Double Phase Conjugate Mirror Research Articles

Background light reduction method with a double phase conjugate mirror and a phase plate for optical inter-satellite communications

In optical intersatellite communication, the background light consisting mainly of sunlight decrease transmission rate with increasing bit error. A spatial filtering system using a double-phase-conjugate mirror (DPCM) and a phase plate is proposed to block the background light, which cannot be filtered out by using wavelength filters, polarization filters, or dousers. In this system, the effect of wavefront distortion caused by the phase plate and the compensation effect of the distortion by phase conjugate light from the DPCM are effectively combined for spatially separating signal light and background light. We demonstrated the reduction of the background light by the proposed system and optimized the phase plate for maximizing this reduction effect. As a result, it was clarified that the transmission rate can be drastically improved up to 11-fold.

Dynamically reconfigurable characteristics of a double phase conjugate mirror using Sn2P2S6 crystals and their application to optical inter-satellite communication

A double phase conjugate mirror (DPCM), created by two mutually incoherent beams entering photorefractive nonlinear materials, can generate a phase conjugate beam whose reflectivity may be greater than 100%. Even though the conditions of the incident beams are changed, the DPCM can be dynamically reconfigured by using a Sn2P2S6 crystal with a high response speed. These features of the DPCM are advantageous, particularly in an optical inter-satellite communication system. In particular, use of the phase conjugate beam from the DPCM offers wavefront compensation and amplification in satellite communication. In addition, the dynamically reconfigurable DPCM using a Sn2P2S6 crystal relaxes the acquisition accuracy of the signal beam in the system. In this study, the temporal and spatial operating characteristics of the DPCM using a Sn2P2S6 crystal were first clarified. Next, an inter-satellite system based on the DPCM was proposed, and it was demonstrated that our system significantly improves the tolerance of the acquisition accuracy and tracking time.

Optical interconnection between different spatial modes using double phase conjugate mirror with polarization alignment system

We propose a fundamental interconnection method using a polarization alignment system for waveguides having different spatial modes. In addition, as an example for the verification of the fundamental operation, we demonstrate an interconnection between a photonic crystal fiber and a laser that have obviously different spatial modes. The polarization alignment system operates synergistically with a self-written waveguide formed with a double phaseconjugate mirror. This technique enables us to interconnect a photonic crystal fiber with a laser source without complicated and time-consuming optical alignment. In this method, although it is not necessary to perform an external control for interconnection, the waveguide most suitable for connection is formed autonomously in a Sn2P2S6:Sb crystal developed for this purpose. There was a marked reduction in the polarization dependence of coupling efficiency, compared with that observed using a stand-alone double phase-conjugate mirror.

Double Phase Conjugate Mirror with Sb-Doped Sn2P2S6 Crystal for Different Incident Angles, Crystal Thickness, Total Incident Intensity and Polarization

We developed photorefractive Sb-doped Sn2P2S6 samples of different crystal thickness to investigate the properties of the media for double phase conjugation. The Sn2P2S6:Sb crystals have high coupling coefficient of 20 cm−1, and show fast response time of 1.3 ms at total incident intensity of 0.3 W/cm2. The maximum phase conjugate reflectivity of 32% was observed with a 4 mm thickness sample. To investigate the characteristics of Sn2P2S6:Sb for double phase conjugation, on the basis of the coupled wave equations, we showed the dependence of phase conjugate reflectivity on incident angle, crystal thickness, total incident intensity and polarization. From these results, the optimum incident condition was clarified for obtaining the highest phase conjugate reflectivity.

Experiments on Double Phase Conjugate Mirror with a Photorefractive Sb:Sn2P2S6 Crystal

We present various optical basic properties of a 1.5% doped Sb:Sn2P2S6 crystal experimentally and demonstrate a double phase conjugate mirror (DPCM). We measured the reflection, absorption and scattering of this crystal. The large coupling strength Γ L = 8.2 was obtained by the experiment of two-wave mixing. In addition, we succeeded in the formation of the DPCM and achieved the high diffraction efficiency of 42%. This crystal was suitable for DPCM applications of high speed response.

Double-phase conjugate mirror in nominally undoped Sn_2P_2S_6

Coherent oscillation is achieved in the geometry of a double-phase conjugate mirror in nominally undoped Sn2P2S6 with 647 nm radiation of a Kr+ laser. The specific temporal dynamics of oscillation with modulated intensity and periodic 0-pi-0-pi variations of the oscillation wave phase is similar to that observed earlier using a semilinear oscillator geometry. The described oscillator ensures submillisecond delay time in the appearance of the phase conjugate wave.

Wave mixing in nominally undoped Sn2P2S6 at high light intensities

The intensity dependence of the photorefractive response of Sn2P2S6 is studied for the Kr+-laser wavelength of 647 nm and pump-beam intensities of up to 10 W/cm2. A considerable enhancement of the two-beam coupling gain factor with increasing intensity at a grating spacing of ≃1 μm is attributed to a light-induced increase of the effective trap density. The large gain reached at high intensities is applied for the build up of a double phase conjugate mirror with a sub-millisecond switch-on time.

Numerical Simulation of Self-Pumped Phase Conjugate Plane–Curve Loop Mirror Based on Photorefractive Nonlinearity

We deal with computer simulation of a transient process in a self-pumped phase conjugate plane—curve loop mirror based on BaTiO3. In optimal circumstances the nonlinear reflectivity and fidelity of such a mirror respectively achieve 0.80-0.90 and 0.95-0.98. The generation of conjugate wave-front occurs due to scattering from the dynamic hologram which is produced in the region of self-intersection of forward and backward beams. In such a model the scenario of passing to unstable generation regimes is similar to the self-pumped phase conjugate plane—plane loop mirror and substantially differs from a single-crystal double phase conjugate mirror.

Generalized derivation of equations for four-wave interaction in photorefractive media

The emergence of optical waves out of optical noise (fanning) in a photorefractive medium is considered in an unconventional manner, i.e., using the Lorentz lemma and ensemble averaging. The derivation of equations for four-wave interaction in the scheme of a double incoherent phase-conjugate mirror is described in detail. Numerical calculation based on the resulting system of equations provides insight into the effect of noise on the oscillation threshold and makes it possible to discover the “higher threshold” effect in saturation-free noisy materials.

Single-frequency operation of a broad-area laser diode by injection locking of a complex spatial mode via a double phase conjugate mirror

We demonstrate what is believed to be the first phase-coherent locking of a high-power broad-area diode to a single-frequency master laser. We use photorefractive double phase conjugation to lock the diode in a self-optimized complex spatial mode while the photorefractive crystal diffracts that complex mode in a near-diffraction-limited beam.

Dynamics of a photorefractive response and competition of nonlinear processes in self-pumping double phase-conjugate mirrors

The dynamics of formation of a nonlinear response of a double phase-conjugate (PC) BaTiO3 mirror is calculated. It is shown that because of competition between processes of different types (related to the presence of several PC channels, the local and nonlocal components of the photorefractive nonlinearity), the transient and dynamic lasing regimes for this mirror can be substantially different. It is found that the development of lasing begins with the successive formation and phasing of dynamic holograms of two different types (two PC channels). It is shown that even under optimal conditions, the lasing regime is not stationary due to competition between processes of different types, and the parameters of output fields fluctuate in time in a nontrivial way (due to the presence of the in-phase and out-of-phase components). Several scenarios of transition to the dynamic chaos are described.

Stabilization of the photorefractive double phase-conjugate mirror inBaTiO3

Photorefractive simple and double phase-conjugate mirrors very often exhibit strong instabilities in their reflectivities. One of their possible origins can be the competition between phase conjugation and its generating process, photorefractive beam fanning. However, by suitably choosing the incident beam widths and directions, we suppressed this competition and replaced it by the enhancement of each process by the other. This technique allows to obtain a double phase-conjugate mirror in BaTiO{sub 3} that establishes itself in seconds for beam intensities on the order of the tenth of a milliwatt. The reflectivities reached are on the order of 30% and stable within 3% for hours.

Adaptive self-aligning, bi-directional interconnection using double phase conjugation in Rh:BaTiO 3

The results of an experimental demonstration of a very efficient adaptive interconnection between two low power lasers, using a double phase-conjugate mirror (DPCM) in Rh:BaTiO 3 are presented. Using low power incident beams, high phase conjugate reflectivity (∼120%) and, simultaneously, high coupling transmission efficiency (∼50%) of the double phase conjugation process have been measured. After subtracting the losses in the crystal, this diffraction efficiency corresponds to an almost complete deflection of one input laser beam into the direction of the other one. We compare our experimental results on phase conjugate reflectivity and coupling transmission efficiency with analytical predictions of DPCM and find they are in agreement. The build-up time of the interconnection channel was measured taking into account the competition of DPCM with self-pumped phase conjugation. The robustness of the interconnection to out-of-plane angular changes of the direction of one of the input beams is demonstrated. The influence of the laser beam modulation on the interconnection efficiency is studied. We show that the interconnection efficiency is very good even at very strong modulation of the input beams.

Experimental study and theoretical modeling of bidimensional beam fanning: application to DPCM

Abstract Features like efficiency and stability of the double phase conjugate mirror (DPCM) in BaTiO 3 obviously depend on the beam fanning (BF) experienced by each incident beam. We propose here a new approach to DPCM optimization which consists in studying the BF phenomenon for each interacting beam. This allows one to choose the best angular configuration of the incident beams in DPCM. The investigation is experimental, by recording the fanning patterns, and also theoretical, by interpreting numerically these patterns on the basis of the coupling gain maximization in the two-wave mixing theory. We focus on the influence of the incident beam width and incidence angle.

Double phase conjugate mirror in CdTe:Ge

Abstract The coherent oscillation in double phase conjugate mirror (DPCM) geometry at 1.064 μm with ac or dc electric field applied to CdTe:Ge sample is reported. The threshold of oscillations and the conversion efficiency are studied at different electric field, total light intensity and intensity ratio of the pump beams. The conversion efficiency up to 50% is reached with rather poor fidelity which can be improved by means of a cylindrical telescope. A new architecture of all optical interconnect is proposed. An auxiliary illumination is used to control the coupling strength of the sample, switching it either below or above the threshold of oscillation. A prototype of optical switch is built that can connect one emitter/receiver to two other emitters/receivers. A switching time less than 5 ms is achieved at the intensities of the recording and erasing beams less than 100 mW/cm 2 .

Two-zone double phase conjugate mirror

An analysis of photorefractive (PR) oscillators consisting of two facing four-wave mixing regions is carried out. The threshold and operation conditions of different phase conjugate mirrors are discussed with the help of the grating action method. In particular, the threshold and the operation of the two-zone double phase conjugate mirror (DPCM) is compared with other related PR oscillators, the interconnected ring, and different connected DPCM.

New approach of beam fanning in BaTiO3 to improve a DPCM

We propose here a new approach of a Double Phase Conjugate Mirror (DPCM) optimization in terms of stability and based on the study of the beam-fanning phenomenon for each interacting beam. This allows to chose the best angular configuration of both incident beams in DPCM. We took into account the influence of different parameters such as the power, the width and the angle of incidence of the laser beam for the two opposite entry faces perpendicular to the ferroelectric axis. In order to interpret the experimental data, we have developped a bidimensional model based on a Two Wave Mixing process, which is known to amplify the seed scattered by inhomogeneities of the crystal. The method is successfully used to determine the optimum DPCM configuration.

Insight into a new geometrical approach to beam fanning in BaTiO_3

The fanning behavior of the two interacting laser beams in a double phase-conjugate mirror (DPCM) obviously influences the characteristics of the mirror. We present a two-dimensional experimental study and model of the beam-fanning patterns obtained in nominally undoped and cobalt-doped (20 parts in 106)BaTiO3 samples. We propose a numerical method based on beam coupling theory that allows us to determine, for each crystal, which directions will develop significant fanning. We thus interpret the beam-fanning patterns by focusing on the influence of the incident beam’s width and incidence angle. The time evolution of the intensity in each of these directions is also studied and modeled. This model has been applied to DPCM to enable both incident beams to generate fanning in a common direction, with good results in terms of response time, efficiency, and stability.

Reciprocity and phase conjugation: application to the study of the double phase conjugate mirror

We develop a theoretical model that shows that the overlap integral of two counterpropagating waves is an invariant along propagation in a reciprocal optical system. We use this property to implement a method that allows a quantitative measurement of the conjugation fidelity of a phase conjugate mirror. Applied to the double phase conjugate mirror (DPCM) this invariant can be rewritten as an equality between the ratio of the conjugation fidelity of the two ports of the DPCM and the ratio of the phase conjugate transmission of the two beams. If restricted to a plane wave case this relation becomes the well known equality of the diffraction efficiencies in both directions. We use the model to implement an experimental characterization of the DPCM, measuring its conjugation fidelity, and demonstrating that it is a reciprocal optical system.

Polarization-independent double phase conjugate mirror

We present a polarization-independent double phase conjugate mirror (DPCM) working at telecommunication wavelength 1.32 μm. To do this, InP:Fe photorefractive crystals are used. A 2Z-DPCM configuration is realized in order to eliminate conical diffraction and to reduce the instabilities. With this device, the conjugation efficiency remains almost unchanged when the polarization state of the incident beam varies. Dynamic measurements demonstrate that the device is well-adapted to relatively slow variations of the polarization, induced by thermal variations for example.