Research On Collimation And Beam Shaping Of Laser Diode And Its Application Based On Spatial Light Modulator

Laser diodes have been widely used in the field of high precision interferometry with large range due to their characters of small size,large power,tunable wavelength and easy to modulate.However,with the defects of large divergence angle,uneven distribution of the light intensity,and intrinsic astigmatism,the output beam of the laser diode cannot be directly used for interferometry.As the phase distribution of the fundamental mode Gaussian beam is close to plane wave within the Rayleigh distance,coupled with the advantages of good collimation and symmetrical spot shape,it is and ideal beam for interferometry.In this thesis,the laser diode beam is shaped to the non-astigmatic fundamental mode Gaussian beam using a spatial light modulator based on the principle of complex amplitude modulation to meet the application requirements of large distance interferometry.The research status of the shaping methods of laser diode beam and the beam shaping algorithm based on the spatial light modulator are described.A scheme of laser diode beam shaping based on a spatial light modulator is designed and a laser diode beam shaping and collimation method based on complex amplitude modulation is proposed.The laser diode beam is shaped to the fundamental mode Gaussian beam with the beam shaping hologram designed according to the complex amplitude modulation algorithm.Then the hologram for eliminating the astigmatism is designed according to the optical field distribution model of the laser diode to compensate the astigmatism existing in the beam,and the laser diode beam is shaped to the fundamental mode Gaussian beam without astigmatism finally.In order to evaluate the quality of the shaped beam,the M~2 factor of the shaped beam is measured according to the hyperbolic fitting method and the proximity of the shaped beam to the fundamental mode Gaussian beam is evaluated.A diode laser beam shaping system including a beam shaping module and a beam quality analysis module is designed.The beam shaping module shapes a diode laser beam into a fundamental mode Gaussian beam by loading a designed hologram on a spatial light modulator.The parameters for evaluating the beam quality are measured by the beam quality analysis module and the hologram for eliminating the astigmatism is designed according the measured parameters of waist size and waist position.The fundamental mode Gaussian beam without astigmatism is obtained by loading the synthetic hologram generated by adding the beam shaping hologram and the astigmatism eliminating hologram.An experimental setup of the diode laser beam shaping system with spatial light modulator has been constructed to verify the feasibility of the proposed method.Firstly,the spot images of the diode laser at transmission distances of 1 m,5 m and 20 m before and after beam shaping indicate that the divergence angle of the beam is significantly reduced and the spot shape become symmetrical,which proves the feasibility of the system preliminarily.Secondly,the experiment for analyzing the beam quality of the shaped beam has been performed.With the beam shaping hologram loaded on the special light modulator,the beam quality is significantly improved.The experimental results show that the M~2 factor in the x and y directions of the shaped beam can reach to 1.034 and 1.043,respectively,which are comparable with the beam of the helium-neon laser,and the astigmatism is 1694.662 mm.By superimposing the astigmatism eliminating hologram,the astigmatism of the reshaped beam is reduced to 0.903 mm and the M~2 factors in both directions are basically maintained to be 1.106 and 1.063,respectively.All the experimental results indicate that the shaped beam is approximate to the ideal non-astigmatic fundamental mode Gaussian beam,which is available for large-distance and high-precision interferometry.The designed beam shaping system has been applied to the large-distance interferometric measurement.The displacement measurement experiments in a range of 15?m with a step of1?m and a repeated displacement measurement experiment of 10?m have been carried out at distances of 5 m and 8 m,respectively.For the displacement measurements at a distance of 5 m,the experimental results show that the average error is 9.059nm with a standard deviation of54.851nm for a step of 1?m,and the average error is-18.723nm with a standard deviation of52.839nm for a step of 10?m.For the displacement measurements at a distance of 8m,the experimental results show that the average error is 6.532 nm with a standard deviation of61.677nm for a step of 1?m,and the average error is 5.781nm with a standard deviation of64.485nm for a step of 10?m.All the experimental results indicate that the shaped beam can meet the requirement of large distance and high precision interferometry,which verifies the effectiveness of the proposed beam shaping system in this dissertation.