1064-nm and 1319-nm light respectively generated by two lasers is combined and injected into a doubly resonant sum-frequency generator. The optical path length of the sum-frequency generator is adjusted responsive to feedback of the 1319-nm light to maintain 1319-nm resonance. Feedback of 1064-nm light is concurrently used to adjust the 1064-nm laser responsive to the optical path length to maintain 1064-nm resonance. Light output from the sum-frequency generator is compared to the sodium D_2a wavelength i.e., approximately 589 nm, and the 1319-nm laser is responsively adjusted to eliminate any differential. This abstract is provided to comply with the rules requiring an abstract, and is intended to allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.