An optical source uses feedback to maintain a substantially fixed spacing between adjacent wavelengths in a set of wavelengths in a wavelength comb output by the optical source. In particular, a set of light sources in the optical source provide optical signals having the set of wavelengths. Moreover, the optical signals are output at diffraction angles of an optical device in the optical source (such as an echelle grating), and optical detectors in the optical source determine optical metrics associated with the optical signals. Furthermore, control logic in the optical source provides control signals to the set of light sources based on the determined optical metrics.