ABSTRACT
In this chapter, we turn our considerations to the simplest model of
a laser that includes all the essential features of any practical laser.
The one-photon losses (due to coupling to the environment), linear
gain (due to inverted atoms) and saturation (due to nonlinear loss),
that are experienced by a single-mode (laser) cavity are all included
in the calculations and their role in the laser properties is fully
explored.We point out that all lasersmust have these three elements
to operate as a laser. If there is only a linear gain, then of course there
can be no output, since an output causes a loss. With both gain and
loss, but not saturation, the laser intensity is either near zero (well
below threshold), or else it rises infinitely (well above threshold).
Neither case is very useful as a model of a real laser, which has a
finite output above threshold. This is why a saturation mechanism is
needed, such as an intensity dependent nonlinear loss. This simple
model used here is then obtainable in the limit of a high-Q laser operating not too far above threshold.
