Abstract

If a monochromatic plane wave is incident on the outside of a multimode fibre under the conditions described in Ref. 1 a single fibre mode can be excited (2,3). This mode has some radiation coupling but, depending upon, the cladding thickness, may be either guided or leaky (4). The emergent mode structure may then be observed at the fibre end with a microscope (Fig. l). The observed mode will be a combination mode of the LP_γμ type (6) unless the angular resolution is very high. For such a mode there are 2γ cycles of intensity azimuthally and μ cycles radially. It is now possible to represent each IP mode by a point on a diagram of mode propagation angle versus γ, since the propagation angle ⊖ = cos-1 (β/kn_o) is the angle that the Poynting vector of the exciting plane wave makes with the fibre axis. The results of such an experiment on a Corning fibre with core and cladding diameters of approx.

© 1975 Optical Society of America