[Physics] Fiber Optic Conveyor

[Doug Marett]

Hi Olivier, (resending, as the last message was too big)

   I am not an expert on this, but from my understanding, if the fiber
optic cable has a very wide multimode core, then the propagation might be
considered to be by internal reflection. However, with the single mode
fiber, it it going to be far more like a waveguide, and the light will be
largely constrained to travel in a straight line. So the dominating factor
on speed will be the refractive index of the cable. I am not too familiar
with the air core fiber, but the claim from the link I sent you was that
the speed was very close to the vacuum speed, so I presume they managed to
get the air core fiber to act as a waveguide so that the speed depends
mostly on the refractive index of air. The core looks from the image like
it is about 20um across, which is wider than the normal IR single mode
core, but then again, the wavelength on the light in air is going to be
longer, so maybe that has something to do with it.
   To answer your other question about the speed of light in the fiber
optic cable, for air core I only know the example like I sent you. However,
for the glass single mode fiber (like smf-28) I can actually give you a
real answer to that question. See below, I threw together a quick setup as
shown in the picture below:[image: Inline image 2]

I did a simple test, I set up a fiber optic laser, which split a beam in
two at a coupler, and one beam went 1 meter to a detector (PD1), the other
went 1001 meters to PD2. So the path difference is 1000m. I then measured
the propagation delay using triggered capture on an oscilloscope, and after
turning the laser ON, measured the difference in the arrival time of the
pulses. The "turn on" had a lot of spikes in laser intensity, which were
useful in making the comparison. As shown in the inset of the oscilloscope
trace, the average difference in the rise/fall of the laser signal between
the two photodiodes was ~ 5uS. So doing a simple calculation, this would be
1000/5E-6 = 2E8 m/s, or approximately C/1.5 (the refractive index is
actually 1.4676 from the spec sheet).
So there is your very approximate answer - about 2E8 m/s in single mode
SMF-28 fiber.

Doug
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