掺镱硼酸钙氧钇飞秒激光器及在拉锥光纤中产生跨倍频程超连续光

跨倍频程超连续光谱的产生是光学频率梳系统中测量载波包络相移频率的关键.本文采用拉锥单模光纤作为非线性光谱展宽介质, 将半导体激光(LD)抽运的掺镱硼酸钙氧钇(Yb:YCOB)振荡器输出的飞秒激光耦合到该拉锥光纤中, 通过飞秒激光在光纤中发生的相位调制、四波混频等非线性效应将光谱展宽至超过倍频程的范围.振荡器输出的飞秒激光脉冲宽度为130 fs, 中心波长为1052 nm, 重复频率为76.8 MHz, 平均功率为620 mW, 耦合进单模拉锥光纤后获得了光谱覆盖范围从550 nm至1350 nm的跨倍频程超连续光谱, 最大输出平均功率为323 mW, 耦合效率达到52%.为进一步实现全固态飞秒激光光学频率梳提供了重要基础.     

飞秒激光在光子晶体光纤中产生超连续光谱机制的实验研究

报道了利用零色散在780nm处的光子晶体光纤与纳焦耳量级的飞秒激光脉冲相互作用的实验结果.实验使用35fs，中心波长810—840nm，单脉冲能量可达14nJ的飞秒激光光源获得了超过一个倍频程的平坦超连续光谱（500—1100nm）.在不同功率、不同中心波长、不同啁啾和有无直流成分的多种飞秒脉冲激光的条件下，研究了超连续光谱的产生情况.并对一系列现象进行了对比，分析了超连续光谱产生的机制. 关键词： 光子晶体光纤 飞秒脉冲激光 超连续光谱     

基于多光子脉冲内干涉相位扫描法对飞秒激光脉冲进行相位测量和补偿的研究

研制了一套基于多光子脉冲内干涉相位扫描方法的可以同时对飞秒激光脉冲进行相位测量和补偿的实验系统装置.实验中,通过自主研发的LabVIEW程序控制液晶空间光调制器和光纤光谱仪,对待测飞秒激光脉冲施加相位扫描,并同时记录受到调制的飞秒激光脉冲的倍频光谱,得到了多光子脉冲内干涉相位扫描(MIIPS)轨迹图.通过MIIPS轨迹图的三次测量和迭代运算,还原出了经过预先啁啾调制的中心波长约为810 nm、重复频率为1 kHz的飞秒激光脉冲的光谱相位,测量精度在0.1 rad以内.根据测量结果,利用液晶空间光调制器对该飞秒激光脉冲进行相位补偿,得到了近似傅里叶变换极限的飞秒激光脉冲.这一装置将在多光子显微成像、脉冲整形、飞秒激光光谱学等众多领域发挥重要作用.     

飞秒激光烧蚀硅表面产生等离子体的发射光谱研究

对中心波长为800 nm,脉宽为100 fs的激光脉冲烧蚀空气中硅(111)产生的等离子体发射光谱进行了时间和空间分辨研究. 结果表明,在等离子体羽膨胀初期(小于50 ns时间范围内),等离子体发射光谱主要由连续光谱构成,此后连续光谱强度逐渐减弱,线状光谱开始占主导地位;在羽体膨胀过程中离子谱线的存在时间短于原子谱线的存在时间. 由时间分辨发射光谱发现在羽体膨胀过程中等离子体辐射波长存在红移现象,波长红移量随时间演化呈二次指数衰减. 最后给出等离子体发射光谱谱线强度的时空演化规律. 关键词： 飞秒激光 脉冲激光烧蚀 等离子体 发射光谱     

基于液晶空间光调制器的太赫兹波频谱调制

频谱可调制的太赫兹波具有广泛的应用价值。利用一台纯相位式的液晶空间光调制器对飞秒激光脉冲进行空间整形,通过改变飞秒激光脉冲的横向空间分布,实现太赫兹波频谱的调制。在实验中,利用光泵浦整流方式产生太赫兹波,并利用太赫兹时域光谱系统对太赫兹信号进行探测。通过GS算法在液晶空间光调制器上加载不同的相位图,获得了不同的飞秒激光脉冲横向空间分布。通过改变探测距离和飞秒脉冲的空间分布参数,实现了太赫兹波频谱的调制。还利用菲涅尔衍射算法对这一过程进行了理论模拟,理论模拟结果与实验结果吻合的较好,这充分说明了基于飞秒脉冲空间整形的太赫兹光谱调制技术的可行性。     

整形飞秒激光脉冲在熔融石英中的成丝控制研究

利用基于液晶空间光调制器的飞秒激光脉冲整形技术,对飞秒激光在熔融石英中形成等离子体丝的过程进行优化控制研究。实验结果表明:通过脉冲整形可以在固体介质中的指定位置产生等离子体丝。实现了整形脉冲在熔融石英中成丝起点的长距离可控移动,最大移动量达到5.4 mm。通过求解(3+1)维非线性薛定谔方程,对整形脉冲在熔融石英中的成丝过程进行理论模拟研究,得到了与实验一致的结果。研究结果表明:等离子体丝起始位置是由整形飞秒脉冲的中心峰值强度和包络分布决定的。     

对超短脉冲强激光在大气通道中产生的三次谐波偏振特性及白光光谱调制特性的研究

研究了超短脉冲强激光在大气中传输产生电离通道后辐射的三次谐波强度随激光偏振的变化.此外，还研究了产生的白光辐射在短波长上的有规律的谱调制，分析了白光谱调制的产生机理. 关键词： 超短脉冲激光 大气通道 超连续辐射 三次谐波     

飞秒激光诱导锗等离子体辐射光谱特性

由于飞秒激光脉冲宽度小于靶材电子—晶格热弛豫时间,飞秒激光烧蚀靶材过程以及诱导击穿产生的等离子体膨胀动力学过程与纳秒激光作用过程不同,因此研究飞秒激光诱导等离子体发射光谱特性对于研究飞秒激光烧蚀机制以及飞秒激光诱导等离子体的膨胀动力学过程非常重要。Ge材料是一种常用的中远红外探测器以及光学元器件材料,对中心波长为800 nm,脉宽为50 fs的激光脉冲烧蚀空气中Ge靶材产生的等离子体发射光谱强度的时间和空间演化规律研究,并探讨了飞秒激光脉冲能量对等离子体发射光谱强度的影响规律。实验结果表明在等离子体羽膨胀初期,飞秒激光诱导Ge等离子体发射光谱主要由线状光谱和连续光谱构成,在200 ns时间内连续光谱强度逐渐减弱,线状光谱开始占主导地位。通过探测Ge等离子体的时间分辨发射光谱,随着等离子体的快速膨胀,等离子体发射光谱强度随着时间的增加呈现先增加后下降变化,在335 ns达到最大。通过探测Ge等离子体的空间分辨发射光谱,随着距离Ge靶材表面的位置增加,等离子体发射光谱强度随远离Ge靶材表面距离增加呈现先增加后下降变化,在0.8 mm位置达到最大。由于存在等离子体自吸收机制,等离子体发射光谱...     

PMMA材料在线偏振和圆偏振光下的超连续光谱特性分析

飞秒激光在透明材料加工过程中会出现超连续光谱现象。在阐述超连续光谱产生的原理的基础上,为了分析PMMA材料在不同偏振光下产生的超连续光谱的阈值,设计了线偏光和圆偏光及不同能量加工PMMA材料的实验方案。利用光谱仪对产生的超连续光谱信号进行采集及处理,分析出不同能量下的线偏振(TE和TM)和圆偏振两种偏振态的超连续光谱的变化规律,并对比了相同能量下线偏振和圆偏振的超连续光谱的区别。实验中采用脉宽160 fs、中心波长为775 nm的飞秒激光,实验结果表明,同一偏振下能量越大,光谱谱宽越宽;通过对比不同能量下的光谱特性,观测出产生超连续光谱的阈值, 线偏振光的阈值为0.46 μJ,圆偏振光的阈值为0.586 μJ;对比相同能量下的线偏振和圆偏振光,线偏振的谱宽比圆偏振的宽。     

1040nm泵浦的光子晶体光纤超连续光谱产生性能退化研究

光子晶体光纤已经被广泛应用于由飞秒脉冲激光源产生超连续光谱。当激光源的重复频率较低时,由光子晶体光纤产生的超连续光谱随时间的变化过程较为缓慢,通常不被注意到。而在天文光谱仪定标等应用中,需要使用GHz至几十GHz量级的高重复频率激光源。此时,可观察到光子晶体光纤的超连续光谱产生性能在有限时间内产生显著的退化。在1 040 nm飞秒激光泵浦条件下,通过测试三种不同气孔占空比的光子晶体光纤的超连续光谱产生性能演化,发现超连续光谱的退化进程随光纤气孔占空比的增大而加速。观察发生光谱退化后的光子晶体光纤样品,发现在光纤上超连续光被产生的区段出现多个不同颜色的亮点,呈现有方向性的光泄露现象。针对光泄露现象,通过测量光纤的吸收光谱线,证实了实验中超连续光谱退化的主因并非是光纤熔融石英材料中大量非桥氧色心产生。针对光泄露具有方向性这一特征,提出了经由多光子吸收作用在光纤纤芯中形成长周期光栅的理论。为探究影响光子晶体光纤超连续光谱产生性能的退化的因素,以达到光谱退化抑制的目的,首先通过改变了光纤的拉锥参数,期望增强光纤熔融石英材料的光子耐受性。实验结果证实了该方法的有效性较为有限。其次,从保持激光源的平均功率,降低激光脉冲的峰值功率和保持激光脉冲的峰值功率,降低激光源平均功率两个方面入手,对激光源进行调制。实验结果证明,光纤单位时间内接受的高峰值功率脉冲总量是影响其超连续光谱产生性能的最重要因素。在天文光谱仪定标的应用中,对超连续光谱光功率的需求并不高,使用斩波器降低光子晶体光纤入射光的平均功率是减缓超连续光谱产生性能退化过程的有效且简单可行的方法。     

Intensity modulation of filaments by shaped laser pulses in fused silica

We show the intensity control of filamentation in fused silica by temporally shaping the femtosecond laser pulse.The arbitrary control of filamentation intensity has been obtained by the feedback control based on the genetic algorithm, and the peak intensity of filament has changed from about 670 to around 2100(charge-coupled device counts). This modulation is in qualitative agreement with the simulation results. It is shown that the control of the intensity is realized by modulating the peak power of the shaped pulse.     

温度对飞秒激光脉冲在NaCl溶液中成丝产生的超连续谱的影响

利用飞秒激光脉冲在NaCl溶液中成丝,研究了溶液温度对飞秒激光成丝过程中产生的超连续谱的影响.发现在激光脉冲能量较低时,溶液温度对超连续谱的影响几乎可以忽略;而在激光脉冲能量较高时,随着NaCl溶液温度的升高,超连续谱呈现出被压缩的趋势.此外,在激光脉冲能量较高的情况下, NaCl中会产生大量的气泡.通过分析,得出了飞秒激光在溶液中成丝产生的气泡是影响超连续谱发射的主要因素.     

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.     

Control of femtosecond filamentation by field-free revivals of molecular alignment

We show that the filamentation dynamics of a femtosecond laser probe pulse can be readily controlled by properly matching it to the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation with an advancing ultrashort pump pulse. Several features of the filamentation process including supercontinuum generation, the length of the plasma channel generated in the wake of the filament, the associated secondary radiations and the multiple filamentation pattern are all easily modified by tuning the cross phase modulation induced by the field-free revivals of molecular alignment, through the delay between the pump and the probe pulses. We show that molecular alignment can also be used to generate conical waves with extremely short intensity spike called shocked X-waves and to further tune the frequency of a few-cycle laser pulse in the wake of a self-guided intense filament.     

Multidimensional control of femtosecond pulses by use of a programmable liquid-crystal matrix

We report the automated generation of high-fidelity spatiotemporally shaped femtosecond pulses by use of an optically addressed, two-dimensional liquid-crystal spatial light modulator. A single input pulse was divided into many independent regions, and each region was shaped temporally. By changing the imaging geometry we accomplished either real-space or wave-vector shaping.     

Coherent enhancement of broadband frequency up-conversion in BBO crystal by shaping femtosecond laser pulses

An optimal feedback control of broadband frequency up-conversion in BBO crystal is experimentally demonstrated by shaping femtosecond laser pulses based on genetic algorithm, and the frequency up-conversion efficiency can be enhanced by ∼16%. SPIDER results show that the optimal laser pulses have shorter pulse-width with the little negative chirp than the original pulse with the little positive chirp. By modulating the fundamental spectral phase with periodic square distribution on SLM-256, the frequency up-conversion can be effectively controlled by the factor of about 17%. The experimental results indicate that the broadband frequency up-conversion efficiency is related to both of second harmonic generation (SHG) and sum frequency generation (SFG), where the former depends on the fundamental pulse intensity, and the latter depends on not only the fundamental pulse intensity but also the fundamental pulse spectral phase.     

基于光纤中超短脉冲非线性传输机理与特定光谱选择技术的多波长飞秒激光的产生

高集成、高可靠性宽调谐飞秒激光源在超快光谱学、量子光学及生物成像等研究与应用领域具有重要价值.如在生物多光子显微成像中,具有适中能量的宽调谐飞秒激光源不仅可满足多种生物组织荧光激发所需的峰值功率与激发波长,而且也可以显著提升非线性荧光产生效率、成像分辨率以及增大成像穿透深度.采用自主研发的高可靠性全保偏光纤飞秒激光器作为抽运源,基于低色散光纤中高峰值功率飞秒激光脉冲非线性传输引起的光谱加宽机制,本文开展了多波长全光纤飞秒激光产生技术研究.通过采用中心波长在980, 1000,1050, 1070与1100 nm的带通滤波片选择性地对单模光纤输出光谱中最左边与最右边光谱旁瓣进行滤波,在上述中心波长处分别可获得203, 195, 196, 187与194 fs的激光输出.本文提出的基于全光纤飞秒激光脉冲在单模光纤中非线性传输引起的光谱加宽机制与特定光谱选择技术的实验方案为高集成、高可靠性宽调谐飞秒激光源的实现提供了新的研究途径.     

基于固体介质的倍频程连续光谱产生的研究进展

超快激光经过透明介质时由于非线性作用光谱会得到展宽,甚至能够产生超过一个倍频程的相干超连续光谱,这样的光源能够压缩得到几个甚至单个光周期的超短脉冲,并在现代超快科学的各个领域得到了广泛应用.实验中已经在气体、液体和固体中都观测到了光谱的展宽,目前较为成熟的方法是使用充满惰性气体的空芯光纤和具有高非线性效应的固体材料展宽光谱.但空芯光纤由于芯径限制无法用于高能量激光脉冲的光谱展宽,而固体材料又容易被高功率密度的脉冲激光损坏.随着激光技术的发展其脉冲能量不断提高,一种新的、利用多片薄固体介质实现光谱展宽的方式被提出.多片薄的非线性介质可以实现光谱展宽的逐片累积,而且避免了激光在介质中因自聚焦产生过高功率密度带来的损坏.目前使用这种方法已经在实验上得到了近毫焦尔量级的倍频程光谱,覆盖了近紫外到中红外的整个区域,并实现了脉冲压缩.本文简要回顾了超快激光在固体中光谱展宽的发展历程,概述了新型薄片固态介质产生超连续光谱的原理,对近年来使用此新方法的实验进行了简要分析,并对其发展前景进行了展望.     

Filamentational instability of partially coherent femtosecond optical pulses in air

The filamentational instability of spatially broadband femtosecond optical pulses in air is investigated by means of a kinetic wave equation for spatially incoherent photons. An explicit expression for the spatial amplification rate is derived and analyzed. It is found that the spatial spectral broadening of the pulse can lead to stabilization of the filamentation instability. Thus optical smoothing techniques could optimize current applications of ultrashort laser pulses, such as atmospheric remote sensing.     

Spatio-temporal evolution scenarios of femtosecond laser pulse filamentation in fused silica

We investigated the evolution of femtosecond laser pulses at different wavelengths corresponding to normal, zero, and anomalous regimes of group velocity dispersion (GVD) in fused silica. The laser pulse filamentation in different GVD regimes under the same similarity parameters was first considered. It was established numerically that the scenario of the pulse filamentation depends both on temporal factors, which are determined by pulse GVD and self-phase modulation, and spatial factors associated with Kerr self-focusing and plasma defocusing. In presence of strong normal GVD the dispersive stretching causes, a pulse power decrease followed by lowering of the intensity in filament, electron density reduction in plasma channel, and suppressing of the refocusing. For zero GVD the multipeak regime of radiation propagation is realized in the filament as a result of recurring self-focusings of powerful pulse tail, which was defocused in laser plasma. When GVD is anomalous a sequence of ??light bullets?? with duration about 10 fs forms in the filament. And the peak intensity in ??light bullet?? stays the same ?? 5 × 10¹³ W/cm². In the regime of anomalous GVD power is transferred from the pulse edges to its center, where the repeated self-focusings occur and form a ??light bullet?? sequence.