Optical Fiber Communications 4th Edition by Gerd Keiser

Exercise 10

Assuming that Rayleight scattering is approxi­mately isotropic (uniform in all directions), show that the fraction S of scattered light trapped in a multimode fiber in the backward direction is given by
where NA is the fiber numerical aperture, n is the core refractive index, and NA/ n represents the half-angle of the cone of captured rays. If NA = 0.20 and n = 1.50, what fraction of the scattered light is recaptured by the fiber in the reverse direction.