Broadband High-power Laser Systems In General And Key Technologies

The research activities on high energy density science (HEDS) are fundamental and challenging. These research fields include inertial confinement fusion (ICF) by laser driving, condensed state physics at extreme conditions, generation of high intensity X-rays and particle beams, and laboratory astrophysics. The ultrahigh intensity and high power laser is the most important research tool for these researches and it is also a huge synthetic scientific engineering. The laser drivers are required to be flexible in parameter adjustment, compatible in multiple functions. This is a challenge for the high-power laser system development and is the motivation of this thesis.The thesis promoted and demonstrated an overall technique scheme for laser driver based on system wide broadband laser pulse, which could meet the requirements of different users by combining the possibilities of high energy, multiple pulse durations, multiple wavelength and the flexibility for switch in different configurations. The theoretical and experimental researches on a series of key techniques, such as generation of femtosecond laser pulse with 1 micrometer wavelength and chirped pulse amplification of high energy broadband laser pulses, were put forward and the practical progress was obtained. There are 8 concerned papers been published on Optics Letters, Chinese Physics Letters and SPIE proceedings. The spotlights of this thesis are as follows:1. A new overall technique scheme (Hi-FPN) for high power laser driver based on whole broadband laser pulse was promoted and demonstrated, which could meet the requirements of HEDS research by combining the possibilities of high energy, multiple pulse durations, multiple wavelength and the flexibility for system configuration reassembling.The technique status of current existed high intensity solid-state laser systems was analyzed according to the requirements of HEDS. A new type of high power laser system named Hi-FPN was promoted, which denoted a new type of high energy (no less than 1-kJ energy and with high frequency conversion efficiency) laser which has the ability to generate strictly synchronized femtosecond pulse, picosecond pulse and nanosecond pulseon the same laser facility. A series of basic scientific problems such as pulse energy conversion and broadband beam control in Hi-FPN were carried out. The laser system configuration and the key techniques and their approaches were determined. The research results showed that although Hi-FPN had some scientific and technical details need to be concerned further, but it raised a creative choice for high power laser driver development in future.2.A new method to generate 1μm ultra short broadband laser pulse was validated theoretically and experimentally. This method called "super continuum injection and optical parametric amplification" have made the best result in this field and showed the possibility to provide the seed pulse for Hi-FPN.To get a high energy output from Hi-FPN laser, phosphate Nd³⁺:glass medium is necessary in present conditions. So generation of a 1 μm ultra short broadband laser pulse was the most important issue. The existed methods could not meet the requirement. A new method was introduced in this paper which used the femtosecond pulse in 800nm from mature Ti:sapphire laser both as the pump source for an OPA system and as an inducing source for the generation of super continuum which act as the seed pulse of OPA. The preliminary experimental results showed that more than 4-mJ energy and less than 100-fs pulse could be obtained with good beam quality. This is the best result in the world in the field of 1μm OPA system. Much better results are expected by optimizing the experimental conditions. This research built a fundamental basis for the future Hi-FPN system.3. Based on previous researches, the integrated theoretical frame of broadband laser pulse amplification dynamics was founded and was suitable for different bandwidths. One-hundred-joule laser pulse was obtained in a CPA demonstrating experiment to validate the theoretical analysis and numerical simulations. This output is the highest CPA output energy in China and demonstrates the technical feasibility of PW glass laser system.Firstly, the bandwidth of laser pulse was divided into five levels. These were narrow band, quasi broad band, broad band, big broad band and ultra broad band. Based on the former investigation results, a relatively complete theory frame of amplification dynamics wasfounded. These were exact theory model (or standard theory model), practical theory model and the simplified theory model. During the founding of amplification equations, relevant approximations were adopted according to different levels of bandwidth, different spectral broadening and energy level relaxing mechanism.An Nd³⁺ doped phosphate glass laser amplification system was involved in the research to validate the theoretical analysis and numerical simulation results. The highest CPA output energy in China was obtained (more than 100J) and a real-time temporal-spectral measurement system was developed to measure the pulse spectrum, duration and chirp rate simultaneously.To get large bandwidth, high energy and hundreds of femtosecond level laser pulses, the gain line width of Nd:glass (about 20 nm) should be made the best use of. Three methods for spectral shaping were developed to compensate the gain narrowing effect. The experiments are still in process.The carried out experiments producing femtosecond broadband laser around 1 μm wave band demonstrated this new type overall technique preliminarily.The overall and key techniques of the new type high power broadband laser mentioned in this paper is innovative and just in its preliminary stage thus it has some uncertainties and need further demonstration, even in all over the world. Besides, the exploring results obtained have not taken into account of some important processes, such as nonlinear self-focusing, gain saturation and dispersion. At the same time, some engineering issues such as load ability, stability and reliability have not been examined because of the restriction of experimental conditions. This thesis is just a beginning. Further efforts will be done to validate and perfect the Hi-FPN techniques.