Research On Laser Beam Quality Measurement Method Based On Scattered Light Imaging

With the increasing need for high quality lasers,an accurate and rapid beam quality measurement method is highly demanded.In general,traditional methods have problems dealing with complex structure and time-consuming.The laser beam quality measurement method based on scattered light imaging described in this article not only solves these problems,but also can measure the beam quality of any laser,rather than being limited to fundamental mode Gaussian beam.Therefore,this article focuses on the two critical issues in this method: the laser scattering in the medium and the collection of scattered light intensity.The feasibility of this method is verified through experiments.First of all,this paper studies the scattering theory,and it is concluded that Rayleigh scattering is one of the conditions for this method to be feasible.According to the Rayleigh scattering theory,a Gaussian beam linear scattering model is established,in which the scattered light intensity is linearly related to the original light intensity and the light intensity distribution is circular,similar to a point light source.The analysis brings us the conclusion that the scattered light intensity distribution of the laser can be used to characterize its original light intensity distribution.Then,a method for collecting scattered light intensity is proposed.In order to acquire the accurate result of the beam quality,a Rayleigh distance is required both before and after the waist position.Besides,the beam waist width needs to occupy at least ten pixels.As a result,a focusing system is used to shorten the Rayleigh distance and cylindrical imaging system is designed to realize different magnification in the horizontal and vertical directions.For the reason that the depth of field of the cylindrical system directly affects the measurement accuracy of the laser beam width,analysis based on geometric optics is present to determine the parameter values of the cylindrical system.Finally,the characteristics of the scattering medium are analyzed through experiments,and the best scattering medium for Rayleigh scattering of the laser is obtained.The scattered light intensity distribution maps of the laser propagating in the scattering medium are collected to analyze and remove the random noise and scattered noise in the image,as well as the aberration introduced by the cylindrical imaging system.The beam quality factor can be derived from the processed light intensity distribution maps.The M2 factor is also obtained via the traditional caustics method to make a comparison with the method proposed in this article to verify the correctness of this method.The results shows that the difference between the measurement results of the two methods is less than 3%.In addition,experimental analysis of different focal scattering systems and cylindrical imaging systems with different depths of field is carried out to study the influence of experimental system parameters on the experimental measurement results.