Consistent with the present disclosure, a transmitter is provided that includes first and second stages of wavelength locking circuitry. The first stage includes a tunable optical filter that sweeps through the spectrum of a WDM signal at a predetermined rate. A first photodiode senses a tapped portion of the output of the tunable filter. The remaining light is fed to the second stage, which includes a second optical filter, typically having a fixed transmission characteristic. A second photodiode senses the light that passes through the second filter. By sweeping the WDM spectrum the tunable filter can be used to identify the peaks in the WDM spectrum, with each peak corresponding to an optical signal wavelength and occurring at a particular time interval during the sweep. Thus, each optical signal wavelength can be associated with a particular time interval in the sweep, and, if no peak is identified during the sweep, a fault can be identified as either a laser failure or that the optical signal wavelength has drifted or “hopped” to another optical signal wavelength. Once having identified that an optical signal has hopped, the optical source outputting that optical signal can be appropriately controlled to output light at the correct wavelength.