Measuring the return flux for a sodium beacon created by combining two laser beams

Scientists at the Starfire Optical Range have been researching mesospheric sodium beacons for adaptive optics since 1992. Since then, we have developed four different sodium-wavelength lasers, all of which were based on diode-pumped, sum-frequency Nd:YAG oscillators. In 2016 we combined light from two commercial sodium-wavelength lasers, which use resonant-frequency doubling of light from a Raman fiber-amplifier, to form a single beacon. We used standard polarization techniques to combine the two lasers and produce a single beam with both left-hand and right-hand circularly polarized light. To avoid competitively pumping sodium atoms of the same velocity class, we tuned each of the lasers slightly off of the peak of the Doppler-broadened sodium line. However, the return flux from this beacon was lower than we expected, even when we accounted for the effect of detuning. In this paper, we propose an explanation for this phenomenon and show results from computer modeling and on-sky tests to support our claim. We believe the sodium atoms, which were optically pumped by the shorter-wavelength laser between the D2a F=2, m=+2 and F=3, m=+3 states, were down-pumped by the longer-wavelength laser to the D2b F=2, m=+2 and F=1, m=+1 states. Thus, these atoms were no longer available for optical pumping, which reduced the efficiency of the lasers in producing return flux. Addendum: We hope to show we can mitigate this effect by increasing re-pumping of the sodium D2b line on the shorter-wavelength laser.