Research On The Methods For Improving The Efficiency Of Sodium Guide Star

Sodium laser guide star(LGS)is one of the key technologies of modern ground-based large adaptive optics telescopes,which can greatly improve the sky coverage problem because of the lack of bright natural stars.To guarantee the accuracy of centroid extraction and wavefront reconstruction,the wavefront detector of adaptive optics system requires the guide star to have enough brightness and a small angular size.Unfortunately,in practical use,affected by factors such as low laser power,low coupling efficiency of the laser mode with the sodium atom,bad beam quality and optical aberrations during laser propagation,it is difficult to get an ideal sodium guide star spot.Based on these problems and the need of engineering applications,the dissertation makes a research on the optimization of sodium laser launch system to improve the efficiency of the guide star and accomplishes the following research work.First,the basic principles and the applications of adaptive optics and two kinds of laser guide star techniques are briefly introduced.The research work on improving the efficiency of sodium guide star in China and world wide is summarized.First,from the perspective of sodium atom physics and its interaction process with lasers,the factors that affected the sodium D2 line transition and emission are analyzed and summarized.Then,from the perspective of laser beam propagation,three factors that affect the process,which are beam quality,optical aberrations and atmospheric turbulence,are discussed.The dissertation chooses four specific aspects to optimized the launch system,which are optical pumping,optical repumping,correction of aberrations in laser expanding system and a method based on Rayleigh scatter light to correct partial turbulence in the laser uplink path.A sodium laser guide star launch and receive experimental platform is constructed which include the 589 nm laser,the expanding and launching optical system and the 2m optical telescope.Field experimental verifications of several techniques are completed on this platform.In order to pump the sodium atom,circular polarization laser is needed to excite the atom.Field experiments are conducted by inserting a quarter waveplate with a special angle into the optical path,the resonance backscatter photons are enhanced by 31.9%.In order to re-pump the atom and make full use of sodium atoms at F=1 ground state,electronical optical modulator is used to modulate about 10% of the laser power from D2 a to D2 b to excite the sodium atom with two spectral lines.Field tests show that this optimization technique can effectively improve the number of returned photons by approximately 25.6%.In order to eliminate the affection of aberrations in the optical path on the far-field distribution of laser energy,the method using adaptive optics to correct these aberrations is proposed.The method compensates the fixed aberrations in the laser expanding and propagating path,the aberrations induced by assembly procedure and aberrations caused by mechanical displacements during long time use.Two techniques are used to accomplish the field experiments.The first technique employs an interferometer and a 37-channel deformable mirror(DM)to form a closed loop,which can test and correct the home made main expanding optical system.A Shack-Hartmann wavefront sensor is used to assist the experiments.Field results show that,after the 0.38λ rms aberrations existed in the launch optics are corrected,the laser power distribution at sodium layer is effectively improved,the central peak intensity of guide star spot increases by 35.5% while the FWHM size decreases by 13.5%.The other technique is based on stochastic parallel gradient descend non-wavefront algorithm,which employs a camera and the deformable mirror to form a closed loop to correct the aberrations in the launch optics.Despite the DM’s flaw,the central peak intensity still increases by 7.2% and the FWHM size decreases by 10.9%.Finally,a new method based on Rayleigh backscatter to sense and correct partial atmospheric turbulence in the sodium laser uplink way is proposed.During the laser’s propagation to the sodium layer,it is inevitably scattered by atmosphere molecules and the scattered light also carries the turbulence information below it.Therefore,the laser launch system can use its own Rayleigh scatter to measure and correct the atmospheric turbulence below it,while the laser pulse mode satisfies the range gate detection requirements.Otherwise,another pulsed laser with different wavelength can be added into the launch system to generate a Rayleigh beacon.The second method is preliminarily analyzed and designed,parameters such as the laser power,the sampling height and depth and the scale of adaptive optics are computed with given input parameters.The feasibility of the method is demonstrated.Based on this,a method which can simultaneously correct aberrations in launch optics and partial atmospheric turbulence is presented.This technique can not only overcome the affection of most atmospheric turbulence and obtain the near-theory distribution of the laser spot,but also can increase the flexibility of the sodium laser guide star system as well as the launch aperture.The methods researched in the dissertation all can effectively increase the number of returned photons or decrease the FWHM angular size,while feature high feasibilities,and have significant meanings in enhancing the performance of adaptive optics systems and enlarging the detection scope of astronomical telescopes with high resolution imaging ability.