Study Of Structured Sodium Laser Guide Star

Sodium laser guide star adaptive optics is used for compensation of optical the wavefront aberration of astronomical target and a powerful tool to improve the imaging resolution of ground-based optical telescope.Nowadays it has already attained near diffraction-limit images of many astronomical objects.But there are some inherent problems that need to be solved,such as the spot fragmentation and the spot wandering of laser guide star(LGS),lack of brightness of LGS,cone effect and small field of view of compensation.In order to solve the problems above,this paper proposes two kinds of structured sodium laser guide stars: the ring-like laser guide star and the array laser guide star.This paper firstly introduces the basic principles of the sodium laser guide star adaptive optics system and the basic theories by required by this study.And based on these basic principle and basic theories,this paper studied the two kinds of structured sodium laser guide star through numerical simulation and experimental research.The contents are as follows:1.This paper describes the properties of turbulence-resisted vortex beam,establishes the numerical simulation model of generating d ring-like LGS through vortex beam,simulates the process that upgoing vortex beams go through atmospheric turbulence in the troposphere and ultimately generate ring-like LGSs in the sodium layer,and then calculates spot wandering variance of ring-like LGSs generationg by vortex beams with different topological charges(TCs).The simulation results show that the spot wandering variance of ring-like LGS decreases with the increase of the TC of upgoing laser,and fell 31% compared with the variance of the spot wanderingof traditional LGS at TC=10.Accordingly simulation results demonstrate the feasibility of reducing the LGS wandering by generating ring-like LGS.2.Referring to foreign literature,we designed and built a sodium laser guide star simulator based on liquid crystal spatial light modulators in the laboratory.The principle of design of the device is explained,and the design parameters of the device are given.The holograms used to to simulate the launch telescope and the atmospheric turbulence are given.And we simulated ring-like LGS spot wandering based on the simulator.We give some examples the intensity distribution of the simulated LGS spot.We statistically calculated the spot wanderings of the the simulated LGSs generated by upgoing laser with different TC.The experimental results show that: the spot wanderings of the simulated LGSs show a downward trend with the increase of TC,and decreased 25% at TC=4 compared with the variance of the spot wandering.In the experimentally allowed error range,experimental results are in accordance with the the results of numerical simulation of this paper,which further validates the feasibility of the the concept of ring-like LGS.3 This paper independently presents an adaptive opticics system based on array laser guide star.Based on this system,we established the theoretical model and conducted numerical simulation.We simulated the process that tructured laser beam array sampled atmospheric turbulence during uplink propagation.According to spatial distribution of LGSs in the image of array LGS,we extracted the information of the atmospheric turbulence that sampled during the uplink process of array LGS.Based on the extracted information of atmospheric turbulence wavefront reconstruction is well performe.And residual error of wavefront compensation is 11%.This preliminary validated the feasibility of array laser guide star concept.The error of wavefront reconstruction is analyzed and the model error in the primary model is pointed out.Finally,the full text is summarized,and the next step of study is prospected.