To obtain seed pulses of large energy, high contrast and broad spectral bandwidth for the high power laser system, a new cross-polarized wave (XPW) generator based on the gadolinium gallium garnet (GGG) crystal is designed. For the injected fundamental wave (FW) pulses exceeding tens of millijoule, the output XPW characteristics of conversion efficiency, spectral broadening and contrast improvement are theoretically analyzed. We also investigated the influence of spatial distribution characteristics of the injected chirped FW pulses on the output XPW. With incident FW pulses of 8 order super gaussian spatial profile for instance, conversion efficiency of ~73%, spectral broadening ratio (SBR) of ~2.7 (from 40nm to 108nm) and contrast boost by >7 orders of magnitude can be obtained. The impact of the FW spatial modulation on XPW was sensitive to the modulation depth, which is significant for spatial quality control of the laser beam in high power laser systems.
Plasma mirror is an effective approach to improve the temporal contrast of high power ultra-short laser system, while it might deteriorate the focal spot, which is reported in some experiments using plasma mirror. In order to investigate such far-field degradation by plasma mirror, we established a spatiotemporal multi-step focusing propagation algorithm based on the formula of plasma expansion and wave-front modulation model. The influence of plasma expansion time, amplitude and spatial frequency of wave-front error on focal spot degradation are quantitatively analyzed. The simulation results reveal that the far-field focal spot degradation by plasma mirror is caused by the non-uniform plasma expansion due to the wave-front error and the wave-front error with higher amplitude and lower spatial frequency has relatively greater effect on the focusing ability. From the perspective of high-contrast ultra-intense output capability, the requirement on the spatiotemporal quality of the pulse is put forward to avoid the far-field focal spot degradation when using plasma mirror in high power ultra-short laser system.
To ensure a high signal to noise contrast ratio, lots of challengeable work must be done during the construction of a petawatt level laser system. In this report, we analyse the effects on the contrast ratio by the optical element manufacture errors expressed as the peak-valley value (PV value) and the PV gradient value, the chromatic aberration and group delay in system design. Using the Fourier transformation method with the random phase attached on the laser beam in frequency domain, it is proved that for manufacture errors, PV gradient value is more tolerable than that of PV value. At the terminal end of a petawatt level laser system, there exist, in pulse compressor, spectral clip, grating manufacture errors and non-uniformity of the diffraction efficiency that will affect the final SN contrast ratio of the laser system. Since the spectral clip here is soft that can benefit the promotion of the contrast ratio. But for manufacture errors of the large size grating, when PV = 1/5 wavelength, and PV differential gradient about 1/75 wavelength per centimeter. The terminal SN contrast ratio is restrained. When focused on the target, simulation for SN contrast ratio near the focal region caused by the residual distortion is taken. Calculation shows that, for a 20 microns focal spot, to maintain the 108:1 contrast ratio across the whole focal spot, residual wavefront distortion should be compensated to PV value less than 0.2 wavelength.
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.
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.