Pre-correction For The Up-link Beam Of The Sodium Laser Guide Star

Adaptive Optics system?AOS?compensates the wavefront aberrations of atmospheric turbulence by wavefront sensing and correcting,thus makes the large ground-based telescope resolving power to near diffraction limitation.For increasing the sky coverage of the wavefront sensing in astronomical adaptive optics system,the artificial Sodium Laser Guide Star?LGS?,has become one of the the main solutions for this issue.However,atmospheric turbulence on the uplink path spreads the spot size,and saturation effect in Sodium atoms decreases the brightness of the Sodium LGS for low utilization of the laser when used for wavefront sensing.Aiming at these problems,combined with the demands of the Sodium LGS Laser Launch System?LLS?on Lijing1.8-meter telescope when using Sodium LGS for wavefront sensing,content of this dissertation is as follows.We established a model for laser propagation in atmosphere,analyzed the effect of laser launch telescope's diameter,dynamical aberrations of atmophseric turbulence in the uplink path,statical aberrations of the LLS and beam quality M² on the power density distribution.A general technique to optimize the laser launch telescope's design was devised.We then calculated the spot size and photon return according to the relationship between the power density and the coupling efficiency.It establishes a foundation for the designing of the following AOS.Pre-correction by Rayleigh LGS of the Sodium laser in the uplink path is used to eliminate the spreading of the dynamical aberrations for the Sodium LGS spot.In order to overcome the saturation effect of the Sodium atoms which decreases the photon return,and control the LGS power density distribution at the Sodium layer from Gaussian type to“top-hat”type with a optimal value,a new method,which is combining pre-correction and beam shaping is firstly proposed,this method applies phase correction with laser beam shaping to control the power density distribution at Sodium layer.Then,the spot size and photon return of the“top-hat”Sodium LGS are calculated,and are combined to analyses the performance of the AOS.Under strong turbulence?Fried parameter r₀ of 5cm?and for a quasi-continuous wave Sodium laser?power of 15W?,the measurement and temporal error of the AOS is decreased by 36%with our technique for a high power laser and poor seeing.At the same time,the sampling frequency is optimized by the photon return,which not only improve the potential of the AOS,but also advise for the repetition rate designing of the upgrading sodium laser.At the same time,the LLS with pre-correction and beam shaping was analysed,and an experimental bench and project was designed by the?300mm laser launch telescope.The Rayleigh LGS determines the accuracy of wavefront sensing,for fulfil the SNR of each sub-aperture,the altitude of the Rayleigh scattering is 9km with a range gate spanning 750m below and above the region of altitude,the sub-aperture of Hartmann is 6×6.The technology of coupling and splitting beam in common with laser launch and Rayleigh scatter receive path choose polarization splitting.The pre-correction is aimed at far field of the Sodium LGS spot,and the technology roadmap is with two deformable mirror,one is for pre-correcting of the phase cause by turbulence,the other is for intensity?or amplitude?modulating,the two deformable mirror lie in Fraunhofer diffration area.The 45-actuator deformable mirror used for pre-correction and the 21-actuator deformable mirror used for beam shaping,the performance is expected to 20%improved.At last,a next generation of 300mm diameter LLS and beam transfer optics for Lijiang 1.8-meter telescope was analyzed and designed.Compared to 200mm LLS,the 300mm LLS was optimized and the laser parameters can be measured.With this LLS,an experiment for measuring Rayleigh scattering on the uplink path was conduct,and then support for selecting of the altitude and range of the Rayleigh scattering.