Crosstalk in Multiple-Beam Waveguides

Arranging many optical channels spatially resolved in the same waveguide is a simple means for high capacity transmission.1,2 All channels can be modulated in the same frequency band as long as the crosstalk is kept below a certain threshold. One source of crosstalk is the inevitable scattering from the focusing and directing elements.3 Diffraction from the ideal beams is negligible.4 Y e t we shall see that diffraction must be considered once the beams are distorted by the focusers. In all likelihood, these focusers would consist of mirrors because lenses of the size needed are apt to have imperfections within the material. The scattering characteristics of high quality front surface mirrors and lenses of the best quality have been measured recently.5,a A comparison shows that the lens scattering was about one order of magnitude larger. Directional changes can easily be accomplished by using periscopic mirror arrangements of the kind shown in Fig. I. 7 Neglecting aberrations, we consider only the first order focusing effect of these periscopes, which is that of thin convex lenses. Two methods of multiple channel transmission have been suggested.2 We discuss these two basic methods with respect to their susceptibility 55