Femtosecond pulse-shape modulation at nanosecond rates

We report high-rate, computer-controlled femtosecond pulse shaping by use of an electro-optical gallium arsenide optical phased-array modulator with 2304 controlled waveguides. It provides fast modulation speed of both spectral phases and amplitudes. Limited by the driving electronics of our current setup, we were able to update a pulse shape in approximately 30 ns. This technique paves the way toward individual shaping of every single pulse of typical femtosecond mode-locked oscillators.     

啁啾脉冲堆积的宽带整形脉冲三倍频方案分析

针对啁啾脉冲堆积方法获得的宽带整形激光脉冲,采用KDP晶体Ⅰ／Ⅱ类角度失谐的三倍频方案,定量分析了影响其三倍频转换效率的主要因素,并对三倍频前后的时间波形及频谱分布进行了比较分析。在此基础上,与时间相位调制的宽带整形脉冲三倍频的相应结果进行了比较。研究结果表明,对于啁啾脉冲堆积方法获得的宽带整形激光脉冲,在实现较高的三倍频转换效率的同时,其时间波形和频谱分布基本保持不变。对于具有相同带宽和波形的时间位相调制整形脉冲,其三倍频转换效率明显低于啁啾脉冲堆积宽带整形脉冲。     

Laser pulse spectral shaping based on electro-optic modulation

A new spectrum shaping method, based on electro-optic modulation, to alleviate gain narrowing in chirped pulse amplification （CPA） system, is described and numerically simulated. Near-Fourier transform-limited seed laser pulse is chirped linearly through optical stretcher. Then the chirped laser pulse is coupled into integrated waveguide electro-optic modulator driven by an aperture-coupled-stripline （ACSL） electricalwaveform generator, and the pulse shape and amplitude are shaped in time domain. Because of the direct relationship between frequency interval and time interval of the linearly chirped pulse, the laser pulse spectrum is shaped correspondingly. Spectrum-shaping examples are modeled numerically to determine the spectral resolution of this technique. The phase error introduced in this method is also discussed.     

Optimizing two-photon fluorescence of Coumarin dye by combined temporal–spatial pulse shaping

Two-photon fluorescence (TPF) process is an important research subject and can be optimized by pulse shaping techniques. In addition to temporal femtosecond pulse shaping by spectral phase modulation, we take into account of spatial configuration in the shaping process. The TPF of Coumarin 500 increases 40% with this additional modulation step. This spatial modulation results in not only transverse spatial profile variation but also effect of temporal redistribution at focus. We show that this spatial modulation is an important dimension for pulse shaping and optimization for TPF.     

Ultrafast control of coherent population transfer with a train of femtosecond pulse pairs

We investigate the ultrafast control of coherent population transfer in a Λ-type three-level system with a train of pump-Stokes femtosecond pulse pairs, where the pulse sequences can be produced either by optical delay line or by pulse shaping with sinusoidal phase modulation. It is shown that when the pump and Stokes pulses in each pair are applied in the counterintuitive order, similar to that in the stimulated Raman adiabatic passage technique, due to temporal quantum interference (besides optical interference in the case of overlapped subpulses), ultrafast control of coherent population transfer can be achieved by scanning the inter-pair time delay or by changing the sinusoidal phase modulation parameters. This method has potential applications in ultrafast control of chemical reactions and quantum information processing.     

Arbitrary pulse shaping based on intensity-only modulation in the frequency domain

We propose a novel technique to implement arbitrary pulse shaping of an ultrashort pulse by intensity-only modulation in the frequency domain. The intensity-only modulation is realized by nonuniformly spaced sampling in the frequency domain. By properly designing the sampling function, multiple pulses in the time domain will be generated with one of which being the desired waveform. The desired waveform is then selected by a time window. Both the optical amplitude and phase distributions of the output waveform can be controlled. Theoretical analysis is presented. An example showing the generation of a rectangular pulse is provided.     

飞秒激光脉冲整形技术及其应用

通过频域上控制飞秒激光脉冲幅度、相位和偏振在时域上可以获得几乎任意形状的飞秒激光脉冲,这种飞秒激光脉冲整形技术可为研究光与原子分子非线性相互作用提供一种全新实验技术手段.本文介绍飞秒激光脉冲整形发展历史、技术方法、控制方式及其相关应用,并展望飞秒激光脉冲整形技术未来的发展方向.     

Tailored terahertz pulses from a laser-modulated electron beam

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.     

Mechanism of pulse shaping in femtosecond lasers based on the high-order soliton

The mechanism of pulse shaping in Kerr-lens mode-locking Ti : sapphire lasers is investigated. Considering the balanced interaction between self-phase modulation, introduced by Kerr effect in Ti : sapphire, and group-velocity dispersion provided by prism-pair, pulse-splitting reflected in the intensity autocorrelation trace and pulse-compression display that the pulses outputting from different positions in laser cavity have different durations. We have concluded that the solitonlike pulse shaping that results from the competition between intracavity self-phase modulation within the Ti : S and negative group-velocity dispersion play dominant role in pulse evolution in Kerr-lens mode-locking lasers.     

All-fiber incoherent frequency-to-time mapping method for microwave signal generation with baseband transmission and multicasting support

We present a proof-of-principle experiment for achieving simultaneous distribution of baseband radio-frequency data and up-conversion with broadcasting support over a passive optical network. The technique is based on an incoherent frequency-to-time mapping method for pulse shaping. Specifically, we synthesize the spectral density function of sliced ASE noise from an EDFA with a periodic Mach–Zehnder fiber interferometer optical filter. By using external intensity modulation combined with propagation in an optical fiber, after photodetection, the resultant averaged temporal pulse profile resembles the shape of the incoherent source. The photodetected signal contains both the baseband data and an up-frequency converted copy with central wavelength for the microwave carrier into the ultra-wideband range and tuning capability by selection of the fiber length.     

基于铌酸锂双折射晶体的皮秒拍瓦激光系统光谱整形

为补偿皮秒拍瓦激光系统中钕玻璃宽带放大引起的增益窄化,提出了一种基于铌酸锂双折射晶体的高能量光谱整形方法.在相同强度调制下,对比了BBO、铌酸锂和石英3种晶体,针对1053 nm激光,选用了高双折射率、大口径且不易潮解的铌酸锂作为整形晶体.理论分析了晶体厚度、倾斜角、面内旋转角对强度调制的影响,发现它们分别决定调制的带宽、中心波长及深度.并对整形过程中晶体引入的光谱相位进行了分析,发现各阶色散量随晶体厚度、倾斜角、面内旋转角变化的规律,因此可通过上述参数控制各阶色散量.在此基础上,开展了中心波长为1053 nm、带宽为10 nm、调制深度为80%的光谱整形实验和相位测量实验,实验与理论分析相一致.针对神光Ⅱ皮秒拍瓦激光系统,利用上述整形方案,国内首次实现了1700 J, 6 nm (FWHM)的高能宽带激光输出,有效补偿了增益窄化.研究结果对国内基于钕玻璃放大系统的宽频带激光装置的工程研制具有重要意义.     

LC-SLM像素型结构对4f脉冲整形系统的影响

液晶空间光调制器的离散型相位调制和像素间隙是4f脉冲整形系统的主要失真原因,基于以上因素的分析,建立了完善的4f脉冲整形系统数学模型,数值模拟了不同相位调制变化率、不同像元尺寸、不同间隙尺寸以及不同色散关系下的离散型相位调制和像素间隙导致的失真情况。模拟结果表明:离散型相位调制会导致整形脉冲中出现副本脉冲,相位调制变化率越大或像元尺寸越大均会导致这种副本脉冲越强,输出目标脉冲失真越严重;像素间隙也会导致整形脉冲中出现副本脉冲,间隙尺寸越大会导致这种副本脉冲越强,输出目标脉冲失真越严重;两种副本脉冲均呈等间隔分布,其周期由每个像元对应的频谱宽度决定,非线性光谱色散会使副本脉冲啁啾化且强度变小。     

LC-SLM像素型结构对4f脉冲整形系统的影响

液晶空间光调制器的离散型相位调制和像素间隙是4f脉冲整形系统的主要失真原因,基于以上因素的分析,建立了完善的4f脉冲整形系统数学模型,数值模拟了不同相位调制变化率、不同像元尺寸、不同间隙尺寸以及不同色散关系下的离散型相位调制和像素间隙导致的失真情况。模拟结果表明：离散型相位调制会导致整形脉冲中出现副本脉冲,相位调制变化率越大或像元尺寸越大均会导致这种副本脉冲越强,输出目标脉冲失真越严重;像素间隙也会导致整形脉冲中出现副本脉冲,间隙尺寸越大会导致这种副本脉冲越强,输出目标脉冲失真越严重;两种副本脉冲均呈等间隔分布,其周期由每个像元对应的频谱宽度决定,非线性光谱色散会使副本脉冲啁啾化且强度变小。     

变栅距光栅实现啁啾脉冲光谱整形

为了充分利用放大介质的增益带宽,获得脉宽更短,功率更高的输出脉冲,需要将输入到主放大链的种子脉冲进行光谱整形来补偿放大过程中的增益窄化效应。提出了利用变栅距反射光栅实现中心波长1053nm,谱宽6nm啁啾脉冲的光谱整形。运用严格的光栅衍射耦合波理论分析光栅的衍射特性,发现该方案不会引入相位畸变。分别计算和分析了刻槽深度、入射角大小、光栅周期以及入射光波长的变化对衍射效率的影响,通过选取适当的光栅参量可获得0.5%~84%的光谱调制深度。     

钛宝石超短脉冲放大过程中的增益窄化效应

本文研究超短脉冲的放大过程中产生的一个很重要的非线性效应──脉冲频谱的增益窄化效应(Gain Narrowing).分析了它的产生机理,同时指出了增益窄化效应在制约由于自相位调制效应引起的脉冲频谱展宽现象时所起的脉冲整形作用,得出了决定这种整形效果的种子脉冲参数和对放大器性能的要求.     

Ultrafast optical pulse shaping: A tutorial review

This paper presents a tutorial on the field of femtosecond pulse shaping, a technology that enables generation of nearly arbitrary, user defined, ultrafast optical waveforms, with control of phase, amplitude, and polarization. The emphasis is on Fourier transform pulse shaping, the most widely applied technique. Selected pulse shaping applications are described, with specific discussion of coherent control of quantum and nonlinear processes and of lightwave communications. Two new areas of pulse shaping research, namely, hyperfine spectral resolution pulse shaping and pulse shaping applications in ultrabroadband RF photonics, are discussed and illustrated with examples taken from the author's laboratory.     

Compensation of self-phase modulation in chirped-pulse amplification laser systems

Small amounts of self-phase modulation can lead to significant pulse distortions in chirped-pulse amplifiers. However, we show the surprising result that the strong chirp of the pulse can be exploited to remove these distortions completely by linear pulse shaping before amplification.     

Acousto-optical shaping of ultraviolet femtosecond pulses

This work demonstrates a simple method for ultraviolet (UV) acousto-optical pulse shaping of both spectral amplitude and phase. A fused-silica acousto-optical modulator is used to ensure high transmission and a high damage threshold at 400-nm center wavelength. The technique eliminates complications associated with the parametric transfer of the spectral phase of near-infrared pulses through a nonlinear process out to UV wavelengths, by separating the frequency doubling and shaping processes. Three illustrative applications of phase control are presented: the compensation of material dispersion, the generation of multiple pulse trains, and the generation of arbitrarily shaped pulse trains. Self-diffraction frequency-resolved optical gating is used to characterize the success of the technique.     

基于PAM4制的高速垂直腔面发射激光器研究进展

短距离光互联技术在云计算、5G通信、物联网技术等方面有重要的商业应用价值。基于高速垂直腔面发射激光器(Vertical-cavity surface-emitting laser,VCSEL)与多模光纤组成链路、采用直接调制检测、并使用如四电平脉冲幅度调制(Four-level pulse amplitude modulation,PAM4)等的高阶调制模式是现阶段短距离光互联链路方案的首选。本文首先介绍了短距离光互联应用的研究现状;第二部分介绍了VCSEL的发展、结构以及动态参数;第三部分介绍了PAM4调制方法及伴随使用的各种电子技术(均衡,前向纠错,脉冲整形);第四部分介绍了提高单链路速率的波分复用(Wavelength division multiplexing,WDM)技术;最后对以高速VCSEL、多模光纤、直接调制检测、PAM4调制以及波分复用技术的短距离光互联方案应用前景做了总结和展望。     

薄膜靶整形强激光脉冲的理论分析和数值模拟

超强超短脉冲激光广泛应用于粒子加速以及新型X射线辐射源产生。较长的激光脉冲上升前沿直接影响激光应用效果。等离子体薄膜靶作为新型光学介质开关,可以有效降低超强激光脉冲前沿上升时间,优化激光等离子体相互作用参数。采用一维理论分析和粒子模拟方法研究了等离子体薄膜靶实现超强激光脉冲整形的机制。研究结果表明,薄膜靶通过对激光脉冲的非线性调制,可有效实现脉宽缩短和脉冲陡化;对比单层靶调制结果,选择参数优化的双层靶,可进一步优化脉冲整形效果,获得更短脉宽和更高振幅的激光脉冲;对于峰值振幅高于薄膜靶击穿阈值的超强激光,脉冲上升前沿可得到明显陡化,薄膜靶的击穿是产生这种脉冲整形效果的直接原因。