[time-nuts] Smart fiber-optic cable ( a reference to Hp's smartclock )

[Bob Camp]

Hi

The transit time of the system is made up (more or less) of:

Forward:

1) electro to optical (likely an LED is involved)
2) transit over the fiber
3) optical to electro (probably a photo diode is involved)

4) electro to optical (likely a different LED is involved)
5) transit over the same fiber in the opposite direction
6) optical to electro (likely a different photo diode)


For a short link, the variation in 1,3,4 and 6 are often significant compared to 2 and 5. In a bi-directional transit, there is no need to model 2 or 5. Since it's the same fiber, it's got the same delay. Any limitation comes from the fact that you have different "stuff" in 1,3, 4 and 6. 

Yes there are other ways to do this. All that I know cancel out the fiber delay. If you want to go a bit crazy, you can use multiple pieces of fiber of matched length and path. For some things having two sets of data can help you out. 

Bob
 
On Feb 4, 2013, at 6:22 PM, Stanley <timenuts at n4iqt.com> wrote:

> My thinking is the temperature has the largest effect on change in delay of the cable and more data points would better to predict this change. The round trip change in delay would collate with a average of the temperature along the cable but not necessary as accurate, think cable buried at different depths under asphalt and grass. It is just noise if we can not correct for it.
> 
> Stanley
> 
> 
> ----- Original Message ----- From: "Bob Camp" <lists at rtty.us>
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Monday, February 04, 2013 4:39 PM
> Subject: Re: [time-nuts] Smart fiber-optic cable ( a reference to Hp's smartclock )
> 
> 
>> Hi
>> 
>> Consider that cost to manufacture the cable goes up as you put stuff in it. You not only need sensor packages, you also need to connect them so they can report data. Unless the sensors are optically powered and linked, they would compromise the inherent lighting immunity the fiber provides.
>> 
>> It's a *lot* easier to do the same thing with a round trip traverse of the entire optical link. This is already done by at least one supplier of these parts. It may be done by several others as well.
>> 
>> Cable delay is not the limiting issue on most short link systems. It's not clear that data rates would be improved by delay modeling.
>> 
>> Bob
>> 
>> 
>> On Feb 4, 2013, at 5:09 PM, Stanley <timenuts at n4iqt.com> wrote:
>> 
>>> If a fiber-optic cable had temperature sensors either installed with or embedded inside of this could make for better modeling changes in delay making more accurate transfer of time and frequency possible. With fiber to tower installs now under way to provide more data at cell towers why not backup GPS frequency and time transfer with the same medium ? Would this also increase the data rate of the cable ? That is faster rates due to the better timing uncertainty.
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