A system includes an external light supply to a silicon photonic chip. The light supply includes a primary light source, a secondary light source, an optical coupler, an optical splitter, and a dark sensor. The optical coupler is to combine any output from the primary light source and any output from the secondary light source into an input to the silicon photonic chip. The optical splitter is located in an optical path between the primary light source and the optical coupler, and is to divert part of the output from the primary light source. The dark sensor is to receive the diverted part of the output from the primary light source and to selectively activate the secondary light source based on the diverted part of the output from the primary light source.