Effects of initial frequency chirp on the linear propagation characteristics of the exponential optical pulse

In this paper, the linear propagation characteristics of the exponential optical pulse with initial linear and nonlinear frequency chirp are numerically studied in a single mode fibre for \be₂<0. It can be found that the temporal full width at half maximum and time-bandwidth product of exponential pulse monotonically increase with the increase of propagation distance and decrease with the increase of linear chirp C for C<0.5, go through an initial decreasing stage near \zeta=1, then increase with the increase of propagation distance and linear chirp C for C\geq0.5. The broadening of pulses with negative chirp is faster than that with positive chirp. The exponential pulse with linear chirp gradually evolves into a near-Gaussian pulse. The effect of nonlinear chirp on waveform of the pulse is much greater than that of linear chirp. The temporal waveform breaking of exponential pulse with nonlinear chirp is first observed in linear propagation. Furthermore, the expressions of the spectral width and time-bandwidth product of the exponential optical pulse with the frequency chirp are given by use of the numerical analysis method.     

线性啁啾光脉冲在具有纵向增益光纤中的自相似传播研究

非线性薛定谔方程是光脉冲在光纤中传播时所遵循的基本方程.本文利用分步傅里叶方法从数值结算的角度研究了具有线性啁啾的光脉冲在具有横向增益光纤中的自相似传播特性.研究表明:当群速度色散系数和光脉冲的啁啾系数符号相同时,由于在传播过程中自相位调制和群速度色散分别起主要作用,不论何种形式的光脉冲在传输过程中均会受到压缩,但随着传播距离的增加压缩的光脉冲会展宽;埃尔米特高斯输入和正弦输入在传播过程中的光强分布是对称的,拉盖尔高斯输入在传播过程中会发生能量交换而表现出明显的偏转;当群速度色散系数和光脉冲的啁啾系数符号不相同时,在传播过程中群速度色散始终起主要作用,脉冲一直展宽.这些研究结论可能为未来光纤压缩器、光纤放大器以及新型THz调制波源的制备提供新的理论基础和方法.     

线性啁啾光脉冲在具有纵向增益光纤中的自相似传播研究

非线性薛定谔方程是光脉冲在光纤中传播时所遵循的基本方程.本文利用分步傅里叶方法从数值结算的角度研究了具有线性啁啾的光脉冲在具有横向增益光纤中的自相似传播特性.研究表明:当群速度色散系数和光脉冲的啁啾系数符号相同时,由于在传播过程中自相位调制和群速度色散分别起主要作用,不论何种形式的光脉冲在传输过程中均会受到压缩,但随着传播距离的增加压缩的光脉冲会展宽;埃尔米特高斯输入和正弦输入在传播过程中的光强分布是对称的,拉盖尔高斯输入在传播过程中会发生能量交换而表现出明显的偏转;当群速度色散系数和光脉冲的啁啾系数符号不相同时,在传播过程中群速度色散始终起主要作用,脉冲一直展宽.这些研究结论可能为未来光纤压缩器、光纤放大器以及新型THz调制波源的制备提供新的理论基础和方法.     

色散渐减光纤中的脉冲展宽研究

讨论了高斯脉冲在五种不同群色散剖面光纤中的传播特性和初始啁啾对脉宽的影响。结果发现:与单模光纤相比,色散渐减光纤(DDF)中的脉冲展宽较慢,并能有效抑制展宽,当β2C>0时,脉冲展宽速度与无啁啾时相比较快,当β2C<0时,脉冲有一初始窄化过程,不同群色散函数对啁啾脉冲有不同的影响。     

光脉冲在标准单模光纤中演化形成孤子的实验研究

利用二次谐波频率分辨光学门脉冲分析仪从实验上分析研究了具有啁啾的10 GHz光脉冲在不同输入功率下演化形成孤子的规律和特点。实验发现:输入脉冲在光纤中传输3.5个色散长度时,其时间宽度、频率啁啾和时间带宽积都随着输入功率的增加而减小。当输入功率大于一阶孤子功率的理论值时,光脉冲能够演化形成孤子;脉冲在随后传输过程中其宽度基本保持不变,波形、频率啁啾和时间带宽积仍随着传输距离的变化而变化;输入功率越高,形成的一阶孤子脉冲的宽度越窄。当输入功率小于一阶孤子功率的理论值时,输出脉冲的时间宽度随着传输距离的增加而增加,频率啁啾随着传输距离的增加而减小,光脉冲不能演化形成孤子。     

Propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium

We investigate the propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium. For certain chirp strength and chirp width, an incident 2π nonlinear chirped pulse will split into optical precursors and a stable self-induced transparency soliton. This is caused by the particular Fourier spectrum that includes not only central resonant frequency components but also high-frequency and low-frequency sidebands. Moreover, the effects of chirp parameters on the evolution of nonlinear chirped pulses are also discussed.     

Effect of initial chirp on near-infrared supercontinuum generation by a nanosecond pulse in a nonlinear fiber amplifier

Theoretical and experimental research on the effect of initial chirp on near-infrared supercontinuum generation by a nanosecond pulse in a nonlinear fiber amplifier is carded out. The complex Ginzburg-Landau equation is used to simulate the propagation of the pulse in the fiber amplifier and the results show that pulses with negative initial chirp produce the widest supercontinuum and pulses with positive initial chirp produce the narrowest supercontinuum when the central wavelength of the pump lies in the normal dispersion region of the gain fiber. A self-made line width narrowing system is utilized to control the initial chirp of the nanosecond pump pulse and a four-stage master oscillator power amplifier configuration is adopted to produce a high power near-infrared suppercontinuum. The experimental results are in good agreement with simulations which can provide some guidance on further optimization of the system in future work.     

色散和非线性效应对高斯脉冲综合影响的理论分析

从光孤子传输所满足的非线性薛定谔方程出发,采用单独分析和综合分析对比,分析了影响高斯脉冲传输的色散、非线性等因素,结果表明：二阶色散参量只会影响脉冲的幅值,对脉冲形状影响不大,而输入脉冲的啁啾则是使脉冲发生形变的主要原因|在啁啾、三阶色散和五阶非线性的共同作用下,它们对脉冲都会产生较大的影响,且存在相互影响和制约作用,在某一临界值,对脉冲存在着较高的压缩增益效应和“整形”作用.从理论上提出了改善高斯光脉冲在光纤中传输特性的解决方案,这对光纤孤子通信的实践过程具有一定的理论借鉴意义.     

高非线性光纤产生自相似抛物线脉冲的特性研究

基于脉冲在光纤中传输所满足的非线性薛定谔方程,采用数值计算的方法对脉冲在高非线性光纤中演化成自相似抛物线脉冲的传输特性进行了研究。结果表明,脉冲在不同初始能量和啁啾参量的情况下,高非线性光纤产生自相似抛物线脉冲,且随着归一化长度的增大,脉冲的自相似指数从0.0796减小到0.0720。进一步的研究表明:啁啾高斯脉冲在光纤传输过程中,脉冲逐渐展宽,同时正啁啾越大,其自相似演化结果越差;参数N=8的正负啁啾脉冲分别演化成平顶自相似抛物线脉冲以及三角形脉冲。     

飞秒啁啾Gauss型脉冲在稠密Λ型三能级原子介质中的传播

利用由预估校正(PC)- 时域有限差分(FDTD)法求得的不含慢变包络近似(SVEA)和旋转波近似(RWA)的全波Maxwell-Bloch方程的数值解, 研究了飞秒啁啾Gauss型激光脉冲(以下简称啁啾脉冲)在稠密Λ型三能级原子介质中的传播.研究表明,啁啾系数(C)的正负及大小的变化对脉冲传播特性有显著的影响,而且这个影响与脉冲面积的大小密切相关.面积小于4π的啁啾脉冲,在介质中传播时不发生分裂,且啁啾脉冲逐渐演化为一个近似的无啁啾(C=0)脉冲,这一特点不随啁啾系数的改变而     

Significance of prechirping on long-haul path-averaged soliton impulse in re-circulating loop at 10 and 20 Gb/s with TOD

In this paper, we have investigated the performance of first- and second-order path-averaged soliton long-haul transmission link including the impact of third-order dispersion (TOD) at varied chirp. Here, the varied chirp is considered keeping in view the inadvertent frequency chirp imposed on all practical sources of short optical pulses. The propagation of strongly chirped pulses in loss-managed long-haul path-averaged soliton transmission network has been shown. The investigations reveal that in first-order (N=1) path-averaged soliton transmission link at 10 and 20 Gb/s, SPM effect on the rising and falling edges of a pulse results in spectral broadening for all values of induced chirp. On the contrary, spectral narrowing of the pulses is observed in second-order negatively chirped path-averaged soliton pulses. The effect of the nonlinearity changes from narrowing to broadening of pulses if the sign of the initial chirp is changed to positive. The results ascertain that the system is capable of transmitting a pulse up to the distance of 24,500 km at bit rates of 10 and 20 Gb/s. Investigations have been carried out by varying the chirp factor in the range −1 to 1 and −1 to 0.4] for 10 and 20 Gb/s, respectively, to demonstrate the robustness of the long-haul soliton link. The observations establish that the pulse width (full-width at half-maximum (FWHM)) remains within the optimal range even at the transmission distance of 24,500 km without and at discrete values of the chirp factor.     

空气中飞秒脉冲激光器互相关信号的理论研究

利用色散脉冲传播理论和空气折射率Ciddor公式,推导了空气中飞秒脉冲激光器的互相关函数;根据互相关函数结果表达式取决于激光器光源的光谱分布,建立了在空气中不同光程路径差及环境参量变化下,不同光谱分布的飞秒脉冲激光器的脉冲之间互相关模式的数值模型.结果表明:随着传播距离的增大,互相关模型具有稳定啁啾及线性加宽;环境参量的变化只引起互相关图案的移动,并不产生任何额外线性加宽或啁啾.     

掺镱光纤放大器中自相似脉冲传输特性研究

研究了高阶色散和增益色散影响下,自相似脉冲在掺镱光纤放大器中的传输特性.从解析求解和数值模拟两方面分别阐述了初始脉冲能量、三阶色散和偶极子驰豫时间对脉冲传输质量的影响.结果表明,当传输考虑三阶色散效应时,伴随着脉冲啁啾非线性区域的扩大,啁啾中心随着色散系数取值的大小和正负前、后移动,脉冲峰向一侧延迟,中心位置发生漂移.增益色散对脉冲放大起到滤波作用,对自相似脉冲具有压窄效应.证实了具有一定驰豫时间的增益介质能够维持自相似孤子波的稳定传输.     

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。     

Optical fibre-grating pulse compression

Numerical simulation is used to consider non-linear pulse propagation in fibres and subsequent pulse compression in a dispersive delay line. It is shown that for small initial pulse powers the conventional non-linear Schrödinger equation (NSE) is quite accurate to describe the process of pulse propagation in fibres. In this case initially symmetrical pulses undergo squaring and spectral broadening in fibres, and frequency chirp is linearized over most of the pulse, while shapes of the pulse, spectrum and frequency chirp remain symmetrical at the output of the fibre. There is a certain optimum fibre lengthZ _opt which is determined by the initial pulse parameters and fibre characteristics for pulse compression in the dispersive delay line. When the fibre lengthZ>Z _opt the optical wave breaking effect distorts the linearity of the frequency chirp and thus deteriorates the quality of the compressed pulse. The region of NSE approximation accuracy is determined. It is demonstrated that at increase of the initial pulse power (initial pulse width makes no difference) the NSE approximation becomes inaccurate. So the pulse dynamics in the fibre were described by the modified NSE derived in the higher-order approximation of the method of slowly varying amplitudes from Maxwell's equations. In this case the shock wave appears at the trailing edge of the pulse, which accelerates the wave breaking process. This results in a decrease of the optimum fibre length and deterioration of compressed pulse parameters, compared with the NSE case. Spectral windowing of the extreme Stokes components of the pulse spectrum permits significant improvement in the quality of the compressed pulse. The main features of the compression of pulses with asymmetrical initial shape are also considered.     

脉冲啁啾对于阿秒脉冲的影响

采用单电子近似和软核势模型, 通过数值求解一维含时薛定谔方程, 理论研究了当脉冲分别带有正、负啁啾的情况下所产生的阿秒脉冲.分析了不同脉冲啁啾特性对阿秒脉冲的强度和宽度的影响. 研究结果表明, 无论是正啁啾还是负啁啾, 随着啁啾量的增加, 都将使激光脉冲由产生单个阿秒脉冲趋向于产生阿秒脉冲链. 正啁啾和负啁啾对于阿秒脉冲宽度的影响是不同的, 负啁啾对于阿秒脉冲宽度影响很小, 适当的负啁啾有利于缩小阿秒脉冲的宽度; 而正啁啾脉冲产生的阿秒脉冲较无啁啾时展宽, 且随着啁啾量的增加, 其阿秒脉冲宽度迅速增大.     

Femtosecond soliton generation induced by intrapulse stimulated Raman scattering in optical fibers

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Higher-order effects on self-similar parabolic pulse in the microstructured fibre amplifier

By considering higher-order effects, the properties of self-similar parabolic pulses propagating in the microstructured fibre amplifier with a normal group-velocity dispersion have been investigated. The numerical results indicate that the higher-order effects can badly distort self-similar parabolic pulse shape and optical spectrum, and at the same time the peak shift and oscillation appear, while the pulse still reveals highly linear chirp but grows into asymmetry. The influence of different higher-order effects on self-similar parabolic pulse propagation has been analysed. It shows that the self-steepening plays a more important role. We can manipulate the geometrical parameters of the microstructured fibre amplifier to gain a suitable dispersion and nonlinearity coefficient which will keep high-quality self-similar parabolic pulse propagation. These results are significant for the further study of self-similar parabolic pulse propagation.     

啁啾光脉冲在类时间透镜介质中的传播

利用空间－时间类似和光线光学方法得到啁啾高斯光脉冲在类时间透镜介质中传播的解析解，讨论了光脉冲宽度和啁啾参数随传播距离的演变，并与光脉冲在克尔介质中传播的特征作了比较。     

三五阶非线性下啁啾超高斯脉冲的啁啾和频谱

利用三五阶非线性效应下的扩展非线性薛定谔方程,在忽略光纤色散的情况下,计算模拟了以二阶情形为例的啁啾超高斯光脉冲的啁啾和频谱。结果表明,脉冲无预啁啾时,正五阶非线性增大啁啾量,负五阶非线性在减小啁啾量的同时还改变啁啾曲线的形状。当预啁啾与五阶非线性系数同(异)号时,总啁啾增加(减小)。随着超高斯脉冲阶次的增大,总啁啾量增大,脉冲中心附近无啁啾的范围变宽,整个有啁啾的范围变小,总啁啾中预啁啾所占比重增大。脉冲无预啁啾时,正负五阶非线性分别可以增大和减小频谱展宽。预啁啾若增强非线性所致啁啾,则可能使频谱峰值结构加强,谱峰数目增多。当超高斯光脉冲的阶次或最大非线性相移增大时,在某些宽大的频谱峰上还可能出现许多精细谱峰。