Optimization of Sodium Guide Star Return using Polarization and/or Modulation Control

Large ground-based telescopes require adaptive optics (AO) to correct for distortions introduced by atmospheric turbulence. In order to function, the AO system must track a bright point source. Although a natural star may be used, full sky coverage requires an artificial beacon created with a laser. The most common type of laser guide star (LGS) employs the mesospheric layer of sodium atoms at an altitude of ~90 km; a laser at 589 nm excites the Na atoms on the D2 line and the subsequent fluorescence is observed. Because of the small collection solid angle afforded by a ~1 m diameter telescope at a distance of ~90 km, returned photons from the Na LGS are at a premium. A possible way to optimize a Na LGS is amplitude or polarization modulation of the laser beam. For several reasons, photon return is enhanced by optical pumping of the Na atoms. However, the presence of the geomagnetic field can limit the effectiveness of optical pumping by inducing Larmor precession, which effectively depolarizes the atoms. Synchronous pumping via modulated light can counteract this effect, thus enhancing photon return. We propose to investigate, using computer simulations, various modulation schemes for enhancing return.