[time-nuts] 10 MHz over optical fiber?

[w3kl at w3kl.com]

All:  Sorry for jumping in abruptly on this ongoing thread.  I have been meaning to chime in ever since the discussion got started.
 
I apologize in advance if I am dragging up an issue that has already been discussed and put to rest.
 
That being said, have you given consideration to the effects of chromatic dispersion in the fiber?
 
A good, telcom grade, dispersion shifted (designed for the 1550nm wavelength band, where absorption of the light by water is small), single mode fiber has a chromatic dispersion of approximately 1 to 10 ps/nm/km.  The concern therefore is changes in timing induced by changing wavelength content of the laser or LED used to drive the fiber - for example, due to temperature changes, etc.
 
If you've already considered this, then please ignore this.  Otherwise, it would be interesting to work through a "what if"?
 
jeff

Jeffrey K. Okamitsu, PhD, MBA
+1-609-638-5402

--- On Wed, 11/26/08, Paul Boven <p.boven at xs4all.nl> wrote:

From: Paul Boven <p.boven at xs4all.nl>
Subject: Re: [time-nuts] 10 MHz over optical fiber?
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Date: Wednesday, November 26, 2008, 12:02 PM

Hi Didier, everyone,

Didier wrote:
> By the time you get the signal from the maser through 34 km of fiber
optic,
> what you get at the end might be not better than a decent GPSDO...

That's exactly what I'm trying to work out before I'll actually
start
building something like that. The JPL device (thanks for the hint, James
P. Lux) is very interesting, see:
http://tycho.usno.navy.mil/ptti/1988/Vol%2020_02.pdf

The JPL system aims to achieve better than 1E17 in 1000s, whereas the
H-maser in their publication is quoted as 1E15 in the same averaging
time. They also state that: 'when stabilities higher than 1E15 are
required the link must be actively stabilized'. So, in a nutshell - If I
were to build such a system, the first incarnation would be without such
a compensation system.
If such a compensation does turn out to be necessary I'd probably not
use a 50% mirror but simply loop the received data back to the
trasmitter of the SFP at the receiving end, and use a fiber pair instead
of a single fiber.

Regards, Paul Boven.

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