Research On Generation Of Tunable Few-cycle Infrared Pulse In Optical Parametric Amplification

Few-cycle infrared laser pulse has attracted numerous attentions around the world in the past decades for the applications in ultrafast spectroscopy, attosecond science and strong field physics. Due to its outstanding characteristics including high gain efficiency, broad gain bandwidth, high wavelength tunability and neglectable thermal effects, optical parametric amplification has become one of the most popular technique for the generation of ultrashort intense pulses ranging from visible to mid-infrared wavelengths. In order to meet the requirements of broadband spectrum, wavelength tunability and millijoule energy for few-cycle pulses generation, our work is focused on the following aspects.The dual-crystal OPA scheme with group-velocity-mismatch compensation is investigated. By making use of the dispersion properties of BBO and BaF2 crystals, the temporal overlap between pump, signal and idler pulses in ultrafast OPA is optimized, this optimization can increase both gain efficiency and bandwidth of the system. DOPA system is capable of efficiently generating few-cycle pulses at non-degenerate wavelength range including 1200-1500 nm and 1700-2100 nm. For signal pulse centered at 1300 nm, the output spectrum supports a sub-three-cycle duration with a conversion efficiency over 25%, meanwhile the signal-to-noise ratio, pulse contrast and beam quality are improved owing to the better temporal overlap. In addition, mid-infrared idler pulses centered at 2100 nm are produced with sub-two-cycle duration and high CEP stability.The dual-crystal OPA scheme is experimentally investigated. The principle of GVM-compensation is confirmed, and the experiment fits well with the simulation result. After further optimization of system parameters, broadband signal pulses with 35?J energy are obtained at non-degenerate 1300 nm. The pulse duration without any compression is 20 fs, the spectrum FWHM bandwidth is 210 nm, supporting a sub-three-cycle TL duration of 12 fs, the beam is nearly Gaussian shaped at the focal point. Broadband pulses ranging from 1150 nm to 1400 nm are generated in the DOPA system. Since the temporal overlap between ultrashort pulses are optimized, the scheme can be potentially applied in a system with multiple nonlinear crystals and the energy conversion efficiency can be further increased.The dual-pump OPCPA scheme is investigated. Chirp-compensation scheme is employed to improve the phase-matching bandwidth in OPCPA, by using two pump pulses with negative chirp and delay, the instantaneous frequency of chirped pump and seed pulses are adjusted in order to fit the phase-matching curve, ultrabroadband gain is achieved around degeneracy. The simulation results show that the output signal spectrum covers 1300 nm to 2100 nm, corresponding to a sub-two-cycle TL duration of 9.0 fs. A near-TL duration of 10.1 fs can be obtained by simply compensating the linear temporal chip of output signal pulse. Meanwhile the conversion efficiency is 22.6%, accompanied with high beam quality and high focusability. By further stretching the input pump and seed pulses and optimizing the delay, this scheme shows excellent energy scalability.The filamentation scheme with a multi-focal-length beam is investigated for the generation of extended laser filaments and few-cycle pulses. The spatial phases of several beams with varying focal lengths are superposed and introduced by a spatial light modulator, an initially collimated beam is therefore modulated to a multi-focal-length beam. The propagation characteristic of a multi-focal-length beam is strictly manipulated by the initial spatial amplitude and phase, and limited energy are efficiently used during filamentation. With only 2-mJ input pulse energy, the achieved filament length is doubled compared to a single-focal-length beam. The output spectrum ranges from visible to mid-infrared wavelength, supporting nearly three-cycle TL duration. This scheme is potentially available in other mediums with higher nonlinearity for generating ultrabroadband supercontinuum.