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Lecture9.

# Lecture9. - Let's go back to the reflection and refraction...

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Lecture 9 7.7 TIR cont. 7.8 Metals 8 Geometric Optics The evanescent wave Without a transmitted wave we have problem The boundary condition cannot be fulfilled We need a transmitted wave which does not carry energy across the boundary (on average) E t = E ot e " # y e i k t x n i n t sin \$ i " % t & ( ( ) * + + " = k t # n i n t \$ % & ( ) 2 sin * i + 1 Let’s go back to the reflection and refraction problem What happens if a the gap is very small such that the eveanescent wave reaches another medium ? Frustrated total internal refection Prism Coupler As we have seen in both descriptions for the waveguide only certain modes will be guided effectively The modes have different wave vectors in x-direction If we want to couple light into a waveguide using the evanescent wave only certain angle of incidence will work

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Coupling light into a waveguide (light ray picture) Waveguide (fiber) Can we couple light directly from the top ? No, because effective incoupling can only be achieved for directions which result in total internal reflection in the fiber. This means the angle in air is larger than 90 o ! p ! n n p sin " p = n f sin " n n p n f k x = " = k f n f sin # n Constructive interference " = k f n p sin # p We adjust the angle !
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Lecture9. - Let's go back to the reflection and refraction...

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