Ian McMichael

Volume holographic memory systems: techniques and architectures

Although the concept of using multiplexed holography for data storage has been considered for some time, recent advances in several critical device technologies along with developments in storage materials have greatly enhanced the likelihood of successful implementations. We review several basic architectural concepts along with various multiplexing options and associated techniques for holographic data storage.

Cross-talk-limited storage capacity of volume holographic memory

We consider the storage density limited by the cross talk between stored pages during readout. We review some earlier work done in this area and present new theoretical work that characterizes the storage capacity limitations due to the cross talk. The results show that because of the presence of degeneracy noise, storage capacity does not benefit from expanding the reference points from one dimension to two dimensions. An optimum configuration that fully utilizes storage capacity and completely eliminates degeneracy noise is given for an angularly multiplexed volume holographic memory system.

Nondegenerate two-wave mixing in ruby.

We demonstrate energy exchange by nondegenerate two-wave mixing in ruby. Our results are in agreement with the theory of nondegenerate two-wave mixing when the theory is generalized to take into account a complex nonlinear index. Two-wave mixing gain exceeding the absorption and reflection losses is demonstrated, and we show that these experiments provide a simple and accurate method for determining the complex nonlinear index, response time, and saturation intensity of the medium.

Influence of phase masks on cross talk in holographic memory

Interpixel cross talk that is due to diffraction and aberrations is, to some extent, unavoidable because of trade-offs in the design of holographic storage systems. Phase masks have been proposed to avoid problems encountered in Fourier-transform holographic recording, but they have other effects on pixel cross talk. We investigate the effect of using phase masks in the data-bearing object beam on the output interpixel cross talk in a holographic memory system.

Phase shifts of photorefractive gratings and phase-conjugate waves

We present measurements of the two-wave mixing gain as a function of frequency detuning in photorefractive crystals. In many cases, this function is asymmetric, indicating that the phase shift of the photorefractive inde grating with respect to the light interference pattern is not exactly 90 degrees , as is often assumed. In four-wave mixing, the phase of the phase-conjugate wave contains the phase of the pumping and probe waves and a phase shift determined by the interaction taking place in the nonlinear medium. This second term, the phase shift of the phase conjugator, is a function of the type of grating and the phase shift of this grating. The phase shift of the grating obtained from the two-wave mixing measurements compares well with that obtained from four-wave mixing measurements of the phase of the phase conjugator.

Correction of polarization and modal scrambling in multimode fibers by phase conjugation

When polarized light is incident upon a long multimode fiber, the emerging light is randomly distributed among the spatial and polarization modes. We present experimental and theoretical results demonstrating that recovery of the spatial and polarization modes of the incident light takes place only when a phase conjugator at the fiber output preserves polarization on reflection.

Polarization-preserving phase conjugator.

Using a single self-pumped crystal of barium titanate, we demonstrate a method for producing the phase conjugate of an incident wave having arbitrary polarization. Our experimental results show that the phase-conjugate wave produced by this method reproduces both the ellipticity and the helicity of the polarization of the incident wave.

Image distortion in multimode fibers and restoration by polarization-preserving phase conjugation

Image restoration after a double pass through a multimode fiber using a polarization-preserving phase conjugator is demonstrated. Results indicate that the resolution of the restored image is limited by the number of guided modes and that the contrast is restored only when the phase conjugator preserves polarization on reflection.

Diffraction properties of multiple-beam photorefractive gratings

We have analytically solved the problem of N mutually incoherent pairs of beams in photorefractive media, each pair of which shares a common grating. The results are applied to study simultaneous read–write of dynamic photorefractive holograms with beams of comparable intensity. The diffraction efficiency is shown to be a nonlinear function of the read-beam intensity and is nonreciprocal with respect to readout from the two input ports. A complete energy transfer between the two write beams occurs in a finite thickness of the photorefractive crystal, in contrast to the infinite thickness required in the standard two-beam coupling case.

Energy transfer between injection-locked single-mode diode lasers by two-beam coupling in BaTiO(3).

Two-beam coupling in photorefractive BaTiO(3) is used to combine beams coherently from two injection-locked single-mode diode lasers operating at 830 nm. We are able to transfer more than 70% of the power in one beam to the other beam with this method.