High coherence is one of the characteristics of laser, which brings adverse effects to some applications of laser, so it is necessary to suppress the high coherence of laser. Rotating ground glass is the most commonly used coherence suppression method, but when the laser passes through the ground glass, the beam will be seriously scattered and the spot distribution cannot be controlled, which results in low utilization rate of light energy. Different from ground glass, random microlens array is a kind of diffuser with a structured surface. The laser is concentrated in a certain divergence angle after passing through a random microlens array, so it has high energy utilization rate. Previous studies have demonstrated that rotating random microlens arrays can be used for laser coherence suppression. However, different application scenarios have different requirements for laser divergence angle and coherence, so it is necessary to analyze the influence of rotating random microlens array parameters on laser divergence angle and coherence. In this paper, the problems are analyzed and discussed.

Firstly, this paper applies the region division method of Thiessen polygon to the design of microlens array, which obtains the random microlens array with high filling ratio and random variation of sublens unit apertures. Secondly, we analyze the influence of the mean aperture and curvature radius of the sublens element on the divergence angle of the random microlens array by changing the parameters of the sublens element. Finally, we establish a laser complex coherence regulation model based on random microlens array. The modulus of complex coherence is used as the criterion of coherence intensity. We use the modulus of complex coherence as the criterion to evaluate the intensity of coherence and analyze the influence of rotation speed of rotating random microlens array on the complex coherence modulus of laser field.

The simulation and experimental results show that in terms of divergence angle, the average aperture and curvature radius of the sublens element affect the divergence angle of the random microlens array (Fig. 5). In the simulation, the average aperture range of the sublens element is set to be 50?141.14 μm, and the radius of curvature is set to be 1?5 mm. The divergence angle of the random microlens array is 9.1?148.1 mrad. The divergence angle of the random microlens array decreases with the decrease of the mean aperture and the increase of the curvature radius of the sublens unit. In terms of complex coherence modulus, the interference fringe diagrams (Figs. 7, 8, 12, and 13) at different rotation speeds (speed range 0?4800 r/min) are obtained in simulation and experiment in this paper. The complex coherence moduli at different rotation speeds are obtained by calculating the contrast of interference fringe and measuring the ratio of double-hole aperture light intensity (Fig. 15). The modulus of complex coherence decreases with increasing rotation speed of the random microlens array. When the rotation speed increases from 0 to 4800 r/min, the total modulus reduction of complex coherence is about 96.67%. We further analyze the modulus decline trend of complex coherence by calculating the modulus decline percentage of complex coherence at every 60 r/min increase in different speed ranges (Table 2). The modulus decline percentage of complex coherence decreases from 61.52% to 1.17% when the rotation speed increases by 60 r/min, with the decline trend gradually slowing down.

In this paper, we design a random microlens array based on Thiessen polygon arrangement. We establish a laser complex coherence control model based on rotating random microlens array, and analyze the effects of rotating random microlens array parameters on laser divergence angle and complex coherence modulus of laser field. The simulation and experimental results show that, the average aperture and curvature radius of the sublens element affect the divergence angle of the random microlens array. The divergence angle of the random microlens array decreases with the decrease of the mean aperture and the increase of the curvature radius of the sublens unit. The average aperture of the random microlens array used in the experiment is 50 μm and the curvature radius is 1 mm. The divergence angle obtained by simulation based on these parameters is basically consistent with the measured divergence angle. In terms of complex coherence modulus, the rotation speed of the random microlens array affects the modulus of complex coherence of laser light field: the higher the rotation speed, the lower the modulus of complex coherence of laser light field. In the experiment, when the rotation speed of the random microlens array increases from 0 to 4800 r/min, the modulus of complex coherence of the laser light field decreases continuously and the total amplitude of the decline of the complex coherence modulus is about 96.67%. The modulus decline percentage of complex coherence decreases from 61.52% to 1.17% when the rotation speed increases by 60 r/min in different speed ranges. The decline trend of the complex coherence modulus gradually slows down with the increase of the rotation speed.