Application Of Stochastic-Parallel-Gradient-Descent Adaptive Optics Techniques In Beam Cleanup

Adaptive Optics (AO) is a new subject researching the real-time and automatic improvement of light wavefront quality. Stochastic Parallel Gradient Descent (SPGD) algorithm is a gradient descent algorithm by perturbing all variables simultaneously to measure every variable's gradient estimate. The fast convergence speed is the obvious advantage of this algorithm over the traditional optimization algorithms, so a wavefront correction technique based on SPGD algorithm has become a research focus in adaptive optics field these years. Avoiding a series of problems which are encountered in the adaptive optics systems based on wavefront measurements when measuring the distorted wavefronts, this method has a simple and compact system architecture with low cost. Therefore, it has great advantages in the applications where the requirement for working bandwidth is not too high. This paper investigates the SPGD AO technique and the feasibility of applying it to a beam cleanup system to improve the quality of the output beams of lasers. The dissertation's main works and innovations are as follows:1. After the working principle of a SPGD AO system was expounded, its numerical model was built in order to perform the numerical simulations of beam cleanup and laser beams transmissions in atmosphere. Then, the limiting factors to the convergence limit and the convergence speed of a SPGD AO system were analyzed theoretically in detail and the corresponding simulations were performed by using this numerical simulation system. The results indicated that the perturbation amplitude and the gain factor both had great influence on the convergence limit as well as on the convergence speed. So the requirements for the convergence limit and the convergence speed were needed to be balanced in practical applications to optimize the system's whole performance.2. Two new methods in algorithm realizations, the subzonal coupling and the multilevel correction, were proposed. A high-resolution wavefront corrector and a virtual low-resolution one were controlled by SPGD algorithm to work synchronously in the subzonal coupling method. The low-resolution wavefront corrector was used to corrct rapidly for the larger-scale and slower-changed wavefront distortion components to improve the convergence speed of the SPGD AO system. After the correction for the large-scale wavefront distortion was completed by a low-resolution wavefront corrector, the low-resolution wavefront corrector was replaced by a higher-resolution one to correct for the smaller-scale wavefront distortion in the multilevel correction method. This method avoided the interference between two coupling wavefront correctors which were working together. The numerical simulation results indicated that both of the methods, especially the multilevel correction method, not only increased the convergence speed but also kept the convergence limit at a good level.3. A set of experimental SPGD AO system was set up, and methods of choosing the values of the perturbation amplitude and the gain factor in experiments were proposed. With the sharpness of a far-field spot's image measured by a CCD camera as the system performance metric, a SPGD control program was run on a computer to controll a 37-element deformable mirror to correct for wavefront distortion. By introducing ever-worsening phase distortion through configuring the surface profile of the deformable mirror with different voltages in sequence, and then correcting for those introduced dynamic phase distortion by controlling the same deformable mirror with SPGD algorithm, the dynamic process of a high-energy laser beam cleancup with a SPGD AO system was emulated. The experimental results showed that the laser beam quality was always kept at a high level rather than deteriorate after starting cleanup process, which means that the target of beam cleanup was achieved. To increase the working bandwidth, a photodetector was used to measure the laser power passed through a pinhole located in the focal spot. Then a SPGD AO system with iteration rate of over 100 Hz was set up with the laser power through the pinhole as the system performance metric. The experimental results of correcting for the dynamic artificial turbulence with this new system showed that the laser power through the pinhole with the turbulence corrected was 2.2 times as large as that without the correction and that the temporal frequency component of 0 Hz of the laser power was increased. It was proved that this system with the existing devices had got the capability of correcting for dynamic wavefront aberrations.The minimum of the perturbation amplitude could be determined by measuring the relation between the perturbation of performance metric and the perturbation amplitude, and making the measurement value of the perturbation of performance metric be twice greater than the measurement noise. The maximum of the perturbation amplitude could be determined by measuring the relation between the probability of the positive performance metric values and the perturbation amplitude, and making the probability be greater than some certain threshold value. The fit value of the gain factor could be determined by making the product of the gain factor and the perturbation of performance metric be one.4. Cleanup experiments on the output beams of a solid-state laser with MOPA architecture were firstly performed with the SPGD AO technique. To avoid the influence of the total laser power fluctuation on the measurement of the system performance metric, another photodetector was added to the above SPGD AO system to measure the total laser power output, and the power in bucket of a far-field spot was used as system performance metric. To obtain the laser beam to be corrected, an indicator laser beam with wavelength of 532 nm was sent through the Nd:YAG crystal in a laser amplifier. The power of the indicator laser output from the amplifier didn't increase, but the beam quality would decline because its wavefront was distorted thermally. The beam cleanup results for the indicator laser output from the amplifier showed that the SPGD AO system could improve the laser beam quality. But with the increasing of the pumping current applied to the amplifier, the thermal wavefront distortion became strong and the quality of the beam corrected also declined. Then, a beam of laser with wavelength of 1064 nm was sent through the same laser amplifier to set up a solid-state laser system with master oscillator power amplifier (MOPA) architecture. The cleanup results for the output beam of the laser system also showed various degrees of the improvement of the laser beam quality with different distortions.To sum up, a SPGD AO system is feasible to the extra-cavity beam cleanup, and the beam cleanup effect is related to the degree of wavefront distortion to be corrected for, the spatial resolution of the SPGD AO system and the system's working bandwidth.