[Physics] Fiber Optic Conveyor

[Doug Marett]

Hi Olivier,

    The Physics server keeps scrubbing the images that I attach, so here is
a link to the image of the fiber optic cable speed of light test if it did
not come through.

http://www.conspiracyoflight.com/misc/light%20speed%20FO%20cable

Doug

On Sun, Nov 13, 2016 at 10:35 PM, Doug Marett <dm88dm at gmail.com> wrote:

> 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
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.tuks.nl/pipermail/physics/attachments/20161114/b7ac1e58/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: light speed FO cable.jpg
Type: image/jpeg
Size: 139181 bytes
Desc: not available
URL: <http://mail.tuks.nl/pipermail/physics/attachments/20161114/b7ac1e58/attachment.jpg>