On the pulse broadening in dielectric multimode waveguides

Abstract

This paper presents theoretical results concerning pulse-broadening in multimode fibres. All the calculations are based on the geometric optic hypothesis. The results are given for different kinds of input functions in the case of a loss-free guide and an energy equidistribution. Subsequently, material and interface losses are included. The influence of angle-dependent energy distributions is also investigated. For all these cases analytical expressions are obtained. Experimental results are presented: they are in good agreement with theory. IntroductionRecently, there has been an increasing interest in dielectric multimode waveguides for possible applications in communication transmission systems, after attempts of producing such waveguides with amazingly low kilometric attenuation have been successful [1]. Because of their larger light guiding cross-section, multimode waveguides seem -contrary to monomode waveguides-to be easier to handle and thus to involve less expenditure in practical application.As far as monomode waveguides are concerned, there exist already very distinct ideas of the transmissible bit rates, whereas the conceptions for multimode waveguides are not yet of a definite nature. Therefore, the objective of this paper is to contribute towards the clarification of this topic. As in the case of many problems encountered in physics, where it is not necessary to use the exact theory for their description, the rigorous electromagnetic theory has not been applied for the treatment of multimode waveguides, but a simplified model in the sense of ray optics has been chosen for this purpose. The paper once more reviews this model; afterwards problems of pulse distortions in both loss-free and lossy waveguides are dealt with in the formalism of this modelIn all considerations special emphasis is laid on analytical expressions and relations which suggest more universal interconnections than computer solutions. The theoretical part is followed by a description of experiments. There is good agreement between the experimental results and theoretical solutions obtained.

Geometrical theory of energy launching and pulse distortion in dielectric optical waveguides

“…It is convenient in this case to make use of the simpler geometrical theory, which is a very good approximation of the modal one; in the geometrical analysis we take into account not only rays that cross the fibre axis (meridional rays) [1,2], but also those that, along their path, never meet the axis (skew rays). Part of these rays, as it will be shown in the following section, may be guided by the fibre also for angles with its axis larger than the critical angle for meridional rays.…”