An optical coherence tomography system uses an optical source that comprises a series of gain waveguides that generate light at the frequencies at which the interference signal is to be sampled. In this way, the optical source generates a discretely tuned optical signal. This has the advantage that the tuning can be directly controlled by a controller that is also used to synchronize the sampling of the interference signal. This avoids the need for separate frequency clock synchronization. In embodiments, the gain waveguides are fabricated from one or more semiconductor edge emitting bars. In some implementations, the gain waveguides comprise periodic structures that define the frequency of operation of the waveguide. However in other implementations, the combiner comprises a dispersive element, such as a diffractive grating, that provides frequency specific feedback to each waveguide.