Radiation Lifetime in a Cavity

Laser radiation in a cavity will have a finite lifetime because of losses at the cavity walls. The decay of a radiation mode can be expressed as

For cavity dimension L and reflection coefficient a, the time to make n reflections is

After n reflections, the radiation intensity is

which gives for the lifetime:

Using the series approximation

leads to

This cavity lifetime is one of the parameters which determines the pumping power which must be used to maintain a given level of radiation from a laser.