Studies On Beam Control In High Power Ultra-short Pulse Ti:sapphire Laser System

Over the past ten years, high-power ultra-short pulse Ti: sapphire laser technologies based on chirped pulse amplification(CPA) have been rapidly developed and a number of large facilities have been built, increasing peak power density is always pursued aim for scientists. This dissertation was based on the SILEX-I (Super Intense Laser for Experiment on the Extremes) ultra-intense ultra-short pulse Ti:sapphire laser built at Research Center of Laser Fusion, CAEP. Two major factors affecting peak power density, compressed pulse quality and focused spot quality, were studied, respectively.Using numerical simulation and experimental methods, spectral properties such as spectral gain narrowing and dispersive control were mostly studied. The far-field description methods of high power ultra-short pulse lasers and the spatio-temporal characteristic of lager-aperture off-axis parabolic focusing systems were also studied. The main research topics included:1. The development, application and general status of ultra-high ultra-short pulse laser systems were summarized in this thesis, the key scientific and technological points in beam control of ultra-short pulse lasers, such as spatial quality, temporal quality, spectral gain narrowing compensation and dispersive control, et al. were analyzed, and the corresponding beam control techniques were elaborated.2. The SILEX-I ultra-intense ultra-short pulse Ti:sapphire laser was briefly depicted. Besides, the key techniques in this facility, such as gain narrowing, spatial quality, temporal quality, propagation of chirped pulse, ASE in lager-aperture crystal, spatio-temporal characteristics in grating compression and the focus of ultra-short pulse were analyzed and discussed.3. Theoretical analysis and experimental study of active spectrum control were completed. The gain of the regeneration amplifier in CPA system is higher than 106, gain-narrowing effects are generally produced when laser propagates through the gain medium in the regeneration amplifier, leading to a broadened pulse duration after compressed. With the active spectrum control, the compressed pulse of near the transform limit can be obtained. Among the controlling ways, acoustic-optic programming dispersive filter(AOPDF) can be used to fulfill the gain spectral amplitude and phase shaping, and decrease the gain-narrowing effects.4. Dispersive characteristics of stretcher and compressor were studied more systematically and detailedly. In a chirped pulse amplification system, the stretcher and thecompressor are the key components, the dispersive of the stretcher and the compressor and the matching degree of dispersive will determinate the pulse width and the signal-to-noise ratio of the pulse after compressed. Based on the research of some scientists, dispersive formulas were deduced renewedly and detailedly, and by ray tracing methods, validity of the formula was testified. Using ray tracing method and formula method, the stretcher models with a folded telescope and Offner triplet were studied in detail to optimize the high power ultra-short pulse laser system.5. The influence of initial chirp and high order dispersive on the stretched pulses were analyzed. It indicated that the incident angle of compressor must be larger than the incident angle of the stretcher to compensate high-order dispersive in CPA systems. The influence of residual dispersive on compressed pulse under different conditions and different matching modes were studied in detail. Optimum methods and dispersive compensation techniques were studied too, it showed that mixed dispersive compensation methods could get Fourier transform limited pulses. Finally, experiments about pulse compressions in SILEX-I laser were carried out. To ensure a good alignment of the compressor gratings a diagnostic device was suggested with simultaneous monitoring both temporal and spatial behaviors.6. Using statistical method and numerical simulation, the far-field description methods of high power ultra-short pulse lasers were established. The research results point out that FWHM of focus cannot be simply used to give the peak power density. Strehl ratio or the encircled energy within FWHM are more appropriate.7. The spatio-temporal characteristics of lager aperture OAP focus in high-power ultra-short pulse laser system were studied carefully. From simulations, It concluded that OAPs not only determine the focal spot as a focal element, but also affect the temporal property. Mislignments of OAP could lead to elongated pulse durations, and thus decrease the power density.