[time-nuts] 1 pps Accuracy in two locations

[Magnus Danielson]

Hi Bruce,

All those numbers is typical of 1310 nm laser, as there is a dispersion
zero in the 1310 nm window, but the downside is significant damping. In
the 1550 nm window you have lower damping, so you can run a factor of 10
times longer roughly with the same power. 1310 is only used for short-haul.

Now, you can use dispersion compensation fibre if needed.

Also, one should not forget that lasers themselves is far from
temperature stable, so as they drift, they probe the dispersion curve on
a different spot and that makes delay shift.

All this is covered in classical literature.

Two-way time-transfer is able to track out most of these shifts, but
calibrating the remaining offset is where the limit is.

Cheers,
Magnus

On 2020-01-16 12:08, Bruce Griffiths wrote:
> Except for some low tempco single mode fibers the delay tempco is on the order of 10ppm/K:
> https://library.nrao.edu/public/memos/edtn/EDTN_168.pdf
>
> Bruce 
>> On 16 January 2020 at 23:29 Magnus Danielson <magnus at rubidium.se> wrote:
>>
>>
>> Hi,
>>
>> On 2020-01-15 23:34, Attila Kinali wrote:
>>> On Wed, 4 Dec 2019 09:40:34 -0000
>>> <martyn at ptsyst.com> wrote:
>>>
>>>> I'm always being asked to provide equipment that can produce two 1 pps
>>>> outputs aligned to each other to within a few ps.
>>>>
>>>> These two 1 pps pulses are not in the same location and could be 100 metres
>>>> to a few km away.
>>> As others have written, getting down to a few ps is not feasible, at least
>>> not with the amount of money your customers are likely willing to pay.
>>> To get down to these levels you will need to pull fibres from one location
>>> to another and using special circuitry to activly compensate variation
>>> in length due to temperature changes and vibration, even for burried fibres.
>>> Just to put into perspective what your customers are asking for: in 1ps
>>> light travels 300µm in vacuum/air or ~150µm in fibre/coax.
>> Let me correct that a little.
>>
>> For fibre the relative dielectrics of the silica glass is just about
>> 2.25 giving the index just about 1.5, which then gives the 300 um / 1.5
>> to about 200 um. I am known to indicate the length of 1 ns in fibre
>> betwen my index finger and thumb, roughly 2 dm, giving the delay for 1 m
>> to be about 5 ns, letting the round-trip-time for 1 m be 10 ns which is
>> a very handy number for rule of thumb conversions for fibre. If you look
>> in more detail, the actual property depend on the wavelength being used
>> and the temperature of the fibre, as this changes the actual delay.
>> While first degree compensation is trivial in two-way systems, you end
>> up having calibration issues.
>>
>> Coax is less easy. If you have the normal RG58 crap, it aligns to about
>> the same numbers as fiber, as the dielectrics is about the same.
>> However, for more phase-stable cables with lower dielectric loss one
>> simply has less dielectrics to start with, such as foam or other form of
>> support for center conductor. That gives the relative dielectric go
>> towards 1 and thus the velocity factor with that. It's much more a "it
>> depends".
>>
>> Other than that, I agree with the general analysis of Attila, it is
>> close to my experience, and I've been working on these things
>> commercially for over 10 years now. If you want to know how things works
>> (or rather not work) in a telecom, it is even more painful than this.
>>
>> So end conclusion being, if you required precision of 1 ps from a timing
>> system, you are likely going to have one very expensive system and it
>> will be a pain to operate, it may be worth considering if you are doing
>> it the right way. I've seen requirements in the 10s of ps for a fixed
>> system setup, but that is while challenging kind of doable, but then
>> that requires quite a bit of additional control loops and knowing what
>> one does.
>>
>> Cheers,
>> Magnus
>>
>>
>>
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