Saturable absorbers for low power optical phase conjugation

Properties of a few saturable absorbers as candidates for low power optical phase conjugation (OPC) are examined. The specific systems studied are the dyes eosin, erythrosin B and Rose Bengal, doped in gelatin, polyvinyl alcohol and boric acid glass films. A method for quickly checking saturated absorption is suggested and demonstrated. Examples for OPC with these systems are cited and limitations are briefly discussed. An erratum to this article is available at .     

Low power phase conjugation in thin films of saturable absorbers

Organic dyes with high triplet yield are used as saturable absorbers with very low saturation intensity. Phase conjugation is performed in a thin film by a cw laser. The theory of degenerate four wave mixing in saturable absorbers is verified and imaging capabilities are demonstrated.     

Saturable absorbers with concentration-dependent absorption recovery time

The ground-state bleaching of highly concentrated rhodamine 6G solutions in methanol is studied with intense picosecond light pulses. The ground-state recovery time changes with concentration from about 3.9 ns at low concentration (10^–5 mol/dm³) to about 1 ps at high concentration (0.6 mol/dm³). The shortening of the absorption recovery time is determined by the concentration dependent quenching of theS ₁-state population due to unbound dimers and by the intensity dependent longitudinal and transverse amplified spontaneous emission.     

Partial compensation of Kerr nonlinearities by optical phase conjugation in optical fiber transmission systems without power symmetry

We investigate the suppression of Kerr nonlinearities by optical phase conjugation (OPC) in a fiber transmission system without power symmetry by including all the manifestations of the Kerr nonlinearities. We found that OPC cannot completely compensate for all Kerr nonlinearities in absence of power symmetry, and some nonlinearities may become even worse if one of the nonlinearities is fully compensated. Therefore, it is necessary to optimize the compensation system to eliminate the most dominating nonlinearity. Simulation results are presented to demonstrate the interplay of various kinds of nonlinearities. The need of proper design for transmission systems to suppress the dominating nonlinear effect is shown when all manifestations of Kerr nonlinearity are not compensated simultaneously.     

Effects of two-photon optical bistability upon phase conjugation

We have investigated phase conjugation by degenerate four-wave mixing, in circumstances where optical bistability can occur by two-photon resonant interaction. The solutions of the probe equation coupled with the two-photon equation of state have been obtained and studied. The two-photon phase-conjugated reflectivity is shown to exhibit multistability as well as hysteresis.     

Dispersion of characteristics of optical nonlinear response used for phase conjugation

The frequency dispersion and decay time dependence for the third-order nonlinear optical response in a substance and the dependence of its value on material parameters are analyzed on the basis of an anharmonic-oscillator approach for a variety of mechanisms of optical nonlinear response. Translated from Preprint of the P. N. Lebedev Physical Institute, No. 37 (1995).     

Non-reciprocal transmission via phase conjugation in multimode optical fibres

We demonstrate phase conjugate correction of modal distortion of a single mode beam on double-passing a passive highly multimode fibre. Phase conjugation is achieved in a self-pumped photorefractive BaTiO₃ crystal which conjugates only the e-polarised input component. We show that only a small fraction of the multimode fibre output is required to faithfully conjugate the single mode input beam. This technique enables the demonstration of a novel non-reciprocal transmission device, which for example has immediate application in gain-grating holographic resonator designs with multimode fibres as the active gain medium.     

向列相液晶中光学相位共轭技术的研究进展

对向列相液晶中光学相位共轭技术的研究进展进行了介绍,分析总结了向列相液晶中光学相位共轭过程的机理与实验方法,并对向列相液晶中的光学相位共轭技术的应用前景进行了展望。     

Compensation for the self-steepening effects in optical fiber communication system using midway optical phase conjugation

Effects of self-steepening (SS) of chirped Gaussian pulses on optical fiber communication system using midway optical phase conjugation (OPC) are analyzed. Dynamic evolution of the ultrashort pulses is simulated numerically. It is found that OPC cannot compensate for pulse waveform distortion due to SS.The initial chirp of pulses and dispersion can counteract SS and improve the compensation performance for the distortion.     

Optical backpropagation for fiber-optic communications using optical phase conjugation at the receiver

A fiber-optic system design with optical backpropagation that uses an optical phase conjugator, high-dispersion fibers, and highly nonlinear fibers is investigated. The proposed technique outperforms the midpoint optical phase conjugation and digital backpropagation with the same step size.     

偏振相移与相位共轭结合的散射光聚焦技术研究

散射介质对光的随机散射作用是制约其光学聚焦和成像的重要因素，光学相位共轭技术能够通过对散射光场共轭还原实现透过散射介质的光学聚焦和成像，其中散射光场相位的获取是共轭还原的核心。阐述了偏振相移的基本原理，将偏振相移与相位共轭技术相结合，设计了新的基于偏振相移的数字光学相位共轭系统。采用633 nm的HeNe激光器作为系统光源，毛玻璃作为散射介质开展散射光聚焦实验研究。实验结果表明:该装置能够成功实现透过散射介质的光学聚焦，其中聚焦点与背景光强的比值可达约400倍。     

Saturable optical resonators with distributed Bragg-reflectors

Saturable optical resonators with distributed Bragg reflectors are analyzed by using the mode coupling theory. Hysteresis and nonlinear characteristics are given as a function of the absorption coefficient, the length of the absorber region, and the coupling coefficient of the corrugated waveguide. The calculated results show that the saturable optical resonator is useful as an optical gate, optical amplifier, and optical memory in integrated optical circuits.     

Turbulence-aberration correction with high-speed high-gain optical phase conjugation in sodium vapor

Optical aberrations that are due to high-speed turbulence in the aero-optical regime are corrected with optical phase conjugation based on coherent population trapping in sodium vapor. Experimental measurements of an unheated, forced helium jet in air have demonstrated aberration correction by a factor of 7.8 at a forcing frequency of 18kHz with an optical power gain of 32.     

Optimization of compensation for high spatial frequency in distorted wavefront using optical phase conjugation

A method is proposed to optimize the recording structure of the photorefractive volume grating to compensate high spatial frequency in the distorted wavefront by optical phase conjugation. Based on the coupled-wave equation, the diffraction efficiency of the recorded grating formed by the scattered beams in different recording structures is simulated. The theoretical results show that the recorded modulations with high spatial frequency can be significantly improved in the small recording angle. In the experiment, three recording structures with the recording angles of 7.5°, 30°, and 45° are chosen to verify the compensation effect. Compared with the reconstructed image in the large recording angle of 45°, the signal to noise ratio of the image recorded at 7.5° increases to 3.2 times of that at 45°.     

Transmission performance of optical OFDM signals with low peak-to-average power ratio by a phase modulator

We have experimentally generated optical orthogonal frequency-division multiplexing (OOFDM) signals by a phase modulator (PM). The generated OOFDM signal can tolerant higher nonlinear effects in fiber than that generated by an intensity modulator because of its lower peak-to-average power ratio (PAPR). It is shown that, by using a PM, the PAPR of the OOFDM signal has a 2-3 dB reduction and the input power in fiber can be improved over 3 dB.     

Saturable absorbers based on tetrahedrally coordinated transition-metal ions in crystals (Review)

Published data on spectroscopic characteristics of saturable absorbers based on crystals doped with tetrahedrally coordinated transition-metal ions (Cr²⁺, Cr⁴⁺, Cr⁵⁺, V³⁺, Cr²⁺, Fe²⁺) in addition to their use as passive Q-switches for solid-state lasers emitting in the visible and near-infrared regions are reviewed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 5–50, January–February, 2009.     

Holography,coherent light amplification and optical phase conjugation with photorefractive materials

This paper presents a review of photorefractive effects in electro-optic materials and their applications. Materials properties giving rise to increased photo-sensitivities are discussed and compared with experimental data for a series of newer materials. The formation of volume holograms in these crystals, and accompanying optical effects such as beam coupling and phase conjugation, are discussed. Theoretical results derived from charge transport parameters are compared with our original data obtained in “dynamic holography” experiments with Bi₁₂GeO₂₀, Bi₁₂SiO ₂₀ and KNbO₃:Fe²⁺. The optimum parameters for applications such as volume hologram storage, coherent light amplification, optical phase conjugation etc. have been determined, and possible applications are discussed.     

基于光参量放大相位共轭特性的图像修复与增强

非线性光学相位共轭技术可将经过散射介质后产生畸变的光学波前进行修复.本文基于光参量放大(OPA)过程的光学相位共轭(OPC)特性,进行了光学相位共轭图像修复和增强的实验研究.基于大能量532 nm皮秒抽运激光和大口径非线性光学晶体KTiOPO_4(KTP)(Ⅱ类相位匹配),对经过牛奶乳浊液后已无法识别的1064 nm近红外光学图像,进行相位共轭修复,修复后的图像分辨率达12线/mm,此外,结合OPA过程的光学增益特性,实现了超过17 dB的光学图像增强,为现有三波混频光学相位共轭修复畸变所获图像增益的最大值.在此基础上,峰值信噪比较修复之前有160%的提升.考虑到光参量过程所具有的波长可调谐特性,在实际应用中,可根据需要,选择与生物组织的光学治疗窗口相匹配的成像波长,从而保证更长的穿透深度,提升生物组织成像和医学无损检测的效果.     

Suppression of Gordon-haus effect and interaction of dark soliton by optical phase conjugation

The optical phase conjugation is put forward for the first time to suppress the Gordon-Haus effect caused by amplifier spontaneous emission noise, and soliton interaction in the dark soliton transmission system. The simulation results show : If the conjugator is sited at proper situation, it can suppress the Gordon-Haus effect or the dark soliton interaction effectively.