Ultra-high coupling efficiency of MEMS tunable laser via 3-dimensional micro-optical coupling system

This paper reports a 3-dimensional (3D) micro-optical coupling system for improving coupling efficiency in the Littrow configured micro-electro-mechanical system (MEMS) tunable lasers. In the coupling system, an optical fiber acts as a rod lens for light convergence in the vertical plane, while a deep-etched silicon parabolic mirror confines the light in the horizontal plane. Compared with previous MEMS lasers without any light focusing or only one-directional focusing mechanism, the proposed 3D micro-optical system allows longer external cavity length and provides higher coupling efficiency. A prototype is fabricated on a SOI-wafer with an etching depth of 100 μm. The laser obtains a coupling efficiency as high as 76.5%, which is much higher than typical value of 3% - 50% in previous designs. The laser has dimensions as small as 3 mm × 3.2 mm in single-chip integration. It achieves large tuning range of 48.3 nm within 1 ms tuning speed.