Study On Coherent Beam Combination Technology Of Slab Lasers

Recently, many significant breakthroughs present to the high brightness solid state laser field, such as the disk laser, the fiber laser and the slab laser. However, the output power and beam quality of the single laser are always limited by nonlinear effects or thermal effects. Coherent beam combination (CBC), which would expand the system power and remain the beam quality, is an effective way to achieve high-energy solid-state lasers at this stage. The slab laser amplifier has many advantages in potential applications, such as high power, single-frequency and high energy pulse laser. According to this, the coherent beam combination technology of the slab lasers has important practical significance and broad future. In this thesis, the theory, experiment and simulation of the CBC technology are explored by building an one hundred watts system based on two slab laser amplifiers, while serveral factors, such as the beam quality and the duty cycle, are discussed, which provides a reference and recommendations for high brightness scaling of the solid-state laser.Firstly, the basic theory of the CBC system is studied and verified in the experiment at low power. The structure, component, calculation methods and the evaluation criteria are described by building the simplest model of CBC system. The active phase locking method such as the multi-dithering technique and the stochastic parallel gradient descent algorithm (SPGD) are introduced. Furthermore, a new optimization algorithm: double stochastic approximation based on logic comparison algorithm (DSAL) is proposed. The active phase locking CBC are validated at low-power platforms. Meanwhile the passive phase locking CBC system based on the mutual injection locking is realized. The performance and the effect on target of the CBC laser are discussed with the experiment.Secondly, the conduction-cooled end-pumped slab laser (CCEPSL) modules with different structures are developed, based on which the corresponding four-pass laser amplifiers are built. The design process and testing programs of a hundred-watt level slab laser oscillator are described. From the measurement and simulation, it could be concluded that the ends absorption and the dissymmetrical edge cooling of the module are the main source inducing the wave-front distortion, according to which, corresponding solutions are proposed to guide the module improvements. The four-pass slab laser amplifiers based on three different modules are designed and implemented. The highest power about 71.2W for the continuous wave (CW) laser and 67.5W for the pulse laser are realized. Finally, the plane polarized, single-frequency, CW laser with above 50W output is supplied, and the beam quality of which could achieve less than 2 times of the diffraction limit.After that, the CBC of two slab laser amplifiers is realized. The active phase locking with two slab laser amplifiers based on the multi-dithering technique and the SPGD algorithm are demonstrated, which obtain about 102W laser output totally. The average peak intensity of the far field pattern has been improved as a factor about 1.7 after close loop, while the long time exposure of the fringe contrast achieves about 87%. During the experiment, several factors impacting the CBC effect are observed, such as the convergence rate of the algorithm, the beam quality of unit laser and the duty cycle of the launch system.According to the result of the experiment, the numerical simulation model of the CBC system based on high power slab laser amplifiers is established. The wave-front aberration within the single slab laser is simulated by Zernike polynomials. The error of the phase locking, tilt and the correction residual error are expressed as Zernike polynomial coefficients obeying a special distribution function. The simulation of the CBC system with multi-slab lasers shows that the beam quality of the CBC system would be significantly decreased compared to cell laser, if in which the low-level wave-front aberrations are dominate.Finally, the improvement and application about the CBC system based on MOPA structure are discussed. The experiment about the phenomenon of coherent second harmonic proves that the second harmonic could be coherent beam combined by phase locking the fundamental laser. Meanwhile, the CW and pulse output of the cell laser could be used in generating the mid-infrared lasers by optical parameter oscillator. It is indicated that the CBC technology has huge potential applications in nonlinear frequency conversion field.