[time-nuts] 10 MHz over optical fiber?

[Magnus Danielson]

Tom Van Baak skrev:
>> Yes. I know of several commecial systems. If you only need to do a short 
>> jump, then using fairly basic E/O-O/E equipment should work well 
>> enought. It all depends if you want/can to roll your own or need to buy 
>> a finished product (aka "buy this, and you will be fine!").
> 
> Magnus, what's the typical noise floor, tempco or drift of cheap
> (i.e., non JPL-level) fiber distribution systems like this? Is it less
> than regular coax, or phase stabilized heliax? At 100 m lengths?

The temperature stability is fairly good, but what Bruce posting didn't 
expose is the temperature stability of the laser. The problem is that 
the laser changes frequency (not wavelength as the optics people say, as 
that depends on the refraction index of the material) with temperature. 
This is a fairly dramatic effect over distances. The fiber delay changes 
by three factors: thermal expansion of fiber (lengthwise), change of 
wave-solution as dielectric and physical expansion occurs, and 
wavelength of light. The lasers for WDM networks typically has a peltier 
cooler for stabilizing temperature.

Choice of fiber and laser is also important. Multi-mode fiber is not 
good at all since the dispersion provided by the many different 
wavepaths in the fiber will shift in a not so forgiving fashion. For 
shorter runs like 100 m and with not too high requirements it will be 
fine, but tune up your requirements and you want to go single-mode.

The laser-type is also important. Cheap lasers exhibit a huge number of 
frequencies and when looking at an optical spectrum analyzer it can be 
hard to decide which has most energy... there is typcially a shape but 
the spikes are so many and close that you can't say point out with 
certaintly which will be the strongest peak... it will shift. Also, the 
dispersion that is gained from the aggregate of those peaks is not nice. 
  WDM lasers like those for 1550 nm has a much cleaner spectrum which 
will gain you alot in the dispersion field.

Long distance communication is very dispersion-sensitive and both 
chromatic and non-chromatic dispersion is being considered. While 
inter-symbol interference as such is not that important to a pure sine 
distribution, the stability aspect of dispersion is.

If you want optimum performance for longer taus, there is really no 
option but to do two-way time transfer compensation. Typically the 
process is fairly slow on properly laid fiber. For a pure frequency 
transport, just looping the signal back to the source and do phase 
measure and phase-adjust so that the sent phase compensate the phase 
error will work. This way you can continuously compensate the variations 
out. Such systems is used for instance in DSN and similar systems with 
multiple arrays of antennas at microwave frequencies.

Anyway, I beleive I had the sum of errors become about 85 ps/nm/K/km. 
Could be wrong, it's just a number stuck in my head.

Does this effect occur in real life? Well, naturally I have an anecdote 
to share on that. In Sweden the national power company had put fibers on 
their high voltage cables running through the country. They needed it 
for their internal use, but could of course provide service to others, 
which they do. They have a SDH network on top of that and can then 
provide SDH based services, alongside WDM channels. However, about 7 in 
the Monday mornings in February their SDH network failed as the sync 
went totally bananas. They called in a synchronisation expert which I 
happend to know. He measured the wander and picked up a very nice 
deviation with a huge swing right there in the morning hours. What 
happend? Well, what happens 7 in the Monday mornings which is 
exceptional? That's when they start up all the heavy machinery in the 
industry, they increase heating since it is cold, people wake up and 
turn TVs on etc. etc. etc. Many of the things which have been off or 
turned down during the weekend is starter up. This makes the current go 
up, this makes the power cables go warmer (they will expand quite 
noticably!) and the fiber being pulled on the cable will change 
temperature. As the heater and fiber run over a considerable length 
together, many 100s of km, and the shift in temperature is several C 
then it is not strange that they experienced a huge shift in delay. 
Having heard my comments on this, the synchronisation expert pointed 
this out. They ended up to move the fibers to the relatively unloaded 
ground wire at the top, and have a much more quieter system.

This shows that uncompensated delay even in a good fiber system can eat 
you for lunch.

The depth by which you have buried your fibers is another issue. Deeper 
means higher stability in temperature, as mother wander (the sun) will 
contribute alot at tau = 43,2 ks.

For shorter runs, one way transfer can work well. The requirements on 
stability may be an issue if you have high requirements. If you do a 10 
m run, few would have a problem on an uncompensated system. Off the 
shelf components could probably satisfy most needs even for 100 m or so.

Interestingly enought, there is a Siemens patent for a fiber based 
oscillator. The short term stability of silica fiber is actually quite 
good. It is the long term aspects which can eat you out if you do not 
deal with it.

Consider also things like laser gyroscopes. The stability of the fiber 
is sufficient to detect even small shifts frequency due to Sagnac effect.

Cheers,
Magnus