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。     

Trends in ultrashort and ultrahigh power laser pulses based on optical parametric chirped pulse amplification

Since the proof-of-principle demonstration of optical parametric amplification to efficiently amplify chirped laser pulses in 1992,optical parametric chirped pulse amplification(OPCPA)became the most promising method for the amplification of broadband optical pulses.In the meantime,we are witnessing an exciting progress in the development of powerful and ultrashort pulse laser systems that employ chirped pulse parametric amplifiers.The output power and pulse duration of these systems have ranged from a few gigawatts to hundreds of terawatts with a potential of tens of petawatts power level.Meanwhile,the output pulse duration based on optical parametric amplification has entered the range of fewoptical-cycle field.In this paper,we overview the basic principles,trends in development,and current state of the ultrashort and laser systems based on OPCPA,respectively.     

利用非线性脉冲预整形实现脉冲快速自相似放大

报道了一种通过预先对信号光脉冲非线性整形, 进而在光纤放大器中实现自相似演化的方法. 利用透射光栅对和普通单模光纤组成被动脉冲整形装置, 预先优化脉冲的时域宽度和光谱质量, 使脉冲在光纤放大器中快速演化到自相似子. 研究中首先通过数值模拟对比, 说明了非线性整形对脉冲实现快速自相似放大的关键作用, 提高了放大器输出脉冲质量, 减小了去啁啾脉冲宽度. 实验中, 通过优化非线性整形输出, 在2.2 m 长的掺Yb³⁺光纤中, 在一定抽运功率范围内均实现了脉冲自相似放大, 去啁啾后得到脉冲宽度60 fs 的变换极限脉冲输出. 这种非线性脉冲预整形方法有效减小了 自相似演化所需光纤长度, 同时降低了自相似放大对种子源质量的要求, 首次将全正色散光纤锁模激光器成功用于自相似放大, 促进了当前自相似放大系统的全光纤化. 关键词： 非线性整形 光纤放大器 自相似放大 飞秒激光     

Generation and amplification of dual-color femtosecond pulses by a noncollinear optical parametric up-conversion process

Noncollinear optical parametric up-conversion generation and amplification are realized in a thick β-barium borate (BBO) crystal, and a couple of visible femtosecond up-conversion laser pulses can be achieved by a femtosecond pulse at 800 nm as the pump sources. The theoretical and experimental results indicate that there exist phase-matching conditions for dual-color noncollinear parametric up-conversion generation and amplification, and their wavelengths can be tuned by rotating the BBO crystal. This parametric up-conversion generation and amplification can be attributed to three and five-wave mixing in a thick BBO crystal, and it shows the potential application on optical parametric chirped pulse amplification (OPCPA) to generate multi-color ultraviolet or visible femtosecond laser pulses pumped directly by femtosecond fundamental laser pulses without frequency-doubling or tripling.     

氟化氪短脉冲激光的放大和组束研究

为充分利用氟化氪(KrF)准分子激光放大器的长泵浦时间,探索提高激光输出效率的方法,开展紫外超短脉冲在KrF准分子激光器中多脉冲放大和组束的实验研究。采用双脉冲放大方案研究激光脉冲时间间隔对输出能量的影响,确定延时时间,提高脉冲总能量并有效抑制自发辐射(ASE)。实现了单次放大4个紫外超短脉冲,获得了近4倍于单脉冲放大的输出能量。并探索紫外超短激光脉冲的组束技术,成功应用光学角多路的方法将两个亚皮秒的紫外激光脉冲进行精确组束。     

Multi-terawatt laser system generating 12-fs pulses at 100 Hz repetition rate

We have developed a laser system generating high peak-power ultrashort pulses based on the chirped pulse amplification of a Ti:sapphire laser. The pulse duration of the laser is reduced to 12 fs, owing to the successful compensation of gain narrowing during amplification with novel optics. The pulse energy and the repetition rate are 40 mJ and 100 Hz, respectively, as a consequence of sufficient energy extraction from a laser medium in the final stage of a multipass amplifier chain, which is designed to balance focusing with a thermal lens against defocusing with convex mirrors without cryogenic cooling for Ti:sapphire crystals. The laser pulses delivered from this laser system are suitable for generating an intense attosecond pulse train in a vacuum ultraviolet or extreme ultraviolet region.     

Stacking chirped pulse optical parametric amplification

Stacking chirped pulse optical parametric amplification based on a home-built Yb³⁺-doped mode-locked fiber laser and an all-fiber pulse stacker has been demonstrated. Energic 11 mJ shaped pulses with pulse duration of 2.3 ns and a net total gain of higher than 1.1 × 10⁷ at fluctuation less than 2% rms are achieved by optical parametric amplification pumped by a Q-switched Nd:YAG frequency-doubled laser, which provides a simple and efficient amplification scheme for temporally shaped pulses by stacking chirped pulse.     

High-resolution indirect pulse shaping by parametric transfer

The phase and amplitude profile of a shaped pulse in the visible is transferred to a pulse in the near-infrared via an optical parametric amplification (OPA) process. Complex shaped pulses, such as multiple-pulse trains and pulses with high-order phase chirp, are produced at 1.2mum . Theoretical conditions necessary for high-fidelity parametric shape transfer are discussed. Similar schemes can be implemented for other OPA systems pumped at near-infrared wavelengths to generate high-resolution shaped pulses in the mid-infrared.     

Amplification of frequency-modulated soliton-like pulses in inhomogeneous optical waveguides with normal dispersion

The possibility of effective amplification of self-similar frequency-modulated pulses (FMPs) in longitudinally inhomogeneous active optical waveguides is studied. Peculiarities of the dynamics of parabolic pulses with a constant frequency modulation rate are considered. An optimal profile of variation of the group velocity dispersion was obtained in correspondence with optimal amplification of a similariton-like pulse. The use of FMPs in amplifying and longitudinally inhomogeneous optical waveguides with a correspondingly matched profile of normal dispersion of group velocities is shown to be capable of providing for an amplification of subpicosecond pulses up to energies above 1 nJ.     

Proposed beatwave experiment at RAL with the Vulcan CPA laser

A chirped pulse amplification (CPA) laser configuration capable of driving a plasma beat wave into saturation before modulation instabilities can grow is reported. The proposal is based on generating a single sub-ps, broad bandwidth pulse (~16 nm) and stretching and filtering to select two wavelength components (separation ~7 nm). The two spectral components are temporally stretched (to >100 ps) and separated (by ~1 ns). The pulses are then amplified sequentially in a single Nd:glass chain to greater than 15 J per pulse. Using a single-pass reflective grating compressor, the pulses are compressed (from 2 to 5 ps) and automatically synchronized. The system is capable of introducing a chirp, such that the compressed pulses can compensate for relativistic detuning of the plasma wave. The optimum laser amplification and recombination configuration for generating a saturated laser-driven beatwave is presented, and options for future work are discussed     

Optical parametric amplification of a shaped white-light continuum

Phase-locked two-color sub-40-fs double pulses in the visible are produced by noncollinear parametric amplification of white light tailored in a pulse shaper with a liquid-crystal mask. The carrier phase between the pulses is conserved during the amplification process and can be adjusted, as can the temporal separation and the center of wavelengths of the pulses.     

Femtosecond thin-disk Yb:KYW regenerative amplifier

We report a compact thin-disk Yb:KYW regenerative amplifier system. Two different concepts are investigated to obtain either subpicosecond pulses with up to 160 μJ or a pulse energy of 20 μJ with a pulse width of about 300 fs. The first concept uses intra-cavity group-velocity dispersion compensation with Gires–Tournois interferometer mirrors to avoid pulse stretching during amplification. The onset of nonlinear effects in this concept inhibits the generation of shorter pulse durations at this energy level. Shorter pulses can be achieved with the second concept, which is based on dispersive pulse stretching during amplification and uses pulse compression after amplification with a grating compressor. Repetition rates up to 45 kHz are demonstrated.This revised version was published online in May 2005. The Article Category was removed.This revised version was published online in August 2005 with a corrected cover date.     

Relic crystal-lattice effects on Raman compression of powerful x-ray pulses in plasmas

Powerful x-ray pulses might be compressed to even greater powers by means of backward Raman amplification in ultradense plasmas produced by ionizing condensed matter by the same pulses. The pulse durations contemplated are shorter than the time for complete smoothing of the crystal lattice by thermal motion of ions. Although inhomogeneities are generally thought to be deleterious to the Raman amplification, the relic lattice might, in fact, be useful for the Raman amplification. The x-ray frequency band gaps can suppress parasitic Raman scattering of amplified pulses, while enhanced dispersion of the x-ray group velocity near the gaps can delay self-phase-modulation instability, thereby enabling further amplification of the x rays.     

小宽带堆积啁啾脉冲传输放大特性

 采用啁啾脉冲堆积的方法可获得ns量级宽带整形激光脉冲,对堆积啁啾脉冲时间波形的影响因素及传输放大后脉冲波形与光谱形状的特性进行了研究。结果表明：在传输过程中脉冲的演化规律与窄带脉冲的演化规律基本一致,而且传输放大过程也不会改变脉冲的调制结构,但光谱形状在传输放大过程中发生了变化,初步认为是由非线性效应造成的;小宽带堆积啁啾脉冲具有与窄带脉冲基本一致的的增益水平。     

Compact, high-throughput expansion-compression scheme for chirped pulse amplification in the 10 fs range

A novel pulse stretching-compression system suitable for kHz-rate chirped-pulse amplification of ≈10 fs pulses in Ti:sapphire systems is demonstrated. The material dispersion of the system components used for pulse selection and isolation broadens the wide-band seed pulses by a factor of ≈300, allowing amplification up to the submillijoule energy range. The compressor consists of a pair of prisms and newly-developed chirped multilayer dielectric mirrors for compensating high order dispersion. Using this simple scheme a recompression of unamplified pulses down to 15 fs with a throughput as high as 80% is demonstrated.     

Fiber based high repetition rate, high energy laser source applying chirped pulse amplification

We demonstrate the potential of a high-gain Yb-fiber amplifier system to provide ultrashort pulses with high energies. 100 μJ pulses generated at a repetition rate of 32 kHz exhibit nearly diffraction limited output (M²≈1.7) at a 1060 nm center wavelength. Using chirped pulse amplification, temporally streched laser pulses from a femtosecond oscillator could be compressed after amplification to subpicosecond pulse duration. The achievable intensities are high enough to create plasmas which can efficiently convert laser light to the extreme ultraviolet radiation. In a multimode Yb-doped fiber amplifier we obtained average powers of up to 22 W and single pulse energies up to 130 μJ.     

Implementation of the direct locking method for long-term carrier-envelope-phase stabilization of a grating-based kHz femtosecond laser

We have stabilized the carrier-envelope phase (CEP) of amplified femtosecond laser pulses from a grating-based chirped-pulse amplification femtosecond laser by the direct locking method. Long-term CEP stabilization in the oscillator was achieved by employing a double-feedback loop to control both the pumping power and the cavity dispersion. Large CEP drift, induced during amplification, was compensated by adjusting the grating separation in the pulse compressor, and the CEP stabilization was maintained for four hours with a phase jitter of about 180 mrad. After pulse compression to 5.5-fs pulses in a filamentation pulse compressor, CEP-stabilized laser pulses were applied for high-harmonic generation to confirm the CEP stabilization.     

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.     

Self-similar amplification in fiber Bragg gratings written in fiber amplifiers

We show, by using numerical simulations, that self-similar pulses with a duration on the order of few nanoseconds and an energy on the order of 10?μJ can be obtained at the output of a fiber Bragg grating (FBG) written in a fiber amplifier. The evolution of the amplified pulses is determined by the combined effect of Kerr nonlinearity, normal-dispersion, gain, and gain saturation, which limit the pulse energy. The output pulse mainly depends on the initial pulse energy rather than on the initial pulse profile. The reduced group velocity in FBGs can significantly increase the total gain for a given amplifier length. Hence we find that the proposed amplification scheme can be highly advantageous for amplification of nanosecond-scale pulses in fiber amplifiers.     

Femtosecond hybrid mode-locked semiconductor laser and amplifier dynamics

We describe the generation of femtosecond high power optical pulses using hybrid passive-active mode-locking techniques. Angle stripe geometry GaAs/AlGaAs semiconductor laser amplifiers are employed in an external cavity including prisms and a stagger-tuned quantum-well saturable absorber. An identical amplifier also serves as an optical power amplifier in a stretched pulse amplification and recompression sequence. After amplification and pulse compression this laser system produces 200 fs, 160 W peak power pulses. We discuss and extend our theory, and supporting phenomenological models, of picosecond and subpicosecond optical pulse amplification in semiconductor laser amplifiers which has been successful in calculating measured spectra and time-resolved dynamics in our amplifiers. We have refined the theory to include a phenomenological model of spectral hole-burning for finite intraband thermalization time. Our calculations are consistent with an intra-band time of approximately 60 fs. This theory of large signal subpicosecond pulse amplification will be an essential tool for understanding the mode-locking dynamics of semiconductor lasers and for analysis of high speed multiple wave-length optical signal processing and transmission devices and systems based on semiconductor laser amplifiers.