Research On Laser Beam Homogenization Method Of Random Microlens Array With High Energy Utilization

Laser is widely used in many fields,such as medical treatment,laser lighting,laser detection,satellite communication and so on.The energy of the laser output spot is generally Gaussian distribution,which can not meet the actual requirements in some special applications,such as laser welding,an optical system of exposure machine,laser drilling,laser projection.Therefore,it needs to convert Gaussian light into flat top light to meet the application requirements.There are many ways to realize laser beam homogenization,such as liquid crystal spatial light modulator,aspheric lens group,optical diffractive element,etc.,but it can not meet the requirements of miniaturization,integration,wide band,low cost of preparation at the same time.The microlens array is a continuous surface structure of refraction type,which can be used for laser beam homogenization.Meanwhile,the microlens array,which has high practical value,has the advantages of high diffraction efficiency,small volume,less stray light,suitable for wide band,and system integration.Nevertheless,when the traditional periodic microlens array is used to homogenize the laser beam with high coherence,the input beam is divided into several sub beams,which are superimposed on each other.Due to the coherence of the laser,the interference fringes will be produced after passing through the periodic microlens array,which seriously affects the effect of the homogenization spot.In order to solve the above problems,the combination of photolithography and wet etching to prepare a new type of miniaturized,integrated and wide band random microlens array is proposed,which can realize laser beam homogenization,break the periodicity of microlens array,reduce the coherence in the homogenization process,and obtain high energy utilization of homogenized spot.The main work of the paper is as follows:(1)The development status of laser beam homogenization at home and abroad is studied,and the random microlens array structure is designed.The sub lens unit of microlens array is mainly determined by the parameters of aperture and vector height,so it can be randomly broken by changing the aperture and vector height.The phase difference between sub beams is randomly distributed after passing through the microlens array,so as to improve the spot uniformity of high coherence laser beam after passing through the microlens array.(2)A new method of fabricating random microlens array by combining photolithography and wet etching is proposed.It can realize the low cost and integrated forming of continuous and irregular microlens array.Based on the study of lithography technology and the ratio of etching solution,honeycomb,single-sided and double-sided random microlens arrays with surface roughness as low as 13 nm were fabricated.(3)The wide band applicability of the structure is tested and verified.The spot uniformity and the energy utilization rate of beam homogenization at 650 nm,532 nm and 405 nm were measured.For the honeycomb random microlens array,the uniformity of spot is 80%,and the energy utilization rate is 86%;For single-sided and double-sided random microlens arrays,the spot uniformity is over 89% and the energy utilization rate is better than 86%.The results show that the random microlens array can be miniaturized and integrated,suitable for wide band,and can achieve higher energy utilization and spot uniformity.