertures are suitable for three-dimensional display applications. For the computation of near-field hologram structures, algorithms that involve large Fourier transforms are time consuming. A simple method is presented that shortens the generation process.

Binary gratings as beam splitters with asymmetric signals.

In the framework of scalar diffraction theory, binary digital holograms and Dammann gratings generate symmetric intensity distributions as diffraction patterns. If asymmetric signals are of interest, either an offset from the optical axis has to be introduced or more quantization levels have to be used. We show that a synthesis on the basis of rigorous diffraction theory permits binary elements with subwavelength features to realize on-axis asymmetric signals.

Design of antireflection gratings with approximate and rigorous methods.

High-spatial-frequency gratings can be used as an alternative to thin-film antireflection coatings to reduce the reflectivity at the boundary between two different media. In the case of one-dimensional gratings, the conditions on the grating structure can be approximately determined by the effective medium theory (EMT) in combination with the thin-film theory. For two-dimensional gratings, which can be used to reduce the polarization sensitivity, a corresponding EMT does not exist.

Increase of the space-bandwidth product of computer-generated near-field holograms by optical means.

Computer-generated near-field holograms are well suited to display numerical data in three-dimensional pictorial form. The available computer capacity and recording facility frequently limit the attainable space-bandwidth product. An optical technique that can improve it and provide comfortable viewing conditions is presented.

Binarization of diffractive elements with nonperiodic structures.

An error diffusion and an iterative concept that is based on the Fourier-transform algorithm are applied to binarize nonperiodic diffractive elements that generate speckle-free diffraction patterns. These methods are used successfully to calculate periodic elements and are adapted to specific constraints in the case of nonperiodic elements. Techniques to avoid stagnation are described, and optical experiments are presented.

Error-diffusion algorithm in phase synthesis and retrieval techniques.

The treatment of phase synthesis and retrieval with the help of projection algorithms often suffers from a stagnation problem. A bandwidth constraint introduces undesired first-order zeros. From the incorporation of error diffusion the stagnation can be avoided.

Iterative techniques to integrate different optical functions in a diffractive phase element.

Diffractive optics allows the incorporation of several optical functions, e.g., wave shaping and focusing, in one element.

Signal synthesis for diffractive optical elements in hybrid systems.

Generally, a diffractive optical element/digital hologram is used as a component in a system. For a specific system output, an appropriate signal must be realized as a diffraction pattern. We present methods to determine such signals, whereby the characteristics of the system are considered.

Laser beam scanning using computer-generated holograms.

A new laser grating scanner concept with attractive capabilities is described. The grating used in this scanner is a computer-generated hologram. The constructional parameters of this type of computer-generated hologram are discussed.