Synthesis of periodic femtosecond pulse trains in the ultraviolet by phase-locked Raman sideband generation

We demonstrate a new technique for femtosecond-pulse generation that employs ultrafast modulation of a laser field phase by impulsively excited molecular rotational or vibrational motion with subsequent temporal compression. An ultrashort pump pulse at 800 nm performs impulsive excitation of a molecular gas in a hollow waveguide, and a weak delayed probe pulse at 400 nm is scattered on the temporal oscillations of its dielectric index. The resultant sinusoidal phase modulation of the probe pulse permits probe pulse temporal compression by use of both positively and negatively dispersive elements. The potential of this new method is demonstrated by the generation of a periodic train of 5.8-fs pulses at 400 nm with positive group-delay dispersion compensation.     

C. 超短激光光源

本文对超短激光脉冲的产生、压缩技术的最新发展做了详细评述。     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Optimal generation of single-dispersion precompensated 1-fs pulses by molecular phase modulation

We show how an optimal control approach can be combined with the pump-probe technique for pulse compression by molecular phase modulation in hollow-core fibers to generate single 1-fs pulses in the visible. Varying the intensity and duration of the Gaussian-shaped pump pulse at the input induces optimal rotational response of the molecules. The probe pulse, which scatters off of the resulting time variation of the refractive index, is shaped at the input for optimal compression at the output, including dispersion to all orders.     

自相位调制及群速色散对超短脉冲在准周期LiNbO3 Fibonacci超晶格中传输的影响

从LiNbO₃ Fibonacci超晶格的结构特点出发,数值模拟了超短脉冲在LiNbO₃ Fibonacci超晶格内传输过程中,自相位调制及高阶群速色散效应对脉冲形状和频谱的影响．结果表明,超短脉冲在Fibonacci超晶格中传输时,由于自相位调制(SPM)与群速色散(GVD)耦合效应的影响,脉冲形状畸变和频谱展宽均是不对称的,随着晶格长度的增加,脉冲将会发生分裂,以至成为两个独立的子脉冲,同时也会出现脉冲频谱上的分裂. 关键词：     

Self-phase modulation via similariton solutions of the perturbed NLSE Modulation instability and induced self-steepening

The perturbed nonlinear Schrodinger equation (PNLSE) describes the pulse propagation in optical fibers, which results from the interaction of the higher-order dispersion effect, self-steepening (SS) and self-phase modulation (SPM). The challenge between these aforementioned phenomena may lead to a dominant one among them. It is worth noticing that the study of modulation instability (MI) leads to the inspection of dominant phenomena (DPh). Indeed, the MI triggers when the coefficient of DPh exceeds a critical value and it may occur that the interaction leads to wave compression. The PNLSE is currently studied in the literature, mainly on finding traveling wave solutions. Here, we are concerned with analyzing the similarity solutions of the PNLSE. The exact solutions are obtained via introducing similarity transformations and by using the extended unified method. The solutions are evaluated numerically and they are shown graphically. It is observed that the intensity of the pulses exhibits self steepening which progresses to shock soliton in ultra-short time (or near t = 0). Also, it is found that the real part of the solution exhibits self-phase modulation in time. The study of (MI) determines the critical value for the coefficients of SS, SPM, or high dispersivity to occur.     

19飞秒激光脉冲的产生与测量

本文介绍产生19飞秒激光脉冲的被动锁模四棱镜补偿群速度分散的环型染料激光器的特点;讨论了二次谐波强度自相关法和二次谐波干涉自相关法。用该激光器产生的超短而高稳定的光源对同步扫描条纹相管作了评价。     

锥形镀膜光纤探针中飞秒激光脉冲的传输

采用三维时域有限差分法对飞秒激光脉冲照明下锥形镀膜光纤探针内的光场进行了计算,研究了飞秒脉冲在探针中传输时的时域、频谱和相位特性,并分析了不同锥角和不同长度的探针对这些特性的影响.研究结果发现飞秒脉冲在探针中传输时出现了脉冲展宽、幅值振荡以及频谱分裂等现象,并且利用频谱和相位的变化特性初步解释了脉冲的展宽和脉冲振幅随时间周期性变化等现象. 关键词： 时域有限差分法 飞秒脉冲 光纤探针     

新型全光纤高能量飞秒光脉冲源的数值模拟

数值模拟了自相似脉冲的产生与压缩,得到一种产生高能量飞秒光脉冲的新方法.结果表明:利用掺铒光纤对光脉冲进行自相似传输,可得到含线性频率啁啾的高能量自相似光脉冲;自相似光脉冲经过空芯光子带隙光纤的一级线性压缩和高非线性光纤的二级非线性压缩,可获得高峰值功率的飞秒光脉冲;压缩过程中存在最佳光纤长度;喇曼自频移和自陡效应对脉冲压缩产生不利影响.     

Compression of single-cycle mid-infrared pulses by Raman-active molecular modulators

We show theoretically how high-order stimulated Raman scattering in the impulsive pump-probe regime can be used for generation of single mid-infrared (MIR) single-cycle pulses. The propagation of MIR probe pulses in a hollow waveguide filled with a Raman-excited gaseous medium, with a probe delay in the maximum of the molecular oscillations, results in spectral broadening covering almost 2 octaves. The spectral phases of this broadening can be compensated for by use of an output glass window with anomalous dispersion in the MIR. The spectral and temporal characteristics of the output pulses and the mechanism of pulse compression are studied by use of numerical and analytical solutions, and compression of a 70-fs input pulse at 4 microm to a single-cycle 6.5-fs output pulse is shown.     

SPM induced limitations for 40 Gbps chirped Gaussian pulses in optical channel

An analysis of self-phase modulation (SPM) induced nonlinearities in a 40 Gbps link with chirped Gaussian pulses of different duty cycles has been reported in this paper. In the present analysis only SPM effect has been considered to control the pulse propagation behavior through the fiber by appropriate selection of pulse width, peak power and channel length to suppress the other channel impairments caused by group velocity dispersion (GVD) and third order dispersion (TOD). The effect of SPM on the Q-factor for the transmission of 40 Gbps chirped pulses with different initial pulse widths and input peak powers has been investigated. It has been observed that a wider pulse having maximum negative chirp can withstand the nonlinear effect of SPM for relatively higher ranges of input peak power.     

Subfemtosecond compression of periodic laser pulses

It is shown that a recently proposed technique of molecular modulation can be used to compress mode-locked laser pulses into the subfemtosecond domain. The method relies on driving a molecular oscillation with a periodic pulse train. The coherent molecular motion, in turn, modulates the laser frequency and allows pulse compression by group-velocity dispersion of the same molecular medium.     

Spectral compression of femtosecond pulses via frequency intermodulation

A new technique of spectral compression of femtosecond pulses, based on their frequency intermodulation in the frequency mixing process, is demonstrated experimentally. In the exploited phenomenon of frequency intermodulation the phase of the reference pulse is superimposed on the signal instead of phase self- or cross-modulation used in traditional spectral compressors. In the process of frequency mixing a twelve-fold aberration-free spectral compression of 100 fs pulses has been achieved experimentally.     

Spectral modulation of ultra-broadband femtosecond laser pulses based on surface plasmon resonance

The surface plasmon resonance (SPR) for spectral modulation of the femtosecond laser pulses with 110 nm ultra-broad bandwidth is demonstrated on the basis of the development of ultrashort pulse laser sources which supports good spatial resolution and high peak intensity. Employing the femtosecond surface plasmon polariton pulses launched by a Kretschmann configuration, whose reflectivity curve has the characteristic of the ultra-broad bandwidth, we observe a frequency-dependent loss with greater attenuation at the peak of the spectrum profile than in the wings, which is very useful for adequate spectral modulation. The SPR for the spectral modulation is investigated in theoretical and experimental aspects. The arbitrary spectral modulation of the femtosecond laser pulses can be fulfilled by controlling and optimizing the SPR of the gold film. The experimental result agrees well with the calculation.     

Evolution of spatial modulation of broadband laser pulses with different pulse durations in medium with non-instantaneous Kerr nonlinearity

We experimentally investigate the evolution of spatial modulation of broadband laser pulses with different pulse durations through a medium with non-instantaneous Kerr nonlinearity. By modifying the input laser pulse width, we observe the nonlinear growth of the spatial stripes that arise from interference with different modulation frequencies, and then obtain the gain spectra of the spatial modulation which clearly reveal the influence of relaxation effect on spatiotemporal modulation instability. We find that all gain spectra have the similar fastest growing frequency, but they have tails at higher frequency region in which the growth rate of gain is bigger for shorter pulse duration. The experimental results are confirmed by the perturbational analysis.     

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.     

相干原子系统的失谐对脉冲调制的影响

研究了一对非共振脉冲在相干介质中的时空传播,着重考究了失谐对相干脉冲相位调制的影响。计算结果表明当脉冲在介质中稳定传播时,脉冲的最终振幅与双光子脉冲共振的情况下相同,只由介质的初始状态决定,与失谐无关。而失谐仅仅只对脉冲的相位进行调制。     

氧化石墨烯被动锁模掺镱光纤激光器多脉冲现象的实验研究

利用氧化石墨烯作为可饱和吸收体,在被动锁模全正常色散掺镱光纤激光器中研究了多脉冲的现象,在同一抽运功率不同偏振态下,实验获得了矩形脉冲谐波锁模、耗散孤子谐波锁模、准谐波锁模,脉冲峰值周期性调制,脉冲簇、脉冲束、混沌多重脉冲的多脉冲现象,插入激光腔内的2nm窄带滤波器具有限制增益带宽、对脉冲塑形、诱导多脉冲产生的作用,调节偏振控制器相当于改变腔内增益,是实现不同类型多脉冲现象的主要原因,本实验研究有利于加深对多脉冲动力学行为在正常色散区域氧化石墨烯锁模掺镱光纤激光器中的理解。     

拉曼增强型自相位调制再生技术

自相位调制可用于信号脉冲的整形,可实现归零码(RZ)信号的再生。以高非线性光纤(HNLF)为介质,在高非线性光纤中实现了拉曼集总放大和信号脉冲的自相位调制。从理论上分析了同向拉曼泵浦增强信号的自相位调制,大大降低了所需的信号光强度。论证了在色散长度(LD)远大于HNLF长度(L)时由自相位调制(SPM)导致的频谱展宽,它与脉冲的啁啾无关,频谱的功率密度与脉冲的强度无关。因此在高非线性光纤之后加入偏移载波频率的带通滤波器可实现归零码信号的再生。集中拉曼放大可增强再生性能。     

Impact of two-photon absorption on self-phase modulation in silicon waveguides

We study the effects of two-photon absorption on the self-phase modulation (SPM) process in silicon waveguides while including both free-carrier absorption and free-carrier dispersion. An analytical solution is provided in the case in which the density of generated carriers is relatively low; it is useful for estimating spectral bandwidth of pulses at low repetition rates. The free-carrier effects are studied numerically with emphasis on their role on the nonlinear phase shift and spectral broadening. We also consider how the repetition rate of a pulse train affects the SPM process.