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This paper presents a 3-dimensional micro-optical system for improving the optical coupling efficiency in the external cavity tunable lasers. It is constructed by an optical fiber and a curved mirror; the former acts as a rod lens to transform the light in one direction while the latter converges the light in the other direction. Compared with the previous MEMS lasers that have no light focusing or only in one direction, this 3D micro-optical system allows for long external cavity length and high coupling efficiency, which are critical for wide tuning range. A prototype is fabricated on SOI wafer by DRIE with an etching depth of 75 µ m. It obtains a coupling efficiency as high as 46.5% with a total external cavity length of 220 µ m. In contrast, the coupling efficiency is only 13% when the fiber rod lens is absent, and is 1% when no light transformation is involved. Compared with conventional laser optical systems that employ external ball lenses and other optical components, this 3D micro-optical system possesses the advantages of simple packaging, large alignment tolerance and easy mass production.