Self-pumped Phase Conjugation Research Articles

Automatic turbulence mitigation for coherent free-space optical links using crystal-based phase conjugation and fiber-coupled data modulation.

There are various performance advantages when using temporal phase-based data encoding and coherent detection with a local oscillator (LO) in free-space optical (FSO) links. However, atmospheric turbulence can cause power coupling from the Gaussian mode of the data beam to higher-order modes, resulting in significantly degraded mixing efficiency between the data beam and a Gaussian LO. Photorefractive crystal-based self-pumped phase conjugation has been previously demonstrated to "automatically" mitigate turbulence with limited-rate free-space-coupled data modulation (e.g., <1 Mbit/s). Here, we demonstrate automatic turbulence mitigation in a 2-Gbit/s quadrature-phase-shift-keying (QPSK) coherent FSO link using degenerate four-wave-mixing (DFWM)-based phase conjugation and fiber-coupled data modulation. Specifically, we counter-propagate a Gaussian probe from the receiver (Rx) to the transmitter (Tx) through turbulence. At the Tx, we generate a Gaussian beam carrying QPSK data by a fiber-coupled phase modulator. Subsequently, we create a phase conjugate data beam through a photorefractive crystal-based DFWM involving the Gaussian data beam, the turbulence-distorted probe, and a spatially filtered Gaussian copy of the probe beam. Finally, the phase conjugate beam is transmitted back to the Rx for turbulence mitigation. Compared to a coherent FSO link without mitigation, our approach shows up to ∼14-dB higher LO-data mixing efficiency and achieves error vector magnitude (EVM) performance of <16% under various turbulence realizations.

Image fidelity improvement in digital holographic microscopy using optical phase conjugation

With respect to digital holography, techniques in suppressing noises derived from reference arm are maturely developed. However, techniques for the object counterpart are not being well developed. Optical phase conjugation technique was believed to be a promising method for this interest. A 0°-cut BaTiO3 photorefractive crystal was involved in self-pumped phase conjugation scheme, and was employed to in-line digital holographic microscopy, in both transmission-type and reflection-type configuration. On pure physical compensation basis, results revealed that the image fidelity was improved substantially with 2.9096 times decrease in noise level and 3.5486 times increase in the ability to discriminate noise on average, by suppressing the scattering noise prior to recording stage.

Discovery of a self-pumped, phase-conjugate mirror with high speed, high image quality, and large accepted incidence area

A study for the incidence geometry to extend the accepted incidence position is presented and demonstrated. High-speed and high-quality self-pumped phase conjugate mirrors (SPPCM) can be formed with a counter-direction incidence with respect to the master light for the Cat-SPPCM. In addition to a counter-directional Kitty-SPPCM, when the incidence position is changed, a Kitty-SPPCM with high-pass filtering and a different SPPCM similar to the Bridge-SPPCM can be found. The discovery of these three kinds of optical phase conjugators extends the accepted incidence position range and angle, helpful for applying the SPPCM in various new applications.

All-solid-state Nd:YAG lasers with self-pumped multiwave-mixing phase conjugate cavities

Several configurations of the laser self-pumped phase-conjugate cavities with multiwave mixing directly in the laser medium based on a single Nd:YAG laser element pumped by multikilowatt laser diode 2D stacks are experimentally studied and compared. In the laser configurations with six- and eight-wave mixing there are generated a high-intensive self-Q-switched 200-ns laser pulse followed by low-intensive free-running lasing. An increase of the number of waves taking part in multiwave mixing from six to eight leads to increase in the laser maximum output energy and optical-to-optical efficiency from 0.55 up to 1.1 J and from 8.3 up to 18% respectively. In the laser configuration with ten-wave mixing the energy characteristic is similar to the same in the laser configuration with eight-wave mixing, but the temporal and spectral characteristics are different from the same in the laser configurations with six- and eight-wave mixing. In last laser configuration not only the first pulse, but also all the train of pulses is generated in high-intensive TEM00 diffraction-limited (M 2 = 1.3---1.5) single-frequency self-Q-switched mode via writing and erasing the gain gratings by the intracavity radiation.

Self-pumped phase conjugation of light beams carrying orbital angular momentum

We investigate the properties of angular momentum carrying vortex beams, reflected by a phase-conjugating mirror. It is shown that a self-pumped photorefractive phase-conjugating mirror is suitable to produce stable, high-fidelity phase conjugation of vortex beams. We prove that the topological charge of the vortex beam is maintained, and thus the angular momentum in the laboratory frame of reference is reversed, as it is expected by the time reversal property of the phase-conjugating mirror. The three dimensional interference pattern in front of the phase-conjugating mirror is studied and applications in optical traps are suggested.

Beam coupling in photorefractive liquid crystal light valves

Photorefractive liquid crystal light valves (LCLVs) are hybrid devices that combine a nematic liquid crystal layer with a thin monocrystalline Bi12SiO20 (BSO) photorefractive crystal in the form of a cell wall. The device behaves as an optically addressed spatial light modulator, where the photoconductive layer is made of the BSO crystal. Differently from conventional types of spatial light modulators, usually working in retroreflective configuration, the photorefractive light valves work in transmission, thus allowing new applications related to the coupling of the optical beams when they pass through the liquid crystal layer. Here, we review some recent experiments of beam coupling in photorefractive LCLVs. After a characterization of the device in terms of its spatial resolution, which is related to the features of pattern formation in an optical feedback configuration, we present two-beam coupling and optical amplification in single pass experiments. Then, we develop a theoretical model by taking into account the Raman–Nath diffraction of the incoming beams over the thin liquid crystal layer. By using two or more light valves in cascade, we show, both experimentally and theoretically, that the two-wave mixing gain can be enhanced by the Talbot effect related to the multi-passage of the beams through the successive layers of the nematic liquid crystal. Finally, we show that self-pumped phase conjugation can be realized by placing the light valve in a tilted feedback configuration. In this case, the four-wave mixing is spontaneously created through the scattering of the signal beam onto the feedback induced grating.

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.

A NOVEL TYPE OF OPTICAL OSCILLATOR BASED ON A LIQUID CRYSTAL GAIN MEDIUM

A nonlinear optical medium results by the collective orientation of liquid crystal molecules tightly coupled to a transparent photoconductive layer made of a thin photorefractive BSO crystal. For this new kind of liquid crystal light-valve, we have recently demonstrated signal beam amplification and self-pumped phase conjugation. After giving a description of the device and its properties, we show here that the liquid crystal light-valve, when inserted in a ring optical cavity, has a gain large enough to overcome the losses, thus resulting in unidirectional field oscillation. The wide transverse size and the high nonlinearity of the liquid crystal medium allow us to explore dynamical regimes where a huge number of longitudinal and transverse modes are simultaneously amplified in the cavity. Up to date, such regimes have remained inaccessible for all the known types of optical cavities. In particular, we report the observation of high amplitude spatiotemporal pulses appearing in random space points and confined along the three space directions. Then, we present a theoretical model that takes into account the longitudinal dependence of the field in the liquid crystal optical oscillator, and show that spatiotemporal pulses confined in 3D space directions arise from the random superposition of many longitudinal and transverse modes oscillating at different frequencies.

Self-pumped phase conjugation in a liquid crystal light valve with a tilted feedback mirror

We report self-pumped phase conjugation in a photorefractive liquid crystal light valve with a tilted feedback mirror. This is a new scheme for the self-generation of a phase-conjugate replica and can in general be applied to thin nonlinear layers, opening the way to distortion-correction applications of thin media.

Preliminary training of a self-pumped loop phase-conjugate mirror based on a photorefractive crystal

It is shown by the example of a loop self-pumped phase-conjugate (SPPC) mirror based on a photorefractive crystal (PRC) BaTiO3 that formation of a phase-conjugate (PC) wave in a SPPC mirror can be considerably accelerated by using a preliminary training of the mirror. For this purpose, it is necessary to direct preliminary an auxiliary (training) optical field on the SPPC mirror, which contains some information on the properties of the input signal whose wave front will be conjugated later. This procedure provides the writing of static refractive-index gratings in the PRC already at the training stage. The presence of these gratings ensures a much more rapid (by 6–20 times) production of volume refractive-index gratings required for the efficient conjugation of the signal radiation. Several variants of static and dynamic SPPC mirror training procedures are simulated and their efficiencies are compared.

Photorefractive dynamic holography using self-pumped phase conjugate beam

Dynamic holography in photorefractive materials using self-pumped phase conjugate beam of the object beam itself as the other writing beam is proposed. Our detailed theoretical analysis shows four-fold increase in the diffraction efficiency of dynamic holograms if recorded using this geometry even in photorefractive crystal like BTO (having low optical activity) without applying external field. Detailed theoretical analysis is given.

Highly efficient phase-conjugation of a 1 µm pico-second Laguerre-Gaussian beam

We have demonstrated highly efficient Laguerre Gaussian beam generation by using a ring self-pumped phase conjugate mirror in the pico-second regime for the first time. The phase conjugate reflectivity was typically ~55%. We have also investigated the conservation of optical angular momentum.

Wavelength dependence of visible and near-infrared photorefraction and phase conjugation in Sn_2P_2S_6

The photorefractive properties of Sn2P2S6 crystals in the wavelength range from 633to1064 nm are investigated. Conventional yellow and modified brown crystals with a variation of nonstoichiometric defects are examined. Brown crystals respond much faster and exhibit substantially higher photorefractive gain than yellow crystals, up to 18 cm−1 at 780 nm and 8.5 cm−1 at 1064 nm. Ring-cavity self-pumped phase conjugation is demonstrated using both types of crystal. The phase-conjugate response of brown Sn2P2S6 is 2 orders of magnitude faster than in yellow Sn2P2S6 or Rh-doped BaTiO3, with a grating recording time below 50 ms for 30 mW power at 860 nm. Various thresholding effects are analyzed to determine the optimal wavelength range for the two types of crystal. We find an optimum in the range of 650–950 nm for yellow and in the range of 850–1100 nm for brown Sn2P2S6.

Sn2P2S6 Crystals for Fast Near-Infrared Photorefraction

Optical and physical properties of Sn 2 P 2 S 6 ferroelectric crystals suitable for photorefractive applications in the red and near-infrared spectral range are reported. The main advantage of Sn 2 P 2 S 6 is its high beam-coupling gain combined with considerably faster response in the near infrared compared to conventional oxide-type photorefractive materials. We show that modifying the growth conditions and doping with Te can efficiently improve the photorefractive properties of Sn 2 P 2 S 6 . Photorefractive two-beam coupling gain at 860 nm increases from about 2 cm − 1 in conventional “yellow” Sn 2 P 2 S 6 to about 7 cm − 1 in Te-doped samples and 14 cm−1 in modified “brown” samples. We examine the self-pumped optical phase conjugation properties of Sn 2 P 2 S 6 in a ring-cavity scheme. Phase-conjugate signals with amplitudes close to the theoretical limit and rise times of about 10 ms for 4 W/cm 2 pump intensity at 780 nm and 50 ms for 2.5 W/cm 2 pump intensity at 860 nm are obtained. *Present Address: Laboratoire Matériaux Optiques, Photonique et Systèmes (LMOPS), University of Metz and Supélec, F-57070 Metz, France.

Self Pumped Optical Phase Conjugation at 1.06 µm in Te-doped Sn2P2S6

We demonstrate self-pumped optical phase conjugation in Te-doped Sn2P2S6, a semiconducting ferroelectric crystal, using a 1.06 microm wavelength cw Nd:YAG laser. The photorefractive gain of this crystal has been increased to Gamma = (3.9+/- 0.4) cm-1 by Te doping. We observed self-pumped optical phase conjugation in a ring cavity scheme with phase conjugate reflectivities of more than 40 percent and a very fast phase conjugate rise time below 100ms at a light intensity of 20 W/cm2. This is more than two orders of magnitude faster than in any other photorefractive crystal, as e.g. in Rh-doped BaTiO3.

Grating Decay in Reduced Barium Titanate Crystals

Under inhomogeneous illumination, a refractive index grating will form in a barium titanate crystal through the photorefractive effect. Due to its large electro-optic coefficients and self-pumped phase conjugation, a photorefractive barium titanate crystal is of a special interest for many applications such as optical signal processing, optical phase conjugation and optical information storage. In general, the grating stored in a barium titanate crystal will fade in the dark soon. That is a barrier for the applications of optical information storage. Here we made a series of reduced BaTiO3 crystals and investigated the optical absorption, the dark decay and the light-induced decay. We found that the decay time of the grating can be lengthened by the reduction treatment. The grating can be kept in the crystal reduced around compensation point for more than a month. At high intensity, the speed of writing/erasing a grating can be determined by the optical absorption.

Observation of strong stimulated photorefractive scattering and self-pumped phase conjugation in LiNbO3:Mg in the ultraviolet

We report on the observation of diffusion-dominant photorefraction and light-induced nonlinear forward and backward scattering in highly Mg-doped LiNbO3 at 351 nm. We also demonstrate what we believe to be the first continuous-wave self-pumped phase conjugation via stimulated photorefractive backscattering in the ultraviolet.

Self-pumped phase conjugation in microspheres

When a microscopic sphere is illuminated by a laser beam, interference fringes inside the sphere lead through the Kerr effect to a holographic grating. This grating is capable of phase conjugation, which is particularly strong when optical resonance takes place. According to numerical simulation, based on the Green function method, phase-conjugation reflectivity R = 0.5 can be achieved in a single silica sphere of approximately 9-microm diameter with realistic laser power. Such spheres can be aligned together to become a phase-conjugation mirror of large area and high reflectivity.

Highly Efficient Mutually Pumped Phase Conjugators Using BaTiO3:Ce

High-efficiency mutually pumped phase conjugation (MPPC) has been realized in a BaTiO $_3$ :Ce crystal in the bridge geometry. The laser source is a dye laser operating from 580 to 650 nm. We have observed that, with a beam ratio of 1, self-pumped phase conjugation (SPPC) appears and competes with MPPC at short wavelengths while it disappears at long wavelengths. When the beam ratio is slightly higher than 1, SPPC no longer occurs. Phase conjugate reflectivity near 500% has been obtained at a beam ratio of 70. Our experiments demonstrate that Ce-doped BaTiO $_3$ crystals, which have already been found efficient in SPPC experiments, are also efficient for the generation of MPPC.

Dark decay behaviors of photorefractive gratings in self-pumped phase conjugation with Ce:BaTiO 3 at 532 and 790 nm wavelengths

Dark decay behaviors of photorefractive gratings in self-pumped photorefractive phase conjugation (SPPC) with Ce:BaTiO 3 are studied by measuring the decay of SPPC reflectivities at 532 and 790 nm wavelengths at room temperature. Four-wave mixing by stimulated photorefractive backscattering (SPB-FWM) mechanism and four-wave mixing by total internal reflection (TIR-FWM) mechanism coexist in SPPC at each wavelength. SPB-FWM dominates in SPPC at 532 nm whereas the reverse is true at 790 nm. The stronger the effect of SPB-FWM is, the faster the SPPC grating decays. It is found that two deep traps, i.e., Fe-ion deep trap and Ce-ion deep trap, both make contributions to the SPPC grating. It is also deduced that SPB-FWM forms the charge grating in Fe-ion deep trap, while TIR-FWM builds up the charge grating in the Ce-ion deep trap.