Photorefractive Self-pumped Phase Conjugators Research Articles

Optical Bistability in a Photorefractive Self-Pumped Phase-Conjugator

We demonstrate the bistable behavior in a photorefractive self-pumped phase-conjugator (SPPC) theoretically and experimentally. In the method, an incoherent erase beam is used to erase the four-wave mixing (FWM) region of the SPPC, and thus the threshold of the SPPC depends on the intensity ratio of the erase beam to the sum of the coherent beams in the FWM region. Even a white light can be used to control the reflectivity as a switch.

Photorefractive properties and self-pumped phase conjugation in a series of BaTiO3 crystals

This paper presents the experimental results of two-wave mixing and self-pumped phase conjugation (SPPC) in barium titanate (BaTiO3) crystals doped with Rh or Ce. It has been shown that the main parameters of SPPC depend very strongly on the concentrations of dopants, the incident angles and the crystal cut used. High gain in two-wave mixing corresponds to low phase-conjugate reflectivity in all samples. It is impossible to maintain the highest reflectivity and gain simultaneously, because the gain will definitely be limited when we try to get the highest reflectivity, and vice versa. Some qualitative explanation of the experimental results is also given.

Compression and broadening of phase-conjugate pulses in photorefractive self-pumped phase conjugators

Pulse propagation and shaping are investigated in photorefractive self-pumped phase conjugators in both transmission- and reflection-grating regimes. The dispersion properties of self-pumped phase conjugators are analyzed by taking into account both the grating dispersion and the angular dispersion. The complex transfer functions are obtained by treating the crystal as a linear dispersive medium. We show that the pulse width as a result of the self-pumped phase conjugation is much wider in the reflection regime than in the transmission regime. The experimental results are consistent with the results calculated for the transmission-grating regime, indicating that this type grating is the dominant mechanism in the case of a femtosecond self-pumped phase conjugator.

Origin of frequency shift and temporal instability in photorefractive self-pumped phase conjugators

The physical origin of frequency shift and dynamic instability in a photorefractive self-pumped phase conjugator is analyzed by use of four-wave mixing and multi-four-wave mixing. We show that the frequency shift and the instability result from the requirement of the maximal energy transfer between the interacting beams. Two examples of numerical results in a cat self-pumped phase conjugator are given, which are in good agreement with previous experimental results. A simple method is proposed to change an unstable phase-conjugate output to a stable one.

Pulse evolution in a photorefractive self-pumped phase conjugator with transmission grating geometry

The effect of grating dispersion on pulse evolution in a photorefractive self-pumped phase conjugator is analyzed. The broadening factors and the time-dependent phases of self-pumped phase-conjugate pulses of unchirped and chirped Gaussian pulses are obtained. When the grating dispersion and the angular dispersion cancel the material dispersion, the self-pumped phase-conjugate pulse has a minimum width, which is determined by the first-order dispersion of the grating. The self-pumped phase conjugation of an unchirped Gaussian pulse broadens and is linearly chirped whereas the self-pumped phase conjugation of a chirped Gaussian pulse is compressed when the dispersion-induced chirp cancels the initial chirp.

Incoherent all-optical NAND-, NOR-, and XOR-gate using photorefractive fanning effect

An all-optical computing device is proposed to perform incoherent digital operations, including the NAND, NOR, and XOR gates. Instead of using interferometer-based methods, making use of self-generating noise gratings and the erasure of them by another beam in two photorefractive self-pumped phase conjugators (SPPC), the device is able to process mutually incoherent as well as mutually coherent inputs. Changing the orientations of two polarizers and a photorefractive crystal, we are able to perform incoherent parallel NAND, NOR, and XOR operations with this proposed setup.

Frequency shifts and dynamic instabilities in photorefractive self-pumped and mutually pumped phase conjugation

Frequency shifts and dynamic instabilities in photorefractive cat self-pumped phase conjugators and bridge mutually pumped phase conjugators are studied by use of a two-dimensional model. Intrinsic electric fields inside the crystals induce the frequency shifts and dynamic instabilities observed in these experiments. In cat mirrors, for small values of the electric field the phase-conjugate reflectivity and the frequency shift are constant. With a further increase in the electric field the reflectivity and the frequency shift become periodic through Hopf bifurcation. For a large value of the electric field both the reflectivity and the frequency shift fluctuate chaotically. In bridge mirrors, for small values of the electric field the phase-conjugate reflectivity is stable, and no frequency shift exists. With a further increase in the electric field the reflectivity and the frequency shift become periodically oscillating in time. For a large value of the electric field both the reflectivity and the frequency shift fluctuate chaotically. The phase-conjugate outputs of the two beams oscillate in an almost synchronous manner.

Dispersion compensation for a femtosecond self-pumped phase conjugator

Dispersion compensation for a photorefractive self-pumped phase conjugator with femtosecond pulses is analyzed. The self-pumped phase conjugator consists of a pair of dynamic gratings coupled by total internal reflections at the crystal surfaces (cat conjugator). The negative angular dispersions of refraction at the air-crystal interface and the gratings inside the crystal compensate for the positive dispersion of the finite crystal path. The experimental results show that with partial dispersion compensation the width of the self-pumped phase conjugation at 450 nm of femtosecond pulses is narrower than that of the transmitted pulses.

Stationary Photorefractive Self-Pump Phase-Conjugation with Stimulated Backward Scattering in LiNbO3:Fe Crystals

Stationary photorefractive self-pumped phase-conjugation with one four-wave-mixing interaction region, stimulated by the photorefractive amplified backward scattering, is obtained in LiNbO3:Fe crystals, in which the strong backward scattered light is gotten by putting a scattering particle on the backface of the sample. Even though its reflectivities are not high (its temporal and stationary reflectivities are 10% and 3%, respectively), due to the low electrooptic coefficients of LiNbO3:Fe crystals, our experimental results prove that this mechanism is effective and simple comparing with the "cat" self-pumped phase-conjugator.

Wavelength selectivity of the complex grating structure formed in a photorefractive phase conjugator

Experimental results show that the wavelength selectivity and the shape of the detuning curve of the complex grating structure formed in a photorefractive self-pumped BaTiO3 phase conjugator depend strongly on the coherence length of the incident beam. It is observed that the wavelength selectivity becomes larger as the coherence length of the incident beam is decreased. With a coherence length much longer than the dimensions of the conjugator crystal a wavelength selectivity of 0.026 nm is measured. It is shown that this very small wavelength selectivity and the experimental detuning curves are in agreement with a simple model that takes into account the interference between light reflected in 2k gratings and light that has traveled one round trip in the internal loop of the conjugator crystal. The findings explain how the optical feedback generated by the dynamic gratings in the phase conjugator can increase the coherence length of a multimode laser.

Implementation of a novel optical half-adder

A novel technique of performing optical logic operations and a compact optical half-adder based on beam coupling in photorefractive self-pumped phase conjugators are proposed and demonstrated.

−C curve configuration (−C 3) photorefractive self-pumped phase conjugator

A novel configuration for a self-pumped phase conjugator has either only one internal loop, or beam path, which curves into one corner or two loops of similar form or two loops of quite different form depending on the exact geometry when the incident beam enters the barium titanate crystal at an acute angle to the −C axis. Phase conjugation is demonstrated over a wide range (30 degrees) of incident angles and over a range of 4 mm in lateral position with efficiencies up to 30%. We characterise the dynamic response, compare our configuration with that of the “Cat” and propose a tentative four-interaction region mechanistic model.

High efficiency self-pumped phase conjugation at 633 nm in cerium-doped barium titanate crystals

Photorefractive self-pumped phase conjugation (SPPC) behavoir is examined at 633 nm in cerium-doped barium titane crystals. A reflectivity as high as 86.8% is reported. The buildup times are an order of magnitude faster than the corresponding values in Co: BaTiO 3 and Cu: KNSBN crystals with input powers. A possible mechanism responsible for the high self-pumped phase conjugation reflectivities in the negative incident angle range is discussed.

Impurity enhanced self-pumped phase conjugation in the near infrared in ‘blue’ BaTiO 3

Photorefractive self-pumped phase-conjugation is examined at near-infrared wavelengths using doped ‘blue’ BaTiO 3, and reflectivities are reported as high as 76% between 720 nm and 1004 nm due, as is believed, to a backward stimulated photorefractive scattering mechanism. Data also suggests that it is possible to stabilise the phase-conjugate reflectivity by vibrating the crystal.

Instabilities in coupled photorefractive ring cavities and self-pumped phase conjugators

In the preceding paper, we used the mean-field approximation to formulate the time evolution equations for two coupled unidirectional photorefractive ring resonators. Linear stability analysis based upon the steady-state solutions of the mean-field equations showed that the coupled system may become unstable in the unstable off- and steady-state operation regimes. In this paper, we investigate the temporal behavior and nonlinear dynamics of the coupled system. The results indicate that the coupled system exhibits self-pulsation and chaos in the unstable off-state and steady-state operation regimes. The instability is explained in terms of the competition between the nonlinear gain and loss. Numerical results of the transient equations are presented and discussed. The coupling between unidirectional ring resonators is extended to explain the instability in self-pumped phase conjugators.

Ring self-pumped phase conjugator using total internal reflection in photorefractive strontium barium niobate

We report the design and operation of a photorefractive ring self-pumped phase conjugator using total internal reflection in strontium barium niobate. Phase-conjugate reflectivity and fidelity are measured and compared with those of the conventional total-internal-reflection phase conjugator.

Tungsten bronze crystals for photorefractive self-pumped phase conjugation

Abstract We review the current status of photorefractive self-pumped optical phase conjugation in the tungsten bronze family of ferroelectric crystals. The tungsten bronze ferroelectrics have emerged as an important class of materials for performing optical phase conjugation in addition to the well known pyroelectric, piezoelectric, and electro-optic applications. The large electro-optic coefficients, open lattice structure, compositional flexibility, and good optical quality of these materials has made possible a new set of self-pumped phase conjugate mirrors. Our initial interest in this class of crystals arose from a number of enlightening discussions with Professor L. Eric Cross who pointed out the multitude of applications possible with the tungsten bronzes. We would like to acknowledge his assistance and encouragement by dedicating this paper to Professor Cross on the occasion of his 65th birthday.