Monolithic single and/or dual detector structures are fabricated on the emitting surface of a VCSEL and/or on a lens or glass substrate configured to be positioned along the axis of emission of an optical light source. Each monolithic detector structure includes one or two PIN detectors fabricated from amorphous silicon germanium with carbon doping or amorphous germanium with hydrogen doping. The monolithic detectors may additionally include various metallization layers, buffer layers, and/or anti-reflective coatings. The monolithic detectors can be grown on 1550 NM VCSELs used in optical transmitters, including lasers with managed chirp and TOSA modules, to reduce power and real estate requirements of the optical transmitters, enabling the optical transmitters to be implemented in long-reach SFP+ transceivers.