Gaussian light homogenization microlens array is different from the structure of traditional optical elements, which makes it have many excellent effects and functions. Common applications of microlens array include: laser homogenization, Gaussian light shaping, Gaussian light homogenization, wavefront detection, fiber coupling, laser cutting, etc. Among them, laser homogenization, Gaussian light shaping and Gaussian light homogenization are the most common. Because, whether used in laser processing or lighting, fiber coupling and Gaussian light homogenization can greatly improve the effect and process controllability. However, there are many types of microlens arrays on the market, with complex parameters and different manufacturers. So how to select a suitable microlens array based on your own laser homogenization requirements? This paper briefly introduces the homogenization scheme of dual arrays, and the selection of microlens array for Gaussian light homogenization shaping.

The optical path diagram of Gaussian light shaping scheme for laser homogenization of double microlens is as follows:

LA1, LA2, FL and FP are respectively microlens array 1, microlens array 2, focusing lens and imaging focal plane from left to right. It can be seen from the above figure that the structure of microlens array is flat and convex. The plane is opposite, and the convex surface is the structure with microlens. The incident Gaussian laser to be homogenized and shaped is collimated from the left and then incident into the microlens array. The sub-unit aperture of the two Gaussian light homogenizing microlens arrays should be consistent and close. The spacing between microlens arrays is equal to the focal length of microlens array 1, which is generally slightly larger. The distance between the microlens array and the focusing lens FL will slightly affect the divergence angle of the homogenized light spot, but the influence is small, and is generally ignored in the selection calculation. In practice, this distance can be adjusted to fine-tune the effect and size of the microlens array. The end is the effect end of Gaussian light homogenization of microlens array.

The relationship between the size of the laser homogenization spot of the microlens is as follows: (p_LA is the aperture size of the sub-unit of the lens array)

D_FT=p_LA·f_FL·（f_LA1+f_LA2-a₁₂）/(f_LA1·f_LA2)

Under the conditions shown in the upper light path, a₁₂=f_LA2, the above formula can be simplified as follows:

D_FT= p_LA·f_FL/ f_LA2

Therefore, according to the formula, the required microlens array parameter range and target direction can be obtained simply. It is assumed that the homogenization requirement is to form a 10 * 10mm square spot at 300mm. Then there is: D_FT=10mm, f_FL=300mm, and the ratio of aperture to focal length of microlens array can be calculated by taking the formula: pLA/fLA2=0.03333. If it is a rectangle, calculate the long and short sides respectively.

In theory, the smaller the aperture of the microlens, the better the homogenization effect. However, if the aperture is too small, there will be diffraction interference, and the final homogenization effect of hardness is not so good. Therefore, it is generally recommended that the caliber should be slightly enlarged when selecting models. The aperture requirement of the microlens is determined, and the corresponding focal length can be calculated according to this pLA/fLA2=0.03333, and the acceptable microlens parameter range can be obtained at the same time.

In the optical path, the main function of the focusing lens at the end of the microlens is to improve the homogenization effect and adjust the position and size of the Gaussian light homogenization, which is not necessary. If the required light spot is far away or large, it can be removed.

It should be noted that because the periodic microlens array is selected, there are certain requirements for the light source itself. If it is a light source with small M2 and high coherence, the Gaussian light shaping and homogenization effect of microlens array is not so good. For this light source, it is recommended to use diffractive shaping homogenizing elements to achieve high homogenization. Of course, you can also pass light through the scattering plate first and then incident it to the microlens array homogenizer group, but the effect is not cost-effective, and is generally not recommended.

In the process of using the dual-piece microlens array, it is particularly important to align the centers of the two microlens arrays. This can be adjusted by rotating the lens and moving it slightly. Due to the area accuracy of the microlens array and the quality of the light source, the homogenized light spot will have a certain amount of speckle, which is a normal phenomenon. Due to the weak speckle, the final use is generally not affected. However, speckle removal requires high accuracy, costs a lot and is not cost-effective. This is a common phenomenon of microlens array shaping, both at home and abroad.