hi zulkarnain
A desirable feature of a laser is the constant amplitude. Right after the laser turns on, the amplitude varies for a while and then gets stabilized to a constant. We call the frequency before the laser gets stabilized the relaxation oscillation frequency.
for your explanation, plese visit;
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-977-ultrafast-optics-spring-2005/lecture-notes/chapter4.pdf
with regards