The amount of light power that is transmitted within a semi-infinite circular optical fiber when it is illuminated obliquely by a coherent beam of light is determined from an electromagnetic-theory analysis. The limit λ→0 is not classical geometric optics, i.e., not that found by tracing all rays along the fiber. Instead, the limit λ→0 corresponds to that of treating all rays as if they were meridional, i.e., as if they cross the fiber axis, ignoring rays skew to the axis. Thus, ray tracing is incorrect for fibers illuminated by coherent light. However, the acceptance property of an optical fiber illuminated by coherent light is very simply found from meridional ray tracing, if the dimensionless quantity
is much greater than unity, where ρ is the fiber radius, λ the wavelength of light in vacuum, and n1, n2 the refractive indices of the fiber and its surround, respectively.
© 1973 Optical Society of America
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