Laser Systems for the Generation of Sodium Layer Guide Stars

Laser generated guide stars in the mesosphere at 90 km provide an effective beacon for adaptive optics schemes which compensate the effects of atmospheric turbulence. Atomic sodium, the species with the highest product of integrated density and cross section, requires an exciting laser with a stable wavelength of 589 nm, a spectral bandwidth of ≈3 GHz and a peak power incident on the mesosphere of ≤5 W/cm2 in order to reduce the effects of saturation. There are several other attributes of the laser which are desirable from a point of view of overall adaptive optics system performance and operation ease. These include the following: (1) near diffraction limited beam quality which is needed to make a small laser guide star, (2) pointing accuracy in the arcsecond range with resolution in the subarcsecond range, (3) the ability to detune the laser for background subtraction or retune to another wavelength for polychromatic guide stars1, (4) a configuration which simplifies the laser projection optics and does not require beam paths through the telescope bearings and (5) an effective method of removing waste head from the laser before it enters the dome volume. Finally, while lasers with average power in the ten watt range may be sufficient for observations in the near IR, extension to visible observations will require multiple guide stars with total powers in the hundred watt range. Therefore the ability to scale up in power by an order of magnitude in a straightforward manner is certainly desirable.