Phase-conjugate Four-wave Mixing Research Articles

Polarization controlled hybrid line shape and switching of noise correlation via Autler-Town splitting

We report the experimental observations of the second- and third-order hybrid intensity noise correlation and intensity difference squeezing among two different multi-order nonlinear fluorescence (FL) and two FL signals as well as one of pair of phase conjugate four-wave mixing in a Pr3+:Y2SiO5 crystal, respectively. The degrees of the correlation and squeezing as well as the sharp and broad peaks are studied by changing the polarization of laser fields and gate position in Λ-type three-level systems. We observed the multi-channel hybrid intensities' correlation among the composite and coexisting signals, which turn out to be positive and negative for the cases without and with Autler-Town splitting of FL, respectively. A wide broad peak and a high sharp peak are observed with respect to the degrees of correlation and polarization state controlled by wave plates. Such results may find potential applications in optical transistor and optical hybrid communications.

Polarization-Dressed Correlation and Anti-Correlation Between Phase-Conjugate Four-Wave Mixing and Fourth-Order Fluorescence

We report the two- and three-mode correlation of phase-conjugate four-wave mixing and fourth-order fluorescence in Pr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> crystal. The degree of correlation is controlled by changing the position of detecting time and polarization state of pumping field. The switching between correlation and anti-correlation was observed at different positions of detecting time because of the competition between four-wave mixing and fourth-order fluorescence signals. The obvious variation of correlation degree was also observed at different polarization states, which is interpreted by the different Clebsch-Gordan coefficients and the EIT subsystem induced by polarization dressing effect.

Modulating the correlation and squeezing of phase-conjugate four-wave mixing via the polarizable dressing states.

We report the experimental observation of the intensity noise correlation and squeezing between counter propagating Stokes and anti-Stokes signals in Pr(3+):Y2SiO5 crystals. Both the degree of correlation and squeezing as well as the oscillation frequency of correlation curves are modulated by changing the polarization states and powers of the dressing fields. The double-dressed effect and the triple-dressed effect in V-type three-level, Λ-type three-level and N-type four-level systems are compared. The polarization and power dependencies in these systems are different, and the oscillation frequency of the correlation curve in the triple-dressed process is greater than that of the double-dressed process. Our results show that the correlation and squeezing of photon pairs can be controlled via polarized dark states.

Squeezed states produced by modulation interaction and phase conjugation in fibers

Number-state expansions are derived for the squeezed states produced by four-wave mixing (modulation interaction and phase conjugation) in fibers. These expansions are valid for arbitrary pump-induced coupling and dispersion- and pump-induced mismatch coefficients. To illustrate their use, formulas are derived for the associated field-quadrature and photon-number variances and correlations.

大信号布里渊增强四波混频相位共轭镜

大信号布里渊增强四波混频相位共轭镜

Phase diagrams for semilinear photorefractive coherent oscillator

The sequence of optical phase transitions is considered for a photorefractive coherent oscillator with a conventional mirror and a four-wave mixing phase conjugate mirror. Analogous to phase transitions in solids, the phase diagrams are constructed and defined qualitatively as different states of a whole system (light-induced scattering below the threshold of oscillation, single-frequency oscillation, two-frequency oscillation, and multimode mirrorless oscillation). The attempt of experimental mapping of a phase diagram is presented for a coherent oscillator with Co-doped BaTiO3.

Four-wave mixing in dressed cold cesium atoms

Quasi-degenerate four-wave mixing optical phase conjugation was investigated in cold cesium atoms confined in a magneto-optical trap. Broad and narrow resonances were observed in the corresponding spectrum. The broad resonances reveal directly the dressed levels structure of the composite system formed by atoms and trapping laser field. However, the observed narrow resonances can only be qualitatively understood in terms of Raman processes involving nearly degenerate ground-state levels, indicating the important role played by the level degeneracy.

Optimization of photorefractive two-wave mixing by accounting for material anisotropies:KNbO3andBaTiO3

The influence of the anisotropy of the effective dielectric constant, effective electro-optic effect, drift mobility, and photoexcitation cross section on the photorefractive two-wave mixing gain and speed are analyzed in detail. Theoretical expressions that include all these influences and that are valid for the single-level band model are reported. They can give the necessary guidance for optimizing the interaction geometries and the extrinsic crystal properties for systems based on two- and four-wave mixing or self-pumped phase conjugation. Concrete examples are given for ${\mathrm{KNbO}}_{3}$ and ${\mathrm{BaTiO}\mathrm{}}_{3},$ where all possible two-beams interaction geometries are analyzed in the plane of maximum photorefractive nonlinearity $(bc$ and $\mathrm{ac}$ plane, respectively). It is shown that, besides the dielectric constant and the electro-optic effect, also the anisotropy of the photoexcitation with respect to wave polarization plays a major role and strongly influences the optimum geometry, allowing potentially very large enhancement of the exponential gain. Corrections to the standard expressions for photorefractive two-wave mixing amplification in the depleted pump regime are also given. They apply in the case of asymmetric incidence and/or under the presence of anisotropic photoexcitation.

Nonlinear optical properties of GaSb and GaInAsSb and their application for phase conjugation in degenerate four-wave mixing

The nonlinear optical properties of, and phase conjugation in, the bulk semiconductor GaSb were investigated at 2.1 µm by use of the Z-scan and the degenerate four-wave mixing techniques. Measurements were also carried out near the fundamental bandgap of the quaternary compound Ga0.87In0.13As0.11Sb0.89. Z-scan measurements as a function of sample temperature, in conjunction with theoretical modeling, identified the predominant sources of the medium nonlinearity to be nonequilibrium free carriers generated through two-photon absorption in GaSb, while it is saturation of real transitions near the fundamental band edge in Ga0.87In0.13As0.11Sb0.89 that was identified as the primary cause of this nonlinearity. Degenerate four-wave mixing phase-conjugate reflectivity of as much as 14% has been achieved in GaSb.

Phase-conjugate four-wave mixing with partially coherent laser fields

Phase-conjugate four-wave mixing with partially coherent laser fields

Degenerate phase-conjugate four-wave mixing in a nearly-Doppler-free two-level atomic medium

We present experimental observations of phase-conjugate degenerate four-wave mixing in a two-level atomic system (the ${3s}^{2}{S}_{1/2}, F=2, {m}_{F}=\stackrel{\ensuremath{\rightarrow}}{2}{3p}^{2}{P}_{3/2}, F=3, {m}_{F}=3$ transition in a diffuse, collision-free, thermal beam of atomic sodium) driven by cw, narrow-band $(\ensuremath{\sim}250 \mathrm{kHz}),$ stabilized pump and probe fields. The primary measurements that we report are of the peak signal strength of the four-wave-mixing interaction and the bandwidth of the four-wave-mixing spectrum (as the laser frequency is tuned through the resonant frequency) as a function of the intensity of the pump field. We compare these measurements with the predictions of a theoretical analysis in which extension to include the effect of the small transverse velocity $(\ensuremath{\sim}3 \mathrm{m}/\mathrm{s}\mathrm{e}\mathrm{c})$ of the atomic beam is critical. These measurements provide a direct verification of the theory of this important nonlinear interaction.

Phase conjugation in four-wave mixing of cw radiation under conditions of strong self-interaction

An experimental investigation is made of the influence of strong self-interaction of the reference waves on phase conjugation in four-wave mixing of cw radiation in a medium with a thermal nonlinearity. It is shown that the thermal lens formed by the Gaussian reference beams reduces the nonlinear reflection coefficient appreciably (several-fold) and causes a deterioration in the phase conjugation quality.

Influence of Thickness of Methyl-Orange-Doped Polyvinyl Alcohol Film on Phase Conjugate Matched Filtering

Phase conjugate matched filtering was performed with optical phase conjugation in Fourier-transforming backward degenerate four-wave mixing. A methyl-orange-doped polyvinyl alcohol (MO/PVA) film was used as a phase conjugator. The influence of the film thickness upon decay of a correlation signal due to phase mismatching was investigated. The thickness was varied from 15 μm to 90 μm. Reduction of the film thickness relaxes the tolerance for phase mismatching, which also occurs when a large reference picture is placed in a pump beam or when a reference picture shifts from the optical axis of the pump beam. Using a MO/PVA film of 15 μm thickness, we obtained allowable angles of 3.4° for a horizontally expanded reference picture and larger than 4.3° for a vertically expanded reference picture, assuming about 40% reduction of a correlation signal is allowable.

Phase conjugation in optical communication links with photorefractive Fe:KNbO 3

Self-aligning of optical links and good wavefront matching in heterodyne detection can be achieved through the use of phase conjugation. We demonstrate an optical self-aligning link between two mutually incoherent laser sources using photorefractive Fe:KNbO 3 as a double incoherent phase conjugate mirror. The performance of a heterodyne detection system for coherent optical communication based on double incoherent phase conjugation and four-wave mixing phase conjugation is also presented.

Polarization dependence and efficiency in a fiber four-wave mixing phase conjugator with orthogonal pump waves

We study the influence of pump separation on polarization dependence and on conversion efficiency in a fiber-based optical phase-conjugator with two orthogonal pump waves at different wavelengths. The polarization dependence is found to be minimized when the pump wavelength separation is small. Furthermore, both theory and experiments show that the efficiency is maximized if the signal wavelength is located close to one of the pumps.

Permanent Electronic Excitation of a Molecular Layer on a Surface through Phase Conjugation

The dynamics of an atom (or molecule) located near the surface of a four-wave mixing phase conjugator is studied. This device consists of a slab of nonlinear material, which is illuminated by two strong counterpropagating laser beams. The atomic dipole field induces a polarization in the nonlinear material, and as a result, a phase-conjugated replica is emitted toward the atom. This effectively leads to a four-wave mixing process at the site of the atom, during which two pump photons are annihilated in favor of an electronic excitation of the atom. The mismatch in energy is emitted as a single fluorescent photon. In the steady state the atom acquires a finite population of the excited level, although there is no direct optical pumping of the atomic transition. It is shown that this population is determined entirely by the Fresnel reflection coefficients of the medium. In case of magnetic degeneracy of the levels, the distribution over the sublevels is furthermore determined by geometrical Clebsch−Gordan coefficients. Three cases of low angular momentum are worked out in detail, as examples.

Temperature dependence of phase conjugation in Ce:LiNbO3

The photorefractive effect in ferroelectric crystal Ce:LiNbO3 as measured by four-wave mixing phase conjugate technique was investigated as function of temperature. Results show that the effect of temperature on the phase conjugate intensity reflectivity of Ce:LiNbO3 is very large. The experimental phenomena has been explained semiquantitatively by using the theory built by Kukhtarve's band transport model and static coupling wave equation.

Optimization of the field of view of a Brillouin-enhanced four-wave-mixing phase conjugator

A Brillouin-enhanced four-wave mixing phase conjugator featuring a field of view that varies by approximately an order of magnitude, i.e., from 2 to 20 mrad, is described. We control this variable field of view by utilizing a slightly noncollinear pumping geometry and by adjusting the pump-pump frequency difference to optimize the phase mismatch for the overall process. A model that describes the interplay between frequency and geometry to the Brillouin-enhanced four-wave-mixing phase mismatch is found to predict the expandable field of view.

Optical transfer function for nonparaxial phase conjugation in degenerate four-wave mixing

The optical transfer function (OTF) related to the phase-conjugate mirror operating as an imaging device is examined. The OTF derived is based on the nonparaxial solution of parametric four-wave mixing; i.e., the OTF is valid for a wide angular spectrum of the probe wave. The influence of the nonuniform pump waves on the Fourier spectrum of the conjugate wave is estimated in an approximation that is valid for the generated wave of weak intensity.

Phase Conjugation of Squeezed Light

Phase conjugation in degenerate four-wave mixing with a squeezed input beam is investigated. We show that the phase conjugate mode is always super-Poissonian and never squeezed, but the combination mode may be squeezed to an extent even greater than the input probe wave.