An optical transmission module or an optical module has a receptacle attached adjacent to an optical component module or package, and a plug configured to connect vertically with the receptacle from above the module. The outer peripheral shape of the housing is designed to fit within an area of the receptacle that faces upwardly to receive the plug in an insertion direction extending vertically downwardly. In the housing of the plug, a ferrule is attached to an end of an optical fiber and a sleeve covers the optical fiber and the ferrule. A knob is used for moving the optical fiber, the sleeve and the ferrule in the lightwave propagation direction, which is transverse to the plug insertion direction. Before connecting the plug, the knob is retracted to pull the ferrule and the sleeve within the plug housing. This prevents contact of the ferrule in the plug with a mating ferrule in the optical component package that extends into the receptacle in optical alignment therewith when the plug is seated within the receptacle. The ferrule in the optical component package is connected to an optical component via another optical fiber. Upon completion of the insertion of the plug, the knob is released and the force of a spring holds the ferrules in contact with on another with sufficient force to maintain a detachable optical connection.