Why must we use mode locked lasers?

Quick question:

If I have a laser cavity with a bunch of harmonics under the gain curve: Why do I not always get a pulsed laser? EG: I mean since these harmonics exist in the cavity arn't they always in phase because the ends are closed and they create standing waves in phase to each other? Where does the out of phase aspect come from ? Mirror imperfections -> implying the length of the cavity is not constant? I mean I am wondering, why not all lasers are pulsed, since there is always going to be more than one frequency under the gain curve to some degree.

Thanks!

• Why should all the harmonics in a cavity be in phase? – Jon Custer Apr 5 '16 at 14:12
• because the ends are fixed, imply there are nodes there so they start out the same – sci-guy Apr 5 '16 at 15:08
• never mind - I got it. – sci-guy Apr 5 '16 at 15:14
• @renegade you mean you found an answer? Why don't you post it? – scrx2 Apr 5 '16 at 19:00

It's pretty straightforward. Consider two laser wavelengths in a cavity of length L: one is $$\frac {L}{N}$$, and the other is $$\frac {L}{N+1}$$. Though the two can have the same phase at the ends of the cavity, they usually don't.