Light signals in an optic fiber bounce off the walls (repeatedly, frequently) of the cable they're in -- the actual physical path the light is taking in this case is approx. 1.5x the path of the cable itself.
AT&T was going to build out a cross country, single mode, circular waveguide network in the 60’s, but was obsoleted due to fiber. Maybe that’s what the HFT ought to due, or maybe air-line coax.
By the way, the uncertainty principle doesn't say we can't have a narrow pulse. Frequency versus time can be used as an analogy to help explain the uncertainty principle. If we know a precise frequency, the signal is poorly localized in the time domain. If a signal is narrowly localized in the time domain, it is poorly localized in the frequency domain. The uncertainty principle is something else, though: it revolves around not being able to make certain related measurements simultaneously, at the quantum level. We can be quite certain about the frequency content of a transient pulse and its exact temporal shape, if it's not something from the quantum domain.