Random phase data masks: fabrication tolerances and advantages of four phase level masks

Experimental study of the effects of a six-level phase mask on a digital holographic storage system

We measure the M/# and the bit-error rate of a digital holographic storage system with a 4f optical arrangement for three configurations: recording at the Fourier plane with and without a phase mask and recording outside the Fourier plane without a phase mask. Unexpectedly, no significant change in the dynamic range was observed when a phase mask was used to record in thick crystals. However, we show that a phase mask is a key component in a 4f digital holographic storage system if high-fidelity holograms with optimum volumetric density are to be stored.

Effects of multilevel phase masks on interpixel cross talk in digital holographic storage

We study the interpixel cross talk introduced to digital holographic data storage by use of a multilevel phase mask at the data-input plane. We evaluate numerically the intensity distribution at the output detector for Fourier plane hologram storage in a limited-aperture storage medium. Only the effect at an output pixel of interpixel cross talk from the four horizontal and vertical neighboring pixels is considered, permitting systematic evaluation of all possibilities. For random two-level and pseudorandom six-level phase masks, the influence of the pixel fill factor, as well as the aperture size of the storage medium, is studied. Our simulations show that, for a given aperture size, a random two-level mask is more susceptible to interpixel cross talk than is a pseudorandom six-level mask. Decreasing the pixel fill factor below 94% with a pseudorandom six-level phase mask makes it theoretically possible to have a system with no errors from interpixel cross talk if one particular 5-pixel pattern is forbidden through modulation coding. Reducing the input fill factor below 85% means that no patterns need to be excluded.

Dammann gratings as phase diffusers in Fourier holography

Fourier-transform holograms of periodic data display intense diffraction orders. To provide accurate storage, the holographic recording medium must have a large dynamic range. Dammann gratings can be used as deterministic phase diffusers to redistribute spatially the irradiance pattern of the hologram and thereby decrease the required dynamic range.

Computer modeling of the effects of apertures in the Fourier-transform plane of Fourier-transform imaging systems

Two computer models are presented that can be used to determine the image formed by a 4-∱ Fourier imaging system with an aperture in the Fourier-transform plane of the system. These models were designed to aid in the selection of system parameters in a Fourier-transform hologram recording system (namely, object size, feature ratio, focal length, and storage area) required to obtain a retransformed image of a given quality. The results of the simulations are verified in the laboratory.

Spectrum leveling by an iterative algorithm with a dummy area for synthesizing the kinoform

This paper describes a new iterative algorithm for synthesizing the kinoform so that the Fourier spectrum of an object is leveled by adjusting the information of a dummy area introduced into the object's domain, spatially isolated from the signal area. Theoretical consideration of the effect of the dummy area derives the required size of the dummy area, and computer simulations prove it to be valid, although restricted to an object composed of binary numbers. Also, it is shown that highly efficient use of the incident light is possible to achieve. Experimental results verify the proposed scheme.

Interferometric spectrum analyzer

Dynamic range is a key performance parameter for spectrum analyzers. The dynamic range of a Bragg cell power spectrum analyzer is generally limited by the dynamic range of self-scanned photodetector arrays. Interferometric techniques can be used to increase the dynamic range; but it is at the expense of increasing the number of photodetectors required, when the interference is introduced in the spatial domain, or a large photodetector bandwidth, when the interference is introduced in the temporal domain. In this paper we describe an interferometric approach wherein a second Bragg cell generates a spatially modulated reference waveform to produce an interference term that has a constant temporal frequency for all spatial frequencies. The advantages of this approach are lower photodetector bandwidth, improved dynamic range, improved cross talk suppression, more efficient use of the Bragg cell time-bandwidth product, immunity to scattered noise, and improved short pulse detectability. The chief disadvantage is the need for a discrete element photodetector array; when such arrays become available in hybrid or integrated packages, an additional advantage will be that of parallel postdetection processing.

Frequency-modulated zone plate for coherent noise reduction

Laser images and holographic reconstructions are highly affected by dust and imperfections present in various optical components of the imaging system, which create noiselike structures in the final images. To reduce this speckle noise, a phase plate based on a frequency-modulated zone plate is proposed. It can be fabricated by holographic techniques, and it does not require a separate Fourier transforming lens. Experimental results are presented showing the capabilities of the phase plate.

Holographic image formation using phase plates with incoherent imaging property

An incoherent image synthesis method using a recently developed grating modulated phase plate has been presented in order to reduce unwanted noise components in the reconstructed images. A study in connection with this method leads us to the analytical derivation of a desirable phase plate with a specified phase sequence. The analytical description presented here, with particular emphasis on obtaining the imaging property in coherent holographic imagery identical to that in incoherent imagery, shows that the derived phase plate enables us to get the reconstructions with far less undesirable irregular intensity fluctuations or noiselike pattern. Some experimental confirmations are given to make sure of the analysis.

High density image-storage holograms by a random phase sampling method

The random phase sampling method that has been proposed to make high quality and high storage density holograms capable of storing image information is described and discussed from the aspect of the quantitative characteristics of its reconstructed image. The method enables the uniform distribution of light energy over the hologram area made on the exact Fourier transformed plane. The details of the characteristics of the method are investigated, especially with respect to the luminance tone linearity, the signal-to-noise ratio, and the resolution to give good agreement with the results of a calculation. In the experiment, reconstructed images with high quality were obtained from holograms of 2-mm diam, which were made by 10(6) sampling and random phase shifting. By using the random phase sampling medthod, an image retrievel model system storing twenty kinds information was developed. This system holds promise of being used in various practical applications such as holographic ultramicrofilm system or a holographic videopackage system.

Comparison of deterministic phase coding with random phase coding in terms of dynamic range

Phase coding techniques are useful for lowering the dynamic range of the recorded spectrum in digitally generated hologram and kinoform memories. Two types of phase coding schemes, random phase coding and deterministic phase coding, which is independent of the input sequence, are compared in terms of the dynamic range of the spectrum. A probabilistic analysis shows that deterministic phase coding is advantageous over random phase coding. It is shown that Frank codes and a specialized form of Schroeder codes are effective for deterministic phase coding.

Speckle reduction by double recorded holograms

A method of speckle reduction in holography for 2-D objects with minimum recording area is presented. The image is incoherently synthesized from the hologram double recorded with a set of random phase plates.

Point Efficiency and Signal-to-Background Ratio in Exponential Holograms for Optical Memories

The reconstruction of a point matrix by holograms with an exponential exposure characteristic is exactly calculated in dependence on exposure parameters, geometry, and position of a point in the image field. Linear and nonlinear distortion of the reconstruction is described in terms of the signal-to-background ratio (SBR). The variation of the SBR across the image field is demonstrated for both phase holograms and exponential amplitude holograms. The results, derived for point holograms in optical memories, are also applicable to the more generalized case of holograms of diffuse objects.