Equal excitation of all modes on an optical fiber

Abstract

In the literature on fiber optics the notions of Lambertian and totally incoherent sources and their efficiency for exciting all waveguide modes are often confused. In this paper a general electromagnetic theory approach is used to establish the nature of the field which does excite all modes equally and independently. The sense in which this source is incoherent, and its Lambertian nature, are exhibited. The relations between modal and asymptotic or geometrical optics formulas for mode counting and description of intensity propagation are clarified for complete fiber excitation.