Eli S. Simova

Dichromated polyvinyl alcohol as a real-time hologram recording material: some observations and discussions.

The real-time hologram formation in dichromated polyvinyl alcohol films has been studied in the presence of external electron donors included in the formulation. The effect of different external electron donors on the photosensitivity and real-time diffraction efficiency of volume holographic transmission gratings has been reported. The electron donors studied were found to be detrimental to both holographic characteristics, even in small concentrations. Some parameters that influence the holographic performance are discussed, and results are given. The photochemical recording mechanism in dichromated polyvinyl alcohol in the presence and absence of electron donors is briefly discussed.

Indoor Wireless Infrared Link with a Holographic Multiple-Spot Diffuser

The emergence of portable terminals is expected to accelerate the introduction of wireless Local Area Networks (LANs). Portable devices require low power consumption, small size and weight. The demand for inexpensive, high-speed links satisfying these requirements has motivated the research on infrared (IR) wireless communications. Indoor wireless optical links can be implemented in different configurations1. High data rates IR systems with directed line-of-sight (LOS) configurations have been reported24. Directed LOS, however, is not suitable for portable terminals, alignment is crucial and full mobility cannot be achieved. A proper choice for portable communications is a non-directed configuration non-LOS, often referred to as a diffuse configuration. Gfeller and Bapst5 have pioneered the wireless IR communications via diffuse radiation. This configuration has also been realized and investigated by other research groups68. The system is tolerant to shadowing and no alignment requirements are imposed on the transceivers. The IR wireless transmission relies on the scattering properties of the surrounding surfaces, i.e., walls and ceiling, which act as distributed optical sources. In the near IR, most of the materials have a reflectivity of 40 to 90%. To achieve an approximately constant received signal power over the entire office space, the IR source radiation has to be distributed on the diffusely reflecting surfaces as homogeneous as possible. Light-emitting diodes produce diffuse radiation, however, a laser diode is required for a wider modulation bandwidth and to meet the power budget at high data rates. The receiver detects attenuated signals after multiple reflections from the ceiling and the walls.

Experimental optical CDMA system based on spectral amplitude encoding of noncoherent broadband sources

The aim of this work is a proof-of-concept demonstration of an optical CDMA system based on spectral amplitude encoding of incoherent broadband sources. A Light Emitting Diode is employed as a source in a multi-mode fiber-optics system, and simple pseudorandom sequences such as m-sequences are used for optical encoding. We propose a multi-pass configuration with a Littrow grating arrangement for the optical encoder and decoder. Since this is a power-limited system, the multi-pass arrangement allows us to greatly reduce the number of lossy optical components and to increase compactness and power throughput. A differential receiver with a simplified design is employed. Measurement results are presented and discussed.

Silver-halide sensitized gelatin derived from Agfa-Gevaert holographic plates

To our knowledge only one processing formula for silver-halide sensitized gelatin (SHSG) derived from Agfa-Gevaert holographic plates has been published in the current literature, which is apparently a result of the extremely high degree of hardening of the gelatin emulsion. We propose a modified processing formula for SHSG derived from Agfa-Gevaert plates. The holographic characteristics of the processed SHSG plates were measured and high diffraction efficiency, as high as 80%, and an almost flat spatial-frequency response within the region of interest were achieved. Some of our observations during the experiments are discussed. The behavior of the gelatin emulsion was consistent with the models for processing dichromated gelatin. We observed swelling rather than the shrinkage expected from the removal of the silver-halide grains. This swelling can be controlled by postbaking.

Holographic 4x4 star-coupler in silver-halide sensitized gelatin

Star couplers are fanin-fanout devices with each input beam split into several output beams with arbitrary intensities traveling at different angles. We demonstrate a 4 x 4 holographic star coupler fabricated as multiplexed holographic gratings in silver-halide sensitized gelatin (SHSG) derived from Agfa-Gevaert plates. The processing formula is modified to achieve high diffraction efficiency and an acceptable modulation transfer function within the region of spatial frequencies of interest. The holographic characteristics of the processed SHSG are measured. The required gratings are recorded simultaneously to avoid reciprocity law failure, which occurs in SHSG as well. The gratings are recorded to ensure equal grating strength, which gives an acceptable intensity ratio between the output beams. The star coupler is examined for use as a broadcast medium in fiber-based local area networks (LAN5), where relatively high loss and inhomogeneity between the outputs could be tolerated.

Light-shaping diffusers for indoor wireless infrared communications via a holographic approach

This paper describes an application of holographic to product diffusers with different optical properties. We report on holographic diffusers for making uniform the infrared source radiation patterns with a variable dispersion angle. The holographic recording geometry provides control over the angular dispersion characteristics of the diffuser. Multiple-lobe and multiple-spot holographic diffusers with 3D scattering patterns have been fabricated. The scattering pattern is achieved by recording a diffuser. We provide some details on the recording set-up and procedure. We discuss some holographic properties of DuPont holographic recording films used in this application. Measurements of the scattering properties are presented for diffusers made in DuPont holographic recording films. For a holographic multiple-spot diffuser, the optical channel frequency response was measured and compared with an opal glass diffuser.

Dichromated polyvinyl alcohol for real-time hologram recording : effect of humidity

Abstract The effect of environmental humidity on the photosensitivity and real-time diffraction efficiency has been studied in dichromated polyvinyl alcohol (DC-PVA) films with and without external electron donors. High relative humidity has been found to significantly improve the photospeed and the real-time diffraction efficiency for almost all of the studied formulations. The results and possible photochemical reactions in the recording mechanism are briefly discussed.

Automated Fourier transform fringe-pattern analysis in holographic moiré

Fourier transform fringe-pattern analysis is a wide-spread technique with many applications to optical metrology. Introducing a spatial carrier into a holographic moire pattern is also a well-known technique to obtain observable moire fringes. Preprocessing of the holographic moire pattern includes optical or digital Fourier filtering to remove the carrier. The pattern is then processed by employing time-consuming and less accurate fringe-counting algorithms. This work presents a new application of the Fourier transform method to automated analysis of holographic moire fringe patterns. The method has the advantage of instantaneous determination of both phase functions in the fringe pattern. Investigations are carried out on the frequency limitation imposed on the spatial carrier. The technique is demonstrated on computer-generated noisy holographic moire patterns. The technique does not complicate the conventional holographic moire arrangement.

Experiment on Optical Code-Division-Multiple-Access Switch System Using Spectral Amplitude Encoding of Light-Emitting Diodes

Significant research is directed toward developing fiber-optic local distribution networks providing high data rate transmission in the local end of the backbone network. Code-Division Multiple-Access (CDMA) technique is capable of utilizing the optical channel bandwidth to provide simultaneous asynchronous access. Time-encoded optical CDMA systems can be classified according to the type of encoding, coherent or incoherent. Incoherent time-domain CDMA has advantages of direct detection and low-cost low-coherence optical sources1–3. Coherent time-domain encoding is more difficult to implement4. Such systems with low-coherence sources have also been proposed5. For time-encoded CDMA, the code length increases with the number of users and spectrum broadening occurs. To overcome the drawbacks of time-encoded CDMA, frequency-encoded systems have emerged based on coherent phase coding of ultrashort pulses6. Users operate at data rates compatible with the electronics, but high-cost mode-locked lasers are required. An optical CDMA system based on incoherent amplitude encoding of low-cost light-emitting diodes (LEDs) and direct-detection receivers has been proposed by D. Zaccarin and M. Kavehrad7–8. A comprehensive overview of the optical CDMA systems has been introduced by K. Iversen9.

Real-time holographic recording in dichromated polyvinyl alcohol films: investigations on some external electron donors and effect of humidity

In this work, we present a study on the effect of four external electron donors in the formulation of dichromated polyvinyl alcohol (DC-PVA) films on the photosensitivity and real-time diffraction efficiency of volume holographic transmission gratings. The studied amines were found to have a detrimental effect on both holographic characteristics, even in small concentration. We found that higher relative humidity significantly improves the photospeed and the diffracted efficiency in DC-PVA films with and without an electron donor in almost all cases. The effect of the environmental humidity on the real-time diffraction efficiency in DC-PVA films with and without electron donors has been studied. Measurements of holographic characteristics are discussed. The photochemical recording mechanism in DC- PVA in the presence of external electron donors is briefly discussed as well. The amines may form a strong chelate ring structure with the chromium ions and, possibly, interfere with the crosslinking action of the photoreduced chromium. Crosslinking of the PVA matrix may occur either by PVA radicals or complexation of PVA with chromium. In both cases, hologram formation includes an electron transfer reaction, and the amines, good electron donors hinder the process by making chelate with the chromium.