All-Optical Switching Tutorial, Part 2

Frustrated total internal reflection (FTIR) is a rather clever technique that can be used to create simple 1x2 switches.

Different media can have different refractive indices – a good example of this is glass (RI ~1.5) vs. air (RI ~1). At the boundaries between these media, a beam of light will be bent, or refracted, through an angle that depends on the ratio of the refractive indices. If the light hits the boundary at too steep an angle, then it will bounce back into the same medium, experiencing total internal reflection. This is what you see happening in the upper left diagram, where the beam of light hits a glass-air boundary.

If the two prisms are forced together, as shown in the lower right-hand diagram, then the boundary disappears and the light is not reflected.

So FTIR could be used for 1x2 switches and could be built in a cascade to scale up to more ports. It’s a wideband technique; you can use it to switch all the channels on a single fiber at the same time, and so it's good as a protection switch.

The 25ms latency is on the high side for protection switching (the goal is complete service restoration in less than 60ms).

Further reading:

  • News Analysis: Optical Switch Comes Into the Light

    Next Page: Gratings and Circulators

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