Matthias Esselbach

Giant momentary readout produced by switching electric fields during two-wave mixing in sillenites

We show theoretically and experimentally that switching an applied square-wave field produces strong and short pulses of the outgoing signal during two-wave mixing in sillenite crystals. These pulses originate from the strong effect of the field on the optical eigenmodes and can be used in new optical schemes based on time-separated recording and readout processes.

Optical information storage using refresh via phase conjugation

Abstract In this paper we discuss the possibility for realizing an optical memory using dynamic refreshment. Via phase-correct back-coupling by means of nonlinear optical phase-conjugation, the information stored in a photorefractive crystal is incessantly read out, transmitted into an auxiliary memory and from this back into the crystal again and in this way refreshed. Practical realizations and first results are presented.

Transient phase measurement using a self-pumped phase-conjugate mirror as an optical-novelty filter

A transient phase-measuring technique that uses a self-pumped phase-conjugate mirror as an optical novelty filter is presented. A change in the reflectivity of the mirror as a function of a change in the incident wave front permits the transient measurement of a two-dimensional phase distribution. This method allows us to investigate fast processes using media with slow response. A simple theoretical model explains the experimental results with sufficient accuracy. The results can be used for the calibration of the measuring system. The described method is used for the measurement of a temporally varying wave front.

Dependence of the refractive index grating in photorefractive barium titanate on intensity

Abstract The interference pattern of light waves creates a refractive index modulation in photorefractive media. This process is relatively well described by the theory (Kukhtarev equations) but deviations are found. Therefore, experimental methods are used in order to characterise the processes. The influence of the absolute intensity as well as the intensity ratio of the interfering waves on the refractive index modulation are studied in a two-wave mixing arrangement. Especially for the dependence on the absolute intensity the interesting relation Δn = f ( I abs )∼ I abs α was found that is not predicted by the theory. Nevertheless, this experimental approximation can be used in calculations in order to minimise the error.

Real-time holographic interferometry with double two-wave mixing in photorefractive crystals

We present a real-time holographic interferometer for which two reference waves of different phases are created by two-wave mixing with a stationary signal wave in a photorefractive crystal. These waves are reconstructions of the stationary signal wave and interfere with the momentary (changing) signal wave in the manner of a holographic real-time interferometer. A fast change (phase or intensity) of the signal wave leads to different intensity changes in both interferograms that are jointly used for evaluation. With an electric dc field applied to the crystal, a high sensitivity for measuring phase changes (down to lambda/50, lambda = 633 nm) is found, and the sign of the phase change can be determined.

Sub-millisecond photorefractive two-wave coupling in Bi12TiO20 at 633 nm

Abstract The unusually fast response of the two-wave mixing process in fibre-like Bi12TiO20 crystals on transient change of the external electric field was experimentally observed. Pulses of the amplification of the signal wave with a rise time of 0.2 ms and a width of 0.5 ms at a total input intensity of 6 mW mm−2 are obtained when applying a rising or decreasing front of an external electric field to the crystal. The two-wave coupling without external field has a response time of 200 ms. A simple calculation based on the bandtransport model qualitatively explains the fast transient amplification but fails to explain the slow response without the electric field.

Phase conjugation in fibre-like BTO crystals with applied electric ac field

Self-pumped and mutually pumped phase conjugation based on the fanning effect is demonstrated in photorefractive Bi12TiO20 (BTO) crystals with applied electric ac fields. We used fibre-like crystals that enable a long interaction region and high applied electric fields. We realized three types of phase conjugate mirrors: internally self-pumped, externally self-pumped and mutually pumped. Experimental results characterizing the spatial and temporal behaviour of the phase conjugated wave, the reflection of intensity and phase patterns, and the quality of the phase conjugation are presented.

Measurement of the electric screening field in Bi12TiO20

In Bi12TiO20 with an applied ac voltage the coupling for two-wave mixing can reach an extreme value at the transient moment tp when the internal electric field is zero. This is because of the changing polarization state of the pump wave during field reversal. The internal field is found to be the sum of the applied field and a time developing screening field arising from charge redistribution. Therefore, at the moment tp the amount of the screening field is equal to the value of the applied field and thus can be measured. Also tracking of the temporal course of the electric screening field is possible. The buildup of screening fields leads to an intensity dependence of tp for a given shape of the ac voltage.

Optical storage of information via refreshing by inverse seeding (OSIRIS)

In this paper we discuss the possibility for realizing an optical memory using dynamic refreshment. Using phase-correct back coupling by means of nonlinear optical phase conjugation the information stored in a photorefractive crystal is continuously read out, transmitted into an auxiliary memory and from this back into the crystal again and is, in this way, refreshed. Practical realizations and first results are presented.

Mathematical simulation of a dynamic storage process

Abstract The possibility of realizing an optical memory with photo-refractive crystals using dynamic refreshment via phase conjugation is discussed. The experimental realization is based on photorefractive barium titanate crystals. Series of refresh cycles, dependent on several parameters, are simulated numerically. It is shown that an extension of the storage time can be reached.