The mean-square fluctuations of log amplitude and phase are studied theoretically for guided beam modes propagating in an asymmetric slab waveguide having random inhomogeneities of the refractive index. The study is based on a method of the Rytov approximation, which has been mainly employed in analyzing the beam fluctuations in an atmosphere. The results show that the mean-square fluctuations of log amplitude and phase increase proportionally to the propagation distance for z/λ > 103, z and λ being the actual propagation distance and the wavelength of light, whereas the log-amplitude fluctuations increase obeying a square law of the distance for z/λ < 102. It is also found that an abrupt reduction of these fluctuations is observed near the center of the slab waveguide where the irradiance of the beam field has a maximum.
© 1976 Optical Society of America
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