Phase conjugation by reflection gratings in electrooptic crystals

Phase conjugation by reflection gratings in electrooptic crystals in investigated in a nonlinear model proposed by Kukhtarev and Semenets [1] which takes into account not only photoconductivity but also dark conductivity. The coupled-wave equations of the model can be solved analytically. To take the boundary conditions into account, numerical calculations are unavoidable. These can be performed, however, without any approximation. Phase mismatch and self-oscillation effects are dealt with in the undepleted pump approximation.Two of us (KHR and GT) would like to dedicate this paper to Prof. Dr. H. Salecker on the occasion of his 65^th birthday     

Phase conjugation in potassium vapour using subnanosecond laser pulses

Phase conjugated waves (PCW) were generated for the first time by degenerate four-wave mixing with subnanosecond laser pulses in potassium vapour near the D ₁ transition at 7699 Å. The light source was a long cavity grazing incidence dye laser. The pulses had a duration of 700–800 ps and a linewidth of 0.2 Å. The experiments were performed in a temperature range of 400–550° C. A maximum reflectivity of 57% has been observed together with temporal pulse shortening. PCW reflectivity has been measured as a function of the temperature, and the backward pump intensity.     

Phase conjugation in a continuous-wave diode-pumped Nd:YVO4 laser

We demonstrate phase conjugation and dynamic holography by degenerate four-wave mixing in a continuous-wave diode-pumped Nd:YVO₄ laser for the first time. A phase-conjugate reflectivity of 1.4% is obtained with a 12 W diode power. The experimental results and the optimizing parameters, such as the cavity mirror reflectivities, are compared with results from a steady-state theoretical analysis. Received: 6 April 1999 / Published online: 24 June 1999     

Phase conjugation via unequal amplitude multiple gratings in photorefractives: A simple shooting method for numerical solution

We present a simple shooting method to obtain numerical solutions of the nonlinear coupled-wave equations for degenerate four-wave mixing in photorefractive crystals when four different types of the index gratings of unequal amplitudes are responsible for phase-conjugation. Our analysis includes the effects of pump depletion and absorption in the medium. Intensities of the four beams, both inside and at the output surface, are obtained as a function of four unequal coupling strengths and absorption coefficient. Intensity of the generated phase-conjugate beam is shown as a function of both the small and wide ranges of signal beam intensity. It is found that the multigrating operation with a careful choice of coupling constants and their combinations may improve the phase-conjugate wave generation. Also the presence of absorption reduces the phase-conjugate intensity more significantly when all the gratings are operative than when a single grating is allowed. Numerical results obtained from the computer calculations are presented in graphical form.     

Study of second harmonic generation by non-linear crystals with phase conjugation

We present the correction of the phase distortion which occurs during the second harmonic generation by non-linear crystals, such as KDP and KTP at high average power laser. This is due to the optical quality and thicknesses of the crystals which in turn influence the quality of the incident laser beam. This phase distortion is corrected by reflecting back the laser beam into the crystal using a phase conjugate mirror. It is found that the conversion efficiency of second harmonic generation without phase conjugation is more than that with phase conjugation. Far field pattern shows that the distortion of the laser beam can be corrected by using the phase conjugate mirror. Fidelity of the beam profile increases significantly with phase conjugation in the case of KDP crystal.     

Powerful laser systems with phase conjugation by SMBS mirror

The method of Stimulated Mandelshtam-Brillouin Scattering (SMBS) is utilized in a powerful laser to obtain better transverse beam quality by phase conjugation. The principle is explained, and the conclusions are substantiated by experimental results.     

Phase measurements of ring dark solitons

A four-frame technique for interferogram analysis is used to measure the transverse phase distribution of nonlinearly split 2D dark beams. Pairs of diametrical phase shifts within each ring are retrieved from the experiment. This result is one of the tests required to denote the formations observed as ring dark solitons.     

Photorefractive mutually pumped phase conjugation with partially coherent beams

We investigate the mutually pumped phase-conjugation process in photorefractive media with partially coherent pump beams. We consider the effects of transmission, reflection, and 2k-reflection gratings as well as the depletion of both pump beams. The theoretical results can be employed to qualitatively explain the experimental observations.     

Self-pumped phase conjugation in Bi12TiO20 crystal without external cavity

We present an experimental study of the self-pumped phase-conjugate-mirror (SPPCM) recording in a Bi₁₂TiO₂₀ sample due to the partial internal reflection of the pump beam from the rear face. SPPCM has been recorded under an external alternating electric field of a square-wave form with response time equal to 170 s at the pump beam intensity of 520 mW/cm². The mechanism of the SPPCM formation is similar to that of photorefractive double-phase-conjugate mirrors. The most effective SPPCM is recorded at the much larger pump-beam-incident angle than it could be supposed from the typical angular dependence of the two-wave-mixing gain factor. Strong dependence of the gain distribution on the pump-beam-propagation angle is observed. It is found that the response time strongly depends on the preliminary history of the sample. Received: 11 August 1998 / Revised version: 14 January 1999 / Published online: 28 April 1999     

Vector phase conjugation in Cr4+:YAG by four-wave mixing with linearly-polarized pump beams

We experimentally demonstrate vector phase conjugation in Cr⁴⁺:YAG saturable absorber at 1.06m with nanosecond pulses using degenerate four-wave mixing with linearly-polarized pump beams of arbitrary strengths. The experimental results show that the phase conjugate wave produced by this method can correct depolarisation induced by a birefringent medium.     

Study of non-linear optical phase conjugation in Ca by resonant degenerate four-wave mixing via bound excited states

. The dependence of the reflectivity on the vapour density, the buffer gas pressure and the pump beams intensity was investigated. The influence of the strong linear absorption was taken into account in a theoretical model that led to a satisfactory interpretation of the experimental results. A maximum reflectivity of 46% was measured under optimized conditions. The effect was also studied through the high-lying bound states . The maximum measured reflectivity was as high as for the first excited state. The effect of the oscillator strength on the reflectivity was demonstrated experimentally and interpreted with the help of a simple theoretical model. Received: 11 June 1996     

Mechanism of phase conjugation via stimulated Brillouin scattering in narrow band gap semiconductors

We develop a theoretical model to study optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in narrow band gap transversely magnetized semiconductors. Threshold value of pump electric field and reflectivity of the image radiation for the onset of OPC-SBS are estimated. The analysis is applied to both cases viz. centrosymmetric (CS) and non-centrosymmetric (NCS) crystals. Numerical estimates made for n-type InSb crystal at liquid nitrogen temperature duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser shows that high OPC-SBS reflectivity (90%) can be achieved in NCS crystals at moderate pump electric fields if the crystal is used as an optical waveguide with relatively large interaction length (L = 5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

Phase conjugate wavefront generation by forced Rayleigh scattering

Phase conjugate wavefront generation with efficiency up to 4% at 355 nm has been demonstrated in liquid media containing nonsaturating absorbers. Experimental results are qualitatively consistent with a simple theory of transient forced Rayleigh scattering which predicts the efficiency to depend upon the solvent, the square of the pump intensity and upon the beam quality but not upon the absorbing species.     

A selection equilibrium theory for “stimulated photorefractive backscattering and four-wave mixing” self-pumped phase conjugation

Received: 7 March 1997/Revised version: 21 July 1997     

Phase conjugation with gaussian beams

We investigate the saturation effects in phase conjugation in the case of gaussian beams. We derive analytical expressions for the phase conjugate reflectivity R_PC for arbitrary values of the pump beams. We show that the results can be drastically different from what is expected from a plane-wave analysis. At last, we show the influence of the overlapping of the three beams.     

White Light Photorefractive Phase Zone Plates

Incoherent white light from an incandescent source is employed to fabricate volume phase zone plates in LiNbO3：Fe, for the first time to our knowledge, which can guide and modulate the input white light or laser light. The diffractive efficiency of the white light volume phase zone plates fabricated can reach as high as 12%. In addition, we test the volume phase zone plates by a probe beam and find that the volume phase zone plate is present in the direction perpendicular to the c-axis and absent in the direction parallel to the c-axis. This directly proves the existence of photovoltaic photorefractive anisotropy of white light.     

Doppler-free spectroscopy and wave-front conjugation by four-wave mixing of nonmonochromatic waves

A new technique to determine the natural and collision width of Doppler-broadened two-photon transitions is shown. The method consists of using the pump with the spectral width considerably exceeding the Doppler one of the transition. Feasibility of effective wave front conjugation is shown for the case of radiation with a large spectral width.     

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. Received: 5 January 2001 / Final version: 25 May 2001 / Published online: 19 September 2001     

Phase transition to superradiance in a multicomponent system

Abtract The study deals with the ground-state energy transition of a multicomponent atomic system from its normal state to the superradiant state when interacting with a multimode quantized electromagnetic field. The threshold conditions are determined.Alexander-von-Humboldt awardee     

Optical phase conjugation and related experiments with surface plasma waves

We report on the experimental observation of optical phase conjugation using surface plasma waves in two different configurations: one involving four surface plasma waves and the other one involving two surface pump waves and bulk probe and conjugate waves. The major nonlinear contribution comes from heating of the metal film. In a related experiment, we have also observed the coupling of a bulk probe wave to the corresponding surface wave through the grating created by two counter-propagating surface plasma waves. In this case, we could observe the creation of surface acoustical waves.