飞秒激光脉冲的谐波频率分辨光学开关法测量研究

建立了一台谐波频率分辨光学开关法(FROG)飞秒脉冲测量装置，利用该装置进行了掺钛蓝宝石飞秒激光脉冲的测量研究.在二次谐波自相关测得的时域和频域信号基础上，结合对信号光强度分布的计算机迭代处理，得到了有关飞秒激光电场、光谱及其相位的信息，所得脉宽与干涉测量的结果基本一致. 关键词： 频率分辨光学开关法(FROG) 迭代计算 飞秒激光 自相关     

超短激光脉冲波形的单次测量技术

提出了一种测量单次超短激光脉冲波形的三阶相关技术.该技术有别于频率分辨光开关法或自参考光谱相位相干电场重构法,技术上采用级联的方式,借助倍频晶体以及和频晶体的非共线频率转换,将双延迟的三阶强度相关信号转换为对应的二维空间强度分布,通过测量双延迟的二维三阶相关信号,无需测量光谱信息,利用解析方法获得脉冲的时间波形;再将它与对应的光谱相结合,利用Gerchberg-Saxton算法,可以准确获得激光场强的时间特性.该测量技术结构简单、算法简便,既适用于单次超快激光脉冲测量,也适用于高重频测量.     

覆盖可见光波长的掺Er光纤飞秒光学频率梳

飞秒光学频率梳波长覆盖范围向可见光波长扩展对于碘稳频激光的绝对频率测量以及光钟研究中钟激光的绝对频率测量都具有十分重要的意义. 本文在自行研制掺Er光纤飞秒光学频率梳的基础上, 采用放大-倍频-扩谱的方案, 实现了激光输出波长向可见光波长的扩展. 掺Er光纤飞秒光学频率梳输出的一部分光激光脉冲, 功率约为8 mW, 首先经掺Er光纤放大器将功率提高到531 mW, 此后利用MgO: PPLN晶体倍频, 倍频后激光的功率为170 mW, 倍频效率为32%, 脉冲宽度为85 fs. 倍频后的激光通过光子晶体光纤进行光谱展宽. 通过优化入射光偏振状态可以实现波长覆盖500-1000 nm, 输出功率为85 mW, 耦合效率为50%. 采用小型化碘稳频532 nm Nd: YAG激光器输出激光与光学频率梳光谱展宽后的激光进行拍频可以获得30 dB的拍频信号. 覆盖可见光波长的掺Er光纤飞秒光学频率梳为可见光范围内激光的绝对频率测量提供了技术手段.     

利用棱镜减小自衍射信号光角色散的研究

自衍射效应在飞秒激光领域有非常重要的应用,如提高飞秒脉冲的时域对比度、作为自参考光谱干涉脉冲测量法的参考光、作为频率分辨光学开关法的信号光等.然而,具有较宽光谱带宽的飞秒激光在自衍射效应过程中产生的信号光存在明显的角色散,这给自衍射效应的应用带来不利影响.本文研究发现通过在自衍射效应的一个光路中加入等腰直角棱镜,当到棱镜的入射角为23fi时可以明显地减小飞秒激光脉冲自衍射信号的角色散.这就为以后将自衍射效应更好的应用到飞秒激光脉冲的研究提供了有用的参考.     

在空间-时间域测量飞秒脉冲

依据飞秒脉冲测量中的空间-时间耦合理论,通过采用空间分辨和光谱分辨在时间和空间域对飞秒脉冲强度和相位进行测量.测量装置主要由两个光学元件组成,实现了空间和光谱干涉.待测脉冲和参考脉冲通过一个绕中心轴转过某一角度的二维衍射光栅,产生倾斜的二维阵列全息图;一个带通滤波片对全息图进行分离,从而每个全息图含有一个唯一的频率.用照相机记录下所有的全息图,可以在时间和空间域再现出待测脉冲,并给出了测量结果. 关键词： 飞秒脉冲 时间-空间耦合 二维衍射光栅 带通滤光片     

用反射式达曼光栅产生飞秒激光双脉冲

采用反射式达曼光栅建立了一种产生飞秒激光双脉冲的新装置.由于采用反射式结构，避免了材料色散和吸收导致的脉冲畸变，并构建了一台二次谐波-频率分辨光学开关装置对产生的双脉冲进行了测量.实验结果表明可以实现脉冲强度相等、时间宽度相同、不同间隔的双脉冲输出.产生双脉冲的装置在飞秒激光领域有着应用的价值. 关键词： 飞秒激光双脉冲 达曼光栅 二次谐波-频率分辨光学开关     

快速扫描频率分辨光学开关装置测量超短激光脉冲

频率分辨光学开关法是目前测量超短激光脉冲的主流方法之一.本文比较了三大类二次谐波频率分辨光学开关系统的特点和适用范围,提出将标准二次谐波频率分辨光学开关法改装成一种快速扫描频率分辨光学开关法(frequency-resolved optical gating, FROG)装置.利用信号发生器输出的正弦信号同步地驱动音圈电机和扫描振镜,其中音圈电机带动直角反射镜往复运动可实现快速的延时扫描,与此同时扫描振镜快速转动进而按照延时顺序将自相关信号光谱反射至面阵相机感光面上的不同位置.该正弦信号还用于触发面阵相机持续曝光,即可拍摄到一幅完整的FROG迹线图,曝光时间可小于1 s.该方案在需要记录较大矩阵FROG迹线图的情形颇具优势,例如可实现色散大的啁啾脉冲和结构复杂的超短脉冲的实时测量.通过测量从自锁模钛宝石激光器输出的飞秒脉冲以及被200 mm厚的BK7玻璃块展宽后的啁啾脉冲的结构,证实了该装置的实用性.     

飞秒时间分辨近场光学系统实现及其应用

结合飞秒光脉冲和近场光学显微镜，成功实现了飞秒时间分辨近场光学系统.系统通过高频声光调制和差频锁相探测，极大提高了信噪比并消除了抽运、探测光本底信号，从而在收集模式下测得了飞秒时间分辨的透射光微弱信号变化.同时获得了80nm的空间分辨和小于200fs的时间分辨测量.利用该实验系统，研究了金纳米结构的热电子弛豫动力学过程，观察到不同位置间热电子弛豫动力学的差异. 关键词： 飞秒近场 扫描近场光学显微镜 飞秒光脉冲 金纳米颗粒     

基于瞬态光栅频率分辨光学开关装置的阿秒延时相位控制

操控多路激光脉冲之间的相对延时（相对相位）对于亚周期相干合成技术意义重大.当周期量级脉冲之间的相对延时接近数十飞秒时,常见的飞秒脉冲测量手段已无法满足脉冲之间相对相位的精确调控需求.本文基于瞬态光栅频率分辨光学开关装置,精确反演出脉冲之间的相对相位.此方案不仅有助于直接产生亚周期（亚飞秒）脉冲,还可应用于时间隐身学和二维相干光谱学等相关领域.     

飞秒激光脉冲的频率分辨偏振光学开关法测量研究

建立了一台频率分辨偏振光学开关（PG FROG）法飞秒脉冲测量装置，利用该装置对“极光Ⅱ号”飞秒激光放大系统进行了测量.在利用偏振光学开关法测得的时域和频域信号基础上，结合对信号光强度分布的计算机迭代处理，得到了有关飞秒激光电场、光谱及其相位的信息；并且对系统工作在不同状态时的激光脉冲进行了测量和比较分析，给出了有关该系统较详细的电场、光谱、相位以及啁啾状况.结果显示，当系统工作在零啁啾附近时，该系统输出的激光脉冲的电场、光谱和相位分布较规则，相位起伏较小；当系统偏离零啁啾状态时，虽然电场和光谱变化不很明显，但相位分布变化剧烈. 关键词： 频率分辨偏振光学开关(PG FROG)法 飞秒激光 自相关     

Amplitude and phase measurement of mid-infrared femtosecond pulses by using cross-correlation frequency-resolved optical gating

We describe a cross-correlation-based frequency-resolved optical gating (XFROG) technique for simultaneously measuring the amplitude and phase of two ultrashort pulses that have different wavelengths but are derived from a common mode-locked oscillator. A measurement is presented in which 4.0-mum mid-IR pulses from a synchronously pumped femtosecond optical parametric oscillator (OPO) are characterized by mixing with the 770-nm OPO pump pulses. Details of the pulse-retrieval algorithm are included, together with examples of pulse data retrieved from the experimentally measured XFROG trace.     

Femtosecond Kerr-lens autocorrelation

An autocorrelation measurement of femtosecond laser pulse duration using the Kerr-lens mechanism is demonstrated. This technique can also be used as a sensitive and absolutely calibratable method for measuring ultrafast optical nonlinearities. A method that uses an electronic spectral-filtering scheme is proposed for determining the frequency chirp of pulses by interferometric autocorrelation.     

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located. PACS 42.65.Re; 42.72.Bj; 32.80.Qk     

单脉冲飞秒激光时域参数测量技术研究

钛宝石飞秒激光放大系统具有重复频率低、脉冲波形复杂等特点，为准确测量其峰值功率，提出对单次脉冲波形和脉冲宽度测量的需求。介绍了单脉冲飞秒激光时域波形和脉冲宽度的测量原理和测量方法，设计了基于频率分辨光学开关法的单脉冲飞秒激光时域参数测量装置。讨论了单脉冲飞秒激光时域参数测量校准面临的问题及解决方法。     

Coherent synthesis using carrier-envelope phase-controlled pulses from a dual-color femtosecond optical parametric oscillator

By using a dual-color femtosecond optical parametric oscillator (OPO), a coherent waveform was synthesized from two coresonant near-infrared signal pulses whose center wavelengths had a separation of 100 nm. Immediately after the OPO cavity the pulses had independent carrier-envelope phase-slip frequencies, and synthesis was achieved by shifting these frequencies using an acousto-optic modulator driven by an internally generated difference frequency. Soliton self-frequency shifted pulses from a photonic crystal fiber and a cross-correlation frequency-resolved optical gating (XFROG) measurement were used to analyze the result of the synthesis experiment and revealed that the synthesized waveform was a train of high-contrast 30 fs pulses.     

Talbot effect of a high-density grating under femtosecond laser illumination

The Talbot effect of a high-density grating under femtosecond laser illumination is analyzed with rigorous electromagnetic theory which is based on the Fourier decomposition and the rigorous coupled-wave analysis (RCWA). Numerical simulations show that the contrast of the Talbot images steadily decreases as the transmitted femtosecond laser pulses propagate forward and with wider spectrum width of the femtosecond laser pulses. The Talbot images of high-density gratings have much higher sensitivity of the spectrum widths of the incident laser pulses than those of the traditional low-density gratings. In experiments, the spectrums and the pulse widths of the incident pulses are measured with a frequency-resolved optical grating (FROG) apparatus. The Talbot images are detected by using a Talbot scanning near-field optical microscopy (Talbot-SNOM) technique, which are in coincidence with the numerical simulations. This effect should be useful for developing new femtosecond laser techniques and devices.     

Generation of 12 fs deep-ultraviolet pulses by four-wave mixing through filamentation in neon gas

Generation of deep-ultraviolet femtosecond pulses by four-wave mixing through filamentation in neon gas was demonstrated. Fundamental (omega) and second-harmonic (2omega) pulses of 25 fs Ti:sapphire amplifier output were focused into neon gas, and 20 microJ pulses with the center wavelength of 260 nm were produced by a four-wave mixing process, 2omega+2omega-omega?3omega through an ~15 cm filament. Additionally, pulses with an energy of 2 microJ at 200 nm were generated, probably by a cascaded process, 3omega+2omega-omega?4omega. The 260 nm pulses were compressed by a grating-based compressor and characterized by a dispersion-free transient grating frequency-resolved optical gating. The estimated pulse width was 12 fs.