Nonlinear Bragg scattering and mode superradiance in an open Fabry–Perot resonator

Mode superradiance (i.e. superfluorescence) in an open Fabry–Perot resonator is characterised by simultaneous emission of counterpropagating coherent pulses even when the mirrors have very low (but finite) reflection coefficients, beginning from $R_{cr}\ll1$. It is shown that for any value of the resonator $Q$ factor, even when $R\gg R_{cr}$ or $R\sim1$, a self-matched $\lambda/2$ grating of the population inversion in a two-level medium does not result in significant Bragg reflection which would exceed the reflection from the mirrors. A consistent allowance for the interference of inhomogeneous conterpropagating waves significantly alters the spatial and temporal dynamics of depletion of the population inversion and, consequently, greatly changes he profile of collective spontaneous radiation pulses, compared with the predictions of the mean field and unidirectional superradiance theories. A new nonlinear equation for superradiance in an open resonator is derived.