Designs of 2-port optical devices are disclosed. The optical devices so designed are amenable to small footprint, enhanced impact performance, lower cost, and easier manufacturing process. The optical device comprises at least two collimators, each of the collimators including: a single-mode fiber, and a multi-mode graded index fiber, aligned with the single-mode fiber in a way that the multi-mode graded index fiber and the single-mode fiber are spliced together, wherein the multi-mode graded index fiber is designed to have a predefined length.