Ruibo Wang

Recording of second-harmonic index gratings in photorefractive (K(0.5) Na(0.5))(0.2) (Sr(0.75) Ba(0.25))(0.9) Nb(2)O(6) crystals.

We propose and demonstrate a novel method for recording second-harmonic volume index gratings in photorefractive crystals. The fundamental component of the index grating is eliminated by means of a pi -phase shifted double-exposure process that uses the Mach-Zehnder interferometric technique. Experimental results for (K(0.5) Na(0.5)) (0.2)(Sr> (0.75)Ba (0.25))(0.9)Nb (2)O(6) crystals are presented and discussed.

All-optical pulse generators for pulse-coupled neurons

We propose and demonstrate an all-optical pulse generator which is based on regenerative pulsation in optical bistable systems. The novel device consists of a nonlinear interference filter, a photorefractive crystal and an optical feedback loop. In such a pulse generator, when the input beam intensity exceeds the switch-on threshold of the system, a train of square-wave pulses will be produced. The firing rate is an increasing function of the intensity of the input beam. The principle of operation and the experimental results are presented and discussed.

Photorefractive properties of Cu-doped KNSBN crystal with fluorine replacing oxygen.

We report the photorefractive properties of a new kind of Cu-doped KNSBN crystal in which a small fraction of oxygen (O) atoms are replaced with fluorine (F) atoms. The replacement of oxygen with fluorine in Cu:KNSBN crystals leads to a much shorter response time of 30 ms at an incident intensity of 5 W/cm(2). We also discuss the effect of electron-hole competition on the coupling constant.

Properties of harmonic gratings in Mn:KNSBN in the process of electrical domain fixing

Photorefractive crystals have many applications in holographic storage, optical communications, and nonlinear optical information processing. One problem is that the photorefractive holographic index gratings can be erased in the readout process. Several non-destructive approaches have been proposed and demonstrated in different photorefractive materials. Based on the initial research of domain fixing on KNSBN crystals, we studied the high order harmonic gratings during electrical domain fixing process in KNSBN crystals. In this paper, we mainly focus on the properties of the first order and second order harmonic index gratings of Mn-doped KNSBN crystal during electrical domain fixing process at room temperature. Domain fixing in Rh:SBN was also investigated, and the results in Mn:KNSBN and Rh:SBN were compared. In Mn:KNSBN crystals, the second order harmonic grating becomes stronger after fixing. This may be the main reason of the high noise in the revealed grating, which affects the image quality and fidelity. In addition, the effect of the accumulated charges in the revealing process was also discussed.

Four-wave mixing with partially coherent waves in photorefractive media. I. Transmission grating approximation

We investigate the effect of beam temporal coherence on four-wave mixing via transmission gratings in photorefractive media. We take into account the propagation of mutual coherence and self-coherence in space and the evolution of mutual coherence and self-coherence with time. Analytical solutions for nondepleted-pump cases and numerical results for general cases are obtained. We find that the mutual coherence of the signal beam and the pump beam can be enhanced or reduced, depending on the coupling constant and the beam ratio of the signal beam and the pump beam. A decrease of the initial mutual coherence can lead to a lower phase-conjugate reflectivity in the nondepleted-pump case. Even with low initial mutual coherence, high phase-conjugate reflectivity can still be obtained in the case with pump depletion. As a result of the shift of the grating profile, partial coherence can lead to an enhancement of the phase-conjugate reflectivity when the signal–pump-beam ratio is small. We also found that when the initial value of the phase-conjugate beam is zero, the phase-conjugate beam and the pump beam are in phase during propagation. Results for beam intensity and mutual coherence are presented and discussed to provide an insight into four-wave mixing with partially temporally coherent waves.

All-optical pulse generator using nonlinear interference filters

Summary form only given.Pulse coupled neural network (PCNN) has been proven to be a useful tool in image transforms that remove unimportant details while improving the over-all quality of the image. In the basic model of the pulse coupled neuron proposed by Eckhorn et al. the pulse generator is the central processing unit. The firing rate of a single neuron is a sigmoidal function of the feeding input. Therefore the original input can be approximately recovered at any time by an average taken over many pulse periods because the pulse frequency is faster for stronger feeding inputs. An all-optical pulse generator using an external feedback loop has been reported previously. Here we present the results of research on a new all-optical pulse generator based on internal feedback by using nonlinear interference filters. We have studied and demonstrated the feasibility of single-channel optical pulse generators for applications in PCNNs.

High harmonic photo-induced gratings in photorefractive media

Photo-induced index gratings can be recorded in photorefractive crystals. These gratings can be employed for applications in optical memory and spectral filters. It is known that the photo-induced index gratings in photorefractive media are not exactly sinusoidal, even though the optical intensity is sinusoidal. In other words, higher harmonic index gratings exist. We propose and demonstrate a novel double exposure scheme which removes the fundamental component and all higher odd-orders of the index grating. As a result, a volume index grating is obtained with a grating period of /spl Lambda//2, where /spl Lambda/ is the fundamental period.

Four-wave mixing with partially coherent waves in photorefractive crystals. I. Transmission grating approximation

We have studied the four wave mixing with partially coherent waves in photorefractive crystals with the transmission grating approximation both theoretically and numerically. The phase conjugation reflectivity depends much on the initial mutual coherence of the signal beam and the pump beam. The beam intensity and mutual coherence of the interacting waves are also obtained.

Recording of high harmonic gratings in photorefractive media

We have proposed and demonstrated a novel scheme for the recording of high harmonic index gratings in photorefractive crystals. By using a double exposure in a Mach Zehnder interferometer with a dual input port, we demonstrated the recording of a second harmonic index grating with a period of /spl lambda//2 in a photorefractive KNaSrBaNb/sub 2/O/sub 6/:Mn (KNSBN:Mn) crystal.