AO4ELT5 Proceedings

Terrestrial combination of multiple sodium laser beams for increased on-sky beacon brightness

Johnson, Robert (Air Force Research Laboratory), Hackett, Shawn (Air Force Research Laboratory), Eickhoff, Mark (Air Force Research Laboratory), Drummond, Jack (Air Force Research Laboratory), Kann, Lee (Air Force Research Laboratory), Buckman, Miles (Air Force Research Laboratory), Richey, Jeff (Air Force Research Laboratory), Reynolds, Odell (Air Force Research Laboratory), Wyman, Keith (Air Force Research Laboratory)

Recent tests at the Air Force Research Laboratory, Star re Optical Range have shown that two sodium-wavelength lasers may be combined at the telescope, then launched into the sky to form a single laser beacon, thus increasing the return flux at the telescope and allowing better performance of the adaptive optics system. In this paper, we present experimental results, which show that two sodium-wavelength lasers, emitting at approximately 589.159 nm, may be combined by typical polarization techniques. We show the deleterious effects of decay to the D2b ground state vice the D2a ground state endemic to this con guration, sometimes called down-pumping, can be reduced in part by repumping the D2b ground state and by offsetting the two laser beams in frequency from the peak of the D2a ground-state excitation frequency. In several tests over a 12-month period, we measured the return flux from the laser beacon while varying the separation in frequency of the two sodium-wavelength lasers to determine the con guration that maximizes the return flux. We discovered the optimum separation was about 200 MHz, which was much farther apart than we had expected. We believe this effect is due to a phenomenon that we call competitive down-pumping.

DOI: 10.26698/AO4ELT5.0161- Proceeding PDF

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