Selective absorption and spontaneous emission in a resonator

A theoretical analysis is made of the influence of spontaneous radiation on the sensitivity of the method of continuous detection of an absorption line in which an absorption cell is placed inside a laser resonator. It is shown that this influence is negligible if the laser modes are coupled not only to the common reservoir of atoms or molecules but each mode additionally acquires energy from its own reservoir. If the contribution of the individual reservoirs to the total energy balance is small compared with the contribution of the common reservoir, the probability of spontaneous emission determines the maximum possible sensitivity of this absorption measurement method.