5 ms·

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 ca

by consz 8y ago

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.

kazinator 8y ago
I think the distance is less of an issue than the temporal smear that the bouncing causes. A clean square pulse turns into a blob.
- hyeonwho4 8y ago
  Temporal smear is caused by the varying refeactive index of glass with wavelength and the fact that a short time pulse must have finite (large) spectral width (by the uncertainty principle), not by the bouncing.
  - kazinator 8y ago
    So, the different path lengths implied by internal reflections don't contribute anything to the smear? 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.
hyeonwho4 8y ago
That's not true. The typical total reflection angle in single mode fiber optic cable is less than 4 degrees. The refractice index of glass, however, is about 1.45. So almost all the slowdown is due to the glass directly slowing down the signal.
- madengr 8y ago
  Though you can still get a group delay (slower than free space) in vacuum filled RF waveguide. When you break down the single mode, it appears to be zig-zagging through the waveguide (bouncing off the walls), and slows down as cutoff is approached. Like in fiber, the modal dispersion is separate from the material dispersion. 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 consz 8y ago

kazinator 8y ago

I think the distance is less of an issue than the temporal smear that the bouncing causes. A clean square pulse turns into a blob.

hyeonwho4 8y ago
Temporal smear is caused by the varying refeactive index of glass with wavelength and the fact that a short time pulse must have finite (large) spectral width (by the uncertainty principle), not by the bouncing.
- kazinator 8y ago
  So, the different path lengths implied by internal reflections don't contribute anything to the smear? 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.

hyeonwho4 8y ago

That's not true. The typical total reflection angle in single mode fiber optic cable is less than 4 degrees. The refractice index of glass, however, is about 1.45. So almost all the slowdown is due to the glass directly slowing down the signal.

madengr 8y ago
Though you can still get a group delay (slower than free space) in vacuum filled RF waveguide. When you break down the single mode, it appears to be zig-zagging through the waveguide (bouncing off the walls), and slows down as cutoff is approached. Like in fiber, the modal dispersion is separate from the material dispersion. 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.