Q-switching

Q-switching creates a short high-energy optical pulse by first storing population inversion while an optical cavity has high loss, then increasing its quality factor so that stimulated emission rapidly extracts the stored excitation. Its duration depends on photon lifetime, excess optical gain, inversion depletion and switching time; it is not fixed by a universal time-bandwidth equality.

For an ideal pulse with negligible stored seed photons and useful output accounting for all cavity photon loss, the extracted energy is $(N_i^{\ast }-N_f^{\ast })V_g\hslash {\omega }_L/\beta$. The factor $\beta =1+g_2/g_1$ converts inversion depletion into stimulated transitions. With over-pumping $r=N_i^{\ast }/N_{\mathrm{th}}^{\ast }$, the nonzero extraction branch satisfies $r\beta \eta =-\ln (1-\beta \eta )$. A fraction $0.95$ of the maximum energy requires $r=-\ln (0.05)/0.95$.