Chirped pulse amplification in single mode Tm:fiber using a chirped Bragg grating

We report femtosecond pulse generation and chirped pulse amplification in Tm:fiber. A mode-locked oscillator operating in the soliton regime produced 800 fs pulses with 5 nm spectral bandwidth, at 40 pJ pulse energy. This oscillator seeded a pre-amplifier that utilizes a Raman soliton self-frequency shift to produce wavelength tunable pulses with 3 nJ energy, reduced pulse duration of 150 fs, and increased bandwidth of 30 nm. For further amplification, the pulses were stretched up to 160 ps using a chirped Bragg grating (CBG). Stretched pulses were amplified to 85 nJ after compression in single-mode Tm:fiber and recompressed with the CBG as short as 400 fs. Compressed pulses were coupled into a highly nonlinear tellurite fiber to investigate the potential of this ultrashort pulse 2-μm fiber source as a pump for mid-IR supercontinuum generation.     

Ultrafast continuum mid-infrared spectroscopy: probing the entire vibrational spectrum in a single laser shot with femtosecond time resolution

Until now, ultrafast IR spectroscopy has been limited by the bandwidth of optical parametric amplifiers, typically 100-400 cm(-1). Here we present the first example of transient IR spectroscopy using a continuum laser source to probe the entire mid-IR region with ultrafast time resolution. The continuum source is based on focusing the fundamental, second harmonic, and third harmonic of 1 mJ, 25 fs, 800 nm pulses in air, generating ～150 fs continuum mid-IR pulses that span the frequency range of <400 to >5000 cm(-1) or, conversely, <2 to >25 μm. We characterize the spectral and temporal properties of dicarbonylacetonato rhodium(I) in hexane. We further demonstrate the versatility of the method by measuring the very fast and broad (>1500 cm(-1)) spectral changes following IR excitation associated with the 7-azaindole-acetic acid heterodimer in carbon tetrachloride.     

All-reflective self-referenced spectral interferometry for single-shot measurement of few-cycle femtosecond pulses in a broadband spectral range

An all-reflective self-referenced spectral interferometry based on the transient grating(TG)effect is proposed for single-shot measuring of the amplitude and phase of ultrashort pulses in a broadband spectral range.Except for a thin third-order nonlinear medium,which was used to generate the TG signal,no transmitted optics were used in the proposed device,and few-cycle pulses in a broad spectral range from deep UV to mid-IR can be characterized.With a homemade compact and alignment-free device,a 5.0 fs pulse at 800 nm corresponding to about two cycles and a 14.3 fs pulse at 1800 nm corresponding to less than three cycles were successfully characterized.     

Supercontinuum generated in calcite with chirped femtosecond pulses

Effects of pulse chirping on the dynamic of supercontinuum (SC) spectra generated by femtosecond laser pulses at 800 nm in calcite are studied. When an 80 fs laser pulse is stretched to more then 130 fs by introducing positive or negative chirp, a strong and sharp spectral peak in the blue wing of the SC spectra is generated. The wavelength of this spectral peak is tuned by introducing chirp and pulse energy.     

Intense self-compressed, self-phase-stabilized few-cycle pulses at 2 microm from an optical filament

We report the compression of intense, carrier-envelope phase stable mid-IR pulses down to few-cycle duration using an optical filament. A filament in xenon gas is formed by using self-phase stabilized 330 microJ 55 fs pulses at 2 microm produced via difference-frequency generation in a Ti:sapphire-pumped optical parametric amplifier. The ultrabroadband 2 microm carrier-wavelength output is self-compressed below 3 optical cycles and has a 270 microJ pulse energy. The self-locked phase offset of the 2 microm difference-frequency field is preserved after filamentation. This is to our knowledge the first experimental realization of pulse compression in optical filaments at mid-IR wavelengths (lambda>0.8 microm).     

Half-cycle pulses in the mid-infrared from a two-color laser-induced filament

Four-wave mixing (FWM) of femtosecond near-infrared laser pulses and its second harmonic in the filamentation regime is shown to give rise to ultrashort field waveforms in the mid-infrared with pulse widths as short as a half of the field cycle and produce ultrabroadband supercontinuum spectra stretching from the mid-IR to the terahertz region. Generation of 7-fs pulses centered at 4.35 μm is demonstrated by a two-color filamentation experiment, where the 25-fs, 800-nm fundamental-wavelength output of a Ti: Sapphire laser is mixed with its second harmonic. The spectral and temporal properties of the mid-IR waveforms, as well as their emission pattern, are consistent with the FWM scenario of frequency conversion generalized to include the Kerr effect and ionization-induced refractive-index modulation.     

All-optical control of the carrier-envelope phase with multi-stage optical parametric amplifiers verified with spectral interference

Intense ultrashort laser pulses with stabilized carrier-envelope phase (CEP) are generated at 800 nm by using multi-stage collinear and non-collinear optical parametric amplifiers (OPAs). The first-stage collinear OPA is directly pumped by the fundamental-wave pulses and tuned to generate idler pulses at 1600 nm, which are further amplified by a second-stage collinear OPA, and then frequency-doubled to generate CEP-stabilized pulses at 800 nm. A non-collinear OPA is used to amplify the CEP-stabilized pulses at 800 nm. The combination of different OPAs can generate and amplify CEP-stabilized pulses at 800 nm without any detrimental influence from the fundamental-wave pulses. The CEP stabilization is verified with a simple and robust spectral interference setup. The stable interference pattern is measured for every single pulse and compared with the unstable pattern from pulses of random CEP. PACS 42.65.Re; 42.65.Yj; 42.25.Kb     

Flexible and rapidly configurable femtosecond pulse generation in the mid-IR

We demonstrate a new experimental approach for flexible femtosecond pulse generation in the mid-IR by use of difference-frequency generation from two tightly synchronized Ti:sapphire lasers. The resultant mid-IR pulse train can be easily tuned, with an adjustable repetition frequency up to 100 MHz, a pulse energy of approximately 1.5 x 10(-13) J, and an intensity noise similar to that of the Ti:sapphire. Rapid switching of the mid-IR wavelength and programmable amplitude modulation are achieved by precision setting of the time delay between two original pulses.     

Frequency conversion of subnanojoule femtosecond pulses in microstructure fibers

We experimentally demonstrate highly efficient multiplex frequency conversion of unamplified subnanojoule femtosecond pulses of Ti:sapphire laser radiation in fused silica microstructure fibers. Nonlinear optical spectral transformation of femtosecond pulses in an array of fused silica threadlike channels in these microstructure fibers results in the generation of isolated anti-Stokes spectral components within the wavelength range of 400–500 nm. An efficiency of frequency conversion of about 20% is achieved for 800-nm pump pulses with an energy of 0.7 nJ and a pulse duration of 70 fs.     

Ionization-assisted guided-wave pulse compression to extreme peak powers and single-cycle pulse widths in the mid-infrared

Ionization-assisted spectral broadening of high-energy 10.6 μm laser pulses in a gas-filled hollow waveguide is shown to yield single-cycle pulses with multiterawatt peak powers in the mid-IR. While the highest quality of pulse compression is achieved in the regime of weak ionization, careful management of complex ionization-assisted spectral broadening of guided-wave fields is the key to compressing the output of advanced high-power mid-IR laser sources to single-cycle pulse widths.     

Generation of Continuum Extreme-Ultraviolet Radiation by Carrier-Envelope-Phase-Stabilized 5-fs Laser Pulses

Coherent extreme-ultraviolet （XUV） radiation is studied by interaction of carrier-envelope （CE） phase stabilized high energy 5-fs infrared （800 nm） laser pulses with neon gas at a repetition rate of 1 kHz. A broadband continuum XUV spectrum in the cut-off region is demonstrated when the CE phase is shifted to about zero, rather than modulated spectral harmonics when setting of CE phase is nonzero. The results show the generation of isolated attosecond XUV pulses.     

Energy-scalable pulsed mid-IR source using orientation-patterned GaAs

Coherent mid-IR sources based on orientation-patterned GaAs (OPGaAs) are of significant interest in diverse scientific, medical, and military applications. The generation of long-wavelength mid-IR beams in OPGaAs using optical parametric oscillation exhibits limitations in the obtainable pulse energy and peak power. The master oscillator power amplifier concept is demonstrated in OPGaAs, by which a mid-IR source based on optical parametric oscillation can be scaled to high energy by amplification of the output of the optical parametric oscillator in an optical parametric amplifier (OPA). A fivefold increase in the pulse energy is obtained using this method by amplifying 3.85μm pulses in an OPGaAs OPA pumped by a Th,Ho:YLF Q-switched laser.     

用于宽带啁啾脉冲光参量放大系统精确同步的新方法

基于非共线光参量放大,以波长800 nm飞秒激光的倍频光为抽运光,以激光二极管640 nm连续波激光为信号光,产生了与800 nm飞秒激光精确同步的无直流本底的1064 nm脉冲光。实验结果显示该1064 nm的光脉冲在脉冲能量和空间光束质量上均可作为1064 nm波长脉冲放大器的种子光。仅通过一级简单紧凑的非共线参量放大就可实现光周期量级的光参量啁啾脉冲放大系统的抽运激光和种子光的精确同步。     

Dramatic Spectral Broadening of Ultrafast Laser Pulses in Molecular Nitrogen Ions

We investigate nonlinear interaction of nitrogen molecules with a two-color laser field composed by an intense800 nm laser pulse and a weak 400 nm laser pulse.It is demonstrated that the spectrum of 400 nm pulses is dramatically broadened when the two beams temporally overlap.In comparison,the spectral broadening in argon is less pronounced,although argon atoms and nitrogen molecules have comparable ionization potentials.We reveal that the dramatic spectral broadening originates from the greatly enhanced nonlinear optical effects in the near-resonant condition of interaction between the 400 nm pulses and the nitrogen molecular ions.     

钕玻璃啁啾脉冲放大器产生百焦耳亚皮秒脉冲

以钕玻璃为增益介质研制了高能啁啾脉冲激光放大器系统,实验演示输出啁啾脉冲能量在百焦耳量级时,光谱宽度保持在4—6 nm.最大能量168 J,相应谱宽5.5 nm,中心波长1054 nm.压缩脉冲宽度最短710 fs. 关键词： 啁啾脉冲放大 钕玻璃 高能拍瓦     

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.     

输出能量4mJ的1kHz飞秒掺钛蓝宝石激光再生放大研究

针对高能量千赫兹重复频率飞秒激光的应用需求,设计了一套采用线性再生腔结构的高效率飞秒钛宝石激光啁啾脉冲放大系统.通过优化腔型设计,在重复频率为1 kHz、单脉冲能量为20 mJ的527 nm激光抽运下,将展宽后的800 nm啁啾脉冲激光的能量放大到5.8 mJ,对应斜效率达到30.7%.进一步通过色散补偿压缩脉冲宽度,获得了单脉冲能量为4 mJ、脉冲宽度为45.7 fs的输出,稳定性测量表明激光的能量抖动仅为0.18%(均方根值).     

Generation of narrowband subpicosecond mid-infrared pulses via difference frequency mixing of chirped near-infrared pulses

The widely used setup for the generation of femtosecond infrared (IR) pulses based on parametric amplifiers (OPAs) and difference frequency mixing (DFM) is extended to produce tunable narrowband mid-IR pulses. The insertion of pairs of silicon prisms after the OPA induces adjustable chirp, which leads to the generation of narrowband pulses in the DFM stage. Rapid tunability of the mid-IR wavelength via a computer-controllable actuator can be achieved in a range of approximately 200 cm(-1) at a bandwidth of the IR-pulses between approximately 15 and approximately 50 cm(-1) and pulse energies up to 0.4 microJ. The narrowband mid-IR pulses are well suited for 2D IR spectroscopy.     

High power supercontinuum generation by dual-color femtosecond laser pulses in fused silica

High power supercontinuum (SC) is generated by focusing 800 nm and 400 nm femtosecond laser pulses in fused silica with a microlens array. It is found that the spectrum of the SC is getting broader compared with the case of a single laser pulse, and the spectral energy density between the two fundamental laser wavelengths is getting significantly higher by optimizing the phase matching angle of the BBO. It exceeds μJ/nm over 490 nm range which is from 380 nm to 870 nm, overcoming the disadvantage of relative lower power in the ranges far from the fundamental wavelength.     

Design of efficient single-stage chirped pulse difference frequency generation at 7 μm,driven by a dual wavelength Ti:sapphire laser

We present simulations for a design of a high-energy single-stage mid-IR difference frequency generation adapted to a two-color Ti:sapphire amplifier system. The optimized mixing process is based on chirped pulse difference frequency generation (CP-DFG), allowing for a higher conversion efficiency and reduced two-photon absorption losses. The numerical start-to-end simulations include stretching, chirped pulse difference frequency generation and pulse compression. Realistic design parameters for commercially available nonlinear crystals (GaSe, AgGaS₂, LiInSe₂, LiGaSe₂) are considered. Compared with conventional unchirped DFG directly pumped by Ti:sapphire technology, we predict a threefold increase in the quantum efficiency. Our CP-DFG scheme provides up to 340 μJ pulse energy directly at 7.2 μm when pumped with 8 mJ and supports a bandwidth of up to 350 nm. The resulting 240 fs mid-IR pulses are inherently phase stable.