Photorefractive BaTiO3 Crystal Research Articles

Wideband chaos from a laser diode with phase-conjugate feedback.

We analyze experimentally and theoretically the chaotic dynamics generated by a laser diode subjected to phase-conjugate feedback. Phase conjugation is obtained from four-wave mixing in a BaTiO3 photorefractive crystal. We demonstrate that the chaos bandwidth first increases linearly with feedback ratio but then saturates to relatively high values. With a single optical feedback, a chaos bandwidth up to about 18GHz is achieved, which is about five times as large as the free-running laser diode relaxation oscillation frequency. Numerical simulations confirm our experimental observations and unveil that the finite depth penetration into the crystal is responsible for the observed saturation.

Image fidelity improvement in digital holographic microscopy using optical phase conjugation

With respect to digital holography, techniques in suppressing noises derived from reference arm are maturely developed. However, techniques for the object counterpart are not being well developed. Optical phase conjugation technique was believed to be a promising method for this interest. A 0°-cut BaTiO3 photorefractive crystal was involved in self-pumped phase conjugation scheme, and was employed to in-line digital holographic microscopy, in both transmission-type and reflection-type configuration. On pure physical compensation basis, results revealed that the image fidelity was improved substantially with 2.9096 times decrease in noise level and 3.5486 times increase in the ability to discriminate noise on average, by suppressing the scattering noise prior to recording stage.

Two-Wave Mixing Amplification of Femtosecond Laser Pulses at 800 nm in Rh:BaTiO3 Photorefractive Crystal

We report the photorefractive (PR) two-wave mixing amplification (TWMA) of broadband femtosecond laser pulses in a Rh:BaTiO3 crystal by one-photon absorption. Two-wave mixing (TWM) is performed in the crystal using 30 fs laser pulses at an 800 nm wavelength. A maximum two-wave mixing amplification gain coefficient of ∼11/cm was achieved using a chirped volume grating written at a grating spacing of 5 µm. The broadband spectrum and the original phase characteristics of the signal pulse are also maintained. Beam fanning for the femtosecond pulse laser is less pronounced than that for the cw laser, facilitating a higher two-wave mixing gain for femtosecond pulse lasers.

Dynamics of photorefractive self-pumped phase-conjugate mirrors with a linear resonator

The phase-conjugation dynamics in a self-pumped phase-conjugate mirror (SP PCM) based on a BaTiO3 photorefractive crystal (PRC) with a linear resonator is simulated numerically. It is shown that a phased array of dynamic holograms is formed in such a SP PCM under optimal conditions, and the interference of the waves scattered by this array provides the very high stationary phase-conjugation efficiency (the nonlinear reflection coefficient and overlap integral exceeding 80% and 95%, respectively). It is found that the development of instabilities is caused both by the violation of phase relations in this array and the formation of additional dynamic holograms in the PRC in the geometry typical of a SP PCM with a semi-linear resonator and by the self-bending effect typical of PRCs.

Optical encryption and decryption using a sandwich random phase diffuser in the Fourier plane

We describe the encryption and decryption of a two-dimensional (2-D) image by using a sandwich diffuser in the Fourier plane of the first lens of a 4-f imaging system. The diffuser is made with two random phase diffusers, sandwiched together. The encrypted image is holographically recorded in a photorefractive BaTiO3 crystal. Decryption is done by generating a phase conjugate image through the phase conjugation technique. During decryption, if only one of the two random phase diffusers constituting the sandwich diffuser is used, it becomes impossible to retrieve the image. Also no decryption is possible if any of the diffusers constituting the sandwich is shifted in-plane with respect to the other. It is possible to generate the image only if the diffusers are sandwiched together again after matching point to point, and then the sandwich diffuser is kept at the same position and orientation in which it was kept during the encryption. Experimental method has been described to overcome the practical difficulty for the generation of the key in the case of the sandwich diffuser. Simulation and experimental results are presented in support of the proposed idea. To evaluate reliability of the technique, the mean-square-error (MSE) between decrypted and original image has been calculated.

Hanbury-Brown and Twiss type double-slit interference with photorefractive fanning light

This paper reports that when an intense extraordinary-polarized laser beam illuminates a photorefractive BaTiO3 crystal, the dynamic beam fanning light is formed to be a thermal-like light source with a long correlation time and wide spectral bandwidth. The experimental results of the first- and second-order double-slit interference with such photorefractive fanning light source, can be understood with the theoretical simulation in terms of Hanbury-Brown and Twiss effect.

Narrowband tuning of an injection-seeded pulsed optical parametric oscillator based on a self-adaptive, phase-conjugate cavity mirror.

Narrowband tuning of a pulsed optical parametric oscillator (OPO) is achieved with a self-adaptive injection-seeded optical cavity employing a phase-conjugate reflector. This approach is used in a novel OPO system based on periodically poled KTiOPO4 and pumped at 532 nm by a pulsed Nd:YAG laser. The OPO is injection seeded at 835-855 nm by a continuous-wave tunable diode laser, which also enables a Rh:BaTiO3 photorefractive crystal to act as a wavelength-selective phase-conjugate reflector, with no need for active control of cavity length. The single-longitudinal-mode tunability and operational simplicity of this OPO system are demonstrated experimentally.

Characterization of a Pico-Second Phase Conjugate Nd:YVO4 Laser System

We present a full characterization of a pico-second pulse generation from a Nd:YVO4 double-pass amplifier with a phase-conjugate mirror based on a photorefractive BaTiO3 crystal with an external loop geometry. With this system, 7.4 W diffraction-limited pico-second output was generated. A corresponding optical extraction efficiency of >20% was achieved.

Photorefractive Optical Image Transfer in Feedback Signal Communication

An optical setup and relevant communication schemes interconnecting optical signals among mutually coherent optical channels are proposed to enhance the functionality of optical devices used for communication purposes. Based on self-pumped phase conjugation and two-wave mixing mechanisms in an undoped BaTiO3 photorefractive crystal, an optical image can be transferred from the signal-input channel to the signal-requiring channel of the emitting station. To overcome noise generated during communication, two different approaches are presented and the associated experimental results also demonstrated. We also discuss how to extend the use of the device to store and retrieve interconnected signals, to prevent communication interruption.

Light-induced absorption in photorefractive BaTiO3 crystals

We present the experimental results on the measurement of temporal and steady-state light-induced absorption change in undoped and rhodium-doped barium titanate (Rh:BaTiO3) crystals at different wavelengths and intensities. Theoretical calculations based on a two-center charge transport model agree well with the experimental results which supplement the earlier studies carried out using photorefractive BaTiO3 crystals. We also report a wavelength-specific light-induced effect that modifies the absorption dynamics in a peculiar manner. This can be attributed to light-induced thermal effects. However, the possibility of additional photorefractive centers becoming active cannot be ruled out.

Anisotropic diffraction of light by volume holographic grating in birefringent photorefractive crystals with extended wavelength range

The properties of anisotropic diffraction of light by volume holographic grating in birefringent photorefractive crystals are discussed. This diffraction takes place when the refractive index for diffracted light is different from the refractive index for incident light. It is found that in some special geometry of wavevector diagram the diffraction becomes less sensitive to the wavelength mismatch of Bragg condition. The wavelength range may extent in several times the range of ordinary isotropic diffraction on the grating of the same spacing and thickness. Theoretical explanation of this phenomena and experimental results of widerange diffraction in BaTiO3 photorefractive crystal are also presented.

Intensity-dependent beam-fanning in high-gain photorefractive barium titanate crystal

A phenomenological multi-wave mixing model has been used along with a more general solution of Kukhtarev's model to explain intensity-dependent beam-fanning in photorefractive (PR) BaTiO3 crystal. The variation of trap density owing to the photoexcitation of charge carriers has been considered to explain the intensity-dependent effect. Theoretical results predicted by the model show a confirmation with experimental results obtained from PR crystal, and also show some of the controlling mechanisms that enable use of beam-fanning for some potential applications.

フォトリフラクティブ２光波混合による画像の強調

A new method of selective image amplification with two photorefractive BaTiO3 crystals is proposed and its amplification characteristics is investigated. The gain of beam amplification by using the method is almost equal to the product of gain obtained from each crystals. The blur of images caused by the amplification reveal very small. Using this method, the images are more emphasized than with single BaTiO3 crystal and amplified images obtained by the selective image amplification are of high quality. It is concluded that this method is very useful for the real time pattern processing.

Motion Detection with an Optical Novelty Filter.

An optical novelty filter based on the two-wave mixing in a photorefractive BaTiO3 crystal is applied to motion detection for the purpose of its application to the robot vision.A simple method is proposed to mimic the reflex vision system that a human being possesses.Our method uses a photo-detector array to detect the output from the optical novelty filter, and the detector element that obtains the highest signal is selected.By simulations using a simple animation, it is shown that the large or the faster objects can be detected at a higher probability than smaller or slower ones.Tracking a moving object is demonstrated with a 2×2 photo-transistor array and a selective circuit including an exclusive gate circuit.

Real-time Incoherent Optical Pattern Recognition with Photorefractive Crystals

Incoherent optical pattern recognition has been demonstrated with a simple setup using photorefractive BaTiO3 crystals. The input patterns to be identified can be transmitted into the system with completely incoherent (both temporal and spatial) light. The setup does not require prefabricated filters for its operation and is thus a real-time system. The working principles of the system involve the photorefractive beam fanning effect and degenerate four-wave mixing.

Reflectivity of photorefractive cross-polarized four-wave mixing in consideration of pump depletion

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.

Motion Detection with an Optical Novelty Filter.

Optical novelty filter based on the two-wave mixing in a photorefractive BaTiO3 crystal is applied to motion detection for the purpose of its application to the robot vision. A simple method is proposed to realize the reflex vision system that a human being possesses. Our method uses a photo detector array to detect the output from the optical novelty filter and the detector element that obtains the highest signal is selected. By simulations using a simple animations, it is showed that the larger or the faster object can be detected in higher probability. Tracking a moving object is demonstrated with a 2×2 photo-transistor array and the selective circuit including the exclusive gate circuit.

Self-focusing in photorefractive BaTiO3 crystal under external DC electric field

Experimental observations of 1D and 2D self-focusing in a BaTiO3 crystal under externally DC applied electric field are reported. Stationary self-focusing behaviour can be obtained with and without additional uniform illumination.

Mirrorless oscillation in photorefractive crystals

In some special wave mixing configurations using photorefractive crystals and forward- and backward-propagating pump waves, one may observe a certain type of self-oscillation without any external feedback, the socalled mirrorless oscillation. Here forward-propagating waves passing through the crystal are enhanced via a parametric amplification process, which in turn causes an enhancement of the backward-propagating waves meeting the same phase-matching conditions. As a result coherent output beams self-develop without any additional input beams apart from the pump beams. Ultimate amplification is only limited by the intensity of the pump beams. In this contribution we describe four configurations which yield mirrorless oscillation for photorefractive BaTiO3 crystals.

External mode locking with feedback from a self‐pumped phase‐conjugator BaTiO<sub>3</sub> crystal

An external mode locking technique using RF modulation by an acousto-optic modulator and feedback from a phase-conjugate mirror is presented. A photorefractive BaTiO3 crystal is arranged in self- pumped phase-conjugator geometry. The setup is completely self- aligning and subnanosecond pulses are easily observed for He-Ne (632.8 nm), Ar 1 (514.5 nm), and He-Cd (442 nm) lasers. © 1996 Society of Photo-Optical Instrumentation Engineers. 15 years. 1 The discovery of the single-beam method for producing a phase-conjugate beam ~PCB!, self-pumped phase conjugator 2 ~SPPC!, has greatly enhanced the poten- tial use of OPC in practical applications. Many of these applications are related to interferometry and real-time im- age processing. In this paper we discuss mode locking of multimode, low to moderate power continuous wave ~cw! lasers, yet another interferometric application, using OPC with a BaTiO3 crystal in an SPPC arrangement. In order to produce mode locking, it is necessary that the phases of the various longitudinal modes of a multimode cw laser be coupled with each other. This is usually achieved by placing an active or passive frequency-shifting element inside or outside the laser cavity. However, in some cases the additional loss in the cavity due to a modu- lator may not be desirable, since it may suppress the lasting action ~specially for low-gain blue UV wavelengths! .I n such cases one can still obtain a pulsed laser by externally modulating the laser beam in amplitude or frequency. In this case laser modes are modulated by the free spectral range ~FSR! of the laser and injected, through the output coupler, into the cavity, causing the coupling of the adja- cent modes. 3-6 Consider a laser beam that is propagating through an acousto-optic modulator ~AOM! whose driving frequency is half of the free spectral range of the laser. The AOM diffracts most of the light into the first order with a fre- quency shift equal to that of the acoustic wave frequency. This beam is subsequently reflected by the SPPC mirror onto its original path, resulting in a phase-conjugate beam with a total frequency change equal to the longitudinal mode separation of the laser. This means that a portion of the light from one mode returns into the cavity that has the frequency of its adjacent mode. In other words, the frequency-shifted portion of the laser beam is delayed by a roundtrip cavity transit time. Different longitudinal modes of the injected PCB that have the frequency of their neigh- boring modes are recombined with their neighboring modes in the cavity. With every pass through the AOM and reflec- tion by the SPPC, adjacent longitudinal modes are locked together in their phases, thus forcing all the modes into a fixed-phase relation. This results in a characteristic, peri- odic pulsation of a mode-locked multimode laser. Assume each laser mode is expressed by a complex am- plitude field and the total field ofN11 axial modes is given by: