A. Aharoni

Beam-correcting holographic doublet for focusing multimode laser diodes.

We design and fabricate a beam-correcting, holographic focusing doublet for laser diodes in the presence of a recording-to-readout wavelength shift. This two-hologram assembly compensates for the strong chromatic variation of the spot size and its lateral position, which are typical of a single diffractive element. The doublet, recorded at 488 nm, successfully corrects the astigmatism of a multimode laser diode beam at 820 nm and focuses it to a near-diffraction-limited 1/e2 spot width with a significantly extended depth of field.

Alleviation of image distortion due to striations in a photorefractive medium by using a phase-conjugated reference wave

Growth-induced striations in photorefractive media scatter transmitted light, degrading their performance in holographic applications. We model this phenomenon by considering the striations to be a permanent hologram within a medium and compare it with experimental results in a striated strontium barium niobate crystal. We show that the construction of the phase-conjugate signal beam by a phase conjugate of the transmitted reference beam alleviates the effects of distortions that are due to striations.

Distortion-free multiplexed holography in striated photorefractive media

Growth-induced striations in photorefractive media constitute phase gratings that scatter transmitted light and therefore degrade the quality of reconstructed images in holographic applications. A simple angular multiplexed holographic memory arrangement in a striated strontium barium niobate sample is considered. For forward-propagating reconstruction the striations distort the readout images and introduce ghost images. For counterpropagating readout, inhomogeneous image reconstruction and interimage cross talk can occur. We investigate these effects experimentally for different reconstruction methods and demonstrate the cross talk and the ghost images with a simple model based on coupled-wave theory. Readout with a counterpropagating plane wave eliminates the ghost images and the distortions but suffers degradation from cross talk and inhomogeneous image intensity. Only reconstruction with a phase conjugate of the transmitted reference beam eliminates cross talk and distortions. The model indicates that negligible material dispersion is a necessary requirement for high-quality phaseconjugation readout.

Phase-conjugate replay for a-axis strontium barium niobate single-crystal fibers

We describe the reconstruction of stored images in a-axis single-crystal fibers of Ce-doped strontium barium niobate. We introduce a new architecture for replay with a phase-conjugated reference wave and describe the effect of noise arising from undesired scatter on the performance of the system.

Limitations of phase-conjugate replay in volume-holographic phase-disturbing media

We examine the consequences of permanent phase distortions on the replay of transmission volume holograms with a phase-conjugated reference wave. In the absence of Bragg degeneracies and significant Bragg mismatch, for negligible dispersion in the spatial frequency response and for negligible variation in obliquity, phase-conjugated reference waves compensate for the effects of phase distortions and permit cross-talk-free multiplexing by using orthogonal reference waves. Spatial frequency dispersion, however, is a source of image distortion and cross talk.

Efficient beam-correcting holographic collimator for laser diodes

We describe a method for recording a beam-correcting holographic collimator for laser diodes in the presence of recording-to-readout wavelength shift. An astigmatism-correcting, beam-shaping hologram recorded with this method at 488 nm displays a high-efficiency, near-diffraction-limited collimation of a diode beam at 820 nm.

Capacity considerations for multiplexed holographic optical data storage

The capacity of volume holographic data storage depends on the multiplexing efficiency of input holographic data arrays or pages. We compare this page-packing capacity of two common approaches: angle, and wavelength multiplexing. For maximal page packing the angular separation between the reference and signal beam incidence in the angle and wavelength multiplexing methods should be 90 degree(s) and 180 degree(s), respectively. We find that in these optimal arrangements both methods can multiplex a similar number of paraxial data-page signals, which is comparable to conceptual storage capacity limits derived from considerations of resolution in three-dimensions. In practice, the high number of multiplexed data pages predicted here will be compromised by material-related noise sources.

A two-tone approach for prolonged readout of multiplexed photorefractive holograms

Photorefractive materials offer important advantages in many optical holography applications, but the recorded images typically remain sensitive to light and therefore erase on readout. Thermal fixing, electrical fixing, two photon absorption during grating formation in the presence of a sensitizing wavelength, and system-level image refreshing procedures have been used to sustain the recorded image during replay. We propose and demonstrate a novel two-tone holography approach that overcomes the unfavorable speed and poor diffraction efficiency of the thermal and electrical fixing, the complexity of refresh techniques, and the high intensities required for two-photon absorption methods. In our two-tone approach, holograms are recorded with short wavelength light, typically in the blue-green spectrum for which the photorefractive material is sensitive, and replayed with longer wavelength light, typically in the red-infrared spectrum, for which the material can be essentially inert. The replay light diffracts off of the recorded gratings, but due to significantly reduced photorefractive sensitivity, it introduces minimal erasure.<<ETX>>

Effect of self-diffraction on erasure dynamics during readout at different wavelengths and geometries in photorefractive materials

Certain memory applications based on photorefractive media require the ability to conveniently fix holograms. An attractive alternative is to sufficiently prolong the readout time. Low photoexcitation at long wavelength augmented by self-diffraction can increase readout time by a few orders of magnitude. The interaction of the writing and readout waves, both of the same wavelength, with the photorefractive space charge field has been investigated theoretically and experimentally. Dynamics of beam coupling and self diffraction, in certain geometries, can lead to an initial increase followed by non-exponential decay of the diffracted signal during readout. Here we study how the difference in gain, lifetime, photoexcitation and absorption at different wavelengths - which require changing incident angles to satisfy the Bragg condition - affect the decay dynamics of photorefractive gratings.<<ETX>>

Recording of an achromatic holographic doublet for focusing laser diodes

A method for designing and recording a holographic achromat, composed of two holographic optical elements (HOEs), is presented. The method is demonstrated with a doublet recorded at 488 nm for application at a central frequency of 820 nm. A near-diffraction-limited focus and high diffraction efficiency are achieved over a comparatively wide spectral range.