啁啾光脉冲的自相位调制效应对压缩光脉冲的影响

 在超强激光系统中，对比度和本底宽度是压缩光脉冲极为重要的指标。在啁啾脉冲放大系统的压缩阶段，在啁啾脉冲宽度为141ps，啁啾系数为1000，B积分值分别为0，1和2的情况下，模拟了自相位调制效应（SPM）对压缩脉冲的峰值光强、脉冲宽度和本底宽度的影响。结果表明：在B积分值为2的条件下，若不补偿非线性色散，压缩脉冲的峰值强度降为无自相位调制效应时的65%左右，脉冲宽度约为种子脉冲宽度的2倍，本底宽度则增加到原来的3倍左右；在B积分值大于0.5的情况下，本底宽度和B积分值近似成线性关系。     

LC-SLM像素型结构对4f脉冲整形系统的影响

液晶空间光调制器的离散型相位调制和像素间隙是4f脉冲整形系统的主要失真原因,基于以上因素的分析,建立了完善的4f脉冲整形系统数学模型,数值模拟了不同相位调制变化率、不同像元尺寸、不同间隙尺寸以及不同色散关系下的离散型相位调制和像素间隙导致的失真情况。模拟结果表明:离散型相位调制会导致整形脉冲中出现副本脉冲,相位调制变化率越大或像元尺寸越大均会导致这种副本脉冲越强,输出目标脉冲失真越严重;像素间隙也会导致整形脉冲中出现副本脉冲,间隙尺寸越大会导致这种副本脉冲越强,输出目标脉冲失真越严重;两种副本脉冲均呈等间隔分布,其周期由每个像元对应的频谱宽度决定,非线性光谱色散会使副本脉冲啁啾化且强度变小。     

高斯脉冲在半导体光放大器中传输的解析表征

采用解析方法,在考虑材料损耗和色散的情况下,详细研究了无啁啾高斯脉冲和啁啾高斯脉冲在半导体光放大器中传输的物理过程,分析了强度增益、脉冲宽度和频率啁啾与线宽增强因子、色散系数、小信号增益特征参数及初始啁啾之间的关系。结果表明:当输入变换极限的高斯脉冲时,色散会引起增益压缩,脉冲展宽和频率啁啾;同样情况下,线宽增强因子越大,脉宽加宽越明显,输出脉冲啁啾越大,且随着线宽增强因子的增大,输出脉冲啁啾极大值向特征参数值较小的一边移动。当输入啁啾高斯脉冲时,初始脉冲啁啾越大,增益压缩越明显,啁啾系数为正时,脉冲单纯展宽,输出啁啾随特征参数的增大而逐渐减小,啁啾系数为负时,初始啁啾与群速度色散导致的啁啾相互竞争,致使脉冲先被压缩后被展宽;脉冲最窄处对应的特征参数随线宽增强因子的增大而先增大后减小,输出啁啾随特征参数的增大而经历振荡后趋于平稳。     

基于光纤光参量放大的光脉冲压窄技术的研究

在高重复频率的光窄脉冲源中,为了获得更窄的脉冲,需要增加泵浦功率|但泵浦功率的提高会使光放大器的增益趋于饱和,放大的自发辐射噪音增强,脉冲光信噪比下降.为此,提出了利用啁啾管理和光纤光参量放大相结合的脉冲压窄方案.将强度调制后的泵浦光通过相位调制引入线性负啁啾,由于经参量过程后在新的闲频光上啁啾会加倍,从而进一步增大了信号的谱宽,并通过色散补偿介质实现了光脉冲压缩.实验给出了10 GHz速率下的结果,通过泵浦光的相位调制在0.5 W平均泵浦光功率条件下得到了脉宽19 ps的光脉冲,结果与理论分析基本吻合.     

基于光纤光参量放大的光脉冲压窄技术的研究

在高重复频率的光窄脉冲源中,为了获得更窄的脉冲,需要增加泵浦功率;但泵浦功率的提高会使光放大器的增益趋于饱和,放大的自发辐射噪音增强,脉冲光信噪比下降.为此,提出了利用啁啾管理和光纤光参量放大相结合的脉冲压窄方案.将强度调制后的泵浦光通过相位调制引入线性负啁啾,由于经参量过程后在新的闲频光上啁啾会加倍,从而进一步增大了...     

超短脉冲在单块晶体中的三次谐波理论模拟

讨论了群速度失配、自相位调制和交叉相位调制、晶体厚度、基波的频率啁啾等因素对超短脉冲单块晶体的三次谐波转换效率、三倍频光脉冲形状以及频谱的影响.结果表明：群速度延迟使三倍频光脉冲展宽；自相位调制和交叉相位调制会降低转换效率,并使三倍频光脉冲形状发生畸变；晶体厚度对三倍频光脉冲频谱展宽有着较大的影响；合适的基频光频率啁啾可以减小三倍频光脉冲的畸变,并有效地提高三倍频转换效率.     

基于光电振荡器和孤子压缩的双波长、高稳光窄脉冲

提出了一种在波长双环路光电振荡器(OEO)中,利用啁啾压缩和孤子压缩效应产生光窄脉冲的有效方案。此方案能够同时产生双波长光窄脉冲。将双环路OEO产生的高质量微波信号调制到系统中的直调激光器和相位调制器上,产生光脉冲和光啁啾。调节直调激光器和相位调制器之间的延时量,从而获得最大的啁啾率。利用色散补偿光纤和高非线性光纤对光脉冲进行啁啾压缩和光孤子压缩,使得光脉冲最终压窄至1.2ps。采用锁相环技术对系统腔长进行有效控制,使光脉冲的长期稳定性得到改善。     

亚10飞秒脉冲的诊断与压缩

采用二次谐波-频谱分辨光闸法对克尔透镜锁模钛宝石激光器输出的飞秒脉冲的振幅和相位 进行诊断，测得其脉冲包络是双曲正割型的，而相位则近似为时间的三次幂形式. 通过对脉 冲频域相位的拟合，诊断出二阶群速度色散是造成脉冲啁啾、导致脉冲展宽的主要原因. 利 用棱镜对补偿二阶群速度色散，进一步将17 fs啁啾脉冲压缩至半高全宽为8.5 fs的近似转 换极限脉冲. 关键词： 飞秒脉冲 二次谐波-频谱分辨光闸法 群速度色散     

双余弦相位调制对超高斯脉冲的频谱压缩

针对具有非线性负啁啾的超高斯脉冲,提出了一种双余弦相位调制方法,采用该方法对高非线性光纤中超高斯脉冲的频谱压缩进行优化。结果表明:由于初始超高斯脉冲的非线性负啁啾与非线性光纤中自相位调制引起的正啁啾失配导致时间相位不平坦,频谱出现分裂和旁瓣,压缩质量受限。通过对超高斯脉冲进行外部的两个余弦相位补偿调制可以减少频谱旁瓣的产生,延长频谱压缩距离,提高频谱压缩质量;其中这两个余弦相位调制的振幅和频率来自于对超高斯脉冲的非线性相移分析;在大范围展宽因子下,超高斯脉冲的频谱压缩质量也可以优化。     

色散补偿和色散位移光纤实现光脉冲压缩

根据超短光脉冲在光纤中传输的非线性薛定谔方程，模拟了不同色散参量情况下色散补偿和色散位移光纤对增益开关半导体激光器产生的光脉冲的压缩，给出了光脉冲在经过色散补偿光纤前后的啁啾曲线。结果表明，使用色散参量D分别为-150，-180和-20ps/(nm·km)的色散补偿光纤可以实现其他脉冲压缩方法的压缩效果，最大压缩因子达到6.09，但色散参量越大，所需光纤长度就越短。此外，脉冲经过色散补偿光纤后线性啁啾几乎为零。还利用色散位移光纤对脉冲进行孤子压缩，脉冲宽度由最初的45ps减小到1.23ps。指出采用这2种光纤相结合的方法可以对光脉冲实现高效压缩。     

Influence of group-delay ripple on timing jitter induced by SPM and IXPM in systems with dispersion compensated by CFBG

An analytical expression was proposed to analyze the influence of group-delay ripple (GDR) on timing jitter induced by self-phase modulation (SPM) and intra-channel cross-phase modulation (IXPM) in pseudolinear transmission systems when dispersion was compensated by chirped fiber Bragg grating (CFBG).Effects of ripple amplitude, period, and phase on timing jitter were discussed by theoretical and numerical analysis in detail. The results show that the influence of GDR on timing jitter changes linearly with the amplitude of GDR and whether it decreases or increases the timing jitter relies on the ripple period and ripple phase. Timing jitter induced by SPM and IXPM could be suppressed totally by adjusting the relative phase between the center frequency of the pulse and the ripples.     

Theoretical model for electromagnetic wave propagation in negative index material induced by cubic-quintic nonlinearities and third-order dispersion effects

We analyze the problem of characterization of electromagnetic (EM) wave that propagates in a negative index material (NIM) and compare to the behavior that observes in a positive index material (PIM), from the conventional technique of characterization of the pulse intensity profile. Firstly, we establish the nonlinear Schrödinger equation (NLSE) and use the Drude model to obtained the NLSE terms. Secondly, we apply the collective variables technique at each frequency range and show that the stability and the light pulse intensity are significantly depending on specific normalized frequencies. We determine the specific frequency range allowing a fear characterization of the intensity profile of light pulse when the frequency increases. We illustrate the effectiveness of this EM wave stabilization by applying the indicated frequency range. This conventional technique in NIM and PIM systems where the second-order dispersion, third-order dispersion, self phase modulation (SPM) and quintic effects originated from SPM called quintic phase modulation act to provoke strong perturbations.     

NOVEL ANALYTIC THEORY FOR ACTIVELY MODE-LOCKED FIBER LASER

The time-domain ABCD matrix formalism is based on the propagation of a Gaussian pulse which is characterized by the pulsewidth and chirp. In this paper, the time-domain ABCD matrix of th SPM (self phase modulation) is perfected so as to acquire more information. With the time-domain ABCD matrices of amplitude modulation, filtration from the optical bandpass, group velocity dispersion and SPM, the detailed characteristics of actively mode-locked fiber laser are described and the steady-state solutions are derived for two simple cases. The application of the formalism is also helpful to understand the mechanism of mode-locking in an actively mode-locked fiber laser.     

Two alternative approaches to electro-optical detection of terahertz pulses

The electro-optical detection of terahertz radiation is analyzed theoretically. While a conventional detection scheme of this type is based on the measurement of the phase shift of an optical pulse arising from its interaction with the terahertz field in a nonlinear medium, it is shown that both the phase and amplitude of the optical pulse vary because of this interaction. The amplitude modulation carries information on the dynamics of the amplitude and phase of the terahertz field and can be used to measure these parameters. With this energy-sensitive detection scheme, avoiding all restrictions on the symmetry type of the nonlinear medium is much simpler than with the phase scheme. Variants of the electro-optical detection technique using periodically poled crystals with the lithium-niobate symmetry and crystals with the zincblende structure are considered. It is shown that spectral sensitivities of the methods based on measurements of the phase and amplitude modulations are related as the frequencies of the laser and terahertz fields.     

有振幅调制和相位畸变光束的远场光谱移动与光谱开关

基于瑞利衍射公式,研究了被光阑衍射的具有振幅调制和相位畸变光束的远场光谱变化规律。结果表明,有振幅调制和相位畸变光束在远场也会出现光谱异变现象和光谱开关效应,光阑截断参数、光束的相位畸变参数、振幅调制参数、光强调制参数对远场光谱的跃迁量和光谱开关产生影响。通过数值计算结果分析讨论了光阑截断参数及光束各参数影响远场光谱的规律。     

An analysis of the intrinsic resonance offset dependence of magnetization generated by stimulated echo pulse sequences for noncoupled spins.

It is demonstrated that the basic radiofrequency pulse train used to generate stimulated echoes (90x-tau TE-90x-tau TM-90x-tau TE-Acq.) is in general characterized by strong amplitude and phase modulations of the transverse magnetization as a function of the resonance offset. Two dephasing techniques which eliminate the modulations are investigated both theoretically and experimentally, and a simple formula is derived for calculating the relative modulation across a spectrum as a function of gradient strength and duration, echo delay, and spectral linewidth.     

非线性色散渐减光纤中高峰值脉冲的激发和传输

本文基于变系数的非线性薛定谔方程,数值地讨论高峰值脉冲在色散渐减光纤中的激发和传输。首先,基于变系数非线性薛定谔方程的Peregrine孤子解,解析和数值地讨论精确的Peregrine孤子在色散渐减光纤中的传输特性。其次,通过输入不同的平面波背景上的局域脉冲,研究高峰值脉冲在非线性色散渐减光纤中的激发和传输。结果显示Peregrine孤子在色散渐减光纤中传输时,会产生一个空间和时间都局域化的高峰值单脉冲,并且当啁啾为负时,脉冲的幅值增加,脉宽被压缩。若光纤系统存在增益,脉冲的幅值也会增加。由于非线性光纤中的调制不稳定性过程,不同平面波背景上的小局部扰动都可激发出高峰值脉冲,除了峰值和宽度略有不同外,激发脉冲的形状几乎相同。     

Performance limit of a multi-frequency probe based coherent optical time domain reflectometry caused by nonlinear effects

The nonlinear effects that limit the performance of the multi-frequency probe(MFP)based coherent optical time domain reflectometry(C-OTDR)are investigated.Based on theoretical analysis and experimental results,compared with conventional C-OTDR,when the probe pulse has power gradient within the pulse width,self-phase modulation(SPM)and cross-phase modulation(XPM)are strengthened in the new C-OTDR scheme.The generation of four-wave mixing(FWM)is dependent on SPM and XPM,and with modulation frequency of phase modulator higher than 40 MHz,the stimulated Brillouin scattering(SBS) threshold can be enhanced by more than 5 dB,which benefits the maximum dynamic range of the MFP C-OTDR.     

Generation of few-cycle laser pulses:Comparison between atomic and molecular gases in a hollow-core fiber

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation(SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation(MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.