Eavesdropping on an optical fiber using evanescent coupling
Channel: Nils Berglund
Date: 2024-01-11

This simulation illustrates how one might eavesdrop on a message transmitted through an optical fiber, using evanescent coupling. A message is transmitted in Morse code through the top fiber. The boxes at the top left and top right show respectively the input signal, and the wave height measured at the other end of the fiber. The bottom fiber is put close to the top one on a certain distance. Evanescent waves that leak from the top fiber are picked up by the bottom fiber, and the measured wave height at the end is displayed in the bottom right box. 
The basic principle of wave guides or optical fibers is total reflection. Light in a medium with a higher index of refraction can be completely reflected on the boundary to a medium with a lower index of refraction, if it propagates at a sufficiently small angle with respect to the interface. Here the index of refraction of the fiber is 2.75, which is quite large compared to what is done in real life. In fact, the fiber has been coated with a layer of intermediate index of refraction, which tends to decrease leakage through the fiber. 
In addition, the medium outside the fiber has been made dissipating, so that the light leaving the fiber does not interfere too much. Four regions outside the fibers in the four corners have been made completely reflecting. This is in order to avoid too much interference due to light escaping the top fiber near the oscillating source at the left side. 
The color hue shows the wave height. There are absorbing boundary conditions. 
The transmission quality through the fibers is not very good, which may me due to the relatively strong bends they contain. While the signal transmitted through the top fiber can still be recognized, it is hard to make the difference between dots and dashes in the leaked transmission. However, in real applications, one would rather use binary code. In addition, owing to the high propagation speed of light, one can use much longer signals for the dots and dashes in case Morse code is used. 

Render time: 1 hour 45 minutes
Compression: crf 23
Color scheme: Twilight by Bastian Bechtold
https://github.com/bastibe/twilight

Music: "Nimbus" by Eveningland@EveninglandMusic

See also https://images.math.cnrs.fr/Des-ondes-dans-mon-billard-partie-I.html for more explanations (in French) on  a few previous simulations of wave equations. 

The simulation solves the wave equation by discretization. The algorithm is adapted from the paper https://hplgit.github.io/fdm-book/doc/pub/wave/pdf/wave-4print.pdf 
C code: https://github.com/nilsberglund-orleans/YouTube-simulations
https://www.idpoisson.fr/berglund/software.html
Many thanks to Marco Mancini and Julian Kauth for helping me to accelerate my code!

#wave_equation #fiberoptics #evanescence