Hyperboloid solgel microlens array fabricated by soft-lithography for optical coupling

It is known that the best way to correct spherical aberrations is the use of aspheric microlenses in optical systems, where a single aspheric microlens can be employed to replace a compound of spherical microlenses in a compact design. However the fabrication of aspheric microlenses is often complex because expensive high-energy beam-sensitive (HEBS) gray scale mask is needed in the fabrication process. In this paper, we reported a cost-effective fabrication method, with a combination of the sample-inverted reflow technique and the soft lithography replication method, to fabricate hyperboloid refractive microlens arrays (MLAs) in the inorganic-organic hybrid SiO2-ZrO2 sol-gel material. The fabrication procedures involved two basic steps. Firstly, a master of hyperboloid MLA was made in photoresist by the sample-inverted reflow technique. Secondly, a negative mold of the master was built by casting polydimethylsiloxane (PDMS) to a silicone elastomer against the master, and then the profile was impressed onto the sol-gel glass. As a result, the fabricated sol-gel MLAs have been obtained with excellent smooth profiles, having negligible discrepancies from the profiles of the ideal hyperboloid MLAs. The root-mean-square roughness values (R-q) of the surface of MLA were measured as 1.2 nm in the central areas and 2.1 nm in the outskirts of the lens. In an application of coupling a laser diode (LD) to a single-mode fibre (SMF), we proposed a two-MLA coupling scheme where two revolved-hyperboloid MLAs were back to back introduced between the LD and the SMF. In this configuration, the coupling efficiency has achieved 83.4% (-0.79 dB).