Time-dependent behavior of photorefractive two- and four-wave mixing

Abstract

A theoretical and experimental study of the temporal response of photorefractive two-and four-wave mixing processes is given. We examine the buildup and the decay of the output signal and the gratings when the input signal beam is turned on and off. For two-wave mixing we have performed an analysis that includes the depleted-pump regime. The experiment was done with a BaTiO 3 crystal. The buildup-and-decay behavior is strongly dependent on the coupling constant and the ratio of the signal and pump intensities. From these measurements we obtain the crystal time constant, which was found to have an intensity dependence with a power of-0.7. The coupling constant is extracted from the data of the whole dynamic process of buildup and erasure and not only from the steady-state point, as is usually done. This is the first analytic study, to the best of our knowledge, for the four-wave mixing decay and buildup. We find that the solution for time-dependent fourwave mixing processes (the phase-conjugate mirror and the double phase-conjugate mirror) with undepleted pumps is identical to that for two-wave mixing with unidirectional optical feedback circuits. This similarity provides a direct way to study the stability properties of the devices. We find, for example, that in the unstable regime of four-wave mixing a self-pulsation of the output corresponds to the frequency detuning of the unidirectional two-wave mixing oscillator.

Full-field particle velocimetry with a photorefractive optical novelty filter

“…7 After detecting an input change, the output falls toward zero intensity within a characteristic time which is determined by the grating time constant g of the photorefractive material. 6 Although the decay of output intensity is known to be best described by an exponentially decaying term weighted by an infinite sum of Bessel functions, 8 in most experimentally relevant situations it can be estimated very well by a purely exponential decay with an effective time constant . 9 With this approximation, the output signal at time t is given by the difference of the current input and the integral over the time-exponential average of the input at all previous times.…”

Ultrasonic vibration modal analysis of ICF targets using a photorefractive optical lock-in

“…The diffracted beam intensity is a direct measure of the grating established, and its diffraction efficiency is determined by the wave-coupling constant (4) where L is the interaction length, 2 is the source wavelength, and 2Ois the angle between the mixing waves. In the undepleted pump approximation, the diffracted (conjugate) beam intensity, by the scattering theory developed by Kogelnik,15 is 14 = 13 exp[-a L /cos 0] Isin Cl2 (5) where 13 iS the backpropagated reference beam intensity and a is the material absorption coefficient. The refractive index modulation amplitude generated in the mixing process is small compared with the average index of the medium, therefore C < 1 and sin(C) C. The intensity ofthe diffracted (conjugate) beam is given by 14=I3exp{-ctL/cosO] CM .…”

Numerical simulations and experimental investigations on transverse effects of two-wave mixing in a photovoltaic photorefractive crystal

“…TWM geometry is used as a building block for more complex optical applications, including optical image processing, optical amplification based on waves mixing, holographic data storage, and dissipative holographic solitons [1][2][3][4][5][6]. This has invited a great variety of studies in TWM phenomenon in order to learn about it and to enhance its response under different external stimulus [7][8][9][10][11][12].…”