Radiation principles and interpretation of experiments on the reflection from amplifying media

The causality concepts are used to prove a generalized radiation principle applicable to absorbing and amplifying media. It is shown that the reflection coefficients of homogeneous plane waves incident from a transparent medium on an interface with an amplifying medium do not exceed unity. This is used to discuss the published experiments. An integral equation is derived and solved for the modes of a laser in which amplification is due to the reflection of light from an active layer.