[time-nuts] time-nuts Digest, Vol 86, Issue 89

[Colby Gutierrez-Kraybill]

> Message: 5
> Date: Sun, 25 Sep 2011 15:26:50 -0700
> From: Hal Murray <hmurray at megapathdsl.net>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] Fast than light neutrino
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> 	<20110925222650.50624800037 at ip-64-139-1-69.sjc.megapath.net>
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> 
> javier.serrano.pareja at gmail.com said:
>>> A fiber-based time-transfer would be nice complementary as it would provide
>>> an independent timing path.
>> Any ideas on how to proceed? This is unknown territory for me. 
> 
> You can get a lot of good ideas from the radio astronomers.  It's been 
> discussed here in the past, but I don't know what terms to use when searching 
> the archives.  I think it was mostly pointers to their papers.  They were 
> interested is much shorter distances.  I think it was 10-20 km.
> 
> 
> The idea is to send a signal in both directions over the same fiber.  If it's 
> the same fiber, the transit times are likely to be the same in both 
> directions.  If you send a pulse out and back, you can assume the time the 
> pulse arrived at the far end was half the round trip time after it left the 
> start.
> 


One method that has worked for millimeter interferometry over the scale of 1-2km is to encode a signal that is 50Hz shorter than a local 10MHz reference (locked to a good standard, Rb usually).  That signal is reflected back on the same fiber (off of the interface at the other end in a very weak signal that is read back, to record the phase, which is then  subtracted from the local 10MHz.  This can allow for femtosecond level accuracy when comparing the phases and sub-millimeter measurement of the fiber itself.  This is done on a separate fiber that is in the same bundle carrying signal.  Losses over a much longer run would be problematic.

Unfortunately, this method would not work well if the fiber is not contiguous.  And, I'm not sure what the erbium doped repeaters do to the phase of the signal passing through them.

- Colby