Low-power polychromatic guidestar through coherent excitation of the 4d state in atomic sodium

Summary form only given. Adaptive optics systems, guided by artificial sodium beacons have reached fundamental limits, because of (i) the low densities of atoms in the mesosphere; (ii) the low level of excitation dictated by the requirement to avoid saturation, and (iii) the monochromatic return which makes it impossible to correct for differential tilt (the largest error-about 90%-in image formation). Different attempts are being made to taylor laser pulse profiles to more efficiently access the Doppler groups of the atomic lines, while avoiding saturation (e.g. only 6.4 mW/cm/sup 2/ for the D2 line), and to assist in the correction for differential tilt by adding Rayleigh or natural beacons. Both problems severely limit the further advancement in this field due to the increased complication and cost. We have presented an alternative, which circumvents these problems by using coherent excitation of sodium with short pulses. The saturation limitation is avoided in this approach in which each pulse totally inverts (3s /spl rarr/ 4s) the interaction volume. Subsequent cascade resonance fluorescence at two wavelengths allows for correction of the tilt.