Generation of sub-5 fs pulses in vacuum ultraviolet using four-wave frequency mixing in hollow waveguides

We investigate the potential of four-wave mixing in dielectric hollow waveguides for vacuum ultraviolet pulse generation with unprecedented short durations. Taking into account higher-order transverse modes and plasma effects we predict the generation of a 2.5 fs pulse at 160 nm using an intense 10 fs, 800 nm pulse and its weaker third harmonic at 267 nm, both coupled to the fundamental transverse mode. Excitation of higher transverse modes allows an increase of the signal energy (up to by a factor of 20) but with a pulse duration of 13 fs (compressible to 7.7 fs).     

Design and fabrication of hollow-core photonic crystal fibers for high-power ultrashort pulse transportation and pulse compression

We report on the recent design and fabrication of kagome-type hollow-core photonic crystal fibers for the purpose of high-power ultrashort pulse transportation. The fabricated seven-cell three-ring hypocycloid-shaped large core fiber exhibits an up-to-date lowest attenuation (among all kagome fibers) of 40 dB/km over a broadband transmission centered at 1500?nm. We show that the large core size, low attenuation, broadband transmission, single-mode guidance, and low dispersion make it an ideal host for high-power laser beam transportation. By filling the fiber with helium gas, a 74?μJ, 850?fs, and 40?kHz repetition rate ultrashort pulse at 1550?nm has been faithfully delivered at the fiber output with little propagation pulse distortion. Compression of a 105?μJ laser pulse from 850?fs down to 300?fs has been achieved by operating the fiber in ambient air.     

Generation of 47 fs Pulses from an Er:Fiber Amplifier

We demonstrate a self-starting erbium fiber oscillator-amplifier system based on the nonlinear polarization rotation mode-locked mechanism. The direct output pulse from the amplifier is 47 fs with an average power of 1.22 W and a repetition rate of 50 MHz, corresponding to a pulse energy of 24 nJ. The full width at half-maximum of the spectrum of the output pulses is approximately 93 nm at a central wavelength of 1572 nm so that the transformlimited pulse duration is as short as 39 fs. Due to the imperfect dispersion compensation, we compress the pulses to 47 fs in this experiment.     

Enhancement and broadening of extreme-ultraviolet supercontinuum in a relative phase controlled two-color laser field

We experimentally demonstrate the generation of an extreme-ultraviolet (XUV) supercontinuum in argon with a two-color laser field consisting of an intense 7 fs pulse at 800 nm and a relatively weak 37 fs pulse at 400 nm. By controlling the relative time delay between the two laser pulses, we observe enhanced high-order harmonic generation as well as spectral broadening of the supercontinuum. A method to produce isolated attosecond pulses with variable width and intensity is proposed.     

The control of the electron quantum paths for the generation of isolated attosecond pulse using a three-color laser scheme

We theoretically propose a three-color laser scheme to enhance the high-order harmonic intensity and generate an isolated attosecond pulse. By adding a 3 fs, 1600 nm laser pulse to a synthesized two-color laser field (5 fs, 800 nm and 10 fs, 1200 nm), the harmonic intensity is effectively enhanced and an isolated attosecond pulse with duration 41 as is generated. In this scheme, the short trajectory is suppressed, the selection of the long quantum path can be achieved. We also investigate emission time of harmonics in terms of the time-frequency analysis and the semi-classical three-step model to illustrate the physical mechanism of high-order harmonic generation.     

High-order harmonics extension and isolated attosecond pulse generation in three-color field: Controlling factors

In the present Letter, we theoretically discuss the optimum conditions for generating ultrashort attosecond pulse in three-color field with a model He exposed to the intense 5 fs, 800 nm fundamental field and the two weak control fields of 25 fs, 400 nm and 25 fs, 1600 nm. Through investigating the controlling factors in HHG spectra generation via manipulating the laser parameters of the three fields, we demonstrate that properly increasing the pulse intensity of 800 nm and 1600 nm fields at the same time with zero phase effects is an effective way to generate short attosecond pulses. Finally, an isolated pulse of 7 as is predicted by Fourier transforming an ultrabroad XUV continuum of 393 eV with phase compensation.     

基于平衡光学互相关方法的超短脉冲激光相干合成技术

相干合成技术是超快光学领域的重要研究方向之一.当单路脉冲激光的连续谱超过一个倍频程时,精确控制其光谱相位(色散管理)是获得亚周期超短脉冲激光的关键.由于常见的脉冲压缩系统存在光谱带宽限制,因此多通道相干合成技术受到了广泛的关注.本文将充气空心光纤展宽后的超倍频程连续光谱分波段独立压缩,并利用平衡光学互相关方法锁定子脉冲之间的相位延迟,获得了4.1 fs的合成脉冲.实验结果表明相干合成技术在高能量亚周期超快光场调控中存在优势.     

正交偏振双色激光场作用下生成的孤立阿秒脉冲

我们理论研究了正交偏振双色激光场作用下的高次谐波发射和孤立阿秒脉冲的产生.当y方向加一束中红外激光脉冲(12.5 fs/2000 nm),x方向加一束强度较弱的激光脉冲(12 fs/800 nm)时,我们得到从250 eV到350 eV的超连续谐波平台,在平台范围内叠加50 eV的谐波,可以得到一个脉宽约为97as的孤立阿秒脉冲. 通过时频分析,我们解释了高次谐波发射的物理机制.     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

Tunable femtosecond near-infrared source based on a Yb:LYSO-laser-pumped optical parametric oscillator

We demonstrate a widely tunable near-infrared source from 767 nm to 874 nm generated by the intracavity second harmonic generation(SHG) in an optical parametric oscillator pumped by a Yb:LYSO solid-state laser. The home-made Yb:LYSO oscillator centered at 1035 nm delivers an average power of 2 W and a pulse duration as short as 351 fs. Two Mg O doped periodically poled lithium niobates(Mg O:PPLN) with grating periods of 28.5–31.5 μm in steps of 0.5 μm and19.5–21.3 μm in steps of 0.2 μm are used for the OPO and intracavity SHG, respectively. The maximum average output power of 180 m W at 798 nm was obtained and the output pulses have pulse duration of 313 fs at 792 nm if a sech2-pulse shape was assumed. In addition, tunable signal femtosecond pulses from 1428 nm to 1763 nm are also realized with the maximum average power of 355 m W at 1628 nm.     

Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR

Noncollinearly phase-matched optical parametric amplifiers (NOPAs) pumped by the blue light of a frequency-doubled Ti:sapphire regenerative amplifier are a convenient source of continuously tunable ultrashort pulses in the visible and near infrared for spectroscopic experiments. We present the underlying principles, report recent improvements and describe the experiences gained from the routine use of a number of NOPAs in our laboratories. We find that the setup can easily be optimized for the given experimental requirements. Typical output-pulse energies in the visible are 5 to 10 μJ and a few μJ in the NIR from 200 μJ regenerative-amplifier pulses at 800 nm. From 460 to 700 nm, pulse lengths between 10 and 20 fs are routinely achieved, while the length increases monotonically from about 20 fs at 900 nm to just below 50 fs at 1600 nm. In all cases this corresponds to a dramatic shortening compared to the length of the pump pulses of around 100 fs. First results show that the 700 to 900 nm region can be accessed with sub-50-fs pulse lengths by use of an intermediate white-light generator in a two-stage setup. Received: 29 November 1999 / Published online: 5 July 2000     

High-power diode-pumped Yb3+:CaF2 femtosecond laser

We report what is believed to be the first demonstration of a high-power passively mode-locked diode-pumped femtosecond laser based on an Yb3+:CaF2 single crystal, directly pumped by a 15-W fiber-coupled laser diode. With a 5-at. % Yb3+ -doped sample and prisms for dispersion compensation we obtained pulses as short as 150 fs, with 880 mW of average power and up to 1.4-W average output power, with a pulse duration of 220 fs, centered at 1049 nm. The laser wavelength could be tuned from 1040 to 1053 nm in the femtosecond regime. Using chirped mirrors for dispersion compensation, the oscillator provided up to 1.74 W of average power, with a pulse duration of 230 fs, corresponding to a pulse energy of 20 nJ and a peak power of 85 kW.     

Generation of femtosecond pulses at 1350 nm by Cerenkov radiation in higher-order-mode fiber

We demonstrate a method of generating short pulses at 1350 nm by exciting Cerenkov radiation in a higher-order-mode fiber with a 1064 nm femtosecond fiber laser. We measure a 106 fs, 0.66 nJ output pulse. Cerenkov radiation in fibers allows for energy transfer between a soliton and a dispersive wave, providing an effective and engineerable platform to shift the wavelength of a femtosecond source. With appropriate design of the higher-order-mode fiber, this method of generating short pulses at 1350 nm can be extended to other wavelengths and to higher pulse energies.     

Passively mode locked femtosecond Tm:Sc2O3 laser at 2.1 μm

We report on the passive mode locking of a Tm3+:Sc2O3 laser at 2.1 μm using a semiconductor saturable absorber mirror based on InGaAsSb quantum wells. Transform-limited 218 fs pulses are generated with an average power of 210 mW. A maximum output power of 325 mW is produced during mode locking with the corresponding pulse duration of 246 fs at a pulse repetition frequency of 124.3 MHz. A Ti:sapphire laser is used as the pump source operating at 796 nm.     

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.     

Isolated attosecond pulse generation from atom radiated by a three-color laser pulse

We theoretically investigate high-order harmonic and attosecond pulse generation from helium atom in a three-color laser field, which is synthesized by 10 fs/800 nm Ti-sapphire laser and a two-color field consisting of 30 fs/532 nm and30 fs/1330 nm pulses. Compared with harmonic spectrum generated by a monochromatic field, the harmonics generated from the synthesized three-color field show a supercontinuum spectrum with a bandwidth of 235 eV, ranging from the 154 th to the 306 th order harmonic. This phenomenon can be attributed to the fact that the ionization of atoms as well as motion of ionized electron can be effectively controlled in the three-color field. Therefore, an isolated 46-as pulse can be generated by superposing supercontinuum from the 160 th to the 210 th order harmonics.     

Ultrafast electron pulses from a tungsten tip triggered by low-power femtosecond laser pulses

We present an experimental and numerical study of electron emission from a sharp tungsten tip triggered by sub-8-fs low-power laser pulses. This process is nonlinear in the laser electric field, and the nonlinearity can be tuned via the dc voltage applied to the tip. Numerical simulations of this system show that electron emission takes place within less than one optical period of the exciting laser pulse, so that an 8 fs 800 nm laser pulse is capable of producing a single electron pulse of less than 1 fs duration. Furthermore, we find that the carrier-envelope phase dependence of the emission process is smaller than 0.1% for an 8 fs pulse but is steeply increasing with decreasing laser pulse duration.     

Femtosecond pulse compression based on second harmonic generation from a frequency chirped pulse

A 49 fs pulse at the wavelength of 800 nm was converted to a 26 fs pulse at the wavelength of 400 nm by temporal stretching (frequency chirping) and second harmonic generation from a chirped pulse with subsequent compression. The energy conversion efficiency of 35% was achieved.     

Extension of Harmonic Cutoff and Generation of Isolated Sub-30 as Pulse in a Two-Color Chirped Laser Field

We theoretically investigate high-order harmonic and isolated attosecond pulse generation in a two-color chirped laser field,which is synthesized by a 9 fs/800 nm fundamental chirped pulse and a 9 fs/1600 nm controlling chirped pulse.Our numerical results show that,by using this method,not only is the harmonic cutoff significantly extended to the 948th order harmonic,but also the bandwidth of the supercontinuum spectrum is effectively broadened to about 1342 eV.In addition,due to the introduction of the chirp,the long quantum path is suppressed and only the short one is selected,and then an isolated 28 as pulse with a bandwidth of 155 eV is obtained directly.     

Extending the High-Order Harmonics Plateau and Isolated Attosecond Pulse Generation in an Intense Three-Color Laser Field

We propose a method to extend the high-order harmonics plateau and generate an isolated sub-10-as pulse by adding a weak control pulse (10 fs, 1600 nm) to modify a two-color laser field (5 fs, 800 nm; 10 fs, 400 nm). The numerical results show that the plateau is extended obviously in the three-color laser field regime. Additionally, the ionization rate and classical returning kinetic-energy maps are calculated to better understand the physical origin of the high-order harmonics generation (HHG). By means of the ionization probability and the time-frequency distributions, more features of the HHG are revealed. Furthermore, our simulations show that the width of the plateau and the relative conversion efficiency of the harmonic spectra are highly dependent on the relative phase. Finally, by adjusting the relative phase and superposing a properly selected range of the HHG spectrum, an isolated attosecond pulse with a duration of 7 as is obtained.