The eigenvalues of 
are
where
The fixed point b is a stable focus since the real parts of the eigenvalues are negative. This focus represents the constant intensity output of a laser, and the oscillation about this steady state is the ringing a laser initially experiences when it is turned on.
Figure 4.17: Phase portrait for the laser rate equation.
The global phase portrait, pieced together from local information about the fixed points, is pictured in Figure 4.17.