A method and apparatus for testing the operability of a signal source formed on a die are described. A pair of modulators are formed on the die and coupled to the signal source. An optical unit is optically coupled to the pair of modulators, which are capable of modulating an optical beam in response to a signal provided by the signal source. The optical unit is capable of detecting modulation of the optical beam. To test the signal source, the signal source is set to generate a signal. If modulation of the optical beam is detected at the optical unit, then the signal source is operable. If modulation of the optical beam is not detected at the optical unit, then the signal source is not operable.