Gain

Before calculating the output power, it is interesting see how the effective gain varies according to the power of the pumping system. Below the oscillation threshold, the effective gain varies exponentially as a function of .

At the threshold and above it when the laser oscillates, the effective gain follows . It is thus fixed at a value determined by the transmission coefficients of the mirrors and the passive losses of the cavity. Figure E10 shows the curve of this evolution as a function of the pump power. When the pump power is zero, the effective gain is 1. This is because the lower energy level of the laser transition is empty so their can be no absorption. Figure E10 also shows the evolution of the effective gain if there is no optical cavity: it rises exponentially.