光纤通信系统中基于光学相位共轭和预啁啾的色散及非线性补偿研究

采用光学相位共轭补偿光纤通信系统的色散及非线性必须满足一个前提条件,即相位共轭器两边线路上的色散和非线性分布(或传输功率分布)必须严格对称,这在现有的一般传输线路（标准单模光纤加集总掺铒光纤放大器）难以实现。提出了预啁啾结合中距相位共轭的补偿方案,并进行了数值计算。结果表明,通过在发送端对输入脉冲进行预啁啾展宽,可有效地减轻非线性效应与色散的相互作用,获得理想的补偿效果;对于皮秒超短光脉冲传输,脉冲内拉曼散射相对于三阶色散对补偿结果的影响很小,因此,频域相位共轭相对于时域相位共轭具有更好的综合补偿性能。该方案简单易行,无需对已敷设好的线路作较大改动。     

利用谱共轭技术补偿光纤传输信号的失真

分析了谱共轭技术用于补偿光纤色散和非线性的基本原理。通过对超高斯脉冲的数值计算表明,谱共轭技术（SPC）较时域相位共轭技术（TPC）能更好地恢复波形失真。为了弥补SPC不能补偿脉冲内拉曼散射的缺点,提出将SPC和TPC结合使用的方法。将其作用于光孤子串脉冲,不仅进一步优化了整形效果,而且还能有效抑制孤子自频移。     

Coherent underwater communication using passive time reversal over multipath channels

Passive time reversal exploits underwater acoustic channels’ spatial and temporal diversity. It can refocus multipath propagated signal at the receiver and can be realized simply by the passive phase conjugation (PPC) method. By the temporal focusing, time delay spread caused by multipath propagation is mitigated for spectral efficient coherent communications. However, the PPC method is unable to eliminate multipath and is limited by channel variations. An adaptive equalizer is therefore needed to compensate residual multipath after refocusing and to track channel temporal variations. Spatial diversity is obtained by using a vertical receiving array. In this paper we used 4-hydrophone array and demonstrated that the adaptive decision feedback equalization in conjunction with PPC significantly decreased the bit error rate.     

Experimental Study of Four-Wave Mixing in Semiconductor Laser Amplifier and its Application to Dispersion Compensation in Mid-Span Spectral Inversion System

Properties of four-wave mixing (FWM) in the semiconductor laser amplifier are studied experimentally in this paper. The optical phase conjugation of the mixing wave is applied to compensate the chromatic dispersion of transmission fiber. It is presented experimental results of the transmission of short optical pulses with 12-ps width over 204 km standard single mode fiber. The results confirm that mid-span spectral inversion (MSSI) employing FWM in a semiconductor laser amplifier is one of potential dispersion compensation techniques for optical transmission over standard single mode fiber with bit rate beyond 40Gb/s.     

Experimental Study of Four Wave Mixing in Semiconductor Laser Amplifier and its Application to Dispersion Compensation in Mid Span Spectral Inversion System

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基于高阶色散管理和相位共轭技术的色散补偿

相位共轭技术能够同时且高效地补偿二阶色散及非线性效应，且该技术同信号比特率、调制方式无关，是最有前景的色散补偿技术之一。理论分析了在高阶色散作用下，超短高斯脉冲信号在中距相位共轭通信系统中的传输演化特性，数值模拟了在二阶、三阶和四阶色散作用下，飞秒高斯脉冲信号在基于中距相位共轭技术的光纤色散管理链中的动态传输过程。结果表明，相位共轭技术和高阶色散管理相结合，不仅可以补偿和复原包括奇数阶和偶数阶色散在内的全部色散和非线性所引入的信号失真和畸变，而且能够减弱时分复用系统中脉冲之间的相互作用，使得信号在传输一个周期后恢复波形，从而提高了相位共轭系统对失真信号的补偿性能。     

Optimal packaging of dispersion-compensating fibers for matched nonlinear compensation and reduced optical noise

A method of packaging dispersion-compensating fibers (DCFs) is discussed that achieves optimal nonlinearity compensation and a good signal-to-noise ratio simultaneously. An optimally packaged dispersion-compensating module (DCM) may consist of portions of DCFs with higher and lower loss coefficients. Such optimized DCMs may be paired with transmission fibers to form scaled translation-symmetric lines that could effectively compensate for signal distortions due to dispersion and nonlinearity, with or without optical phase conjugation.     

时间位相调制对高强度三次谐波转换的影响

在惯性约束聚变研究中,为了利用光谱色散进行光束匀滑以及抑制KDP晶体等大口径光学元件中的横向受激布里渊散射,通常需要对激光脉冲进行时间位相调制.详细讨论了高功率条件下KDP晶体Ⅰ/Ⅱ角度失谐三倍频方案中,时间位相调制对三次谐波转换效率和脉冲频谱的影响.研究结果表明,时间位相调制会导致三次谐波转换效率明显下降.     

Dispersion compensation for a femtosecond self-pumped phase conjugator

Dispersion compensation for a photorefractive self-pumped phase conjugator with femtosecond pulses is analyzed. The self-pumped phase conjugator consists of a pair of dynamic gratings coupled by total internal reflections at the crystal surfaces (cat conjugator). The negative angular dispersions of refraction at the air-crystal interface and the gratings inside the crystal compensate for the positive dispersion of the finite crystal path. The experimental results show that with partial dispersion compensation the width of the self-pumped phase conjugation at 450 nm of femtosecond pulses is narrower than that of the transmitted pulses.     

Temporal contrast improvement in chirped pulse amplification systems by a four-grating compressor and by spectral modifications

The quest for higher peak focused intensities and better temporal contrast drives one to improve the design of all possible stages of the chirped pulse amplification (CPA) system. In this paper, we have analyzed the role of dispersion and spectral profile on the temporal shape and contrast ratio of the output pulse of a CPA system. The simulations indicate that an initial sech² or a Gaussian pulse in the CPA system is best for a good contrast ratio. Incorporating a four-grating based pulse compressor in the CPA system improves the contrast as well as provides the flexibility to compensate the dispersion upto the fourth order. The simulations also detail the effect of spectral profile tailoring on the contrast ratio and peak power.     

Transmission of 10 GHz,3.5 ps Optical Pulses Over 100-km Standard Fiber Using Mid-span Spectral Inversion

Transmission of 3.5ps optical pulses over 100-km standard fiber was demonstrated by employing phase conjugation in a semiconductor optical amplifier to compensate the chromatic dispersion.The pulses were broadened to 5 ps after transmission,which confirms the applicability of this technique to ultra high-bit-rate system beyond 40 Gb/s.     

Transmission of 10 GHz, 3.5 ps Optical Pulses Over 100-km Standard Fiber Using Mid-span Spectral Inversion

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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.     

Tunable sub-10-fs ultraviolet pulses generated by achromatic frequency doubling

Tunable UV pulses shorter than 10 fs are generated by achromatic frequency doubling of a noncollinear optical parametric amplifier. With a suitable two-prism sequence we achieve first- and second-order achromatic phase matching and increase the natural bandwidth of the nonlinear crystal by a factor of 80. Extremely broad UV spectra with a Fourier limit of 2.9 fs are generated in a 360-microm-thick beta-barium borate crystal at a conversion efficiency of 20%. We compensate for the angular dispersion and the first-order chirp of the highly stable UV pulses with a second prism sequence and fully characterize the temporal pulse shape with zero-additional-phase spectral phase interferometry for direct electric-field reconstruction (ZAP-SPIDER). Pulses as short as 7 fs are generated by controlling the higher-order chirp with a deformable mirror.     

利用消衍射透镜列阵及光谱色散平滑实现焦斑均匀辐照

提出将空间域的透镜列阵法和时间域的光谱色散平滑法结合起来实现靶面的均匀辐照. 消衍射型透镜列阵能获得边缘陡峭且顶部较平坦的准近场焦斑，光谱色散平滑则能有效地抹平焦斑内部由多光束干涉引起的细密条纹. 数值结果显示，通过该方案能获得均匀性较好的焦斑. 进一步分析了光谱色散平滑单元中位相调制和光栅的参数对辐照均匀性的影响，发现参数的选取要在焦斑均匀性和能量利用率之间取得合理平衡，以在整体上获得最佳的均匀辐照效果. 关键词： 光谱色散平滑 透镜列阵 衍射 多光束干涉     

Optical phase conjugation and pseudolinear transmission

We describe the use of optical phase conjugation (OPC) to suppress intrachannel nonlinearities that limit pseudolinear transmission. We show that OPC combined with appropriate dispersion mapping is effective in suppressing intrachannel nonlinearities, even in the absence of signal-power evolution symmetry that is generally required by OPC to compensate fiber nonlinearity. An increase in signal power by 5.5 dB is observed when a single OPC is used for 40-Gbit/s pseudolinear transmission over 32 x 100 km of passive-fiber spans.     

啁啾补偿的折返点匹配二倍频

提出了一种啁啾补偿群速色散的倍频方法,通过向入射基频光引入合适的初始啁啾,让其与色散相互作用,以实现对基频光脉冲宽度的主动控制,提高转换效率。研究结果表明,这种方法能显著地提高倍频转换效率,以氘含量12.6%(摩尔百分比)的KD*P晶体对脉宽30fs,中心波长为1.053μm的宽带基频光的折返点匹配宽带二倍频过程为例,当基频光转换极限位置处于晶体中心时,能取得最佳的啁啾补偿效果,转换效率可提高近22%。进一步研究了转换效率和转换带宽与晶体长度的关系。     

高码率光孤子传输系统中用光相位共轭补偿高阶扰动

在由分布式掺铒光纤级联的高码率传输系统中存在不可忽略的高阶扰动,它们引起孤子自频率漂移和孤子变形。本文用光相位共轭补偿高阶扰动,结果表明:由分布式掺铒光纤级联的高码率传输系统中,适当选取光相位共轭周期长度,可以完全消除高阶扰动引起的孤子自频率漂移和孤子变形。     

PMD compensation via real-time phase retrieval from spectral interference

We present a technique for pulse recovery based on real-time measurement of the differential optical phase spectrum from spectral interference patterns. Using a phase retrieval algorithm we can obtain accurate all order polarization mode dispersion (PMD) information for the optical signal and correspondingly compensate the impairment in optical transmission lines. Linear PMD is accurately extracted from measurements, and analytical simulations show recovery of pulses distorted by higher order PMD.     

Adaptive control of pulse phase in a chirped-pulse amplifier

Using experimental feedback, we demonstrate that a chirped-pulse amplifier can adaptively learn to compensate for the higher-order phase dispersion that is inherent in the amplification process. A genetic algorithm-based search routine is used to repetitively update the pulse phase in a programmable pulse stretcher during a plasma breakdown experiment to maximize the magnitude of spectral blueshift. Reductions in pulse duration from 37 to 30 fs and substantially better wing structure are typically obtained as a result of the optimization.