Sodium laser guide star projection for adaptive optics

Abstract

In order to increase sky coverage, adaptive optics (AO) systems for large telescopes will require laser systems to provide artificial reference beacons. The most prominent method for creating an artificial beacon is to project laser light tuned to the 589nm, D2 line of sodium onto the mesospheric sodium atoms at an altitude of 90km. When correcting with AO, the best wavefront measurements are obtained when the image of the sodium beacon is as bright and sharp as possible. Blurring occurs due to spot elongation, as a result of sub-aperture displacement from the projector axis, and from diffraction and seeing effects on the projected beam. Mounting the projector in the center of the telescope minimizes the effect of elongation. Simulations were conducted that show that matching the beam waist to ∼2 times the atmospheric turbulence parameter r₀ minimizes the beacon size. For r₀ = 15cm and a 48cm projector, calculations show the optimum projected waist is 29cm. A prototype projector has been built and operated. Recent experiments have shown that this projector is capable of producing 0.75arcsec beacons under good seeing. In addition, spot elongation of 0.5arcsec was observed corresponding to a sodium layer thickness of 10km. The first experimental evidence for optical pumping in the mesospheric layer were obtained. They show a non-thermal profile for the sodium hyperfine structure (3.5:1 line ratio as opposed to 5:3) when projecting circularly polarized light. This profile indicates that the maximum return per watt is obtained by pumping the F = 2 level with a narrow bandwidth compared with pumping both F = 2 and F = 1 with a broad bandwidth. In addition, evidence shows a 30% increase in beacon brightness when pumping the sodium layer with circularly polarized light over linear. A projector for the 6.5m MMT conversion has been designed based on experience gained with the prototype. Analysis of the Strehl reduction due to wavefront reconstruction error shows a reduction in Strehl of < 1% for the optimal operating parameters at the MMT. This less than the fundamental limit of 0.79 for focus anisoplanatism.