What is the Q-Factor of a laser cavity? What is the energy stored in it?

Question

I've got the deffinition of Q-factor in a laser cavity as
Q=2 x PI x energy stored in cavity / energy loss per cycle.
How do I find the energy stored in the cavity?

Basically, I've got the question:
"The output in a gas laser with a cavity length of 1m transmits 0.5% of the incident light at a wavelength of 488nm. Ignoring other losses in the system, calcuate the Q-factor of the cavity and the cavity lifetime."

(I've also got the cavity lifetime to be, Q= freq x cavity lifetime, for the last bit of the question.)

Thanks.

Answer 1

The .5% is the energy loss per cycle of the *incident* light. That means 95% is reflected. So the loss per cycle is 0.005 times 1/(1+.95) of the total energy in the cavity (since the latter includes both incident and reflected), or 0.00256. According to your definition, then, Q is 2 pi times the inverse of this.