Getting Light Where You Want It

We are all familiar with changing the direction of light using a mirror. Although a mirror looks at first like a very efficient way to change the path of light it is not efficient enough if the light direction must change many times over some distance. Imagine building a version of a periscope where the light has to change direction 100 times. The reflection at the surface must be very high to obtain useful light transmission from one end to the other. Take an example of a typical mirror with a rear surface mirror coating. The loss at the glass-air interface is about 5% and the reflection from the glass-mirror surface is 95%. This means that about 10% of the light is lost at each mirror, therefore there would be only 90% of the light reflected. Only 0.5% of the light would remain after 50 bounces; the rest would have been lost along the way. 

We can also get reflection under certain conditions from a glass-air or glass-glass interface. These losses can be lower than 0.1% per reflection. For the case where each reflection gives 99% of the light back, there will be about 37% left after 100 reflections. That’s a little improvement. None of these losses, however, included the effects of any light lost due to the light beam spreading and not hitting the next surface. We could use a hollow glass tube or capillary to prevent this but we still need better reflection. Under the right conditions a glass rod with a different optical material surrounding it can be produced in a long fiber with a much higher reflection per bounce. 

With today’s optical fiber and thousands of reflections it is now quite common to have an average power loss of less than 5dB per kilometer. This equates to about 32% of the input power remaining at the output end, after 1km. Astonishingly better transmission than a thousand very good mirrors or 10 sheets of window glass.

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