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A method for coded-wavelength multiplexing according to which a signal waves Si (r) is recorded in a holographic medium in a counter-propagating geometry using corresponding writing reference waves Ri (r). The method involves selecting discrete wavelengths λ and encoding reference wave vectors ρl which make up writing reference waves Ri (r) such that the writing reference waves Ri (r) at each wavelength λ are orthogonal. The stored signal waves Si (r) are reconstructed in the form of reconstruction waves Ac (σ) with reconstruction reference waves Rc (r) selected from among the writing reference waves Ri (r). In the event of angular multiplexing of the reference wave vectors ρl, it is possible to use one reference wave to produce a number of reconstruction waves Ac (σ) and generate a mosaic of desired holographic pages.

Volume holography is an important candidate for data storage and data processing applications. Its advantages lie in the inherent high capacity of volume memory, fast access due to optical addressing, and fast transfer rates due to the parallel nature of holography [1]. Multiple holograms, or data pages, may be stored using angular multiplexing, wavelength multiplexing, and related techniques. Capacity is determined by the angular bandwidth of the signal, the number of holographic pages, and the acceptable level of noise due to crosstalk and other sources present in the system. Volume holography is also an important candidate for optical interconnects and wavelength division multiplexing in communications and computing systems. Frequently, the same criteria required for data storage coincide with those required for other applications.

Holographic storage offers many advantages over conventional storage methods, however, readout of information cannot be made non-destructive without special fixing procedures, such as thermal and electrical fixing. Several other techniques enabling non-destructive readout of information have been investigated, including two-photon recording in singe species crystals[1], and sensitizing with a short wavelength and recording and reading with a long wavelength with two species[2,3]. The last method has the advantage of allowing each species to be chosen, so that the photorefractive properties can be optimized while retaining the information during readout.

Summary Holographic data storage systems [1] require sensitive optical materials that provide some means of nondestrective readout [2]. Among the number of techniques for nonvolatile readout available [2]. the sensitazation of a photorefractive medium by illumination at one wavelength, called gating, for writing at a second using a resonant two-photon process, constitutes the most versatile all-optical nondestructive method. Typically, this process consists of absorption of a photon at frequency ω1 to generate an intermediate excited state, followed by absorption of a second at ω2 to generate a free carrier [3, 4, 5]. Gating occurs for absorption of two photons at two different wavelengths. In the present work, we identify the critical physical characteristics of the processes that provide the simultaneous realization of true gating and writing sensitivity at a variety of frequencies, evaluate the refractive index perturbation and the response rate, and investigate the significance of the the intermediate state occupancy and the arisining nonlinear photorefractive behavior at its saturation regime.

Multiple holograms may be stored in volume holographic media through angular, phase-encoded, or wavelength multiplexing.1 We compare the cross-talk for angular and phase-encoded multiplexing.

Phase conjugated light waves reverse phase distortions in media with negligible absorption or for paraxial beam propagation [1]. We examine here a different case in which a volume conjugating medium is itself imperfect, possessing phase distortions and a conjugating efficiency dependent on grating spatial frequency. We show theoretically that for multiplexed signals, cross-talk-free phase-conjugation is obtained only for negligible spatial-frequency dispersion and a phase-conjugated reference or pump wave, and we present supporting experimental results.

Phase-conjugated light waves are known to reverse phase distortions in media with negligible absorption or for paraxial beam propagation.1 We examine here a different case in which a volume conjugating medium is itself imperfect, possessing phase distortions and a conjugating efficiency dependent on grating spatial frequency. We show theoretically that for multiplexed signals, cross-talk-free phase conjugation is obtained only for negligible spatial-frequency dispersion and a phase-conjugated reference wave. We further present experimental results in a severely striated sample of photorefractive strontium barium niobate; using a phase-conjugated reference wave, we demonstrate the alleviation of distortion2 and the elimination of cross talk between closely spaced angularly multiplexed signals.

Most photorefractive crystals display a measure of imperfection, such as striations, curved surfaces, or other phase-disturbing properties, which distort transmitted images and degrade the fidelity of holographic storage. We consider strontium barium niobate crystals with striations perpendicular to the c-axis. In such crystals, extraordinary polarized light experiences the strongest photorefractive effect but scatters off the striae, degrading the image fidelity. Conventional four-wave mixing, using counterpropagating pump beams, does not completely alleviate the problem, as the pump beams are also scattered by the striae. This difficulty can be overcome by using two true phase-conjugate pump beams. We describe hologram storage in striated strontium barium niobate, using a true conjugate of the reference beam generated by internal self-pumping in a second photorefractive crystal. The conjugate of the reference beam generates a conjugate of the signal beam, which then reproduces the stored image with high fidelity. This technique enables the use of imperfect crystals in photorefractive applications. We explore uses of this technique for holographic data storage.

We identify three separate regimes in the evolution of self-pumped phase conjugation in photorefractive media. These are evident from the time evolution of transmission, omnidirectional scatter, and phase-conjugate reflection intensities measured experimentally. Initially, beam fanning occurs, manifested by depletion of the transmitted light to one-third of its initial value. This is followed by a gradual increase in the omnidirectional scatter intensity caused by spreading of the light paths in the crystal, until almost all the crystal is flooded. Only much later does the phase-conjugate beam appear and grow to half of the input intensity at the expense of both the transmitted and scattered light. The three processes persist for different durations: fanning is appreciably faster than the other two processes. Each regime is characterized by different beam spread within the crystal, which is observed in top-view images. We postulate physical mechanisms to describe the observed phenomena.

Principal and complementary space-charge gratings are formed in photorefractive bismuth silicon oxide with use of 785-nm light. An electric field is optionally applied in the direction of the grating for hologram evolution by either drift or diffusion of charge carriers. For write times on the order of the decay time of the principal grating, no complementary behavior is observed. For much longer write times, a complementary space-charge grating is introduced both in the presence and in the absence of the applied field, and is initially hidden due to screening by the principal grating. Uniform illumination in the presence of the field reveals the complementary grating. Time-resolved data show its growth and decay, with a response rate much lower than that of the principal grating. A two-level electron-hole transport model explains the observations; one level participates in the establishment of the principal grating by majority carriers, and the other in the establishment of the extended-lifetime complementary grating by minority carriers. A scheme for multiplexing normal and extended-lifetime complementary gratings is presented.

Extended-lifetime holograms are observed in photorefractive bismuth silicon oxide (BSO) at 785 nm. In the crystal, light beams from a diode laser (~500 μW/cm2) create an index grating with a period of 10 μm. An electric field ~5 kV/cm is optionally applied in the direction of the grating 110 for hologram formation by either diffusion or drift of charge carriers. Different formation procedures lead to different decay processes of the grating. In the first case, the hologram is written with the applied field for 70 s and, upon uniform illumination in the applied field, decays normally, with a time constant ~15 s. In another, the hologram is written without the applied field for 640 s and decays less simply: The initial diffusion grating is extremely weak for this period; however, upon uniform illumination in the applied field, the hologram quickly strengthens to the level of the previous case ~15 s, and then decays more slowly ~2000 s. Two-level electron–hole transport1 can explain these effects: using one level, electrons establish quick-response gratings; using the other, holes establish complementary extended-lifetime gratings. Vainos et al.2 have multiplexed short-lived photorefractive with long-lived photochromic holograms in BSO, leading to many image processing techniques. Multiplexing the normal and extended-lifetime holograms described here may yield similar techniques.

For most regimes descriptions of photorefractive behavior by electron–hole transport with a single species of impurity levels and by electron–hole transport with two species of impurity levels are significantly different. The steady-state space-charge field depends on the model used, and the presence of two response rates and complementary gratings requires the presence of two species. For appropriate grating spatial frequencies, electric fields, and time limits, however, the models give similar results. We describe these regimes of difference and similarity.

A scanned Ar+ laser beam is demonstrated to be an effective way to selectively disorder very localized regions of an AlGaAs‐GaAs superlattice. As the focused laser beam scans across the sample, the superlattice rapidly melts and then regrows. Scanning electron microscope images of the scan cross section show a micron‐sized region of nearly homogeneous composition with a sharp (