Item — HN Reader

That is true. One of the fun things illustrating first-hand how microwave/millimeter wave and fiber are the same thing is if you ever get to handle a 120 GHz band waveguide/feed. Some radar and military stuff operates in the 120 band.

It's so incredibly tiny and narrow compared to a 11 or 18 GHz microwave waveguide. Then imagine continuing to make it narrower while increasing the frequency, follow that to its logical conclusion and you have imagined a 9/125 fiber.

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But it's something of an apparently completely different... phenotype from a small waveguide.

The seemingly arbitrary 300Ghz line was probably more cultural than essential; the instrumentation was just very different. What's the analog for a prism for microwave?

Apparently a block of plastics like Delrin works:

http://www.physics.rutgers.edu/ugrad/205/manuals/micro.pdf

The key part in physics is it's still a wave... And a singlemode fiber is a type of waveguide. A 120GHz signal is a wave, a 171 THz signal is a wave, the latter just has a much shorter wavelength.

300Ghz ends up with a wavelength of 1mm, so it more follows the pattern of bandwidth assignment, which likes nice round numbers given in wavelength.