交叉偏振滤波技术提高飞秒超强激光信噪比的研究

对交叉偏振波滤波技术理论分析的基础上,在"极光-III"装置中采用该技术进行了钛宝石飞秒放大激光脉冲对比度提高的实验研究,不仅显著的抑制了纳秒范围内再生放大器产生的预脉冲,而且也将皮秒范围内的信噪比由10^-5提高到了10^-7,实验测得基波到交叉偏振波的转化效率大于10%.研究表明,该技术可以有效地提高飞秒超强激光的信噪比.     

Influence of spectral phase on cross-polarized wave generation with short femtosecond pulses

In this paper we present the first comprehensive study of the role of spectral phase on cross-polarized wave (XPW) generation using sub-30 femtosecond (fs) laser pulses. XPW generation improves the temporal contrast and shortens the pulse duration of fs chirped pulse amplification (CPA) lasers. For Ti:Sa lasers, compression below 30?fs is non-trivial and therefore never perfect. We therefore systematically analyze the effect of an arbitrary input spectral phase on the output spectrum and efficiency of the XPW process, both theoretically and experimentally. We derive the maximum acceptable value of residual phase for a given initial pulse duration in order to efficiently drive the XPW process for pulse shortening and contrast improvement.     

Temporal contrast enhancement of picosecond pulses based on phase-conjugate wave generation

A practical technique based on the phase-conjugate wave (PCW) generation is proposed to improve the temporal contrast of the picosecond pulses. Our theory predicts the temporal contrast of the picosecond pulses can be enhanced to about the cube of the temporal contrast of the input pulse via the PCW generation, in which the conversion efficiency from the incident pulse to the PCW is about 25%. In a proof-of-principle experiment, the temporal contrast of picosecond pulses was enhanced from 1.7×10³ to 8.6×10⁸ with the conversion efficiency of 10%. This technique is promising to effectively eliminate the background of the ultrashort and ultraintense laser pulses in the future.     

10(-10) temporal contrast for femtosecond ultraintense lasers by cross-polarized wave generation

We take advantage of nonlinear properties associated with chi(3) tensor elements in BaF2 cubic crystal to improve the temporal contrast of femtosecond laser pulses. The technique presented is based on cross-polarized wave (XPW) generation. We have obtained a transmission efficiency of 10% and 10(-10) contrast with an input pulse in the millijoule range. This filter does not affect the spectral shape or the phase of the cleaned pulse. It also acts as an efficient spatial filter. In this method the contrast enhancement is limited only by the extinction ratio of the polarization discrimination device.     

XPW技术在提高超短激光脉冲对比度中的作用

有效提高激光脉冲对比度是推动超短超强激光技术发展的一个关键问题。交叉偏振波（XPW）滤波技术基于非线性晶体的三阶非线性特性,可显著提高超短激光脉冲对比度。结合国际上有关XPW滤波技术研究的最新进展,介绍了XPW滤波特性及实验研究结果。最新研究结果表明,采用这种技术可以将超短激光脉冲对比度提高4个量级。内部转换效率提高到50％。     

饱和功率密度下线性啁啾对交叉偏振波输出特性的影响

交叉偏振波(XPW)技术是一种基于具有各向异性特征的三阶非线性晶体的非线性滤波技术,由于结构简单,稳定可靠,是超强超快激光领域提高时域对比度、压缩脉宽的有效手段之一.在实验中发现,XPW的输出特性受驱动脉冲特性的直接影响,通过理论计算得到了XPW的脉宽和光谱宽度与驱动脉冲的啁啾特性关系,同时利用声光可编程色散滤波器设计实验对理论计算进行了验证.结果表明:实验结果很好地反映了理论计算得出的结论,同时在饱和功率密度条件下,还表现出了一些理论计算没有反映出的新现象,即XPW的光谱展宽突破了驱动脉冲宽度√3倍的限制,最终脉宽也能够压缩至小于入射脉宽的1/√3;此外对于相反线性啁啾的驱动脉冲所产生的XPW信号,其在光谱形状上有明显的偏移差异,同时输出效率也有所不同.最后对这些新现象进行了进一步的分析和理论解释.     

基于交叉偏振波产生的脉冲净化技术研究与应用

本文利用交叉偏振波产生技术(XPW)对800 nm波段钛宝石飞秒激光器输出的激光脉冲进行时域净化, 提高脉冲时域对比度, 并测量验证了10¹¹对比度的脉冲, 达到测量仪器的动态范围极限, 比初始脉冲时域对比度有三个量级的提高, XPW的效率为22%. 同时发现净化后脉冲光谱宽度也得到一定展宽, 进一步利用啁啾镜对和补偿片对净化后的脉冲进行色散补偿, 得到25 fs脉宽的脉冲. 利用该净化后的激光脉冲作为种子注入已有的太瓦级钛宝石啁啾脉冲放大系统中, 在输出脉冲能量250 mJ, 宽度50 fs, 对应峰值功率5 TW的情况下, 在主脉冲前100 ps以外的范围内测量验证了10¹¹的脉冲对比度.     

Wavelength tunable,high energy femtosecond laser pulses directly generated from large-mode-area photonic crystal fiber

Wavelength tunable high energy ultrashort laser pulses are generated from a large-mode-area photonic crystal fiber in anomalous dispersion (AD) regime. A simplified laser cavity design with one fine polished facet of the fiber as a cavity mirror is used. The intra-cavity dispersion compensation is achieved by a grating pair, the spatial dispersed light from which also have optical spectrum filtering effects combined with the limited aperture of the fiber core. The laser system is able to generate ultrashort pulses ranging from 494 fs (with 56 nJ pulse energy) to 1.24 ps (with 49 nJ pulse energy) at 55 MHz repetition rate. The filtering mechanism benefits the generation of high energy pulses with narrowing pulse duration in AD regime. An undulation in frequency and time domain is also observed with the increase of the pump power. Furthermore, this laser system is directly used as seed for supercontinuum generation.     

Nonlinear spectral cleaning effect in cross-polarized wave generation

The underlying mechanism of the spectral cleaning effect of the cross-polarized wave(XPW) generation process was theoretically investigated. This study shows that the spectral noise of an input spectrum can be removed in the XPW generation process and that the spectral cleaning effect depends on the characteristics of the input pulses, such as the chirp and Fourier-transform-limited duration of the initial pulse, and the modulation amplitude and frequency of the spectral noise. Though these factors codetermine the output spectrum of the XPW generation process, the spectral cleaning effect is mainly affected by the initial pulse chirp. The smoothing of the spectrum in the XPW generation process leads to a significant enhancement of the coherent contrast.     

非真空传输的高效交叉偏振滤波设计与产生

采用常规透镜设计了适用于非真空环境中交叉偏振波(XPW)产生的双透镜聚焦系统,在相对较短的距离实现了长焦透镜聚焦的效果,并测量了聚焦后的激光脉冲,发现其没有显著的非线性相位积累,保证了激光光束质量.在非真空中采用双BaF_2晶体得到了XPW系统转换效率22%,光谱1.78倍展宽的净化脉冲输出,双透镜组合聚焦形式使得双BaF_2晶体间距在13—22 cm内可保证20%以上的XPW转换效率,双晶体间距的调节冗余度提高了两个量级,极大地降低了双晶转换效率对晶体间距的依赖.这种正负透镜组合聚焦的光路设计在非真空中实现了高效稳定的XPW输出,为后续的放大应用提供了高对比度、宽光谱的高质量种子源.     

14-fs high temporal quality injector for ultra-high intensity laser

We present a chirped pulse amplification (CPA) Ti:Sa laser generating sub-15 fs pulses with expected high temporal quality. Gain-narrowing in the pre-amplifier is balanced by a variable spectral reflectivity mirror and by a fine adaptation of the saturation conditions. A crossed polarized wave generation (XPW) filter is introduced to enhance the contrast, reduce the pulse duration and improve the spectral quality. The pulses are generated at 10 Hz repetition rate, with pulse energy of 110 μJ and very clean Gaussian spectrum. The temporal contrast is evaluated by a measurement before the XPW filter and calculations of the enhancement by the filter. The potential temporal incoherent contrast is 10¹² and the coherent contrast 10¹⁰. The performance of the system makes it suitable as an injector for petawatt lasers operating in the double-CPA scheme.     

Electrically actuated elastomers for electro–optical modulators

We demonstrate the technique of cross-polarized wave (XPW) generation at 1 μm with a 310-fs optical parametric chirped-pulse amplification system at the millijoule level. We show an improvement of the temporal contrast by three orders of magnitude with an energy transmission of 22%. Additionally, we report that XPW generation preserves the beam spatial quality and shortens the pulse duration by a factor of 3^1/2, resulting in a peak power transmission of ∼35%. PACS 42.65.Re; 42.65.Yj; 42.70.Mp     

Nonlinear spectral cleaning of few-cycle pulses via cross-polarized wave (XPW) generation

The characterization of a temporal filter based on cross-polarized wave generation working in the few-optical-cycle regime is presented. We show that this device dramatically improves the spectral quality of the ultrashort seed pulses, opening the way to the production of sub-10?fs pulses with high incoherent and coherent contrast. The dispersion compensation conditions for an optimized behavior of the process are experimentally and theoretically discussed.     

Suppression of the amplified spontaneous emission in ultraintense and ultrashort lasers by a highly efficient saturable absorber

For ultraintense and ultrashort laser facilities, there are few efficient and reliable methods for suppressing the amplified spontaneous emission (ASE) generated in the last several amplifiers. In the paper, we applied a highly efficient saturable absorber to suppress the ASE. Experiments were carried out to study the absorption property of the saturable absorber and the enhancement of temporal contrast with it in the front-end system of the SILEX-I laser facility with a central wavelength of 800 nm. We also analyzed the effects of the saturable absorber on other laser parameters, such as the spectrum, the spatial beam profile, and energy stability. The results show that the saturable absorber is highly efficient and the transmissivity is as high as 90%. The temporal contrast is improved by more than one order of magnitude. Meanwhile, the saturable absorber has little impact on other parameters. With these advantages, the saturable absorber is an efficient, appropriate and convenient solution to suppressing the ASE generated in the front-end system and also in the last several amplification stages for ultraintense and ultrashort lasers.     

不同入射脉冲强度线性啁啾对BaF2晶体交叉偏振波输出特性影响的数值模拟研究

在超短飞秒激光技术中, 基于BaF₂晶体三阶非线性效应的交叉偏振波技术是目前最重要的提高脉冲时域对比度的手段之一. 针对不同入射脉冲强度下入射脉冲的线性啁啾对BaF₂晶体交叉偏振波的输出特性, 包括转换效率、光谱加宽和时域压缩的影响进行了详细的数值模拟和讨论.特别地, 详细对比研究了非饱和及强入射脉冲情况下线性啁啾对交叉偏振波输出脉冲光谱加宽的影响. 结果表明, 在强入射脉冲情况下, 交叉偏振波的光谱加宽量从非饱和情况下相对于零点线性啁啾时的对称结构, 变得不再具有对称性, 并对产生非对称性的原因进行了详细的物理解释.     

Efficient femtosecond Yb:YAG laser pumped by a single-mode laser diode

Single-mode diodes enable a particularly simple, compact and effective pumping of solid-state laser devices for many specialized applications. We investigated a single-mode, 300-mW laser diode for pumping at 935 nm a Yb:YAG laser passively mode-locked by a semiconductor saturable absorber. Relatively short pulse generation (156 fs), tunable across 1033–1059 nm has been demonstrated. An optical-to-optical efficiency of about 28% has been obtained with 320 fs long pulses. Therefore, contrarily to what previously believed, compact diode-pumped ultrafast Yb:YAG oscillators can reliably and efficiently deliver pulses in the range of ≈ 100–200 fs with few tens of mW, which are very appealing for bio-diagnostics and amplifier seeding applications.     

Master Oscillator-Power Amplifier carbon monoxide laser system emitting nanosecond pulses

To increase a peak power of carbon monoxide laser emitting nanosecond pulses a Master Oscillator-Power Amplifier (MOPA) laser system was developed. The MOPA CO-laser system employed one and the same gain medium of wide-aperture pulsed electron-beam-sustained-discharge CO-laser facility. Amplification parameters include gain and saturation intensity of amplifying media. The MOPA CO-laser system emitted a train of nanosecond pulses with peak power up to ~ 0.1 MW on a single spectral line and up to ~ 0.4 MW with multiline spectrum.     

Numerical simulation of the amplification of picosecond laser pulses in tapered semiconductor amplifiers and comparison with experimental results

We apply a traveling wave model to the simulation of the amplification of laser pulses generated by Q-switched or mode-locked distributed-Bragg reflector lasers. The power amplifier monolithically integrates a ridge-waveguide section acting as pre-amplifier and a flared gain-region amplifier. The diffraction limited and spectral-narrow band pulses injected in to the pre-amplifier have durations between 10 ps and 100 ps and a peak power of typical 1 W. After the amplifier, the pulses reach a peak power of several tens of Watts preserving the spatial, spectral and temporal properties of the input pulse. We report results obtained by a numerical solution of the traveling-wave equations and compare them with experimental investigations. The peak powers obtained experimentally are in good agreement with the theoretical predictions. The performance of the power amplifier is evaluated by considering the dependence of the pulse energy as a function of different device and material parameters.     

Diode-pumped passively mode-locked Nd:Lu0.15Y0.85VO4 laser with a single-walled carbon nanotube saturable absorber

Diode-pumped passively mode-locked Nd:Lu_0.15Y_0.85VO₄ laser with a single-walled carbon nanotube saturable absorber was demonstrated for the first time, to our best knowledge. The laser generated 19-ps pulses with maximum output power of 902 mW for the incident pump power of 6.5 W at the central wavelength of 1064 nm, giving an optical conversion efficiency of 14%. The continuous wave mode-locking conditions of single-walled carbon nanotube saturable absorber were analyzed and discussed.     

Generation of long bursts of high-repetition-rate arbitrarily shaped optical pulses using time lens

A technique for the generation of long ultrahigh-speed bursts of optical pulses with arbitrary shapes is proposed. A laser pulse is temporally chirped by a time lens and then passes through a filter with a reconfigurable periodic spectral response, which produces time-delayed replicas of the chirped pulse and recombines them. As a result of the temporal interference between the replicas, the chirped pulse is broken up into short pulses with the shape determined by the chosen filter response. It is demonstrated that the filter acts on a long chirped optical pulse as a temporal modulator with a periodic modulation function. The modulation frequency and bandwidth of the modulator can be much higher than for commercially available high-frequency modulators. The additional advantage of this modulator is the arbitrary shape of the modulation function. A 2.4 ns burst of nearly flat-top pulses with a repetition rate of about 400 GHz is obtained in numerical simulations. In addition, the technique proposed can act as a pulse repetition rate multiplier and a pulse compressor. A repetition rate of 1.589 THz and an individual pulse width of 212 fs are achieved in simulations for a 9.7 ns sinusoidally phase modulated pulse burst.