啁啾脉冲放大激光系统中展宽器色散的解析算法

时域色散精密控制是超短激光产生及其应用中的关键技术之一,它通过控制各波长的光程产生相对延迟从而改变脉冲宽度.展宽器是啁啾脉冲放大激光系统中对激光脉冲展宽的装置,基于光线追迹法研究光线在展宽器中的传输路径,可计算飞秒脉冲中各波长的光程,进而计算脉冲展宽量并应用于系统设计.由于展宽器的光程表达式复杂,直接对其求导获得色散表达式较困难,目前只能采用数值导数获得近似解,这在计算过程中会引入误差,不利于激光系统精确设计和优化.本文介绍了一种易实现的求解展宽器色散的解析算法,通过归纳展宽器光程表达式特点,引入四个基元函数,将光程表达式分解和反复代换,可得到高阶色散的精确解析值.本文首先对Martinez型展宽器重新光线追迹,获得与Offner型展宽器一致的相位表达式,其次通过解析算法获得了两种展宽器的精确高阶色散值,最后将解析算法与数值算法的结果进行了比较.该解析算法对于啁啾脉冲放大系统的参数设计具有实用价值.     

A novel confocal optical pulse stretcher for laser pulses

We present an analytical and experimental study of a novel confocal optical pulse stretcher (COPS). The simple and passive pulse stretcher consists of two concave mirrors and a scraper beam-splitter and its optical configuration ensures a perfect spatial overlapping of laser pulses at the beam-splitter. The pulse stretcher is compact and suitable for laser pulses of large divergences. The confocal optical pulse stretcher is demonstrated on a pulsed copper vapour laser to convert a 40 ns (1/e² % points) pulse into a 55 ns with reduction of peak power by a factor of 1.375 without loss of pulse energy.     

Analysis of the adjustment precision of the stretcher in the petawatt laser facility

Effects of stretcher adjustment aberration on the residual dispersions of the laser system is analyzed making use of the ray tracing method. It is shown that the adjustment aberrations affect the pulse contrast and we must select an appropriate adjustment precision for the stretcher in the petawatt laser system. And as an example, a proper adjustment precision of the pulse stretcher is also proposed for the SHENGUANG (SG) II laser facility.     

基于耗散孤子种子的啁啾脉冲光纤放大系统输出特性

以不同滤波器带宽下获得的全正色散光纤激光器耗散孤子作为啁啾脉冲放大(CPA)系统的种子脉冲, 研究了光栅对和光纤展宽器CPA系统输出脉冲的可压缩性. 结果表明, 对于大能量耗散孤子种子脉冲, 当CPA系统采用正色散光纤展宽器时, 光纤群速色散与自相位调制之间的相互作用不仅可抑制耗散孤子脉冲光谱调制的影响, 还可使脉冲在光纤展宽器中自相似演化, 从而可提高CPA输出脉冲的可压缩性. 通过优化光纤展宽器长度, 对于耗散孤子种子源, 采用光纤展宽器的CPA系统输出脉冲可压缩性与主脉冲所占脉冲总能量之比均优于采用光栅对展宽器时的情况.     

Tunable light source for coherent anti-Stokes Raman scattering microspectroscopy based on the soliton self-frequency shift

We present a photonic crystal fiber (PCF)-based light source for generating tunable excitation pulses (pump and Stokes) that are applicable to coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The laser employed is an unamplified Ti:sapphire femtosecond laser oscillator. The CARS pump pulse is generated by spectral compression of a laser pulse in a PCF. The Stokes pulse is generated by redshifting a laser pulse in a PCF through the soliton self-frequency shift. This setup allows for probing up to 4000 cm(-1) with a spectral resolution of approximately 25 cm(-1). We characterize the stability and robustness of CARS microspectroscopy employing this light source.     

Theoretical and experimental studies on a compact stretcher with large spectral bandwidth and high transmission efficiency

To ensure safe and effective amplification for the chirped pulse amplification laser facility, many factors such as pulse spectral width, pulse duration, grating parameters and incidence angle parameter are considered carefully in the design of the pulse stretcher. A compact four pass Offner stretcher design is given out. Experiment results show that the stretcher has broad spectral width, large stretching ratio and high transmission efficiency. By comparison, the calculated result of the dispersion effect on the pulse duration is completely consistent with the experiment result.     

利用CARS方法研究超短脉冲激光的脉冲宽度

在理论上提出了利用两光束CARS/CSRS方法测量超短脉冲激光的三阶相关函数。实验上在几种样品中测量了CARS/CSRS的时间特性,得到了泵浦光脉冲,SCDL输出光脉冲以及受激喇曼散射光脉冲的宽度。     

新型无象差展宽器色散量的光线追迹计算法

对无象差展宽器的结构进行了改进,利用光线追迹法计算了其色散量,分析了展宽结果与展宽器各参量的关系,数值模拟了飞秒脉冲的展宽过程,为该展宽器的优化设计与精细调节提供了依据.     

新型无象差展宽器色散量的光线追迹计算法

对无象差展宽器的结构进行了改进，利用光线追迹法计算出其色散量，分析了展宽结果与展宽器各参量的关系，数值模拟了飞秒脉冲的展宽过程，为该展宽器的优化设计与精细调节提供了依据。     

ffner展宽器高倍率展宽脉冲的理论与实验研究

在理论上分析了 ffner展宽器的色散特性 ,给出了展宽脉冲随 ffner展宽器系统参数变化的关系 .据此在实验上建立了一台 ffner飞秒展宽器系统 ,将 19fs的种子脉冲无色差展宽至 6 0 5ps ,测量结果与理论计算一致 .     

展宽压缩器的谱透过率对短脉冲时间特性的影响

为了考察短脉冲激光装置中展宽压缩器的谱透过率对输出脉冲时间特性的影响,通过解析推导得到了激光脉冲通过平行光栅对的谱透过率表达式。结果表明:谱透过率不仅与光栅尺寸及光栅线密度有关,还与光束口径、入射角、光栅间距等参数有关;通常情况下展宽器的截止带通较小,对脉冲的信噪比有较大的影响;而压缩器中由于光束口径很大,有很宽的渐变透过率带通,对压缩后的脉宽及信噪比有利。目前常见的短脉冲激光系统中,限制输出脉冲信噪比的往往是展宽器而非压缩器。     

(O)ffner展宽器曲面镜误差对输出脉冲对比度的影响

在(O)ffner展宽器的设计研究中,对曲面镜曲率半径的误差研究非常重要,它直接影响输出脉冲的质量.因此有必要分析(O)ffner展宽器中曲面镜曲率半径的误差对输出脉冲对比度的影响.参考(O)ffner展宽器的光线追迹模型,建立了(O)ffner展宽器中曲面镜曲率半径存在误差时的光线追迹模型;根据建立的模型.详细分析了曲面镜曲率半径的误差对输出脉冲质量的影响;发现在目前曲面镜曲率半径的加上误差在0.2%～0.5%的情况下,完全可以满足神光Ⅱ拍瓦激光装置中对展宽器的要求;同时发现如果曲面镜曲率半径存在误差时,应该尽量使两曲面镜之间的距离保持为凹面反射镜曲率半径的一半,而不是使两曲面镜保持同心.     

Test of an optical parametric oscillator (OPO) as a compact and fast tunable stokes source in coherent anti-stokes raman spectroscopy (CARS)

An optical parametric oscillator (OPO), as a novel kind of broadband Stokes source, is employed for coherent anti-Stokes Raman spectroscopy (CARS). Compared to the conventional dye laser configuration OPO-CARS offers practical advantages. The tunable OPO allows a fast and comfortable frequency tuning. The excitation bandwidth of about 35 cm^–1 (FWHM) limits the spectral range of effective and stable single pulse CARS generation but can be used to enhance selected spectral structures.     

Optimization of grating size in chirped-pulse-amplification. laser system

We performed calculations for a chirped-pulse-amplification laser system with a sub-100-fs pulse duration including the finite size of gratings in an aberration-free stretcher/compressor design. Both the higher-order phase dispersion and the spectral clipping, which is due to the finite size of the optics in the stretcher and the compressor, are taken into account in the calculation. We discuss the influence of both phase dispersion and spectral clipping on the important parameters of the recompressed pulse, such as pulse duration, pulse contrast, and peak intensity, employing different phase-dispersion compensation methods. The calculations show that the use of a small compressor grating reduces the higher-order phase errors and smoothes the sharp spectral cut-off caused by spectral clipping in the stretcher, which consequently improves the pulse contrast ratio of the main pulse to the pedestal. Received: 1 April 2000 / Published online: 8 November 2000     

Characterization of three-color CARS in a two-pulse broadband CARS spectrum

We demonstrate that a broadband coherent anti-Stokes Raman scattering (CARS) spectrum generated with a typical two-pulse scheme contains two distinct, significant signals: '2-color' CARS, where the pump and probe are provided by a narrowband pulse and the continuum pulse constitutes the Stokes light, and '3-color' CARS, where the pump and Stokes are provided by two different frequency components in the continuum pulse and the narrowband pulse serves as the probe. The CARS spectra from the two different mechanisms show distinct characteristics in Raman shift range, laser power dependence, and chirping dependence. We discuss the potential for a 3-color CARS signal to cover the fingerprint region with reduced photodamage of live cells. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.     

用于OPCPA高展宽比的展宽器

 根据öffner展宽器的原理和设计,进行了光学参量啁啾脉冲放大系统中信号光脉冲展宽的实验研究。通过数值方法,合理地选择了展宽器光学元件的各项参数,即光栅常数为1 800 (102 mm×102 mm),凹面镜曲率半径为1 000 mm,凸面镜与凹面镜之间的距离为653 mm,信号光入射角为51.05°。实验结果表明,优化后的öffner展宽器可以将脉宽为30 fs的信号光脉冲展宽到545 ps, 展宽比为18 167,满足OPCPA放大的需要。     

Femtosecond mode-locked Yb fiber laser for single-mode fiber chirped pulse amplification system

High-peak-power laser systems are based on the chirped pulse amplification (CPA) technique. CPA laser system needs a mode-locked laser oscillator for stable ultrashort pulse laser generation. We report a single-mode fiber CPA system which consists of stable mode-locked Yb-doped fiber laser oscillator with a wide tuning range, chirped fiber Bragg grating stretcher, and two Yb-doped fiber amplifiers. The single-mode fibers can generate high-quality and single-mode beams which are stable for long time.     

An analysis of As_2S_3 chirped fiber grating formed by two-photon absorption effect

When femtosecond laser pulses interfere with chirped femtosecond laser pulses in As2S3 fiber, a chirped fiber grating is formed. An analytical expression is given to describe the chirped grating, and its Bragg reflectivity is calculated. Because of the high photosensitive effect of As2S3 material, the chirped fiber grating has a wide Bragg reflective spectrum and high reflectivity by choosing proper parameters. This indicates that the chirped fiber grating can be used as a stretcher in the femtosecond chirped pulse amplification (CPA) system.     

适用于亚10 fs的共心衍射无像差展宽器

在升级5TW/40fs钛宝石激光系统的过程中,伴随10fs种子源的使用,提出并研制了一种新型的共心衍射无像差展宽器和全能量压缩器。该展宽器的最大特点是通过平面镜反射使光束入射时所形成的衍射点与展宽后的还原点均在凹面镜的球心(即共心结构),凹面镜不存在其它同类方法中的成像问题,展宽函数中不涉及凹面镜的焦距和离轴量。该展宽器可将亚10fs的种子脉冲展宽到纳秒量级,且具有无像差、高带宽、结构紧凑、调整精度高等优点,适合100TW级超快激光系统中脉冲展宽。     

High power all-fiber structured supercontinuum source with variable spectral coverage

An all-fiber structured supercontinuum source based on chirped pulse amplification (CPA) technology and the use of nonlinear photonic crystal fiber was constructed. By experimentally varying fiber stretcher length in CPA laser source, we can change the spectral coverage and therefore the percentage of visible light in the produced supercontinuum at output of nonlinear photonic crystal fiber, while the average supercontinuum power always keeps at watts of level. The all-fiber structured feature and the capability of changing supercontinuum spectral coverage make this CPA system a promising implement for different applications that require different broad band spectrum.