Deastigmatism, circularization, and focusing of a laser diode beam using a single biconvex microlens

Abstract

A single biconvex microlens is proposed to correct the astigmatism and ellipticity of a laser diode (LD) beam and focus it to a smallest circular spot. The microlens has three different profiles in which one cylindrical input surface is to collimate the beam in the fast-axis (y-axis) direction. Output surface, on the other hand, holds two different parabolic profiles in fast- and slow-axis (x-axis) directions to correct the astigmatism and focus the beam into a smallest circular spot. A simulation software is used to design and optimize those lens profiles. Theoretically, the smallest focused spot size is around 4.24 μm in diameter. The three profiles are then transferred to photo-masks to fabricate the microlens on polycarbonate material using an excimer laser dragging method with alignment accuracy of 1 μm. The machined microlens is assembled with the LD using double-sided optically clear adhesive tape. The experimental focused spot is found to be 16.75 μm in diameter. Circularity of the focused spot is demonstrated by a single-shot exposure test on thin photoresist layer that shows a circular-dot diameter of 7.32 μm. The proposed technique has great potential in applications such as beam pen lithography, fiber coupling, and optical read-write head.