[time-nuts] Optical transfer of time and frequency

[Michael Wouters]

On Sat, Apr 30, 2016 at 6:14 AM, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:
> Well, giving the conditions mentioned, doing ranging codes such as those
> used by GPS is very easy and cheap. Doing this in bidirectional isn't too
> hard. Doing a suitably high chip-rate should cost very little.

I've done two-way time-transfer over optical fibre using exactly this
technique. The TDEV is about 1 ps for tau>1s. Not so cheap, about 25K
euro per node (20K signal processing - NI FPGA, 2K laser and power
supplies, 1K detector, 1K RF electronics) in my setup, but that cost
could be greatly reduced since a $100 OEM FPGA could do the signal
processing (I've already done work on this but currently looking for
motivation to finish it off) and a simple, intensity-modulated laser
would probably be fine. A 2K euro budget would be a challenge though.

> The two-way time-transfer is relatively easy, but you will need to do some
> calibration to get the precision needed.
>

At first glance, I would think that you should be able to define the
optical RX/TX path to within 10 cm without any trouble and that gives
you 300 ps accuracy. Even on fibre links, I don't think anyone would
claim an accuracy of better than a few hundred ps.

Cheers
Michael