Spherical concave micro-mirror fabricated using gray-tone optical lithography for vertical coupling

Abstract

Based on gray-tone optical lithography technology combined with the overlay alignment method, a spherical concave micro-mirror is fabricated at the end of a rectangular optical waveguide (ROW) for low vertical coupling loss. The optimal structures of the spherical concave micro-mirrors were designed through ray-tracing simulation. The results indicate that the minimal vertical coupling loss is only 1.02 dB for the ROW core size of 20 μm × 20 μm. The surface roughness of the micro-mirror is considered, and it should be less than 106 nm to ensure that the vertical coupling loss is less than 1.5 dB. The radius of the fabricated spherical concave micro-mirror was measured as 263.3 μm and the surface roughness of the micro-mirror is 29.19 nm. The vertical coupling loss induced by the micro-mirror was measured as 1.39 dB. 1-dB tolerances in the direction of x-, y-, and z-axes are calculated to be ± 6.9 μm, ± 6.3 μm, and 46.2 μm, respectively.

Critical analysis of in-plane free-space light beam coupling using photonic curved micromirrors

“…Silicon-etched spherical micromirrors were reported in both cases of having an out-of-plane or in-plane optical axis in many applications including single-atom detection, optical cavities and tunable lasers, and optical interconnects in addition to vertical coupling. [71][72][73][74] The main purpose was to have an efficient coupling mechanism. It is noteworthy that silicon curved micromirrors fabricated on a wafer top surface were also reported using isotropic chemical etching for the purpose of optical detection of a single atom, 71 a selective polishing method on the top of MEMS tunable vertical-cavity surface-emitting lasers (VCSEL), 72 and ion beam irradiation followed by electrochemical etching for cavity-quantum electro dynamics (QED) studies as well as optical interconnects.…”