A transmitter optical subassembly (TOSA) for transmitting an optical signal into the end of an optical fiber connected thereto comprises a mounting tube containing a convergent lens adapted to focus light from a solid state light source at one end of the tube to an optical fiber terminating at a fiber endface in a fiber receiver at the other end of the tube, the lens being formed of a glass having thermal characteristics compatible with those of the mounting tube and other metal components of the subassembly and the fiber receiver being mounted on the tube at attachment points substantially coplanar with the plane of the fiber endface.