9.6.1 Laser Rate Equations

Our purpose is now to describe the time evolution of the light emitted by a laser. The laser is supposed to work only on one mode.

We use the set of equations (9.29) to which must be added the description of the pumping and of the optical losses of the resonator. The losses are "useful losses" since they represent the light that is emitted by the laser. Let us call dn _loss the number of photons that leave the resonator during the time dt,

The power emitted by the laser is given by formula (9.49.b):

We suppose that the lower level of the laser transition is the ground level, the pumping becomes completely inefficient when this level is empty, so the pumping term in the equation is written as W _pN ₀,

Equations (9.50) become

Equations (9.50), (9.51), and (9.52) are called the laser rate equations; they need two variables ? and n, the variation of which has to be calculated. Their solution is not immediate because of the presence of the stimulated term nN/ ? _rad and can only be made by numerical integration.