NASA's Laser Communication Relay Demonstration Satellite

Why NASA Needs Laser Communication
Astrum

Tight beam communications is coming! It's been a staple of science fiction space war stories. If you are using a laser to communicate with other ships in your squadron, the enemy can't listen in on your conversations. Compared to radio, data rates are higher and energy requirements are lower since you focusing all your energy in one beam, and not broadcasting it to every bug-eyed monster in the neighborhood. Of course, you must be able to point your laser accurately. 

• One degree of misalignment at a distance of 25,000 miles (geosynchronous orbit, for instance) will cause you to miss your target by 70 miles. 
• One second of misalignment (1/60 of a degree) will put you off target by a mile. 
• One arcsecond (1/60th of an arcminute or 1/3600th of a degree) will put you off by 100 feet.

The Webb Space Telescope has a similar problem with aligning its mirrors, but they seem to have figured that out. Then there is the problem of the beam spreading. The beam of a laser beam sent from the Earth might be a quarter mile wide by the time it reaches the moon.  Now imagine how wide that beam would be by the time it got to Pluto. There are some lasers that have tighter beams that would not spread as much.

This is not the first laser communication system in space. There have several others, some still in use. I imagine what makes this one different is in the details.