Unless I'm crazy, Prat's answer is actually unrelated to your question. It's an article about pulse shaping, which is used to, well, change the shape of an ultra-short pulse. Not generate it.
What you want to look for is mode-locking. Essentially it's based on interference. You have a lot of standing waves in a laser cavity and lock them so that they all have the same phase. Then they'll interfere destructively except at a regular interval where they add up to make a powerful but very short pulse. Imagine water in a pool, if you generate waves just right you can get it to splash suddenly and powerfully in certain spots, even though your wave machine itself can't generate such strong splashes.
http://en.wikipedia.org/wiki/Mode-locking
Disclaimer: I use these lasers sometimes, but I'm in signal processing, not physics.