Lecture 22

Lecture 22 - Lecture 22 Lecture 22 Wave picture We can...

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Lecture 22 1 © Jeffrey Bokor, 2000, all rights reserved Lecture 22 Wave picture We can capture much of the wave physics of fibers and waveguides by considering a wave guide with perfectly reflecting walls. The boundary condition on the wall is that the electric field must be zero. The wave equation solutions are then cosines in the transverse direction with an integer number of half-wavelengths between the walls. Different “modes” correspond to varying numbers of these half- wavelengths. From the above diagram, we can see the correspondence between the various transverse modes and the propagation angle for the corresponding rays. This can be expressed as Low order modes propagate at shallower angles than higher order modes. The cutoff angle imposed by c then imposes a mode cutoff. Mode numbers below the cutoff will propagate with low loss, while higher order modes are lost. For reasons we will discuss in a moment, it is often desirable to design the guide such that only the very lowest order mode will propagate, and all higher order modes will be lost. The condition for the second mode (m = 1) to be lost is that The condition for single mode operation is then Take
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Lecture 22 - Lecture 22 Lecture 22 Wave picture We can...

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