Abstract: A signal transmission system in accordance with the invention is implemented using a dual-purpose pin of an optical sub-assembly. The dual-purpose pin is used to propagate an analog signal out of the optical sub-assembly and also for providing an access point where an external element may be coupled to the optical sub-assembly for modulating the analog signal. The optical sub-assembly houses a photodetector, a signal transmitter circuit, and a signal receiver circuit. The photodetector receives light and generates a corresponding electrical signal indicative of the light intensity. The signal transmitter circuit converts the electrical signal received from the photodetector into the analog signal that is transmitted out of the dual-purpose pin. The signal receiver circuit located inside the optical sub-assembly is configured to monitor the modulated analog signal from the dual-purpose pin and generate therefrom, a control signal inside the optical sub-assembly.