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.     

Conclusive evidence of an attosecond pulse train observed with the mode-resolved autocorrelation technique

We report on the direct observation of an attosecond pulse train with a mode-resolved autocorrelation technique. The chirp among the three harmonic fields is specified by analyzing two-photon above-threshold ionization spectra of electrons, resulting in a pulse duration that should be shorter than 450 as, which is, to our knowledge, the first determination of the chirp in the attosecond pulse train with an autocorrelation technique. These results will open the way to full characterization of an attosecond pulse train with its envelope.     

Coherent matter waves for ultrafast laser pulse characterization

A technique for the characterization of ultrashort laser pulses using coherent matter waves is demonstrated. We emphasize the analogy between matter wave packets and electromagnetic wave packets propagating in dispersive media. Due to quadratic dispersion the wave packets are stretched and their temporal structure eventually converges to their spectrum, thus providing a possibility for energy measurements in conjugate space. This is demonstrated theoretically and experimentally and is the basis for our laser pulse characterization technique. We use energy resolved interferometrically recorded photoelectron spectra generated by above-threshold ionization in an autocorrelation setup to characterize ultrashort laser pulses at 800 nm wavelength. This approach is potentially applicable to the XUV wavelength region.     

自相关法测量激光超短脉冲时域分布的解相关

本文论述一种从典型的自相关法测量得到激光超短脉冲时域分布的有效方法,这种方法不必进行另外的光谱测试及假定脉冲形状。     

Pulse shaper assisted short laser pulse characterization

We demonstrate that a pulse shaper is able to simultaneously act as an optical waveform generator and a short pulse characterization device when combined with an appropriate nonlinear element. We present autocorrelation measurements and their frequency resolved counterparts. We show that control over the carrier envelope phase allows continuous tuning between an intensity-like and an interferometric autocorrelation. By changing the transfer function other measurement techniques, for example STRUT, are easily realized without any modification of the optical setup. PACS 42.65.Re; 42.30.Lr; 42.30.Rx     

Temporal characterization of short-pulse third-harmonic generation in an atomic gas by a transmission-grating Michelson interferometer

By use of a transmission-grating-based Michelson interferometer, second-order interferometric as well as intensity autocorrelation traces of the third harmonic of a Ti:sapphire 50-fs laser beam produced in Ar have been measured. The duration of the harmonic is found to be that expected from lowest-order perturbation theory. At this wavelength, the performance of the interferometer with respect to pulse-front distortion and dispersion is found to be satisfactory. This result is a first step toward the use of the interferometer for the temporal characterization of higher harmonics or harmonic superposition forming attosecond pulse trains.     

Two-Photon Ionization of He through a Superposition of Higher Harmonics

We present experimental results and theoretical analysis of two-photon ionization of He by a superposition of the 7th to the 13th harmonic of a Ti:sapphire laser. Solving the time-dependent Schr?dinger equation for He in a coherent polychromatic field, the He+ yield is calculated. From this yield the number of He+ ions produced has been estimated and found in reasonable agreement with its measured value. The present results establish the feasibility of a second-order autocorrelation measurement of superposition of harmonics, and thus they represent the precursor towards the direct temporal characterization of attosecond pulse trains.     

Description of pulse propagation in a dispersive medium by use of a pulse quality factor

We investigated how the duration of short laser pulses evolves in a dispersive material, using rms widths and a propagation law based on a pulse quality factor. Experiments were carried out with femtosecond pulses (10 to 25fs at the temporal waist) propagating in bulk fused silica. Excellent agreement was found between theory and experiment. This approach does not require complete characterization of laser pulses and eliminates the need for any assumption regarding the interpretation of autocorrelation traces. The method is of general validity, and it can be applied to pulses of arbitrary shape.     

In situ third-order interferometric autocorrelation of a femtosecond deep-ultraviolet pulse

The interferometric measurement of an ultrashort optical pulse is demonstrated using three-photon ionization of Xe in a time-of-flight mass spectrometer (MS). This approach allowed measurement of the third-order fringe-resolved autocorrelation trace of an ultrashort optical pulse in the deep-ultraviolet (DUV) region. The pulse can be measured in situ using the same MS that is employed in ultrafast spectroscopy. Sensitivity (>3×10⁷ W) was sufficient to measure a DUV pulse used in such applications. This setup has the potential to measure the temporal duration of a broadband (204–306 nm) ultrashort optical pulse with no additional distortion in the temporal characteristics.     

Phase-amplitude characterization of a high-repetition-rate quantum dash passively mode-locked laser

We apply a novel phase-amplitude characterization method to a one-section quantum dash-based passively mode-locked laser at a 42.2 GHz repetition rate. The method relies on the measurement of the spectral phase of the longitudinal modes by the successive analysis of the correlation signal of a group of three adjacent modes. It provides both the temporal shape of the intensity and the phase of the emitted signal. A pulse of 1.5 ps of width is measured, and a pedestal is exhibited. Extinction ratio limitation is explained by investigating the origin of this pedestal. The accuracy of the method is estimated by comparing the measured autocorrelation signal and the calculated one from the phase analysis.     

Electric-field reconstruction of femtosecond laser pulses from interferometric autocorrelation using an evolutionary algorithm

We present an evolutionary algorithm for reconstructing a femtosecond laser pulse from its interferometric autocorrelation trace and laser spectrum. The algorithm is optimized for the intensity and phase characterization of several-cycle optical pulses. We tested this algorithm with numerically-generated femtosecond pulses and then applied it to experimental data. In the experiment, a negatively chirped 31-fs pulse and a sub-10-fs pulse containing high-order phase distortion were characterized. Frequency-resolved optical gating measurements, performed for comparison, confirm the reliability of our technique.     

Frequency resolved optical gating in the UV using the electronic Kerr effect

A phase-sensitive single-shot autocorrelator (FROG) for the UV spectral region based on the electronic Kerr effect is demonstrated. Since the Kerr shutter technique leads to a third-order autocorrelation function, information about pulse asymmetries and pulse structures along with the temporal phase can be obtained.     

Characterization of ultra-short pulses by cross-phase modulation

We present an improved iterative algorithm which allows to reconstruct the temporal pulse profile, phase and absolute intensity of ultra-short laser pulses from a set of cross-phase-modulated spectra. The changes in the centre of gravity of the recorded spectra allow to reconstruct the intensity autocorrelation independently of the phase retrieval algorithm. Ambiguity problems encountered in a previous scheme are eliminated.     

磁场中氢里德堡波包的自动关联函数

本文采用激光短脉冲激发磁场中氢原子,研究氢里德堡波包在磁场中随时间的演化.结果证实氢里德堡波包在磁场中的运动也与闭合轨道密切相关.讨论了脉冲宽度τ对自动关联函数的影响以及不同初始态下的自动关联函数.     

Reconfigurable wavefront sensor for ultrashort pulses

A highly flexible Shack-Hartmann wavefront sensor for ultrashort pulse diagnostics is presented. The temporal system performance is studied in detail. Reflective operation is enabled by programming tilt-tolerant microaxicons into a liquid-crystal-on-silicon spatial light modulator. Nearly undistorted pulse transfer is obtained by generating nondiffracting needle beams as subbeams. Reproducible wavefront analysis and spatially resolved second-order autocorrelation are demonstrated at incident angles up to 50° and pulse durations down to 6 fs.     

Pulse width measurement of high-order harmonics by autocorrelation

Pulse widths of the 7th and 9th harmonics of a Ti:sapphire (TiS) laser have been measured by an autocorrelation technique using two-photon and three-photon ionization of He and Ar as nonlinear phenomena. The pulse widths of the 7th and 9th harmonics were 47 fs and 27 fs, respectively, where the pulse width of the fundamental pulse was 34 fs. The pulse width of the 9th harmonic depends strongly on the peak intensity of the fundamental pulse. These results can be explained by quantum theory by taking into account the temporal and spatial distribution of the fundamental beam. Received: 15 June 1999 / Revised version: 8 October 1999 / Published online: 2 February 2000     

Simultaneous autocorrelation and synchroscan streak camera measurement of cavity length detuning effects in a synchronously pumped cw dye laser

The cavity matching condition for optimum stable, picosecond pulse generation in a synchronously pumped c.w. mode locked dye laser has been achieved with an accuracy of 2 μm using a synchronously operated streak camera exhibiting a modest temporal resolution of 9 ps. This was possible because of the observed distinct asymmetry of the pulses generated under conditions of cavity mismatch about the zero position. Simultaneous autocorrelation measurements confirmed the streak camera observations.     

啁啾脉冲激光放大的数值分析

 从Maxwell波动方程出发,建立了描述脉冲在啁啾放大系统中传播的非线性薛定谔方程;采用数值计算,系统地分析研究了放大过程中的各种效应对脉冲的影响,给出了啁啾脉冲在频域、时域内及放大压缩后脉冲信噪比的动态演化特征;讨论了展宽器附加色散量在放大过程中的影响。     

Four-dimensional investigation of the 2nd order volume autocorrelation technique

The 2nd order volume autocorrelation technique, widely utilized in directly measuring ultra-short light pulses durations, is examined in detail via model calculations that include three-dimensional integration over a large ionization volume, temporal delay and spatial displacement of the two beams of the autocorrelator at the focus. The effects of the inherent displacement to the 2nd order autocorrelation technique are demonstrated for short and long pulses, elucidating the appropriate implementation of the technique in tight focusing conditions. Based on the above investigations, a high accuracy 2nd order volume autocorrelation measurement of the duration of the 5th harmonic of a 50 fs long laser pulse, including the measurement of the carrier wavelength oscillation, is presented.     

Measurement of subpicosecond optical waveforms using a resonator-based phase modulator

We propose a method for the measurement of periodic optical waveforms based on the use of an electrooptic phase modulator placed in an optical Fabry-Perot or ring resonator. Significant broadening of the modulation spectrum extends the recently developed method of periodic modulation for pulse characterization into femtosecond scales. We numerically demonstrate the characterization of a 300-fs optical pulse. We also present a technique based on the temporal fractional Talbot effect for restoration of the pulse phase profile. After fast linear processing, subpicosecond pulses will be observed on the screen of a real-time oscilloscope. This complete characterization of optical pulses is entirely linear and therefore highly sensitive and simple in implementation.