Ragini Saxena

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.

Perturbative analysis of higher-order photorefractive gratings

Analytic expressions are derived for second- and third-order space-charge fields for a moving intensity pattern and arbitrary field strengths. The dependence on the spatial frequency, the applied field strength, and the velocity of the moving grating is examined. The magnitude of the term of correction to the fundamental harmonic is strongly dependent on experimental conditions and the relative strengths of characteristic fields. For diffusion-dominated charge transport and optimum fringe spacing the cubic correction term is only 5% of the fundamental amplitude, even for a large modulation depth of unity. Limiting cases yield the same results as previous studies.

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.

High-gain nondegenerate two-wave mixing in Cr:YAlO 3

A gain of 22 times was obtained by nondegenerate two-wave mixing in Cr:YAlO(3). To our knowledge, this is the largest cw two-wave mixing gain obtained in a bulk solid-state nonphotorefractive material. The measured gain appears to be limited by beam breakup that is due to spatial nonuniformities in the nonlinear refractive-index change that are the result of inhomogeneities in the crystal. Predictions based on our data indicate that gain in excess of 200 times should be possible in Cr:YAlO(3) if a homogeneous crystal can be obtained.

Dynamics of refractive-index changes and two-beam coupling in resonant media

The dynamics of light-induced change in the refractive index of a resonant medium are examined. For illumination with weak fields, the two relevant relaxation times are T1, the population lifetime and T2, the dipole-dephasing time. The response time of the index change is determined by the slower relaxation time of the medium which is usually the time T1 taken by the excited system to relax back to its thermal equilibrium value. Illumination with two beams of the same frequency that intersect within the medium leads to the formation of a volume grating in the medium that is spatially local. Hence there is no exchange of energy between the beams that write the grating, each beam merely reducing the absorption experienced by the other beam. Illumination with a moving, spatially periodic intensity pattern leads to gain for one beam and additional absorption for the other beam as they propagate through the medium. A complete set of coupled equations describing the intensities and phases of the beams undergoing non-degenerate two-wave mixing in the resonant media is derived using third-order perturbation theory, and the solutions are studied numerically. In particular, the two-beam coupling constant for intensity is shown to depend on the frequency difference between the two beams and on the pressure-induced collisional relaxations in the system.

Theory of phase-conjugate oscillators. I

We have developed a theory for nondegenerate oscillations in optical resonators containing an intracavity phase-conjugate element. The phase-conjugate element consists of a nonlinear transparent medium that is pumped externally by a pair of counterpropagating laser beams of the same frequency and intensity. Phase conjugation of an input beam of slightly different frequency occurs because of nondegenerate four-wave mixing. The theory takes into account linear absorption (or gain) in the medium and is applied to study the threshold behavior of phase-conjugate oscillators. For the special case of no conventional mirrors, the phase-conjugate oscillator reduces to an ordinary phase-conjugate mirror, and our general formulation yields the results of previous studies. Our analysis shows that the parametric gain required for oscillation increases (or decreases) as a result of linear absorption (or gain) in the medium, and oscillation can occur at a frequency different from that of the pump beams in the presence of large linear gain (or loss). The effects of linear absorption (or gain) on the filter operation are also examined.

Mutually pumped phase conjugation in Kerr media and the effects of external seeding

Mutually pumped phase conjugation, a process by which two incident laser beams can mutually pump each other to generate the phase-conjugate replica of the other beam, is studied in a transparent Kerr medium with an electrostrictive nonlinearity. The nonlinear medium acts as a phase-conjugating mirror to both of the incident beams and generates their Stokes-shifted phase-conjugated outputs by the nearly degenerate four-wave mixing process. Our results show that this process will occur only if the coupling strength, defined by the product of the Kerr amplitude gain coefficient, the total input intensity, and the interaction length, is above the threshold value of 2. If the intensity of one of the input beams exceeds the threshold value and the second input beam is weaker than the first, then the weak beam is phase conjugated with a large reflectivity that has a maximum value of the intensity ratio of the strong to the weak input beam. For negligible absorption in the medium, a complete transfer of energy of both the incident beams into the phase-conjugate beams can occur for finite interaction lengths that are only slightly larger than the threshold value. Seeding the oscillation beams tends to lower the phase-conjugate power reflectivities, probably because of increased contribution from the competing two-wave mixing process.

Properties of photorefractive gratings with complex coupling constants

We examine simultaneous write–read of dynamic photorefractive gratings for arbitrary phase shifts between index grating and intensity pattern. The nonlinear behavior of diffraction efficiency with read beam intensity, as well as the nonreciprocity of diffraction efficiency with respect to direction of readout, is examined for various values of the phase shift.

Frequency locking in phase-conjugate ring oscillators

Frequency locking in a photorefractive phase-conjugate ring oscillator was studied by injecting a seed beam into the resonator. For a seed beam coherent with the pump, frequency locking of the oscillator was observed at large seed powers. At lower seed levels, the beat signal displayed multiple harmonics in its frequency behavior. No evidence of locking was found for a seed incoherent with respect to the pump.

Diffraction properties of photorefractive gratings

Volume phase gratings in photorefractive crystals have nonuniform amplitude and phase due to the energy exchanged by the writing beams within the material. Analytic expression is obtained for the diffraction efficiency of a weak reading beam that does not perturb the grating and has a different polarization from the write beams. For a read beam of arbitrary intensity, the diffraction efficiency is a nonlinear function of the read beam intensity and is nonreciprocal with respect to readout from the two input ports. These properties of photorefractive gratings are studied for arbitrary phase shifts of the index grating from the intensity pattern.