Beam Propagation Ratios Measurement Based On Transmissive Liquid Crystal Spatial Light Modulator

In recent years, with the development of research, the technology of laser is developing and the application of laser has been expanded from scientific research field into the area of daily life, industry, medical applications, etc. There are many parameters to evaluate the features of one laser, such as divergence angle?wavelength?power?beam spot radius? energy distribution and so on. During the practical application of laser, beam quality is a critical parameter for laser beams of different modes and energies. Up to now, there is no unified evaluation method on how to evaluate the quality of the laser beam in the scientific community for the reason that different evaluation methods should correspond to different application purpose. In this paper, we use the concept of beam propagation ratios—M2 to evaluate the beam quality of laser to be measured with the help of LC-SLM in order to evaluate the performance of a laser scientifically and accurately. LC-SLM is an abbreviation for Liquid Crystal-Spatial Light Modulator. Spatial light modulators are widely used in many cases where modulation of light is required because of their high optical efficiency, high degree of freedom, programmability, and availability, especially the properties to modulate the characteristics of light wave continuously and dynamically.In this paper, we do some researches on measuring beam propagation ratios—M2 of laser beams based on LC-SLM. The paper's contents and results are divided into the following sections:1. The design principle and method of digital zoom lens(DZL) based on transmissive LC-SLM are introduced in this paper. Besides, we have analyzed the influences of LC-SLM's phase modulation depth, nonlinear relation between applied voltage and phase modulation on the actual focus of zoom lens. Lastly, we also have proposed a method to obtain the actual focus position of zoom lens based LC-SLM. Both theoretical details and experimental results using a transmissive LC-SLM are present in this paper, which proves that the method is feasible and accurate.2. A method utilizing a transmissive LC-SLM to measure beam propagation ratios—M2 of laser beams is presented in this paper. In comparison with the standard measurement method provided by the International Standard Organization(ISO), through using the transmissive LC-SLM based zoom lens, beam propagation ratios were calculated by curve fitting based on the beam spot size collected by a digital camera versus the focal length of the zoom lens without any movements in the experiments. The experiment and comparison results show that the proposed method effectively measures the M2 of a Gaussian beam with low cost and high accuracy using a relatively simple setup.