Your laser is in the high tens of megawatts, perhaps even low hundreds. That's quite capable of shooting pretty much anything out of the sky. You had better keep the beam perfectly on target. And what's the target, anyway? How do you convert that kind power into propulsion with very high efficiency? (If your efficiency isn't good enough your plane melts.)
Choose a wavelength that doesn't penetrate the troposphere. Also, multiple beams focused on the aircraft from different angles will not be focused on the ground.
As for conversion: probably some kind of PV, actively cooled.
