Microbending loss of multimode square-law fibres: a ray theory

Using a simple ray theory, we show that the steady-state microbending loss of multimode square-law fibres is equal to 8¿/¿ dB/m. We have defined ¿ = ¿n/n, and ¿ is the power-spectral density of the fibre axis curvature in any meridional plane at the natural frequency of ray oscillation, expressed in reciprocal metres. The steady-state irradiance distribution in the fibre core is also given.