Spherical Aberration Analysis And Improvement Of Planar Square Aperture Microlens Array

Microlens array has become a hot topic in microoptics recently, and noncircular aperture microlens array has been proposed as a new subject. Photo etching technology, ion-exchanging technology and especial removing Ti film process were applied to fabricate square-aperture planar microlens array (PMLA). The filling factor of array was approximate100%and its imaging resolution ratio was improved greatly. With those high performances, square-aperture PMLA plays an important role in optical fields, such as optical information processing, optical interconnects, information storage, optical communication, optical switch and imaging of laser sensors and so on.However, numerical aperture (N.A.) and focal length (F) of square-aperture PMLA were difficult to design and the aberration, especially spherical aberration, can not be ignored. To solve these two problems, experiments and theory analysis have been done in this paper.Firstly, thermal diffusion theory of ion was used to analyze the influence on N.A and F of PMLA with ion-exchanging time; meanwhile, N.A and F were investigated with different ion-exchanging time but the same size of mask aperture experimentally. Experimental results are comparable with theory analysis. These results can be used as a reference for designing square-aperture PMLA with appropriate N.A and F by setting optimal ion-exchanging time and also as a reference to obtain the best ion-exchanging time as the same time to obtain the big N.A and F.Secondly, we dealt with square-aperture PMLA by annealing treatmen, and the longitudinal and transverse spherical aberration by the method of parallel light incidence and emergent light pupil. These experimental results showed that the refractive index distribution was closer to ideal distribution and the spherical aberration was decreased obviously after annealing technology. Meanwhile, imaging resolution ratios are also improved more greatly.Thirdly, the optical system with controllable focus spot was proposed with two pieces of the square-aperture PMLA which has continuously adjustable F and an aspherical lens. F can be adjusted continuously just by precisely adjusting the distance between the two pieces of square aperture PMLA. According to the high filling factor, fitting N.A and F, continuously adjustable F and small spherical aberration, this square-aperture PMLA has a prospect application in more optical systems, such as, uniform irradiation systems, higher quality imaging systems, long focal length measuring systems and so on. The study of comprehensive imaging and some important measuring will be done in the future.