An optical module that includes a shell, an optical fiber, a coupling portion, and a ferrule is disclosed. The shell installs an optical device, for instance, a multi-mode interference (MMI) device therein. The optical fiber in a tip thereof is optically coupled with the optical device within the shell. The coupling portion has a cylindrical shape with a bore having an axis and secures the optical fiber, where the coupling portion is attached to the shell. The ferrule, which is secured in the coupling portion, has a pillared shape with a diameter that is slightly smaller than a diameter of the bore of the coupling portion. The ferrule has a groove that receives and secures the optical fiber therein. The filler fills the groove and fixes the optical fiber in the groove.