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.     

小波变换重建超短脉冲光谱相位的误差分析

分析了噪声对小波变换重建超短脉冲光谱相位的影响. 在理想情况和附加噪声、倍增噪声， 以及干涉条纹采样量化干扰的情况下，分析了小波变换对几种光谱相位的重建精度. 结果显 示，在理想情况和噪声干扰的情况下，小波变换的相位重建精度都在003rad范围以内. 这 说明小波变换是一种精确可信的光谱相位重建方法，具有很强的抗噪声干扰能力. 另外，将 小波变换的轨迹图类比SHG FROG的轨迹图，可以直接定性判断脉冲的啁啾特征. 关键词： 超短脉冲 相位重建 小波变换 噪声     

基于智能搜寻者优化的频率分辨光学开关重构算法

频率分辨光学开关(frequency-resolved optical gating, FROG)法是目前测量超短激光脉冲的主要方法之一.针对其常用的主成分广义投影重构(principal component generalized projections, PCGP)算法在处理大矩阵FROG谱图时速度会减慢及存在噪音时准确度下降的缺点,本文提出一种基于搜寻者优化算法的FROG算法.该算法在直接测量脉冲光谱分布的基础上,通过搜索脉冲频域相位的几个色散系数,从而恢复脉冲的结构.由于新算法主要在频域上进行运算,流程比PCGP算法简便很多,收敛速度和准确性都有明显改善.通过数值模拟方法重构了多个不同结构的超短脉冲,分析讨论了无噪音和在不同噪音水平下该算法的准确度.计算结果充分展示了该算法重构脉冲的速度快、准确度高的特点,在无噪音条件下其准确度比PCGP提升了3个数量级以上.     

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     

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     

载波-包络相位对于基频光与其自身倍频光脉冲合成的影响

将基频光与其自身的倍频光合成，是一种得到超过一个倍频程的超宽光谱的简单方法，而载波_包络相位(CEP)对合成超宽光谱产生的超短脉冲有重要的影响. 本文从光频梳的角度，根据钛宝石激光器中产生的超短脉冲光谱，计算载波_包络相位对该脉冲与其自身的倍频脉冲合成产生的超短脉冲的影响，说明了调整载波_包络相位对于脉冲合成的重要性. 关键词： 超短脉冲 载波_包络相位 光频梳 脉冲合成     

用SHG-FROG方法测量超短光脉冲的振幅和相位

介绍了二次谐波振荡频率分辨光学快门(SHGFROG)测量超短脉冲的实验系统和二维相位恢复算法,这种方法可以实现对超短脉冲振幅和相位的完全测量.利用矩阵方法数值模拟了几种常见超短脉冲的SHGFROG迹线,并用主元素广义投影算法(PCGPA)从这些无噪音的SHGFROG迹线中恢复了脉冲的振幅和相位,误差接近收敛的标准.研究结果表明,这种测量方法结构简单,算法可靠,是测量超短脉冲的理想方法之一.     

小波变换提取放大超短脉冲载波-包络相位的研究

提出了一种新的超短放大脉冲的载波-包络相位还原方法,应用小波变换从超短放大脉冲的光谱拍频干涉中直接提取载波-包络相位. 消除了传统的Fourier变换方法不同宽度的滤波窗口引入不同程度的相位噪声而产生不确定性的影响,得到了更精确的载波-包络相位信息,对于近单周期超短脉冲特性测量具有重要意义. 关键词： 超短脉冲 载波-包络相位 相位还原 小波变换     

自参考光谱相干法的小波变换相位重建

将小波变换法引入光谱相位相干电场重建法的相位重建，从光谱干涉条纹的小波变换中直接 读取条纹相位，消除了传统傅里叶方法滤波过程中产生的相位噪声. 这种方法将有利于超宽 带、单周期超短脉冲的产生，更适用于超短脉冲的评价. 关键词： 超短脉冲 相位重建 小波变换     

载波相位对超短正弦高斯脉冲的影响

在时间域中研究了载波相位对超短正弦高斯脉冲的影响.给出了超短正弦高斯脉冲的脉冲能量和时间重心的解析表达式和数值模拟结果.结果表明载波相位和正弦函数的参数对超短正弦高斯脉冲的脉冲能量和时间重心有很大的影响.当载波相位等于nπ和2n+1π/2时,超短正弦高斯脉冲的脉冲能量分别达到最小和最大值;当载波相位等于2n+1π/4时,载波相位对超短正弦高斯脉冲的脉冲能量没有影响.当载波相位等于4n+3π/4和4n+1π/4时,超短正弦高斯脉冲的时间重心分别达到最小和最大值;当载波相位等于nπ/2时,超短正弦高斯脉冲的时间重心不发生变化.     

Pulse‐shape instabilities and their measurement

Multi‐shot pulse‐shape measurements of trains of ultrashort pulses with unstable pulse shapes are studied. Measurement techniques considered include spectral‐phase interferometry for direct electric‐field reconstruction (SPIDER), second harmonic generation frequency‐resolved optical gating (FROG), polarization gate FROG, and cross‐correlation FROG. An analytical calculation and simulations show that SPIDER cannot see unstable pulse‐shape components and only measures the coherent artifact. Further, the presence of this instability cannot be distinguished from benign misalignment effects in SPIDER. FROG methods yield a better, although necessarily rough, estimate of the pulse shape and also indicate instability by exhibiting disagreement between measured and retrieved traces. Only good agreement between measured and retrieved FROG traces or 100% SPIDER fringe visibility guarantees a stable pulse train.     

Convergence test for inversion of frequency-resolved optical gating spectrograms

We describe a new and simple method to aid in the analysis of retrieved pulses from inverted frequency-resolved optical gating (FROG) traces. The analysis can separate noise from distortion and shows that distortion is more deleterious to the retrieved pulse than is pure noise. The analysis relies on the fact that FROG traces can be constructed from a single outer product of two vectors, whereas distortion and noise require the sum of a series of outer products.     

Evolving FROGS: phase retrieval from frequency-resolved optical gating measurements by use of genetic algorithms

We present a new technique based on genetic algorithms for retrieving the electric field and the phase of ultrashort pulses from frequency-resolved optical gating (FROG) traces. We have successfully applied a very basic genetic algorithm to the two most common beam geometries: polarization gate and second-harmonic generation (SHG). In the case of SHG FROG, the genetic algorithm returns a lower error than the standard iterative composite algorithm.     

Coherent artifact in modern pulse measurements

We simulate multishot intensity-and-phase measurements of unstable trains of complex ultrashort pulses using second-harmonic-generation (SHG) frequency-resolved optical gating (FROG) and spectral-phase interferometry for direct electric-field reconstruction (SPIDER). Both techniques fail to see the pulse structure. But FROG yields the correct average pulse duration and suggests the instability by exhibiting significant disagreement between measured and retrieved traces. SPIDER retrieves the correct average spectral phase but significantly underestimates the average pulse duration. In short, SPIDER measures only the coherent artifact. An analytical calculation confirms this last fact.     

Amplitude ambiguities in second-harmonic generation frequency-resolved optical gating

We construct field shapes with distinct amplitude profiles that have nearly identical second-harmonic generation frequency-resolved optical gating (SHG FROG) traces. Although such fields are not true mathematical ambiguities, they result in experimentally indistinguishable FROG traces. These fields are neither time-reversed copies nor pulselets with a mere relative phase difference, which are well known nontrivial ambiguities for SHG FROG. We also show that for certain example fields, second-order interferometric autocorrelation is more sensitive to the pulse shape than is SHG FROG.     

Blind frequency-resolved optical-gating pulse characterization for quantitative differential multiphoton microscopy

We use a unique multifocal multiphoton microscope to directly characterize the pulse in the focal plane of a high-NA objective using second-harmonic generation frequency-resolved optical gating (FROG). Because of the nature of the optical setup, femtosecond laser pulses of orthogonal polarization states are generated in the focal plane, each acquiring a different spectral dispersion. By applying an additional constraint on the phase extraction algorithm, we simultaneously extract both the gate and probe pulses from a single spectrogram with a FROG error of 0.016.     

Wavelet-transform analysis for carrier-envelope phase extraction of amplified ultrashort optical pulses

We introduced a wavelet-transform technique into the carrier-envelope phase extraction from the spectral interferogram of amplified ultrashort optical pulses. With this technique, carrier-envelope phases were directly extracted from the ridges of the wavelet transforms. This technique did not need filters; therefore, the uncertainty introduced from filter was avoided naturally. The Wavelet transforms technique produced a two-dimensional topography on which the time-frequency information of the spectral interferogram was shown apparently.     

Feedback-controlled femtosecond pulse shaping

We describe the experimental implementation of feedback-optimized femtosecond laser pulse shaping. A frequency-domain phase shaper is combined with different pulse characterization methods and appropriate optimization algorithms to compensate for any phase deviation. In particular, bandwidth-limited, amplified laser pulses are achieved by maximizing the second-harmonic generation (SHG) of the shaped laser pulses with the aid of an evolutionary algorithm. Real-time measurement of the absolute phases is achieved with spectral interferometry where the reference pulse is characterized by FROG, the so-called TADPOLE method. Using the complete electric field as feedback, arbitrary laser pulse shapes can be optimally generated in two different ways. First, a local convergence algorithm can be used to apply reliable and accurate spectral chirps. Second, an evolutionary algorithm can be employed to reach specific temporal profiles.     

High resolution FROG system for the characterization of ps laser pulses

We report on a frequency resolved optical gating (FROG) system which allows the reconstruction of amplitude and phase of picosecond pulses generated by mode-locked diode laser oscillators and amplifiers. The main challenge of developing such a FROG system is the required high spectral resolution. In the spectrometer a cylindrical grating with 1200 lines/mm and a radius of curvature of 3 m is used. A width of the illuminated area of 15 cm provides in a spectral resolution exceeding 2 GHz (1.4 pm) at 460 nm. The FROG system is, thus, well suited for analyzing ps-pulses with durations of up to 80 ps and a corresponding fourier-limited spectral width of 0.017 nm. PACS 42.55.Px; 42.65.Re; 42.60.Fc; 42.30.Rx