Photorefractive Phase Conjugate 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.

High-gain and high-speed wavefront shaping through scattering media.

Wavefront shaping (WFS) is emerging as a promising tool for controlling and focusing light in complex scattering media. The shaping system's speed, the energy gain of the corrected wavefronts, and the control degrees of freedom (DOF) are the most important metrics for WFS, especially for highly scattering and dynamic samples. Despite recent advances, current methods suffer from trade-offs that limit satisfactory performance to only one or two of these metrics. Here, we report a WFS technique that simultaneously achieves high speed, high energy gain, and high control DOF. By combining photorefractive crystal-based analog optical phase conjugation (AOPC) and stimulated emission light amplification, our technique achieves an energy gain approaching unity, more than three orders of magnitude larger than conventional AOPC. The response time of ~10 μs with about 106 control modes corresponds to an average mode time of about 0.01 ns/mode, which is more than 50 times lower than some of the fastest WFS systems to date. We anticipate that this technique will be instrumental in overcoming the optical diffusion limit in photonics and translate WFS techniques to real-world applications.

フォトリフラクティブ位相共役鏡を用いた高出力ピコ秒ホログラフィックレーザー

フォトリフラクティブ位相共役鏡を用いた高出力ピコ秒ホログラフィックレーザー

Strong lowering of the mirrorless optical oscillation threshold by angular mismatches for nonlocal photorefractive nonlinearity

We show that the introduction of an angular mismatch for the pump waves results, in the case of nonlocal photorefractive nonlinearity, in a strong almost twofold decrease of the threshold value of the coupling strength for the mirrorless optical oscillation. This surprising feature will lead to a strong modification of the threshold and near-threshold behavior of a vast variety of optical oscillators based on the photorefractive phase conjugation and involving finite-size light beams.

High Power, High Quality Phase Conjugate Vanadate Lasers

We presented high quality, high power pico-second output from a diode-side-pumped Nd: YVO4 laser system with a photorefractive phase conjugate mirror. Peak output powers in the range of 2.8-6.8MW were achieved at a pulse repetition frequency range of 0.33-1.0MHz. The corresponding average powers of 18-26W were obtained.

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.

Over 10-watt pico-second diffraction-limited output from a Nd:YVO4 slab amplifier with a phase conjugate mirror

We have produced a high beam quality pico-second laser based on a continuous-wave diode pumped Nd:YVO4 slab amplifier with a photorefractive phase conjugate mirror. 12.8W diffraction-limited output with a pulse width of 8.7ps was obtained.

The study of bridge phase conjugation in photorefractive Tb:Cu:KNSBN crystal

Photorefractive bridge phase conjugation is studied theoretically by two-region coupled-wave theory. The dependence of phase conjugate reflectivity and transmissivity on the input intensity ratio is measured. When two incoherent beams symmetrically enter opposite Tb:Cu:KNSBN crystal faces, the maximum reflectivities are higher than 351%, 236% and 147% for incident angles of 17.5°, 27.5 and 39° respectively; we also obtain a transmissivity higher than 58% for an incident angle of 33.5°. A comparison between the experimental data and theoretical results is also given.

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.

Enhancement of the switched phase conjugate reflectivity in a BaTiO3 crystal

Abstract We experimentally investigated the response of the photorefractive phase conjugation to a pulsed reading beam in an undoped BaTiO 3 crystal with a four-wave mixing geometry. A single longitudinal-mode Ar + laser was used as a light source. The reading beam with a pulse width of ∼1 s was switched on after writing a steady-state grating in the crystal by two recording beams under a suitable condition and the generated phase conjugation was measured. The phase conjugate output was found to be increased by some factors of magnitude over the initial value for a given peak power and period of the reading pulses. A maximum of the enhanced reflectivity can be controlled by adjusting the interacting beam powers. An increase in the diffraction efficiency was also found with the same read-out process, which plays a key role to increase the phase conjugation as our knowledge.

Multi-watt diffraction-limited picosecond pulses from a diode-pumped Nd:YVO4amplifier with a photorefractive phase conjugate mirror

We demonstrate>4 W and 7.2 ps output of a double-pass Nd:YVO4 slab amplifier with a photorefractive ring phase conjugate mirror. A double-pass gain of 190 and optical extraction efficiency of >20% were obtained. The amplified signal beam exhibits a TEM00 profile with beam propagation parameters Mx2<1.4 and My2<1.1.

High Quality 7.5 W Continuous-Wave Operation of a Nd:YVO4 Laser with a Rh:BaTiO3 Phase Conjugate Mirror

Laser oscillation of a diode-pumped Nd:YVO4 phase conjugate resonator with a photorefractive ring phase conjugate mirror has been demonstrated. A 7.5 W continuous-wave (CW) output with high beam quality was obtained. Slope efficiency of 46% and optical extraction efficiency of 33% were achieved. Standard variance of the output power fluctuation was 2.3%.

Photorefractive Materials and Their Applications. High-Power Nd:YAG laser Using Ring Self-Pumped Phase Conjugate Mirror.

A LD-pumped sigzag slab Nd:YAG oscillator-amplifier system using a photorefractive ring self-pumped phase conjugate mirror is developed. The photorefractive phase conjugator is optimized in the point view of high-energy pulse capacity, fidelity of phase conjugation and stability. The laser system generates the IR laser pulses of 600 mJ and the second harmonic pulses of 250 mJ at the repetition rate of 100 Hz. Both the IR and the second harmonic have 1.1 times diffraction limited beam quality.

Photorefractive Materials and Their Applications. High Quality Output of a Broad-Stripe Laser Diode Using a Photorefractive Phase Conjugate Mirror.

A highly bright radiation from a single-stripe high power laser diode with a photorefractive phase conjugate mirror was presented. The laser diode was then operated in a narrow-band and generated a single-lobed far-field.

Two-way two-dimensional pattern transferring upon requesting with BaTiO3 crystal

We propose a setup which is capable of transferring two-dimensional (2D) pictures from one plane to another in either way upon the requesting of the plane which wishes to obtain information from the other one. Since this picture transferring is purely optical, it is free of electromagnetic interference. The setup consists of a high-gain photorefractive BaTiO 3 crystal, and the crystal is set in a configuration of a Cat conjugator. The working principle of the setup relies on the mechanism of photorefractive self-pump phase conjugation and four-wave mixing.

Multi-watt CW output from a double-pass diode side-pumped Nd:YVO 4 amplifier with a Rh:BaTiO 3 phase conjugator

We demonstrate a 7.8 W high beam quality continuous-wave output of a double-pass Nd:YVO 4 slab amplifier with a photorefractive ring phase conjugate mirror. Optical extraction efficiency of 33% was obtained. Build-up time of the output of the amplifier was 10 s. Standard variance of the output power fluctuation was 2.6%.

High power solid-state laser with photorefractive phase conjugate mirror

High power solid-state laser with photorefractive phase conjugate mirror

Compression and broadening of phase-conjugate pulses in photorefractive self-pumped phase conjugators

Pulse propagation and shaping are investigated in photorefractive self-pumped phase conjugators in both transmission- and reflection-grating regimes. The dispersion properties of self-pumped phase conjugators are analyzed by taking into account both the grating dispersion and the angular dispersion. The complex transfer functions are obtained by treating the crystal as a linear dispersive medium. We show that the pulse width as a result of the self-pumped phase conjugation is much wider in the reflection regime than in the transmission regime. The experimental results are consistent with the results calculated for the transmission-grating regime, indicating that this type grating is the dominant mechanism in the case of a femtosecond self-pumped phase conjugator.

Origin of frequency shift and temporal instability in photorefractive self-pumped phase conjugators

The physical origin of frequency shift and dynamic instability in a photorefractive self-pumped phase conjugator is analyzed by use of four-wave mixing and multi-four-wave mixing. We show that the frequency shift and the instability result from the requirement of the maximal energy transfer between the interacting beams. Two examples of numerical results in a cat self-pumped phase conjugator are given, which are in good agreement with previous experimental results. A simple method is proposed to change an unstable phase-conjugate output to a stable one.

Stabilization of unstable steady states in photorefractive phase conjugators

A semiderivative method, which consists of illumination of a photorefractive medium with a weak beam that is incoherent with the interacting beams, is described for stabilization of unstable steady states in photorefractive phase conjugators, such as an externally pumped four-wave mixer, a semilinear self-pumped phase conjugator, and a double phase conjugator.