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Electronics and Communications in Japan (Part II Electronics)

Properties of phase‐conjugate reflectivity of self‐pumped four‐wave mixing

Masayoshi Dohata, Atsushi Okamoto, Koji Enbutsu, Kunihiro Sato

Published Jan 1, 1996• 0 citations
Publisher

Abstract

Abstract The properties of the phase‐conjugate reflectivity are analyzed anew for the self‐pumped four‐wave mixing in which only the two waves, i.e., the forward pump wave and the probe wave, are incident on the photo‐refractive crystal and the phase‐conjugate wave of the forward pump wave is used as the backward pump wave. First, by means of a model in which a Cat‐type phase conjugator is placed outside the interaction region, the equation for the phase‐conjugate reflectivity of self‐pumped four‐wave mixing using a transmission grating is derived. Next, the numerical calculations have been used for the first time to determine the effects of the coupling strength, the probe ratio as the incident intensity ratio of the forward pump wave to the probe wave, and the reflectivity of the Cat‐type phase conjugator on the entire phase conjugate reflectivity. Since the equation describing the phase‐conjugate reflectivity has multiple solutions, the strict analysis of the equation shows that the phase‐conjugate reflectivity has multiple values. Further, the conditions for high phase‐conjugate reflectivity has been studied.

DOI
10.1002/ecjb.4420790701

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Optics Communications•Dec 1, 1997•3 citations
Abstract The properties of the phase‐conjugate reflectivity are analyzed anew for the self‐pumped four‐wave mixing in which only the two waves, i.e., the forward pump wave and the probe wave, are incident on the photo‐refractive crystal and the phase‐conjugate wave of the forward pump wave is used as the backward pump wave. First, by means of a model in which a Cat‐type phase conjugator is placed outside the interaction region, the equation for the phase‐conjugate reflectivity of self‐pumped four‐wave mixing using a transmission grating is derived. Next, the numerical calculations have been used for the first time to determine the effects of the coupling strength, the probe ratio as the incident intensity ratio of the forward pump wave to the probe wave, and the reflectivity of the Cat‐type phase conjugator on the entire phase conjugate reflectivity. Since the equation describing the phase‐conjugate reflectivity has multiple solutions, the strict analysis of the equation shows that the phase‐conjugate reflectivity has multiple values. Further, the conditions for high phase‐conjugate reflectivity has been studied.
We present a plane-wave analysis of a recently demonstrated self-pumped phase conjugator. This device uses four-wave mixing to produce the phase-conjugate replica of an incident optical wave. All the waves are derived from the single incident wave: there are no externally supplied pumping beams. We consider the case of four-wave mixing in two interaction regions coupled by simple reflection. We calculate the phase-conjugate reflectivity as a function of coupling strength, taking into account imperfect coupling between the two interaction regions, and show that there is a threshold coupling strength below which the reflectivity is zero and above which the reflectivity is multiple valued. We also compute the coupling strength per unit length for a photorefractive crystal of barium titanate.
A typical method of generating phase conjugate light using photorefractive crystals is four-wave mixing. It is known that the phase conjugate reflectivity is multivalued in a steady state. In this paper, a time domain response analysis of four-wave mixing is carried out and the method for controlling the bistability of the phase conjugate reflectivity is described. First, a steady-state analysis of the four-wave mixing is carried out and the condition is derived for bistability of the phase conjugate reflectivity. Next, by using two different initial conditions, the time response analysis is carried out for the four-wave mixing. Under one of the conditions, no forward pump beam is incident before and after observation of the bistability for measurement of phase conjugate reflectivity. Under another condition, the forward pump is incident before observation of the bistability. The phase conjugate reflectivity is measured after this pump is cut off when a steady state is reached. For each case, the numerical calculations are carried out. It is shown for the first time that the bistability of the phase conjugate reflectivity can be controlled by the initial condition. Further, the temporal and spatial variations of the diffraction grating generated in the crystal are studied and the process of the phase conjugate reflectivity to exhibit bistability is clarified. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(2): 1–8, 1998
One of the typical methods to generate the phase-conjugate beam is four-wave mixing with a photorefractive crystal. It is known that the phase-conjugate reflectivity shows the bistability in some cases. Analyzing the temporal property of four-wave mixing, we offer a new method to control the bistable phase-conjugate reflectivity, and we perform an experiment of the bistability control by the method. An equation to obtain the threshold value of the coupling strength for showing the bistability is derived by a steady-state analysis of four-wave mixing, and the temporal property of four-wave mixing is studied with two different initial conditions that are used for letting the phase-conjugate reflectivity R converge to an off state (R=0) or to an on state (R>0). The initial condition for the converging off state is that the forward pump beam is turned off throughout. The condition for the converging on state is that the forward pump beam is turned on in advance, and after enough time passes, it is turned off. In both conditions, the backward pump beam and the probe beam continue to illuminate throughout. Whereas both optical arrangements are equivalent after setting up these initial conditions, each phase-conjugate reflectivity converges to different stationary values, R=0 and R>0. Besides, an experiment is performed and we actually control the bistability by this method. Furthermore, the temporal and the spatial variation of the refractive-index grating in the photorefractive crystal is investigated, and the process of the bistability is explained.
We analyze the phase-conjugate reflectivity and the amplification property of cross-polarized four-wave mixing with a photorefractive BaTiO3 crystal in consideration of the pump-beam depletion. The coupled-wave equations of cross-polarized four-wave mixing with ordinary writing beams and an extraordinary reading beam are derived and solved with the grating-integral method. The results of calculations of the reflectivity and the amplification factors will be compared with the result under the undepleted-pumps approximation. In addition, to achieve high conversion efficiency, the optimum conditions for the intensity ratio and the angle configurations of the probe and pump beams are also discussed.
When the nonlinear effect of the crystal is strong in four-wave mixing using a photorefractive crystal, the generated phase conjugate light takes multiple values. In this paper, a new method of controlling the bistable values of the phase conjugate reflectivity, that is, the ratio of the phase conjugate beam to the probe beam, via the erase beam is proposed for use in four-wave mixing using the fanning beam generated in the crystal in place of the forward pump beam. It is extremely difficult to solve analytically the problem of four-wave mixing including the fanning beam. By means of a model in which a part of the probe beam is used for fanning and acts as the forward pump beam in the interaction region, the previous method of analysis for four-wave mixing can be used. First, a steady-state analysis of the phase conjugate reflectivity is carried out. It is shown that the phase conjugate reflectivity has bistable values as function of the intensity of the erase beam. Next, by time response analysis, the intensity of the erase beam is varied and the bistable values of the phase conjugate reflectivity are controlled. Finally, the theory is compared with experiments. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(6): 8–14, 2000
We describe the steady-state enhancement of the phase-conjugate reflectivity that occurs with linear absorption near self-oscillation in four-wave mixing systems. Using a simplified general model, we show that this effect can arise in a variety of phase conjugators including those based on photorefractive and Kerr media. A new expression is introduced for the optimal reflectivity condition, which occurs if the transfer of energy from the strong forward pump to the weak probe is maximized at the entrance to the nonlinear medium.