Laser Guide Star Adaptive Optical systems become an established technique at telescope facilities with large apertures. At these aperture diameters, such as 8m class telescope facilities, the finite distance and vertical extend of an artificial excited guide star result in perspective elongation, which produces errors in wave-front reconstruction and could influence the performance of adaptive optical systems seriously. In this paper, we shall briefly introduce and explain the effect of the perspective elongation, and show some results of theoretical simulation and experiment. First of all, we analyzed how the perspective elongation of sodium LGS changes, and gave the results of simulation which indicated the relation between the perspective elongation and some related parameters. The aberration caused by the elongation was analyzed, and the possibility of aberration correction was discussed. Based on the results of the theoretical simulation, we designed an experiment to observe the perspective elongation. A transmitting and receiving system has been set up. The system consisted of a 300mJ sodium LGS laser, a telescope with an aperture diameter of 450mm, a beam expander with an aperture diameter of 200mm, a LGS detecting device, etc. Based on the pulsed laser and the mobile LGS projector, we operated the experiment at different distance between the telescope and the laser projector. A series of elongated images, corresponding the distance from 5m to 30m, was obtained. The analytic results of the image data agreed with the theoretical simulation. Based on the experimental data, we deduced the aberration of wave-front at 30m separation. According to theoretical simulation of the perspective elongation, the effects including the aberration of wave-front could be achieved, which had been partially verified in the experiment. We suggest that one could improve the reconstruction accuracy in a sodium or Rayleigh LGS adaptive optical system by eliminating the influence of the perspective elongation in advance.
Spectral beam combination (SBC) is a promising method to combine multiple fiber outputs for further power scaling with the capability of maintaining high beam quality, but the beam quality will be degraded with spectral linewidth broadening, because it could result in additional angular spread in the output beam. In this paper, we described theoretical calculation as well as experimental investigation on the influence of spectral linewidth broadening on beam quality. The results show that in single SBC system the spectral linewidth should be limited to less than a few GHz in order to avoid beam quality degradation, but the linewidth requirement could be decreased to more than hundreds of GHz using a pair of parallel gratings, which reveals a feasible way to increase the stimulated Brillouin scattering (SBS)-free power output of single fiber laser for overall output scaling and high beam quality.
We report a coherent combining of four slab laser amplifiers with high beam quality. The long strip laser beam is reshaped into a square beam using adjustable beam expander which removes the enormous astigmatism aberration. A filling ratio of 90% is achieved by two-dimensional splicing. A compact optical system with high sampling frequency is designed to detect the pointing direction of lasers. Fast steering mirror (FSM) driven by piezoelectric ceramics is applied in laser stabilizing. Thanks to the closed loop pointing control, the root mean square error of the optical axis is significantly reduced to be less than 2 microradians. The piston phases of the lasers are locked by an active phase control system based on Field Programmable Gate Array (FPGA) using stochastic parallel gradient descent (SPGD) algorithm. When the total output power of four lasers is 400W, the in-phase peak intensity of the far field spot is increased by a factor of 3.8, reaching 95% of the ideal case. The beam quality of the combined beam is improved by CBC from 1.52x diffraction limit (DL) to 1.26x DL. When the output power is increased to 805W, the phase locking and pointing control still work stably. The results suggest that CBC of solid-state lasers with higher energy could be achieved by using the techniques presented here.