High frequency traders do NOT want to be limited by the speed of light in a fiber - due to its refractive index (1.5 for ZBLAN according to Wikipedia) this is only 66% of the speed of a radio signal. Over 10km this amounts to an 11 microseconds difference - already significant for HFT.

This is why they have been leveling hills to carve a (mostly, with minimal amount of repeater towers) line of sight path between NYC and Chicago, gaining 3ms (1), or set up a line of sight laser link between NYSE and NASDAQ to eke out a handful of microseconds (2), or set up multiple links across the English channel to reduce latency between London, Amsterdam and Frankfurt (3).

1. https://www.nytimes.com/2014/04/14/opinion/krugman-three-exp...

2. https://www.extremetech.com/extreme/176551-new-laser-network...

3. https://sniperinmahwah.wordpress.com/2016/01/26/hft-in-the-b...