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.     

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.     

飞秒激光脉冲宽度和脉冲波形测试技术

飞秒激光在激光核聚变、卫星精密测距、激光微加工等领域具有重要的应用前景,同时也是产生太赫兹波的主要泵浦源。介绍了国内外飞秒激光脉冲宽度和脉冲波形的测试方法,比较了自相关法、频率分辨光学快门法、光谱相位相干直接电场重构法的优缺点。自相关法具有脉宽测量范围广、结构简单等特点,但不具备脉冲波形测试能力。光谱相位相干直接电场重构法对待测激光光束质量要求较高, 不适合大量程范围激光脉宽快速测量。为满足10 fs~5 ps大量程范围超短激光脉冲宽度和脉冲波形的测试需求,采用自相关法及二次谐波频率分辨光学开关法研制飞秒激光脉冲宽度和脉冲波形测试仪,时间分辨率优于2 fs。     

飞秒脉冲测量技术

随着飞秒激光技术的发展，飞秒激光脉冲的准确测量已成为非常重要的研究内容。文章在简要概述几种常见测量方法的基础上，着重综述了近年来发展起来的光学频率光栅开关法（FROG）及自参考光谱位相相干电场重建法（SPIDER），并介绍了文章作者进行的相关工作。     

Tomographic retrieval of the polarization state of an ultrafast laser pulse

We introduce a self-referenced method for determining the complete polarization state of an ultrafast pulse field. The algorithm is based on any well-established technique that measures both the intensity and phase of a single polarization, such as frequency-resolved optical gating (FROG). We demonstrate the retrieval of nontrivial fields generated using a polarization-amplitude-phase ultrafast pulse shaper using four standard FROG measurements.     

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.     

Application of single-beam homodyne SPIDER for the control of complex spectral phase profiles

We investigate the range of application of single-beam homodyne spectral phase interferometry for direct electric field reconstruction (SPIDER) for multiphoton microscopy. Simulations and experimental studies performed on model spectral phase profiles show that the phase reconstruction technique used in this method makes the phase retrieval quality highly sensitive to the complexity of the profile. In addition, we show that the use of iterative processes is likely to deteriorate the phase retrieval quality, especially for strongly varying phase profiles. These effects are illustrated and quantified for sinusoidal and quadratic spectral phase profiles.     

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     

Characterization of a broadband pulse for phase controlled multiphoton microscopy by single beam SPIDER

We present what we believe to be a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER) using only a single-phase and polarization controlled laser beam. Two narrow pulses and one broadband pulse are selected out of an ultrafast laser pulse by a polarization and phase control technique to generate second harmonic generation (SHG) signals, which are equivalent to a spectral shear interferogram in the conventional SPIDER method. The spectral phase of the broadband laser pulse is extracted analytically with double quadrature spectral interferometry (DQSI). An arbitrary spectral phase can be retrieved with great precision and compensated in situ at the sample position of a microscope. This new method requires no separate reference beam and is suitable for nonlinear optical microscopy with a phase controlled laser pulse.     

Advanced methods for the characterization of few-cycle light pulses: a comparison

We discuss and compare four methods for measuring the width and pulse profile of ultrashort pulses. For our comparison, we use stable sub-7 fs pulses from a Ti:sapphire oscillator. Interferometric autocorrelation, spectral phase interferometry for direct electric-field reconstruction (SPIDER), a spatially-encoded variant of SPIDER, and interferometric frequency-resolved optical gating (IFROG) are utilized for characterizing pulses from the oscillator. The methods are found to agree within 5% as far as determination of the pulse width is concerned. However, differences are observed in the satellite structure reconstructed by either method. The current state of the art of measuring ultrashort pulses with these methods is reviewed and current limitations, in particular for characterizing complex pulse shapes, are discussed. PACS 06.60.Jn; 07.60.Ly; 42.65.Re; 42.79.Hp     

频率分辨光学开关法行迹相位还原的时频分析

将小波变换分析光谱相位相干原理应用于频率分辨光学开关(FROG)行迹图的时间-频率分析，不需要传统的迭代算法，从对FROG行迹的脊的分析中，直接得到超短脉冲的相位.对于FROG行迹的脊变化平缓的脉冲得到了精确的相位. 关键词： 超短脉冲 相位重建     

Single-beam homodyne SPIDER for multiphoton microscopy

We report a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER) requiring only a single phase-shaped laser beam. A narrowband probe pulse is selected out of a broadband ultrafast laser pulse by a phase pulse-shaping technique and mixed with the original broadband pulse to generate a second-harmonic generation (SHG) signal. Using another SHG signal solely generated by the broadband pulse as a local oscillator, the spectral phase of the broadband laser pulse can be analytically retrieved by a combination of double-quadrature spectral interferometry and homodyne optical technique for SPIDER (HOT SPIDER). An arbitrary spectral phase at the sample position of a microscope can be compensated with a precision of 0.05 rad over the FWHM of the laser spectrum. It is readily applicable to a nonlinear microscopy technique with a phase-controlled broadband laser pulse.     

Generation of double pulses in-line by using reflective Dammann gratings

Doubled femtosecond laser pulses in-line are needed in the collinear pump-probe technique, collinear second harmonic generation frequency-resolved optical gating (SHG FROG) and the spectral phase interferometry for direct electric-field reconstruction (SPIDER), etc. Normally, it is generated by using a Michelson's structure. In this paper, we proposed a novel structure with two-layered reflective Dammann gratings and the reflective mirrors to generate doubled femtosecond laser pulses in line without transmission optical elements. Angular dispersion and spectral spatial walk-off are both compensated. In addition, this structure can also compress the positive chirped pulse, which cannot be realized with a Michelson's structure. By adopting triangular grating and blazed gratings, the efficiency of the system would in principle be increased as the Michelson's scheme. Experiments demonstrated that this method should be an alternative approach for generation of the double compressed pulses of femtosecond laser for practical applications.     

Spatially resolved amplitude and phase characterization of femtosecond optical pulses

Ultrabroadband pulses exhibit a frequency-dependent mode size owing to the wavelength dependence of free-space diffraction. Additionally, rather complex lateral dependence of the temporal pulse shape has been reported for Kerr-lens mode-locked lasers and broadband amplifier chains and in frequency-domain pulse shapers, for example. We demonstrate an ultrashort-pulse characterization technique that reveals lateral pulse-shape variations by spatially resolved amplitude and phase measurements by use of spectral phase interferometry for direct electric-field reconstruction (SPIDER). Unlike with autocorrelation techniques, with SPIDER we can obtain spatially resolved pulse characterization even after the nonlinear process. Thus, with this method the spectral phase of the pulse can be resolved very rapidly along one lateral beam axis in a single measurement.     

Characterization of sub-6-fs optical pulses with spectral phase interferometry for direct electric-field reconstruction

We demonstrate spectral phase interferometry for direct electric-field reconstruction (SPIDER) as a novel method to characterize sub-6-fs pulses with nanojoule pulse energy. SPIDER reconstructs pulse phase and amplitude from a measurement of only two optical spectra by use of a fast noniterative algorithm. SPIDER is well suited to the measurement of ultrabroadband pulses because it is quite insensitive to crystal phase-matching bandwidth and to unknown detector spectral responsivity. Moreover, it combines highly accurate pulse-shape measurement with the potential for online laser system diagnostics at video refresh rates.     

Characterization of the spectral phase of ultrashort light pulses

The complete characterization of a short light pulse requires the measurement of the value of the electric field as a function of time, or conversely, as a function of the optical frequency. Many different techniques have been demonstrated for this purpose. They fall in two main categories, whether a known reference pulse whose spectrum encompasses that of the unknown pulse is available or not. In the first case, linear techniques such as time-domain or frequency-domain interferometry can be directly used, with the advantage of a high sensitivity. In the latter case, non-stationary filters can be implemented using optical non-linearities, in techniques such as Frequency Resolved Optical Gating (FROG) or Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER).     

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.     

用SPIDER法测量飞秒激光脉冲的光谱相位

介绍了用SPIDER测量光谱相位的实验装置和模拟计算飞秒激光特性参数的原理；提出了Ω 和τ等重要参数的确定方法.实验上用自建的SPIDER进行了掺钛蓝宝石飞秒振荡器输出脉 冲的相位测量，并以此为基础还原出了原输入脉冲的时域形式，模合计算所得的脉宽为107 fs，与利用二次相关法直接测量的结果十分一致. 关键词： 飞秒激光 SPIDER 光谱相位 光谱相干     

改进型零附加相位光谱相位相干电场重构系统对啁啾脉冲的测量

现有的基于光谱相位相干电场重构法(SPIDER)的脉冲测量系统,在测量啁啾脉冲时容易出现误差.本文提出一个改进型零附加相位光谱相位相干电场重构系统(MZAP-SPIDER),来解决上述问题.在实验上,利用改进后的SPIDER系统测量了钛宝石飞秒激光器输出的脉冲及其经80mm长的BK7玻璃块展宽得到的啁啾脉冲.结果表明,该系统能胜任啁啾脉冲的相位测量.     

Collinear type II second-harmonic-generation frequency-resolved optical gating for the characterization of sub-10-fs optical pulses

For the first time to our knowledge, we demonstrate a collinear frequency-resolved optical gating (FROG) technique that is suitable for the characterization of sub-10-fs pulses. This FROG variant does not suffer from geometrical blurring effects, and a temporal resolution of 1 fs can be achieved without the need for additional aperturing. The apparatus is suitable for subnanojoule pulse energies. We apply this technique for the full characterization of pulses from a Kerr-lens mode-locked Ti:sapphire laser.