Photorefractive Beam Coupling Research Articles

BRAGG-MATCHED PHOTOREFRACTIVE TWO-BEAM COUPLING IN ORGANIC–INORGANIC HYBRIDS

This review article presents the concept of photorefractive beam coupling in hybrid photorefractive cells comprising a liquid crystal layer adjacent to inorganic photorefractive windows. The roles of the liquid crystal layer and the photorefractive windows, as well as the overall advantages of this architecture, are explained. The mechanism involved and the required window parameters to break the local alignment symmetry in order to achieve Bragg-matched two-beam coupling in a liquid crystal layer are discussed.

Understanding the optical and electrical properties of a new photorefractive potassium niobate crystal

Optical and electrical measurements have been made on a new codoped potassium niobate crystal that yields a significant increase in linear absorption, photocurrent, and photorefractive beam coupling efficiency. The Ag impurity enters the K site, changing the local field in the lattice. The Fe in the Nb site, perturbed by the replacement of the next near-neighbor K with Ag, is responsible for the enhanced linear and nonlinear properties. The modified material parameters result in an increase of the photorefractive counter-propagating two-beam coupling efficiency.

Pump-induced nonlinear refractive-index change in erbium- and ytterbium-doped fibers: theory and experiment

A 980-nm pump-induced nonlinear refractive-index (n2) change in erbium-doped (20-m) and ytterbium-doped (20-m) fibers has been measured at 1064 nm by time-delayed photorefractive beam coupling in a co-propagating and counterpropagating geometry. It was found that n2 decreases at the wavelength of the probe beam when the pump beam is present. We present a semiclassical theory based on a four-state system that accounts for the pump-induced change of n2. Both theoretical and experimental results show that a significant index change, of the order of 10%, can be obtained for cw pump powers as small as 10 mW.

Fast switching of light propagation in a photorefractive crystal via Pockels effect

We experimentally demonstrate fast switching of light propagation and phase conjugate generation in a BaTiO3:Ce crystal with a high efficiency. The switching mechanism is based on the phase mismatch in photorefractive beam coupling between an incident beam and a fanning beam. The phase mismatch is induced by an externally applied electric field via Pockels effect. In our experiment, we obatin a voltage-generator-determined switching time of the order of 10 μs. The required voltage to reduce the output phase conjugate power by more than 90% is about only 100 V for our 3.8 mm-thick crystal.

New approach to the measurement of the nonlinear refractive index of short (

The nonlinear refractive index n2 of silica fiber (24 m) and erbium-doped fiber (10 m) is measured to within an accuracy of 5% by use of time-delayed photorefractive beam coupling of intense 53-ps, 1.064-microm pulses that experience self-phase modulation in the fibers. The resultant induced grating autocorrelation response yields a value of n2/A(eff) and a calibration standard for the fiber. A phase shift of the order of 0.19pi can be detected and is limited only by laser amplitude fluctuations. A unique advantage of this technique is its ability to measure n2 accurately in short lengths (z < or = 25 m) of fiber, whereas other approaches typically use much longer lengths of fiber (z > or = 100 m).

Evaluation of electro-optic, elasto-optic, piezoelectric, and elastic coefficients of Ba0.77Ca0.23TiO3 through orientation dependent photorefractive beam coupling

We present a demonstration of the influence of the indirect electro-optic effect on the photorefractive performance of Ba0.77Ca0.23TiO3 (BCT). The comparison of the orientation dependence of the two-beam coupling energy transfer gain with theoretical model allows us to obtain information on the electro-optic coefficients, but also an evaluation of the unknown elasto-optic, piezoelectric, and elastic stiffness coefficients of BCT. We found r13T=33 pm V−1 and r33T=170 pm V−1. These values are in good agreement with other published values. For the interesting coefficient r42T, we find a higher value r42T=465 pm V−1 than the one previously published. From this study we also determine that as in the case of BaTiO3, the optimal orientation is given by a 45°-cut BCT.

Theory of critical enhancement of photorefractive beam coupling

We develop the idea of critical enhancement of the photorefractive response near the threshold of parametric excitation of space-charge waves (the spatial subharmonics) taking into account the vectorial character of beam coupling and a fairly strong broadening of the nonlinear resonance owing to light absorption. The results of our calculations are a description of the measurable characteristics of critical enhancement and optimization of the experimental conditions for detection of anomalously high amplification gain factors in cubic Bi(12)SiO(20) crystals.

OPTICAL INVERSE FILTER FOR HOLOGRAPHIC FILTERING BY PHOTOREFRACTIVE FOUR-WAVE MIXING WITH INCOHERENT ERASURE

We analyzed the phase conjugate reflectivity by photorefractive four-wave mixing exposed to incoherent erasure, and found that the incoherent erasure brings inversion relation in amplitude of the phase conjugate beam with the intensity of the erasure even in a large coupling photorefractive material. Since such inversion relation appears in real-time, this function contributes to put forward holographic filtering to real-time processing. For the sake of confirmation of the analysis, we performed an experiment with employing a 45^{\circ} -cut \hbox{BaTiO}_3 crystal which is known as a crystal providing one of the largest photorefractive beam coupling.

Attractors and auto-oscillations for feedback controlled photorefractive beam coupling

We investigate the physics of a novel nonlinear system, feedback controlled photorefractive beam coupling. Inertia of the electronic feedback is found to be an element crucial for permanent operation of this system. Theoretically and experimentally we have found a wealth of periodic and quasi-periodic regimes for observable characteristics of the feedback controlled wave coupling. A good qualitative agreement between theory and experiment is obtained for LiNbO 3 crystals.

Dynamics of feedback controlled photorefractive beam coupling

We investigate theoretically and experimentally the operation of a nonlinear system, feedback controlled photorefractive beam coupling. It is shown that the feedback equations used earlier ( 6p/2 phase shift between the diffracted and transmitted beam components! are satisfactory only during a relatively short initial stage of development. We have generalized these equations by taking into account the inertia inherent in any real feedback loop. The inertial feedback equations ensure the permanent operation of the nonlinear system and lead to a variety of periodic and quasiperiodic regimes. We investigate the distinctive features of these regimes and transitions between them for moderately thick crystals. Good qualitative agreement between theory and experimental data is obtained for LiNbO3 crystals.

Critical enhancement of photorefractive beam coupling

We show that a hybridization of the optical and material nonlinearities takes place near the threshold of the subharmonic generation in photorefractive crystals. It results in a critical (with a singularity) enhancement of the rate of spatial amplification of light waves and leads to a variety of new optical critical phenomena.

Photorefractive Beam-Coupling Measurement of the Nonlinear Refractive Index of Semiconductor Films

The nonlinear refractive index n2 of short lengths (<25 m) of a 20 m silica fiber were measured by photorefractive beam-coupling of 53 ps, 1064 nm pulses that experienced self-phase modulation (SPM) in the fiber. The resultant Induced Grating Autocorrelation (IGA) response yields a value of n2 and a calibration standard for the fiber. By coupling the 1064 nm signal pulse into a semiconductor film and taking the output and coupling it into the calibrated fiber, the modified IGA response, with respect to the fiber alone, results in both the sign and the magnitude of n2 of the semiconductor film. The sign and magnitude of n2 for 1 mm thick GaAs, ZnTe, and CdTe films have been measured by noting the modified IGA response – the temporal shift of the photorefractive beam-coupling response due to SPM in the semiconductor.

Photorefractive Beam-Coupling Measurement of the Nonlinear Refractive Index of Semiconductor Films

The nonlinear refractive index n2 of short lengths (<25 m) of a 20 m silica fiber were measured by photorefractive beam-coupling of 53 ps, 1064 nm pulses that experienced self-phase modulation (SPM) in the fiber. The resultant Induced Grating Autocorrelation (IGA) response yields a value of n2 and a calibration standard for the fiber. By coupling the 1064 nm signal pulse into a semiconductor film and taking the output and coupling it into the calibrated fiber, the modified IGA response, with respect to the fiber alone, results in both the sign and the magnitude of n2 of the semiconductor film. The sign and magnitude of n2 for 1 mm thick GaAs, ZnTe, and CdTe films have been measured by noting the modified IGA response – the temporal shift of the photorefractive beam-coupling response due to SPM in the semiconductor.

Study of transient photorefractive beam coupling in high- and low-molar-mass liquid crystal mixtures

We observed transient signals of Bragg grating formation in high-performance photorefractive high- and low-molar-mass liquid crystal mixtures (HL-LCMs). These transient signals including oscillation-like behavior can be explained by a complex time constant in the case of a finite applied dc field. The time constant for the space-charge field to reach its steady state and the oscillation frequency are estimated.

High optical gain using counterpropagating beams in iron and terbium-doped photorefractive lithium niobate

Exceptionally high values of photorefractive gain coefficient of up to 100 cm-1 have been observed in Fe-doped and Fe/Tb-doped crystals of photorefractive lithium niobate. It is believed that these are the highest observed coefficients of any crystalline photorefractive medium. Accurate measurements of gain coefficient have been possible for the first time by using a specially cut triangular crystal which allows the use of short interaction lengths and the complete elimination of surface reflections by having the beams incident at Brewster’s angle. Experimental results are consistent with a simple model of photorefractive beam coupling up until the onset of noise which is observed to deplete the pump for interaction lengths longer than 1 mm.

Photorefractive beam coupling between orthogonally polarized light beams by linear dichroism in Cu-doped potassium sodium strontium barium niobate

Two-beam coupling between orthogonally polarized waves is demonstrated by use of linear dichroism in Cu-doped potassium sodium strontium barium niobate. A maximum coupling gain coefficient of approximately 4 cm-1 is obtained at a grating spacing of 1 µm and a wavelength of 514.5 nm, where linear dichroism is moderate. The experimental results are compared with a theoretical model that allows a photorefractive grating to have an arbitrary phase shift with respect to an intensity-interference pattern. Fitting results indicate that the grating is not 90°-phase-shifted under open-circuit conditions. The contribution of the bulk photovoltaic field to the photorefractive phase shift is discussed.

Fast mutually pumped phase conjugation induced by a transient photorefractive surface wave

Mutually pumped phase conjugation with a recording time of few milliseconds is obtained in Bi12TiO20 crystals by use of transient photorefractive beam coupling under a dc external electric field. It is demonstrated that the additional acceleration of the positive-feedback-loop formation is required for successful generation of transient phase-conjugate wave fronts. This acceleration is provided by the high-intensity transient photorefractive surface wave that appears immediately after application of the external electric field as the result of coupling of the incident beam with the reflected fanning beams. To the authors’ knowledge, this is the first experimental observation of a transient photorefractive surface wave.

Crystal-temperature dependence of picosecond two-beam coupling gains in semi-insulating semiconductors

Temperature dependences of time-resolved picosecond beam coupling gains and losses in semi-insulating undoped GaAs and CdTe:Ga are investigated experimentally at a wavelength of 1.064 μm. In the temperature range of 283–363 K a weak temperature dependence of the free-carrier energy transfer gain is observed at high fluence excitation, while photorefractive beam coupling gains and nonlinear absorption losses are found to exhibit no clear temperature dependence. A numerical simulation of the dynamics of photoexcited carriers and space-charge fields is also performed in order to examine the experimental results.

Fast mutually pumped phase conjugation using transient photorefractive coupling

Unusually fast mutually pumped phase-conjugation (MPPC) is achieved in a Bi12TiO20 crystal by using transient photorefractive beam coupling under a dc electric field. Pulses of phase-conjugate waves with a buildup time of 4.5 ms and a width of 50 ms at an input intensity of 2×0.8 W/cm2 are obtained after applying a rising front of external electric field to the crystal. A phase-conjugation efficiency as high as 25% of the incident light power is measured.

Powerful, diffraction-limited semiconductor laser using photorefractive beam coupling

We use semiconductor laser amplifiers and a photorefractive crystal to generate a high-power, diffraction-limited laser beam at 860 nm. Using a single flared amplifier, we obtain 1.09 W in a diffraction-limited beam from 2.2 W of pump power. Using an array of flared amplifiers, we also demonstrate efficient beam coupling, showing that this technique is easily extended to semiconductor amplifier arrays.