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A number of attempts have been made in an effort to combine the advantages of line-of-sight and diffuse configurations for indoor optical wireless communications via sophisticated combinations of elements that are characteristic for these architectures. A different approach has been followed in the present investigation, namely, developing a transceiver capable of operating in both configurations. It is proposed that the transceiver design be based on the utilization of two-dimensional arrays of infrared light-emitting devices and photodetectors. Basic design parameters of transceiver optics are derived from considerations about link blockage and system compliance with the unique features of line-of-sight and diffuse methods of transmission.

We propose a wireless optical-mesh network based on optical code-division multiple-access (OCDMA)-coded free-space optical links. The performance of the proposed network under intermediate and strong turbulence channel conditions is investigated for synchronous and asynchronous OCDMA. We show that synchronous OCDMA using complementary Walsh-Hadamard codes operating at a raw bit rate of 622 Mb/s per user can achieve a bit-error rate of [10.sup.-9]. This is achievable in channels with strong turbulence and high temporal correlation, or in channels with intermediate turbulence conditions; reasonable conditions based on channel characteristics inferred from the refractive index spectrum. For asynchronous OCDMA using optical orthogonal codes (OOCs), using a lower bound, we show that asynchronous OCDMA using OOCs cannot achieve acceptable performance for the same channel conditions, due to interference limitations. Index Terms--Optical code-division multiple access (OCDMA), wireless mesh networks (WMNs), wireless optics.

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. Key words: Real-time hologram recording, photosensitive materials, self-developing photosensitive materials for holography, photopolymers, dichromated polyvinyl alcohol.

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.

A number of attempts have been made in an effort to combine the advantages of line-of-sight and diffuse configurations for indoor optical wireless communications via sophisticated combinations of elements that are characteristic for these architectures. A different approach has been followed in the present investigation, namely, developing a transceiver capable of operating in both configurations. It is proposed that the transceiver design be based on the utilization of two-dimensional arrays of infrared light-emitting devices and photodetectors. Basic design parameters of transceiver optics are derived from considerations about link blockage and system compliance with the unique features of line-of-sight and diffuse methods of transmission.

Multispot diffuse configuration (MSDC) for indoor wireless optical communications, utilizing multibeam transmitter and angle diversity detection, is one of the most promising ways of achieving high capacities for use in high-bandwidth islands such as classrooms, hotel lobbies, shopping malls, and train stations. Typically, the optical front end of the receiver consists of an optical concentrator to increase the received optical signal power and an optical bandpass filter to reject the ambient light. Using the unique properties of holographic optical elements (HOE), we propose a novel design for the receiver optical subsystem used in MSDC. With a holographic curved mirror as an optical front end, the receiver would achieve more than an 10-dB improvement in the electrical signal-to-noise ratio compared with a bare photodetector. Features such as multifunctionality of the HOE and the receiver's small size, light weight, and low cost make the receiver front end a promising candidate for a user's portable equipment in broadband indoor wireless multimedia access. OCIS codes: 060.4510, 090.2890, 090.7330.