The surface of the gold film grating appeared to different degrees of carbon burning phenomenon under high energy laser irradiation, which resulted in the degradation of the grating performance. Thus, in this study, the main components and relative contents of organic contaminants in the wall and air at different positions in the chirped pulse amplification system were detected by gas chromatography and mass spectrometry. The organic molecules were volatilized from potential sources such as components and pump oil or dust produced by stray light irradiation of carbon-based materials. The contaminant C12H38O5Si6 was found at multiple sampling sites, indicating that the hydrocarbon molecules in the contaminant formed a chemical bond with the molecular structure of silicon and oxygen on the surface of the optical component. Compared to physical adsorption, this chemical bond adsorption is stronger and more difficult to remove. The effect of long-term vacuum organic contamination on the diffraction efficiency of the gold grating was not significant enough. On the contrary, organic residual contaminants were formed in the laser-irradiated area of the surface of the gold grating, and the diffraction efficiency was significantly reduced to two-thirds of the undamaged area. Many small organic molecules, particles and water molecules were deposited in the grooves on the surface of the gold grating, and carbonization occurs under the action of ultra-short pulse laser. A stress pit appeared in the center area of laser irradiation, causing damage to the surface of the grating.
Spatial-spectral interference carries the spectral phase difference information between short pulses. We propose a new method of time delay retrieval via the slope of spatial-spectral interference fringe in the case of only time delay without high-order spectral phase difference between short pulses. The analytical expression is deduced based on the principle of spatial-spectral interference. The simulation results show that the slope of spatial-spectral interference fringe and the crossing angle between short pulses are both important for the calculation accuracy. This proposed method has advantages of no direction-of-time ambiguity, simple principle and calculation process, which are helpful for the measurement and control of the time delay between short pulses in coherent combination, plasma parameter diagnosis and so on.
We report on a method to enhance the temporal contrast of optical parametric chirped-pulse amplifiers (OPCPAs) by smoothing pump noise. The instantaneous parametric gain in OPCPA couples the temporal modulation on the pump pulses to spectral variations of the intensity of the stretched signal pulses being amplified. In this way, pump noise significantly degrades the temporal contrast of the amplified pulses after recompression. Cascaded second harmonic generation (SHG) is adopted to smooth modulation on the pump pulses in the proposed method. Apparent reduction of modulation on the pump pulses is observed in the experiments. Numerical simulation reproduces the experimental results. Simulation results show that cascaded SHG with stable output 2 ω can enhance the temporal contrast for OPCPAs with four to five orders. It is believed that this new method can be widely adopted to build high-contrast OPCPA systems.
The paper presents the technical design and progress on a special high-power laser facility, i.e. XG-III, which is being used for high-field physics research and fast ignition research. The laser facility outputs synchronized nanosecond, picosecond and femtosecond beams with three wavelengths, i.e. 527 nm, 1053 nm and 800 nm respectively, and multiple combinations of the beams can be used for physics experiments. The commissioning of the laser facility was completed by the end of 2013. The measurement results show that the main parameters of the three beams are equal to or greater than the designed ones.
Chirp pulse amplification (CPA) has been promoted as an effective way to explore the intensity frontier. High order dispersion induced by the stretcher and materials in the CPA system, which deteriorates both the pulse duration and temporal contrast, however, can not be absolutely compensated by the compressor. Placed at the Fourier plane of a 4f zero-dispersion stretcher consisting of a grating, the deformable mirror (DM) has been demonstrated as the modulator to compensate high order dispersion. Using the method of ray tracing, the relation between spectrum and position on DM has been obtained. It shows that the resolution of the deformable mirror can be controlled by adjusting the focal length and incident angle. We have simulated a typical Ti:sappire CPA system to revise the spectral phase by the DM. The result illustrates that if the spectral phase can be compensated, the temporal contrast will be improved by 2 order of magnitude.
The paper presents the development of a sub-petawatt ultrashort laser facility, i.e. the upgraded super intense laser for
experiment on the extremes (SILEX-I). The facility is a multi-stage Ti:sapphire chirped pulse amplification (CPA) laser
system. Cross-polarized wave generation was used to improve the temporal contrast. An adaptive optical system was
utilized to correct wavefront aberrations and to improve focusability before each shot. After upgrading, the maximum
energy is 20.1 J, the recompressed pulse width is 26.8 fs and the peak power is up to 750 TW. The temporal contrast is
around 109. The on-target focal spot size (full width at half maximum (FWHM)) is Φ6.5 μm and the focused intensity is
greater than 4x1020 W/cm2.
Applying the self-diffraction process to clean ultrashort laser pulses temporally is a recently developed effective way to temporal contrast enhancement. In this paper, we attempt to clean ultrashort laser pulses temporally by the self-diffraction process. Experiments were carried out to study the temporal contrast improvement in the front-end system of an ultraintense and ultrashort laser facility, i.e. the super intense laser for experiment on the extremes (SILEX-I). The results show that the maximum conversion efficiency of the first-order self-diffraction (SD1) pulse is 11%. The temporal contrast of the SD1 signal is improved by two orders of magnitude, i.e. to 103, for a 2.4-ns prepulse with initial contrast of ~10. For a 5.5 -ns prepulse with initial contrast of 2×103, the temporal contrast of the SD1 signal is improved by more than three orders of magnitude.
The temporal contrast is an important factor affecting the application of ultra-intense and ultra-short lasers. In this paper, we develop a double chirped-pulse-amplification (CPA) front-end system with an intermediate nonlinear temporal pulse filter to improve the temporal contrast at a sub-petawatt Ti:sapphire laser facility, i.e. the super intense laser for experiment on the extremes (SILEX-I). The temporal pulse filter employs cross-polarized wave (XPW) generation to suppress the amplified spontaneous emission (ASE). The design output energy is 320 mJ for the front-end system. The experimental results show that the output energy of the double CPA system is 360 mJ. The ASE pedestal is suppressed significantly and the temporal contrast is improved by around three orders of magnitude.
Development of a phased-array-grating compressor is a crucial issue for high-energy ultrashort pulse petawatt lasers. To achieve tiled array gratings and increase stability of tiled-grating frames, a new tiled grating frame is designed. In the tiled-grating frame, an integrated support structure is adopted to increase the natural frequency of the tiled-grating and the flexible hinges are used rather than the spring to increase the joint stiffness between the grating and the support frame. The experiment indicates that stability of the tiled-grating can be maintained for more than 1 h and the standard deviation of the tiling error is 35.7 nm, which satisfies the design requirement.
Temporal contrast is an important factor affecting the application of ultraintense and ultrashort laser systems. In this
paper, we employ cross-polarized wave (XPW) generation to improve the temporal contrast for ultraintense and
ultrashort pulses in a 300 TW Ti:Sapphire laser facility, i.e. the super intense laser for experiment on the extremes
(SILEX-I). We designed a double chirped-pulse amplification (CPA) system with an intermediate nonlinear temporal
pulse filter based on XPW generation and the estimated output energy is more than 300 mJ for the new front-end system.
The experimental results show that the output energy of the double CPA system is greater than 370 mJ. The amplified
spontaneous emission (ASE) pedestal is suppressed significantly and the temporal contrast is improved by more than two
orders of magnitude.
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