[time-nuts] 10 MHz over optical fiber?

w3kl at w3kl.com w3kl at w3kl.com
Wed Nov 26 14:49:35 EST 2008


The biggest concern with temperature effects in the fiber would be thermal expansion.  Potentially, as well, changes in waveguide shape, which could add more waveguide dispersion as well as potential polarization mode dispersion.
 
Jeff

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

--- On Wed, 11/26/08, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
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, 2:13 PM

w3kl at w3kl.com wrote:
> 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
>
>   
Without compensation, the delay tempco of 34 km of fiber will be
somewhat more problematic unless the temperature of the entire run is
constant and/or expensive low delay tempco fiber is used.

Bruce

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