How to select a microlens array for laser Gaussian light homogenization to flat-top light

The Gaussian light homogenization microlens array differ from the structure of traditional optical elements, which makes it have many excellent effects and functions. Frequently-used applications of microlens array include: laser homogenization, Gaussian light shaping, Gaussian light homogenization, wavefront detection, fiber coupling, laser cutting, etc. Among whom, laser homogenization, Gaussian light shaping, and Gaussian light homogenization are the most common. The reason is that fiber coupling and Gaussian optical homogenization can vastly improve the effect and process controllability, whether used for laser processing or lighting. 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 laser homogenization requirements? This paper briefly introduces the dual arrays homogenization scheme, and the selection of microlens array for Gaussian light homogenization shaping.

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

LA1, LA2, FL, and FP are microlens array 1, microlens array 2, focusing lens and imaging focal plane from left to right. It can see from the above figure that the structure of the microlens array is flat and convex. The two lenses are placed in opposite directions, with the two convex surface facing outward. 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 the 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-d）/(f_LA1·f_LA2)

Under the conditions shown in the upper light path, d=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 obtain easily. It assumes that the homogenization requirement is to form a 10 * 10mm square spot at 300mm. Then D_FT=10mm, f_FL=300mm and the ratio of aperture to the focal length of the microlens array can calculate by taking the formula: pLA/fLA2=0.03333. If it is a rectangle, calculate the long and short sides separately.

In theory, the smaller the aperture of the microlens, the better the homogenization effect. However, if the aperture size is too small, there will be diffraction interference, and the final homogenization effect of hardness is not very 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 calculate according to this pLA/fLA2=0.03333, and the acceptable microlens parameter range can obtain 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 unnecessary. 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 very 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, however 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. It can adjust 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. It is a common phenomenon of microlens array shaping, both at home and abroad.

Note: The ML series microlens array are cost-effective product line. Mainly used for Medical beauty, Shack Hartmann sensor, High sensitivity imaging, Wavefront sensing, Beam focusing and some other applications. Due to the limited level of our cleanroom in old factory, there are always some defects (bad spots) on the surface of ML series products. The surface accuracy is also not very good.

The MLA, MLA1 and CLA, CLA1 series are high-end product line. MLA, MLA1 and CLA, CLA1 series microlens array have more accurate surface and less surface defects. They could be used for different applications, such as beam shaping (beam homogenization) and Industrial grade light field camera.