At the distance from Pluto to Earth, Earth's diameter amounts to about 1.5 * 10^-4 degrees in angle. Precisely pointing a laser beam at a given target, even a large one, at that distance seems almost impossible. Especially since the space probe probably won't stay perfectly still but will always slightly tumble.
Is the earth visible in an optical telescope from the spacecraft? This should be the basis for automatic tracking... (transmit the laser through the same telescope back to earth). The transmitter on earth could track on the "star" of the laser from the spacecraft.
I've been trying to understand the noise temperature of erbium doped fiber amplifiers- not nearly as good as DSN masers.
