Laser technology for astronomical adaptive optics

In this paper we review the current status of work in the sodium guidestar laser arena from the perspective of an astronomical AO system developer and user. Sodium beacons provide the highest and most useful guidestars for the 8m and larger class telescopes, but unfortunately sodium lasers are expensive and difficult to build at high output powers. Here we present highlights of recent advancements in the laser technology. Perhaps most dramatic are the recent theoretical and experimental efforts leading to better understanding the physics of coupling the laser light to the upper altitude sodium for best return signal. In addition we will discuss the key issues which affect LGS AO system performance and their technology drivers, including: pulse format, guidestar elongation, crystal and fiber technology, and beam transport.