This article describes how the Keck Observatory uses 40W laser pulses to measure atmospheric disturbance, which can then be reduced by changing the shapes of segmented 10-meter telescope mirrors and post-processing the resulting images.
Of course you can’t just go firing 40W lasers willy-nilly into the sky — the FDA considers any visible laser with output greater than 5mW to be hazardous — so the observatory has some interesting safety measures in place:
Our Federal Aviation Administration–approved approach to aircraft safety is to have two spotters outside during all laser operations, with switches to shutter the laser. We also have an IR camera boresighted to the laser that will automatically shutter the laser in the event of aircraft detection.
Prior to each laser observing run, a list of targets and their observation times is faxed to the Laser Clearinghouse at the US Space Control Center. The Space Control Center faxes back a list of any required blackout periods, to avoid illuminating satellites.