 Photon lifetime

The photon lifetime is the exponential decay time of stored energy in an unpumped optical cavity. If a round trip of duration $T$ retains an energy fraction $\rho$, then ${\tau }_c=-T/\log \rho$ and the quality factor is $Q=\omega {\tau }_c$. Field-amplitude decay takes twice this time. Output coupling contributes to the loss as well as absorption and scattering.