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I was reading through a research paper and they showed the following schematic:

enter image description here

I am not exactly sure what is meant by a 45 degree coupler so I looked online but there is no such thing as a 45 degree coupler. So I was wondering if a 45 degree coupler in this context is just a fibre optic cable that has been cut at a 45 degree angle?

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enter image description here

Figure 1. A 45° cut on the fibre end will result in total internal reflection and a view at 90° to the axis of the fibre. (Image original.)

Total internal reflection occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle. For this application the critical angle would have to be < 45°.

Link: Fibre optic surface plasmon resonance sensor system designed for smartphones by Kort Bremer and Bernhard Roth.

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  • $\begingroup$ Alternatively, one could metallize the 45-degree surface to guarantee reflection. $\endgroup$ – Carl Witthoft May 4 at 11:54

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