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.     

A Six-Pulse PASS MAS-NMR Technique Solved by a Phasor Method

The PASS MAS-NMR technique is capable of recovering full intensities of the central resonances of a spectrum by phase adjusting the spinning sidebands. This variant of the original PASS sequence by Dixon uses six π pulses instead of four. The addition of two π pulses provides more flexibility in choosing the spacing between pulses and therefore eliminates pulse overlap and receiver dead-time problems. The nonlinear, underdetermined PASS equations were solved using a graphical phasor method. All PASS sequences were successfully tested. A table of the six pulse delay times for different pitches is presented.     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Reconfigurable generation of high-repetition-rate optical pulse sequences based on time-domain phase-only filtering

We propose and demonstrate a fiber-based phase-only filtering technique for programmable optical pulse shaping, in which the filtering operation is implemented in the time domain by means of an electro-optical (EO) phase modulator. The technique has been applied for generating customized ultrahigh-repetition-rate optical pulse sequences (>40 GHz) from single input pulses by driving the EO phase modulator with a periodic electronic waveform (RF tone). The generated output pulses are replicas of the input pulse and both the repetition rate and the envelope profile of the generated sequences can be controlled and tuned electronically using this approach.     

Few-cycle pulse compression through cascade of bulk media and hollow-core fiber

We experimentally demonstrated a new few-cycle pulse compression technique through the cascade of bulk media and hollow-core fiber (HCF) and this compression system has been intensively studied. The pulses with the duration of ∼5 fs and the energy of 0.33 mJ near 800 nm have been generated by compressing the ∼40 fs input pulse from a commercial laser system. In principle, this technique allows compression of pulses with duration of picoseconds to a few cycles (sub-7 fs) and the output can be above 1 mJ.     

Frequency shifting of sub-100 fs laser pulses by stimulated Raman scattering in a capillary filled with pressurized gas

The technique of Raman conversion of sub-100 fs laser pulses based on excitation of active medium by two orthogonally polarized pulses has been developed for Raman lasers with a glass capillary. 52 fs Stokes pulse at the wavelength of 1200 nm has been generated by stimulated Raman scattering of 48 fs Ti:sapphire laser pulse at the wavelength of 800 nm in hydrogen. 13% energy conversion efficiency has been achieved at pulse repetition rate up to 2 kHz.     

Femtosecond time-resolved optical polarigraphy: imaging of the propagation dynamics of intense light in a medium

A time-resolved imaging technique for visualizing ultrafast propagation dynamics of intense light pulses in a medium has been demonstrated. The method probes the instantaneous birefringence induced by a pulse in the medium. Through consecutive femtosecond snapshot images of intense femtosecond laser pulses propagating in air, ultrafast temporal changes in the two-dimensional spatial distribution of the optical pulse intensity were clearly seen.     

多路窄脉冲功率线路合成

介绍了传输线变压器的结构特点,对其进行了理论分析和实验研究。采用传输线变压器结构将每路幅度约为4.5 kV/50Ω的4路脉冲合成了幅度约8.8 kV/50Ω的脉冲,通过实验证实了采用多根传输线串并联方式构成的传输线变压器装置可以将多路窄脉冲进行线路功率合成。应用此类结构设计了一种紧凑的高压窄脉冲发生器,将36路8 kV/50Ω的脉冲合成了约46 kV/50Ω的脉冲。采用传输线变压器合成器,应确保各路输入脉冲的前沿抖动足够小和高精度同步。     

超短飞秒光脉冲产生技术

本文叙述了群速弥散补偿被动锁模环型染料激光器的特性;研究了产生极短飞秒光脉冲的条件和光脉冲强度分布的非对称性。该技术产生的最短光脉冲宽度为21 fs;光脉冲宽度可调范围从最短到500fs以上。实验和理论数据拟合证明光强分布为非对称双曲正割平方,非对称因子r在7到11之间。     

Parametrically amplified ultrashort pulses from a shaped photonic crystal fiber supercontinuum

By seeding a noncollinear optical parametric amplifier with a photonic crystal fiber supercontinuum, temporally well-defined amplified output pulses have been generated with durations down to 13 fs. The phase of the supercontinuum seed has been characterized by the ZAP-SPIDER technique and can be tailored with a femtosecond pulse shaper. Thus, a very flexible source for arbitrarily shaped, amplified ultrashort laser pulses has been realized.     

Complete pulse characterization at 1.5 mum by cross-phase modulation in optical fibers

Cross-phase modulation in optical fibers has been used for complete characterization of ultrashort pulses by a modified frequency-resolved optical gating (FROG) measurement technique. This technique has been used for characterization of picosecond pulses at 1.5mum with energy as low as 24 pJ, and the results are in excellent agreement with second-harmonic generation (SHG)-FROG characterization. The use of an optical waveguide gives measurement sensitivity comparable with that of SHG-FROG but without any temporal ambiguity in the retrieved pulse.     

Simple electro-optic technique to generate temporally flat-top laser pulses

A simple technique is presented to generate temporally flat-top shaped laser pulses using electro-optic modulator (Pockels cell). It involves splitting of input laser pulse into two halves of equal intensity and then stacking together with appropriate optical delay to get a temporally flat-top laser pulse. It also allows generation of other pulse shapes by varying the relative intensity, delay, and phase between two halves of the input laser pulse. Temporally flat-top laser pulses of duration ~ 9 ns have been generated using ~ 7 ns duration incident laser pulses from a flash lamp pumped Q-switched Nd:glass laser oscillator. The rise and fall-time of the shaped pulse is limited by speed of electro-optic switch (Pockels cell), which is ~ 2 ns in the present case.     

Nanostructuring with spatially localized femtosecond laser pulses

Spatially localized femtosecond pulses have been produced by a combination of scanning near-field optical microscopy with ultrashort pulse lasers. With these pulses direct ablative writing on metal surfaces is demonstrated. Possible applications of this technique for nanostructuring, repair, and production of lithographic masks are discussed.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

亚纳秒脉冲高电压测量探头

为测量紧凑型快前沿高电压脉冲源的输出电压,设计了D-dot电压探头。分别进行了刻度因素标定和频响标定,采用前沿约50 ns的高压脉冲信号对探头进行在线标定确定探头的刻度因素。将探头安装在阻抗为50 的传输线上,用亚纳秒脉冲源进行频响标定,表明该探头的响应约为150 ps。高压实验结果表明该探头能够正确获取高电压快脉冲信号,工作稳定可靠。     

利用脉冲负反馈压缩准分子激光脉冲

 分析了准分子激光脉冲在放大过程中的展宽现象。将饱和开关技术应用于放电泵浦激光器放大腔的放大压缩,发展了脉冲负反馈饱和压缩技术。利用该技术在天光一号系统前端LPX-150激光器上成功获得能量100 mJ、脉宽10 ns的脉冲输出,其空间分布均匀性保持在3%以内。这证明该技术可与无阶梯诱导非相干技术联合使用,在保证光束均匀性基础上进行脉冲压缩。     

Self-compression of millijoule pulses to 7.8 fs duration in a white-light filament

We demonstrate a novel technique for pulse compression of few-millijoule pulses with shorter than 10 fs duration. Our technique relies on spectral broadening in a white-light filament generated in a noble gas. In this filament we observe self-compression of 45 fs pulses down to below 8 fs duration without the need for any additional dispersion compensation. Using input pulses of 5 mJ, we generate compressed pulses with up to 3.8 mJ pulse energy. Therefore this method is much more efficient than previously demonstrated compression schemes. The generated peak powers of more than 100 GW at a kilohertz repetition rate open up a perspective for compression of few-cycle pulses with energies well beyond the capacity of hollow-fiber compressors.     

Microdrilling of metals with an inexpensive and compact ultra-short-pulse fiber amplified microchip laser

We have investigated the ultra-fast microdrilling of metals using a compact and cheap fiber amplified passively Q-switched microchip laser. This laser system delivers 100-ps pulses with repetition rates higher than 100 kHz and pulse energies up to 80 μJ. The ablation process has been studied on metals with quite different thermal properties (copper, carbon steel and stainless steel). The dependence of the ablation depth per pulse on the pulse energy follows the same logarithmic scaling laws governing laser ablation with sub-picosecond pulses. Structures ablated with 100-ps laser pulses are accompanied only by a thin layer of melted material. Despite this, results with a high level of precision are obtained when using the laser trepanning technique. This simple and affordable laser system could be a valid alternative to nanosecond laser sources for micromachining applications.     

Energy extraction improvement in picosecond amplifiers by pulse tilting

We report a novel tilted pulse amplification technique that, in contrast with the chirped-pulse amplification method, allows stretching of ultrashort pulses independently upon the initial pulse bandwidth. A gain factor of 4.5 and good reconstruction of a 15 ps pulse after stretching to 200 ps in a double-pass end-pumped Nd:YAG amplifier have been achieved.     

Self-compression controlled by the chirp of the input pulse

Self-compressed (SC) pulses have been achieved through the filamentation process in air without any additional dispersion compensation, using the input pulse chirp as the control parameter. For any studied input pulse energy (3-5 mJ), we have found two opposite sign input group-delay dispersion values for which SC pulses can be achieved systematically. In addition, we have observed that the energy coupled into the inner core of the filament is always of the order of 20% of the total input pulse energy, which opens the way to a scalable technique to obtain intense short pulses directly from the filamentation process.