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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Noise reduction in photorefractive memory by double mutually pumped phase conjugate mirrors

Hisatoshi Funakoshi, Atsushi Okamoto

Published Jun 17, 2004• 1 citation
Publisher

Abstract

In this report, we demonstrate a noise reduction recording in a photorefractive memory with a dynamic refreshing technique by mutually pumped phase conjugate mirrors (MPPCMs). The dynamic refreshing, that means the continuous rewriting of an original hologram with the optical feedback in synchronism with the readout, largely contributes to the maintenance of the recorded hologram by all-optical process. Two MPPCMs are used in the dynamic refreshing technique; one is built for the optical feedback, the other is used as the storage. The nondestructive readout without any fixing technique can be achieved by the continuous rewriting with the beam resonance between the phase conjugate mirrors. Our technique additionally offers the noise reduction of an input image in the recording process without any external image processing systems. The noise reduction is caused by the characteristics of the MPPCM that the incident intensity ratio required for the MPPCM generation is determined by the coupling strength of the crystal. Moreover, the application of the MPPCM to the photorefractive memory brings great advantages of the high quality image retrieving and the simple optical configuration because the illumination of two incoherent beams into a PR crystal can generate the mutual phase conjugate beams in the MPPCM. We show the noise reduction effect is controllable by the appropriate adjustment of the incident beam intensities. We also experiment on the photorefractive memory with this technique using barium titanate crystals and show the high-quality noise-reduced image can be read out over 10 times longer than the conventional readout technique.

DOI
10.1117/12.528224

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In this report, we demonstrate a noise reduction recording in a photorefractive memory with a dynamic refreshing technique by mutually pumped phase conjugate mirrors (MPPCMs). The dynamic refreshing, that means the continuous rewriting of an original hologram with the optical feedback in synchronism with the readout, largely contributes to the maintenance of the recorded hologram by all-optical process. Two MPPCMs are used in the dynamic refreshing technique; one is built for the optical feedback, the other is used as the storage. The nondestructive readout without any fixing technique can be achieved by the continuous rewriting with the beam resonance between the phase conjugate mirrors. Our technique additionally offers the noise reduction of an input image in the recording process without any external image processing systems. The noise reduction is caused by the characteristics of the MPPCM that the incident intensity ratio required for the MPPCM generation is determined by the coupling strength of the crystal. Moreover, the application of the MPPCM to the photorefractive memory brings great advantages of the high quality image retrieving and the simple optical configuration because the illumination of two incoherent beams into a PR crystal can generate the mutual phase conjugate beams in the MPPCM. We show the noise reduction effect is controllable by the appropriate adjustment of the incident beam intensities. We also experiment on the photorefractive memory with this technique using barium titanate crystals and show the high-quality noise-reduced image can be read out over 10 times longer than the conventional readout technique.
We propose a dynamic refreshing technique in a photorefractive memory with double mutually pumped phase conjugate mirrors. Long-term continuous readout without any fixing technique can be achieved by the high quality image refreshing that uses the resonance of the phase conjugate beams with image information between phase conjugate mirrors. We perform the experiment on the long-term reading of the images in the photorefractive memory with BaTiO/sub 3/ crystals and achieve the continuous reading time over 10 times longer than the time constant of the crystal without image degradation.
Several mutually pumped phase conjugation configurations have been experimentally demonstrated and theoretically studied in photorefractive media.1 Although these materials are very efficient at low operating power, the process is inherently slow due to the need to generate a large number of photo-induced charge carriers.2 This paper examines the possibility of the same process occurring in a transparent Kerr medium which offers the advantage of faster response times. Due to an electrostrictive nonlinearity in the medium, a photon from either of the two input beams can be spontaneously converted into a frequency downshifted Stokes photon and an acoustic phonon, a process better known as Brillouin scattering.3 The presence of phonons due to noise will also help in Stokes scattering the incident photons. Each pair of incident beam and its coherently generated scattered beam will then write a grating that is moving at the phonon velocity. Reading of this moving hologram by the other incoherent input beam may generate the cross-readout phase conjugated beam that is frequency shifted from the read beam by the frequency of the acoustic field. The nonlinear medium thus acts like a phase conjugate mirror to both of the incident beams. Since the read-write processes occur simultaneously, the dynamics of the beam coupling are described by NFWM equations in the nonlinear medium with appropriate boundary conditions. Our calculations show that it is indeed possible to phase conjugate the two mutually incoherent input beams simultaneously when the coupling strength is above a threshold value. If one of the input beams is very weak compared to the other input beam, it is phase conjugated with a very high reflection coefficient. We have also examined the effects of seeding the phase conjugated beams.
We propose and experimentally demonstrate two original methods for suppressing the beam fanning noise of photorefractive amplifiers. The first one involves two wave mixing in a slowly rotating crystal, while the second requires to tilt the crystal with respect to the recording beams. This method applies to materials which exhibit a sharp resonance of the gain versus the pump-probe beam angle. With both methods a large improvement of the S/N ratio is achieved and virtually noise free image amplifiers are demonstrated. In Bi12SiO20, subpicowatt optical signals are detected and amplified. By using combinations of LiNbO3/Bi12SiO20, as storage and amplification crystals respectively a long term readout (more than 20 hours) of a photorefractive memory is achieved.
Phase conjugate reflectivity of the photorefractive four-wave mixing system is usually quite unstable and it is serious issue for practical use of these systems. The cause of this instability is mainly combination of the fluctuation of the phases of input signal and pump beams and the slow response time of the photorefractive crystal. When all input beams in the four-wave mixing system are mutually coherent and the phases of the input beams are not stable, four kinds of gratings are compete each other and the phase conjugate output becomes quite unstable. To avoid this problem, we can make one of the pump incoherent to the signal and another pump1. The phase conjugate output is, however, still unstable due to the rewriting of the grating caused by the fluctuation of the phase of the input beams. In this paper, we report our feedback system to stabilize the power of the phase conjugate output from the four-wave mixing system by controlling the phase of the signal beam. In this system, we detect the phase difference between signal and forward pump beam with lock-in amplifier and control the phase of the signal with a piezoelectric mirror to stabilize the phase difference.
We propose and demonstrate a long-term readout photorefractive memory. It is made up of two photorefractive crystals: the first where the images are stored and the second where the signal is amplified by use of a low-noise self-bent pump-beam technique, making it possible to use very low illumination of the storage crystal. A LiNbO(3)/BaTiO(3) configuration is used with angular multiplexing; this technique permits continuous readout for more than 60 days. The long-term storage capacity and the maximum number of stored images are estimated.