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.     

Spectral-Phase-Modulated Cross-Polarized Wave for Chirped Pulse Amplifier with High Contrast Ratio

We demonstrate a high-quality cross-polarized-wave filter based on spectral phase modulation. Driven by wellcompressed spectral-phase fully-compensated fundamental laser pulses, the filter stretches the pulse bandwidth from 35 nm to 70 nm with a conversion efficiency of 20%. After implementing the filter into a femtosecond TW Ti:sapphire laser system, we generate 40 mJ output pulse energy with pulse duration of 18.9 fs. The temporal contrast of the compressed pulse is enhanced to 10~9.     

Generation of 17-TW 23-fs Pulses with a Two-Stage Ti：Sapphire Amplifier at Repetition Rate 10 Hz

We have developed a two-stage Ti：sapphire amplifier system which can produce 17- TW/23-fs pulses at a repetition rate 10Hz. A birefringent plate is used in the regenerative amplifier to alleviate gain narrowing, while an all- reflective cylindrical-mirror-based pulse stretcher and an acousto-optic programmable dispersive filter （AOPDF） are used to compensate for the higher order dispersion of the system.     

10 GHz, 2.4 ps pulse generation using a single-stage dual-drive Mach-Zehnder modulator

This Letter reports on picosecond pulse generation at a repetition of 10 GHz by using a single-stage electro-optic LiNbO(3) Mach-Zehnder modulator, stressing the simplicity of its setup. It is analytically and experimentally proved that dual-arm modulation with in-phase sinusoidal signals having slightly different amplitudes generated a highly coherent optical comb with a great spectral flatness and a parabolic phase relationship in its spectrum. The generated comb was Fourier synthesized and shaped into an ultrashort pulse train with an optical bandpass filter and a dispersive fiber. The pulse source was highly stable, exhibiting an ultralow timing jitter of less than 130 fs.     

Generation of 10-GHz duty-cycle tunable square optical pulse in an SOA-based mode-locked fiber laser

We demonstrate the generation of 10-GHz optical square pulses by injecting a picosecond pulse train into an SOA-based mode-locked fiber laser. The novel scheme exploits nonlinear effects and gain saturation phenomenon in the semiconductor optical amplifier (SOA). This technique uses gain-compression dynamics between the input pulses and the generated ones in gain-saturated SOA to form square pulses. The center wavelength of the generated optical square pulse can be tuned from 1530 to 1570 nm by adjusting the center wavelength of the optical band pass filter (OBPF) in the SOA-based mode-locked fiber ring laser. The duty cycle of the output pulse can be tuned from 12.7 to 88.4%, which strongly depends on the input power and intra-cavity power.     

Intracavity acousto-optic programmable gain control for ultra-wide-band regenerative amplifiers

We have developed a new Brewster-cut acousto-optic programmable gain control filter to optimize the spectral losses inside a regenerative cavity. This spectral amplitude control combined with an acousto-optic programmable dispersive filter for the spectral phase control optimizes, in a few steps of a direct algorithm, the performance of a 10-Hz CPA Ti:sapphire laser to 18-fs pulse duration, 1.2% RMS energy stability and 10⁷ contrast ratio. PACS 42.65.Re; 42.79.Jq     

Picosecond pulse contrast enhancement by use of polarization rotation in crystals with the second-order nonlinearity

We report on the implementation of intensity-dependent filter for picosecond pulse temporal cleaning based on an effect of pulse polarization rotation during the unbalanced phase matched second harmonic generation. The pulse contrast enhancement by more than two orders of magnitude and filter transmission of ∼25% has been demonstrated at pulse intensities of <1 GW/cm². A simple and reliable all-solid-state filter scheme was integrated in a two-stage picosecond Nd:YAG regenerative amplifier seeded with pJ-energy femtosecond pulses of the Yb:KGW oscillator, and allowed us to reduce the ASE intensity level down to 1.5×10⁻⁸ with reference to the intensity of main 100 ps and 100 μJ pulse.     

Bandwidth expansion and pulse shape optimized for 10 PW laser design via spectral shaping

We demonstrated a scheme of bandwidth expansion and pulse shape optimized to afford 10 PW laser design via spectral shaping, which uses the existing Nd:glass amplifier chain of the SG PW laser. Compared to the amplified pulse with a gain-narrowing effect, the required parameters of injected pulse energy, spectral bandwidth, and shape are analyzed, together with their influence on the system B-integral, energy output capability, and temporal intensity contrast. A bandwidth expansion to 7 nm by using LiNbO₃ birefringent spectral shaping resulted in an output energy of 2 kJ in a proof-of-principle experiment. The results are consistent with the theoretical prediction which suggests that the amplifier chain of SG PW laser is capable of achieving 6 kJ at the bandwidth of 7 nm and the B-integral < π . This will support a 10 PW laser with a compressed pulse energy of 4.8 kJ (efficiency=80%) at 480 fs.     

可编程声光色散滤波器在10 Hz飞秒再生放大系统中的应用

报道了在10 Hz飞秒再生放大超快激光系统中,运用可编程声光色散滤波器(AOPDF)对注入再生放大的种子光进行频谱整形和色散预补偿,使钛宝石激光系统输出光脉冲谱线半峰全宽由原来的38 nm展宽到66 nm,压缩后输出脉冲宽度从35 fs减小到20 fs。实验结果表明,利用可编程声光色散滤波器能够同时独立进行光谱整形和大范围内色散补偿的特性,可以有效抑制啁啾脉冲放大过程中存在的谱线增益窄化效应,补偿钛宝石激光系统中存在的残留色散。这为进一步研制小于20 fs,10 TW量级钛宝石激光系统奠定了基础。     

Phase-dependent loss due to nonadiabatic ionization by sub-10-fs pulses

For the first time to the author's knowledge, the effect of phase-dependent loss that can be used to stabilize the phase of a sub-10-fs laser pulse with respect to the envelope is shown. This loss is caused by the nonadiabatic response of an atom illuminated by a short pulse, which leads to the dependence of the tunneling ionization on the absolute phase of the pulse. The phase selectivity can be employed as a phase-dependent filter within the laser cavity with important applications in nonlinear optics in the single-cycle regime.     

Fiber-array-based detection scheme for single-shot pulse contrast characterization

We propose and demonstrate a fiber-array-based detection scheme for single-shot pulse contrast characterization. The parallel to serial mapping using the fiber array allows the use of a high-sensitivity photomultiplier tube and eliminates the external neutral density filter, both resulting in significantly improved dynamic range. The proof-of-principle experiments show a dynamic range of 2x10(7):1. The demonstrated technique can be readily applied to existing instruments for single-shot pulse characterization.     

Highly efficient nonlinear filter for femtosecond pulse contrast enhancement and pulse shortening

We propose a highly efficient scheme for temporal filters devoted to femtosecond pulse contrast enhancement. The filter is based on cross-polarized wave generation with a spatially suger-Gaussian-shaped beam. In a single nonlinear crystal scheme the energy conversion to the cross-polarized pulse can reach 28%. We demonstrate that the process enables a significant spectral broadening. For an efficiency of 23% the pulse shortening is estimated to 2.2, leading to an intensity transmission of the nonlinear filter of 50%.     

Tunable 12×10 GHz mode-locked semiconductor fiber laser incorporating a Mach-Zehnder interferometer filter

A stable multiwavelength mode-locked semiconductor fiber ring laser incorporating a fiber Mach-Zehnder interferometer (MZI) filter was proposed and experimentally demonstrated. A semiconductor optical amplifier (SOA) serves as an optically controlled mode-locking element due to gain exhaustion caused by external injected optical pulses. Another SOA severs as a constant-gain medium. A fiber MZI filter with a temperature control is incorporated into the fiber ring cavity to acquire a stable and tunable multiwavelength oscillation. Twelve wavelengths are synchronously mode-locked at 10 GHz, pulse width of mode-locked pulse are about 30 ps. Proposed multiwavelength mode-locked semiconductor fiber ring laser has some distinct advantages, such as simple and compact structure, easy integration, convenient tuning, good stability, potential high repetition rate operating, which has potential application in the future WDM communication system.     

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.     

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.     

基于10 m光子晶体光纤的放大自相似锁模振荡器研究

基于单根10 m大模场面积保偏光子晶体光纤, 搭建了带有色散图的放大自相似振荡器; 通过仔细调节腔内色散量的大小以及位于色散补偿端的端镜前的狭缝位置和大小, 实现了稳定的锁模运转, 获得了抛物线形脉冲输出. 输出脉冲的重复频率为8.6 MHz, 脉冲宽度为6.2 ps, 光谱宽度为3.84 nm, 平均功率820 mW, 对应单脉冲能量95 nJ. 这是第一次在自相似振荡器中直接获得重复频率在10 MHz 以下的脉冲输出, 95 nJ也是目前自相似振荡器直接输出的最高脉冲能量. 通过数值模拟证实了在第一个光栅的零级反射处和狭缝滤波后可以分别实现抛物线型脉冲和高斯脉冲的两种锁模脉冲输出.     

光谱扫描滤波法提升飞秒激光信噪比的理论分析

在时间--频率域中,啁啾脉冲被淹没在放大自发辐射等噪声中,且其瞬时频率线性分布,基于此,提出采用光谱扫描滤波的办法来提升高功率飞秒激光系统的信噪比,并以法布里--珀罗干涉仪作为光谱扫描滤波器,对该法提升信噪比的效果进行了详细的理论分析.采用短时傅里叶变换方法,研究了光谱扫描滤波器对自发辐射放大(ASE)的滤波效果,数值分析表明,在时间抖动为-2 ps到2 ps之间、光谱扫描滤波器的通带宽度为0.4 nm条件下,该法能将飞秒激光系统的信噪比提升2个量级,而啁啾信号光以超过90%的透光率通过光谱扫描滤波器;级联光谱扫描滤波方式能进一步提升飞秒激光信噪比.     

滤波位置相关的全正色散掺Yb3+锁模光纤激光器的实验研究

采用非线性偏振旋转锁模,构建了一种全正色散掺Yb³⁺光纤环形激光器.在该激光腔内采用了10 nm带宽的光纤滤波器提供附加的自振幅调制.通过改变滤波器在腔内的位置,实验证明了滤波器位置对高啁啾脉冲的整形过程起到重要的作用.在320 mW的抽运功率下,最终获得平均功率为922 mW,脉冲重复频率为266 MHz,脉冲宽度为62 ps的稳定脉冲输出,单脉冲能量达35 nJ的优化结果.通过数值模拟与实验结果的验证表明,两者相符很好. 关键词： 锁模光纤激光器 全正色散 滤波效应 高啁啾脉冲     

电光双折射扫描滤波法提高啁啾脉冲对比度

提出了一种通过扫描滤波来提高啁啾脉冲对比度的方法,并分析了该方法的原理。与传统的非线性滤波技术不同,扫描滤波方法消除预脉冲时,选通原理是基于光的频率而不是强度。以电光双折射扫描滤波器为例,说明了扫描滤波法的具体应用。通过数值模拟及实验研究,发现电光双折射扫描滤波法可以有效地消除主脉冲以前几百ps的预脉冲,并且主脉冲同时还能保证较高的透过率。     

6.1 MW, 10 kHz Side-Pumped Burst-Mode Nd:YAG Laser

We present a compact high-peak-power, high-repetition-rate burst-mode laser from a master-oscillator power amplifier (MOPA) at 1064 nm for laser-based measurement applications. The oscillator is an 808 nm pulsed laser diode side-pumped acousto-optical (A-O) Q-switched Nd:YAG laser at repetition rates ranging from 10–100 kHz, producing a pulse train with a pulse number of 2–25. The maximum output energy of the oscillator is 15.6 mJ at 10 kHz, whereas it is 1.7 mJ at 100 kHz. After twostage amplifiers, a single-pulse energy of 85.2 mJ with a pulse-width of 14.5 ns is achieved at 10 kHz, which produces a peak power of 6.1 MW. At 100 kHz, the total burst energy reaches 220 mJ with a single-pulse energy of 8.8 mJ in the pulse burst laser system.