Research On Adaptive Amplification Of Ultrashort Pulses And Beam Shaping & Coupling Technologies For Laser Diode Bars

Rare-earth-doped double-clad fiber based high-power and high-energy ultrashort pulses laser systems have been widely used in national defense, industrial processing and military due to their outstanding advantages in high efficiency and compactness. Thus, study on the enhancement of the energy and power of ultrashort pulses is really attractive currently. This dissertation is mainly devoted to the detailed theoretical and experimental researches on:adaptive pulse shaping techniques used for power amplification of ultrashort pulses in chirped pulse amplification (CPA) system, beam shaping of laser diode bars (LDBs), and methods of enhancing the pump coupling efficiency for LDBs with double-clad fibers. The main achievements of this thesis are listed as follows:1. A numerical model for Yb-doped fiber amplifier based CPA system with adaptive pulse shaping is found. With this model, several primary limiting factors in power amplification of the output pulses of the system are compensated respectively; meanwhile, the function of the adaptive pulse shaping in each compensation procedure is also illustrated in detail. For the first time, phase shaping, amplitude shaping and phase& amplitude shaping are compared with each other to verify their different influence on the power amplification, and different feedback parameters of the optimization algorithm are also compared for this purpose. This is of great importance for the setup and debugging of the experimental system.2. The generation of high-fidelity femtosecond pulses is experimentally demonstrated in a fiber based CPA system through an adaptive amplitude and phase pre-shaping technique. A train of pulses is produced with an average power of 12.6W at a 50MHz repetition rate from this fiber CPA system, which are compressible to high fidelity pulses with duration of 170 fs.3. A non-sequential optical model of laser diode (LD) bars is established with ZEMAX software, and a new beam shaping method for LD bars based on beam division and rearrangement by using parabolic reflective mirrors and parallelepiped prisms is optimized and verified with this model. Using this method, a rectangular shaped beam whose fill factor is 1 is obtained with more concentrated energy distribution and improved brightness.4. A compact beam collimator for LD bar is fabricated with a simple method. The fabrication is based on the using of fiber-drawing tower and silica. Theoretical and experimental results show that both the quality and the brightness of the collimated beam are greatly enhanced via the redistribution of intensity in the two axis directions and by eliminating the fill factor for the LD bar.5. Two methods of fabricating tapered SiO2 rods for beam concentrating are presented: using a fiber-drawing tower and utilizing the etching function of hydrofluoric acid. The dynamic procedures of these two methods are analyzed, and the predicted curves of the diameters of the SiO2 tapers versus their lengths are shown, which are of good consistency with the measured results. The simulation and experiment results show that, the taper fabricated in the latter way has a higher coupling efficiency when it is used for beam concentrating of the laser diode bar, which can be as high as 76%. Thus, it is possible to apply this taper in multi-points side pumping of the rare-earth-doped double-clad fiber, which will greatly enhance the output power of the fiber laser and amplifier systems.