Study On The Properties Of The Harmonic Generation And The Propagation Of High Power Laser In The Final Optical System

The study on the properties of the harmonic generation and the propagation of the broadband laser in the final optical system is one of the important contents in the research of Inertial Confined Fusion (ICF). Detailed and systematical studies on the above mentioned aspects are carried on in this thesis. The main contents are as follows.1) Establishment of the physical model for harmonic generation of high-intensity laser and development of the numerical simulation code with visual interfaceThe physical model for harmonic generation of high-intensity laser was built up, which included the effect of the paraxial diffraction, the spatial and temporal walk-off, the third-order nonlinear effect including the self- and cross-phase modulation, the amplitude and the phase of the input fundamental field, the absorption and the reflection of the harmonic generation crystals, etc. The corresponding numerical simulation code with visual interface was developed and verified by comparing with the theoretical and experimental results of the Lawrence Livermore National Laboratory (LLNL), showing the validity and reliability of our code.The development of our code provides a convenient and reliable platform for the analyzing of the experimental results and the optimizing of the schemes of harmonic generation experiment. Our code has been used to analysis the experimental results obtained in CEPA and the numerical results achieved good agreement with the experiment results.2) Study on the harmonic generation of the nanosecond high-intensity broadband laserThe factors, which affected the spectrum and the conversion efficiency of the broadband third harmonic field with cascaded crystals of type I/II/II, were analyzed. A new scheme with cascaded crystals of type I/II/I was proposed and optimized, and the effects of the intensity and the bandwidth of the input fundamental field on the conversion efficiency were discussed. The principle of the broadband harmonic generation with the traditional technology of angular spectrum dispersion was analyzed and the limitation of the technology was pointed out. An improved and simpler method of non-collinear angular spectrum dispersion, which is suitable for the larger bandwidth, was presented and optimized.The nanosecond high-intensity broadband laser is the basis of the beam smoothing technology in temporal domain. Moreover, broadband laser has advantages in controlling the harmful nonlinear effects, avoiding the damage of the optical elements with large aperture and improving the uniformity of the laser beam. Therefore, studies on the harmonic conversion of the nanosecond high-intensity broadband laser are of great importance in the ICF research.3) Study on the harmonic generation of the femtosecond high-intensity broadband laserThe second harmonic generation of high-intensity femtosecond broadband laser with a type I KDP crystal or a type II KDP crystal was analyzed theoretically. The experiment of the second harmonic generation of the femtosecond laser with the center wavelength of 800nm, the pulse width (FWHM) of 30fs and the pulse energy of 30～150mJ by using a type I KDP crystal with the thickness of 1mm was carried out for the first time in China. The conversion efficiency of about 10% was achieved. The experimental results were analyzed and the effects of the initial frequency chirp and the modulations of the intensity and the phase of the fundamental field, as well as the third-order nonlinear effect on the conversion efficiency and the bandwidth of the second harmonic field were discussed. Consequently, the method to improve the conversion efficiency and the bandwidth of the second harmonic field was proposed.Theoretical and experimental studies on the third harmonic generation of the high-intensity femtosecond laser with a single BBO crystal were carried out. Comparison of the third-order effect and the cascaded second-order effect in the third harmonic generation was made. The effects of the intensity of the fundamental field, the thickness of the crystal, the nonlinear phase modulation, the temporal walk-off, the detuned angle and the azimuth angle on the conversion efficiency, the pulse shape and the spectrum were discussed. The corresponding experiment was performed for the femtosecond laser with center wavelength of 800nm, the pulse width (FWHM) of 100fs, the bandwidth is 25nm and the pulse energy of about 6mJ by using a single BBO crystal. The conversion efficiency of nearly 1% was achieved.Because the ultra-short UV laser can be used as the probe light and has distinctive advantages in "fast-ignition" in ICF, studies on the frequency conversion of the high-intensity femtosecond laser are very important.4) Study on the inverse problem of harmonic generationA method for solving the inverse problem of harmonic generation was proposed for the first time based on the split Fourier transformation, the forth-order Runge-Kutta method and the iterative method, namely, to determine the characterization of input fundamental field and the requirement for KDP from the required characterization of output tripling field. The inverse problems of third harmonic generation of the narrowband laser and the broadband laser, and of the second harmonic generation of the femtosecond laser were studied. The calculation efficiency of the iteration method and the accuracy of the calculation results are both high.The proposed method for solving the inverse problem of harmonic generation provides a new way of optimizing the frequency conversion process.5) Study on the nonlinear effects in the final optical systemBased on the Maxwell-Bloch-Langevin equations, the stimulated rotational Raman scattering (SRRS) of the high-intensity laser in long air path was discussed. The effects of the modulations of the intensity and the phase and the bandwidth of the fundamental and the third harmonic pump fields on the threshold condition of SRRS effect and the beam quality of laser were analyzed. Consequently, the methods for controlling the SRRS effect were proposed. Furthermore, the transverse stimulated Raman scattering (TSRS) in KDP crystal with large aperture was also studied. The effects of the intensity, the gain coefficient and the bandwidth of the pump field with the pulse width of 1ns and 3ns on the conversion of the Stokes field were discussed.Based on the modified nonlinear Schrodinger equations, the relationship between the intensity and the location of the self-focusing and the modulations of the intensity and the phase of the high-intensity ultraviolet laser in fused silica was analyzed quantitatively. The definition of the threshold condition of the self-focusing effect for ultraviolet laser was supplemented. The theory of the self-focusing was used to explain the damage of the silica window and the focusing lens in the target chamber in the experiment, and the solution was also presented.The study on the nonlinear' effects in the final optical system has important significance in improving beam quality and diminishing the damage risk of the optical components in the final optical system.6) Study on the beam smoothing and the focusing properties of the broadband harmonic fieldThe beam smoothing techniques were summarized and compared. Numerical simulations of smoothing method for the broadband laser with temporal phase modulation and frequency chirp by using spectrum dispersion were carried out and analyzed.Uniformed irradiation on the target is an essential condition to realize the ICF ignition and has advantages in increasing the compress ratio of the target and decreasing the requirement of laser energy. The study on the beam smoothing and the focusing properties of the broadband harmonic field can provide valuable reference to the experiment.