The key features of excess noise are reviewed and the results illustra
ted using a confocal strip resonator example. The excess noise factor
(Or Petermann K factor) is shown to be a strong function of the effect
ive Fresnel number of the resonator; for Fresnel numbers in the region
of 50, peak values of K are greater than 10(4). The identity between
the excess noise factor and the injected-wave excitation factor is dem
onstrated numerically, and both mathematical and physical interpretati
ons of the high values offered.