Pulse time delay (PTD) and defocus are mainly introduced by transmitted-based large-aperture beam expander systems in ultrashort high power laser systems due to chromatic aberration, which can significantly reduce the focal-spot intensity by spatially enlarge the spot size as well as temporally distort the pulse profile. In this paper we investigate the chromatic aberration and measure how it deteriorate the focal spot size in SG-II 5PW ultrashort laser system. In addition, we propose and design a simple chromatic aberration pre-compensation scheme based on combination of aspherical lens and spherical mirrors. The simulation results indicate that both PTD and defocus dispersion can nearly be fully compensated by applying this compensation scheme with proper alignment in the system without introducing other kinds of wave-front aberrations.
A model is presented to analyze the effect of chromatic aberration of the spatial filter lenses on the compressed pulse duration. The parameters of the SHENGUANG (SG) II 5 PW laser system are demonstrated as examples. The numerical simulation results show that the compressed pulse duration with full aperture is increased by 3 times compared with the ideal compressed pulse duration in the case of chromatic aberration of the lenses. The result of full-aperture measurement at the SG II 5 PW laser system is in good agreement with the theoretical calculation. In the case of full-aperture measurement, the shortest compressed pulse duration is obtained by adjusting the compressor to compensate for the phase aberration introduced by the chromatic aberration of the spatial filter lenses at the SG II 5 PW laser facility. These results are helpful in finding an optimized pulse duration, considering the chromatic aberration of lenses in the femtosecond laser facility, and in obtaining a deep understanding of the effect of chromatic aberration of lenses on the pulse contrast ratio and pulse duration in ultrashort laser systems.
The SG II 5PW laser is designed as an open ultra-short high power laser facility that operates at the wavelength of 808nm. Three optical parametric chirped pulse amplification (OPCPA) stages are used to ensure the uncompressed pulse energy up to 260J. With a four pass zigzag compressor, the pulse width is compressed into less than 30fs and the pulse energy about 150J. By using BBO and LBO crystal, the first two OPCPA amplifiers have been accomplished this year. 35J@21fs outputs have been achieved. Since the largest size of the LBO crystal now is only about 100mm×100mm that is not enough for the needs of the third OPCPA amplifier. In our work, potassium deuterium phosphate (DKDP) as a candidate crystal has been studied theoretically and experimentally. Phase-matching parameters for various deuterium doped rate DKDP crystals are calculated. OPCPA amplifier based on 95% deuterium doped rate is designed and the output characteristics are simulated by OPA coupled wave equations. The results show that DKDP crystals with deuterium doped rate higher than 90% can be utilized in ultra-short high power laser systems that support the pulse width shorter than 30 femtoseconds. Still by estimation, when Quasi-phase-matching techniques and collinear design are used in small signal OPCPA amplification, the greatest efficiency can reach above 55%. By experiment it has proved that the output spectrum width can be more than 80nm.
Laser beam alignment is very important for high-power laser facility. Long laser path and large-aperture lens for alignment are generally used, while the proposed alignment system with a wedge by far-field sampling technique reduces both space and cost requirements. General alignment system for large-aperture laser beam is long in distance and large in volum because of taking near-field sampling technique. With the development of laser fusion facilities, the space for alignment system is limited. A new alignment system for large-aperture laser beam is designed to save space and reduce operating costs. The new alignment for large-aperture laser beam with a wedge is based on far-field sampling technique. The wedge is placed behind the spatial filter to reflect some laser beam as signal light for alignment. Therefore, laser beam diameter in alignment system is small, which can save space for the laser facility. Comparing to general alignment system for large-aperture laser beam, large-aperture lenses for near-field and far-field sampling, long distance laser path are unnecessary for proposed alignment system, which saves cost and space greatly. This alignment system for large-aperture laser beam has been demonstrated well on the Muliti-PW Facility which uses the 7th beam of the SG-Ⅱ Facility as pump source. The experimental results indicate that the average near-field alignment error is less than 1% of reference, and the average far-filed alignment error is less than 5% of spatial filter pinhole diameter, which meet the alignment system requirements for laser beam of Multi-PW Facility.
The non-collinear phase-matching in Potassium Dideuterium Phosphate (DKDP) crystal is analyzed in detail with signal pulse of center wavelength at 808 nm and pump pulse of wavelength at 526.5 nm. By numerical analysis, parametric bandwidths for various DKDP crystals of different deuteration level are presented. In particularly for DKDP crystals of 95% deuteration level, the optimal non-collinear angles, phase-matching angles, parametric bandwidths, walk-off angles, acceptance angles, efficiency coefficients, gain and gain bandwidths are provided based on the parameter concepts. Optical parametric chirped pulse amplifier based on DKDP crystal is designed and the output characteristics are simulated by OPA coupled wave equations for further discuss. It is concluded that DKDP crystals higher than 90% deuteration level can be utilized in ultra-short high power laser systems with compressed pulses broader than 30fs. The disadvantage is that the acceptance angles are small, increasing the difficulty of engineering regulation.
By using the SG-II laser and the ninth-beam as the pump source, the Shenguang-II multi petawatt laser system with three OPCPA stages is designed. Based on the largest size of the commercial gratings provided now by the JY Company, chirped pulses with 260J energy will be delivered after the third stage of OPCPA. When compressed by a four pass compressor, a laser pulse of 150J 30fs (5PW) will be obtained. This laser system is under construction and is expected to be finished in the late of 2015. The paper presents the details of its design and the progress has achieved.
In extremely intense laser system used for plasma physics experiments, temporal contrast is an important property of the ultra-short pulse. In this paper, we theoretically study the temporal contrast degradation due to wave front deviation in large aperture ultra-short pulse focusing system. Two-step focusing fast Fourier transform (FFT) algorithm with the coordinate transform based on Fresnel approximation in space domain and Fourier integral transform method in time domain were used to simulate the focusing process spatially and temporally, in which the spatial distribution of ultra-short pulse temporal contrast characteristics at the focal spot is related to the wave front in large aperture off-axis parabolic mirror focusing optical system. Firstly, temporal contrast degradation due to wave front noise with higher spatial frequency is analyzed and appropriate evaluation parameter for large aperture ultra-short pulse focusing system is put forward from the perspective of temporal contrast. Secondly, the influence of wave front distortion with lower spatial frequency on temporal contrast is revealed comparing different degradation characteristics of various aberrations. At last, a method by controlling and optimizing the wave front to prevent temporal contrast degradation in large aperture ultra-short laser system is proposed, which is of great significance for high temporal contrast petawatt laser facilities.