Production of picosecond, kilojoule, and petawatt laser pulses via Raman amplification of nanosecond pulses

Raman amplification in plasma has been promoted as a means of compressing picosecond optical laser pulses to femtosecond duration to explore the intensity frontier. Here we show for the first time that it can be used, with equal success, to compress laser pulses from nanosecond to picosecond duration. Simulations show up to 60% energy transfer from pump pulse to probe pulse, implying that multikilojoule ultraviolet petawatt laser pulses can be produced using this scheme. This has important consequences for the demonstration of fast-ignition inertial confinement fusion.     

Generation and applications of passively mode-locked picosecond light pulses

A model concerning the generation of picosecond light pulses in solid state lasers is compared with existing experimental observations. The quality of selected single pulses and their favourable properties for investigations with ultrashort light pulses are discussed. Extensive physical applications of picosecond light pulses are reviewed. Emphasis is given to quantitative investigations of non-linear optics and of ultrafast relaxation processes in condensed matter.     

Generator of picosecond laser pulses

The design of the generator of picosecond laser pulses and the results on semiconductor target (ZnSe, CdS, and others) excitation by electric field and electron beam pulses are presented. The maximum power of laser radiation reached 10 kW at pulse durations of 100–200 ps.     

A rapid-scanning autocorrelation scheme for continuous monitoring of picosecond laser pulses

We describe a scheme for rapidly introducing a periodic linear time delay to a train of picosecond laser pulses. By incorporating this scheme in one arm of the Michelson interferometer of a conventional autocorrelator, the second order intensity autocorrelation function of a cw train of picosecond pulses is continuously displayed on an oscilloscope.     

Rapid scanning autocorrelator for measurements of picosecond laser pulses

A rapid scanning autocorrelation interferometer for measurements of picosecond laser pulses is described which uses a rotating prism as scanning device in one arm of the interferometer to permit continuous display of autocorrelation traces at audio frequencies on an oscilloscope. Scan widths of more than 500 ps with high linearity can be achieved. Autocorrelation measurements of picosecond pulses from a synchronously pumped mode-locked dye laser are presented.     

Temporal contrast enhancement of picosecond pulses based on phase-conjugate wave generation

A practical technique based on the phase-conjugate wave (PCW) generation is proposed to improve the temporal contrast of the picosecond pulses. Our theory predicts the temporal contrast of the picosecond pulses can be enhanced to about the cube of the temporal contrast of the input pulse via the PCW generation, in which the conversion efficiency from the incident pulse to the PCW is about 25%. In a proof-of-principle experiment, the temporal contrast of picosecond pulses was enhanced from 1.7×10³ to 8.6×10⁸ with the conversion efficiency of 10%. This technique is promising to effectively eliminate the background of the ultrashort and ultraintense laser pulses in the future.     

Interpulse coherence of picosecond pulses

Interferometric studies of trains of picosecond pulses from dye mode-locked Nd: glass lasers have been made. Both autocorrelation and cross correlation measurements of pulses have been recorded. Even for non-minimum width pulses very high fringe contrast ratios have been obtained for both cases indicating considerable similarity between pulses. It is also concluded that the relative phase shift from pulse to pulse is constant.     

Holographic interferometry of laser produced plasmas using picosecond pulses

Interferograms have been obtained using a holographic system illuminated by picosecond pulses. The technique has been applied to a picosecond laser produced plasma, and the total number of electrons present in the plasma has been measured to be at least 5 × 10¹⁴for a laser energy of 0.1 joule.     

Drilling rate of five metals with picosecond laser pulses at 355, 532, and 1064 nm

Experimental results on picosecond laser processing of aluminum, nickel, stainless steel, molybdenum, and tungsten are described. Hole drilling is employed for comparative analysis of processing rates in an air environment. Drilling rates are measured over a wide range of laser fluences (0.05–20?J/cm²). Experiments with picosecond pulses at 355?nm are carried out for all five metals and in addition at 532?nm, and 1064?nm for nickel. A comparison of drilling rate with 6-ps and 6-ns pulses at 355?nm is performed. The dependence of drilling rate on laser fluence measured with picosecond pulses demonstrates two logarithmic regimes for all five metals. To determine the transition from one regime to another, a critical fluence is measured and correlated with the thermal properties of the metals. The logarithmic regime at high-fluence range with UV picosecond pulses is reported for the first time. The energy efficiency of material removal for the different regimes is evaluated. The results demonstrate that UV picosecond pulses can provide comparable quality and higher processing rate compared with literature data on ablation with near-IR femtosecond lasers. A significant contribution of two-photon absorption to the ablation process is suggested to explain high processing rate with powerful UV picosecond pulses.     

Determination of the shape and duration of picosecond light pulses by bleaching of dyes

A new method is developed to measure the shape and duration of picosecond light pulses in a single shot. A pump and probe technique in a dye sample is applied. Single picosecond light pulses at different frequencies were analysed with various absorbers. Asymmetric pulse shapes with leading edges steeper than the trailing edges were observed.     

Multimillijoule chirped parametric amplification of few-cycle pulses

The concept of optical parametric chirped-pulse amplification is applied to attain pulses with energies up to 8 mJ and a bandwidth of more than 100 THz. Stretched broadband seed pulses from a Ti:sapphire oscillator are amplified in a multistage noncollinear type I phase-matched beta-barium borate parametric amplifier by use of an independent picosecond laser with lock-to-clock repetition rate synchronization. Partial compression of amplified pulses is demonstrated down to a 10-fs duration with a down-chirped pulse stretcher and a nearly lossless compressor comprising bulk material and positive-dispersion chirped mirrors.     

Supercontinuum generated in all-normal dispersion photonic crystal fibers with picosecond pump pulses

The supercontinuum (SC) generation in all-normal dispersion (ANDi) photonic-crystal fiber (PCF) pumped by high power picosecond pulses are investigated in this paper. Our results show that an octave SC may be achieved by pumping the ANDi PCF with picosecond pump pulses. However, the PCF length required may have to be lengthened to several tens of centimeters, which is much longer than that with femtosecond pump pulses. The relatively long PCF gives rise to much higher Raman gain and stronger Raman frequency shift compared to those with femtosecond pump pulses, which in turn not only cause a distorted temporal waveform and an un-flattened spectrum, but also severely degrade the coherence of the generated SC.     

Generation of synchronized femtosecond and picosecond laser pulses in a two-beam-pumped Ti:sapphire laser

<正>Two operating modes,independent self-mode-locking and cross-mode-locking,are presented in a two-beampumped double-cavity dual-wavelength femtosecond Ti:sapphire laser.Synchronization of femtosecond and picosecond laser pulses is achieved by properly adjusting the cavity length matching and distributing the pump laser powers in the two laser cavities,and moreover,a timing jitter of 517 fs between femtosecond and picosecond pulses is obtained,with wavelength tuning ranges around 36 and 22 nm in the femtosecond and picosecond cavities,respectively.     

Generation of tunable picosecond pulses in the ultraviolet region down to 197 nm

Tunable picosecond pulses in the ultraviolet region down to 197 nm with ? 20 kW peak power are generated by the sum frequency mixing of fourth or third harmonic pulses of a mode-locked YAG laser with tunable pulses produced by a LiNbO₃ parametric oscillator pumped by second harmonic pulses of the YAG laser by using a KB5 or KDP crystal.     

皮秒光纤激光脉冲两个关键问题的研究

窄带耗散孤子锁模光纤激光器可以产生接近变换限制的皮秒脉冲,但受非线性相移的限制,输出脉冲重复频率不能通过增加腔长来降低,脉冲能量仅在0.1 nJ以下,严重制约着这类皮秒脉冲的实际应用。提出一种通过耦合器抽取腔内脉冲能量、抑制腔内非线性相移积累,进而允许增加腔长来降低窄带耗散孤子皮秒光纤激光脉冲重复频率的方法。运用该方法,成功地将激光器重复频率由35.2 MHz降低到了1.77 MHz,且脉冲时频特性保持不变。提出了一种基于级间FBG陷波滤波的抑制皮秒脉冲光纤放大中光谱展宽的方法。通过简单地使用级间陷波滤波器,既可窄化第一级光纤放大器后的输出脉冲谱宽,允许采用第二级光纤放大器进一步提升脉冲能量,而且,还可将脉冲重塑为近高斯形,利用高斯脉冲光谱展宽斜率小的特点,允许第二级光纤放大器将脉冲能量提升得更高。利用该方法,在RMS（均方值）谱宽保持0.4 nm以内的前提下,10 ps脉冲经标准单模光纤放大器后,能量可由0.2 nJ可提升到10 nJ以上。     

Blue 489-nm picosecond pulses generated by intracavity frequency doubling in a passively mode-locked optically pumped semiconductor disk laser

We report the generation of blue 489-nm picosecond laser pulses by intracavity second-harmonic generation in a mode-locked optically pumped InGaAs vertical-external-cavity surface-emitting laser. Mode locking achieved by a semiconductor saturable absorber mirror generated 5.8-ps-long sech²-shaped pulses at an emission wavelength of 978 nm and a repetition rate of 1.88 GHz. Intracavity frequency doubling in a 5-mm-long lithium triborate crystal generated blue picosecond pulses with a spectral width of 0.15 nm and an average output power of up to 6 mW.     

Coherent excitation of molecular vibrational modes by high-power ultrashort infrared laser pulses

Coherent dynamics of multiphoton excitation of molecular vibrational modes by subpicosecond IR laser pulses differs greatly from that of picosecond pulse excitation. The resonance response of a molecule is primarily determined by the power broadening rather than the laser carrier frequency. Selective excitation of high vibrational levels is possible with the use of subpicosecond pulses.     

Amplification of picosecond pulses and gigahertz signals in bismuth-doped fiber amplifiers

We demonstrate the capability for amplification of picosecond pulses in two bismuth-doped alumosilicate fibers. A spectrally filtered supercontinuum source is used to provide a train of picosecond pulses at discrete wavelengths within the gain bandwidth of bismuth fiber amplifiers. With a 30 m length of active fiber, a small signal gain at 1160 nm of over 20 dB is observed. In addition, we assess the viability of amplification of high repetition rate signals in such amplifiers, applying a 10 GHz modulation to a continuous wave Raman fiber laser operating at 1178 nm, finding that such signals are amplified without noticeable distortion.     

High quantum yield of photochemical reactions of protoporphyrin-IX induced by powerful ultrashort laser pulses

The action of powerful pulsed picosecond radiation from a Nd: YAG laser (λ=530 nm, pulse energy: 0.01 J, intensity: 2GW/cm²) and an argon laser (λ=515 nm, power: 50 mW) on protoporphyrin-IX dimethylether in three solvents (trichlormethane, carbon tetrachloride, dioxane) has been studied. Under continuous irradiation the quantum yield and resulting products do not differ materially from the ones produced under mercury lamp irradiation. When irradiation is performed by powerful laser pulses of picosecond duration the quantum yield of photodecomposition of protoporphyrin-IX dimethylether inereases substantially: by 10 in dioxane, by 4 in carbon tetrachloride and by 100 in trichlormethane. It is assumed that a quite different mechanism of multistep excitation is responsible for photodecomposition under powerful picosecond pulses.     

Temporal coherent control of superfluorescent pulses

We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).