Can I use a 150mw green laser to scare pigeons and not interfere with air traffic?
There are cases where you can use a laser pointer safely, such as pointing out constellations in a dark field far from any air traffic, but as a general rule of thumb lasers should not be pointed skyward - not even to get rid of irritating sky rats pigeons.  

Should you wind up illuminating an aircraft with your laser you can cause serious problems for the crew - inadvertent or not it could get you a visit from some very unhappy law enforcement officials.



There are many other ways to get rid of pigeons, some of which are used at airports and many of which are more effective (and safer) than your laser idea.
Your local Animal Control group may be able to offer some additional ideas.

(Avoid the "predator bird statue" idea - in my experience pigeons realize it's fake and use it as a toilet.)