An optical sub assembly for receiving an optical fiber transmission line for coupling to an optical device, to a receptacle for use in such a sub assembly and to a method of manufacture of such a sub assembly. The optical sub assembly preferably provides electrical isolation between a receptacle having a nose for receiving the optical fiber and a package housing an optical device. The nose houses a ferrule containing a fiber stub which may be optically aligned with an optical device housed in a package before securing the receptacle to the package. The electrical insulation is provided between the nose and the package by a sleeve extending along a middle portion of the ferrule and an insulating portion positioned between the sleeve and the nose.