Temperature and fiber|
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Posted by Michelot on January 28, 2009, 9:40 am
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(1) Would you have please a real value for a temperature parameter of a specific fiber? I took a coefficient a 30 ps/km/K. That is what I found. (2) I am not sure that we have to write "ps/km/K" or "ps.km.K". Thanks for your opinion. Best regards, Michelot | ||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Phil Hobbs on January 31, 2009, 5:39 pm
Please log in for more thread options That's just the temperature coefficient of optical path length in quartz, about 10**-5/K. (Almost all types of glass are near that value--TC_opl = dn/dT + n*CTE--quartz has a high dn/dT but low CTE, whereas other glasses have lower dn/dT and higher CTE.) Most of the time, if you have a fibre application for which that matters, you'll have a much worse time with etalon fringes and scatter than with temperature coefficient. Cheers, Phil Hobbs | ||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Michelot on February 1, 2009, 3:30 pm
Please log in for more thread options Bonsoir Phil,
Thanks for this interesting reply. > That's just the temperature coefficient of optical path length in
> quartz, about 10**-5/K. (Almost all types of glass are near that > value--TC_opl =3D dn/dT + n*CTE--quartz has a high dn/dT but low CTE, > whereas other glasses have lower dn/dT and higher CTE.) =A0Most of the > time, if you have a fibre application for which that matters, you'll > have a much worse time with etalon fringes and scatter than with > temperature coefficient. This view is a little new for me, let me try this reasoning to see if I understand correctly. I wished to consider the coefficient a 30 ps/km/K. If we can suppose that dn is relatively less than dL, the speed is yet c =3D 3 x 10e8 m/s. So the 30 ps are given by dL =3D 3 x 10e8 x 30 x 10e-12 =3D 9 x 10e-3 m. With L =3D 1 km, the coefficient 1/L x dL/dT is 9 x 10e-6, around 10 ppm/ K. Is it globally correct, or can we improve that? Best regards, Michelot | ||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Phil Hobbs on February 1, 2009, 8:56 pm
Please log in for more thread options Michelot wrote:
> Bonsoir Phil,
> > Thanks for this interesting reply. > >> That's just the temperature coefficient of optical path length in
>> quartz, about 10**-5/K. (Almost all types of glass are near that >> value--TC_opl = dn/dT + n*CTE--quartz has a high dn/dT but low CTE, >> whereas other glasses have lower dn/dT and higher CTE.) Most of the >> time, if you have a fibre application for which that matters, you'll >> have a much worse time with etalon fringes and scatter than with >> temperature coefficient. >
Every kind of common glass I know about has a TC_opl right around there.
> This view is a little new for me, let me try this reasoning to see if > I understand correctly. > > I wished to consider the coefficient a 30 ps/km/K. > > If we can suppose that dn is relatively less than dL, the speed is yet > c = 3 x 10e8 m/s. So the 30 ps are given by dL = 3 x 10e8 x 30 x > 10e-12 = 9 x 10e-3 m. > > With L = 1 km, the coefficient 1/L x dL/dT is 9 x 10e-6, around 10 ppm/ > K. > > Is it globally correct, or can we improve that? > Best regards, > Michelot > > Some are a bit more, some a bit less. There are some near zero TCN (i.e. dn/dT) glasses sold for special uses, e.g. Nd:glass lasers, but nothing that is commonly made into fibre. You might be able to get a special run. If you're trying to preserve timing coherence across some big gizmo e.g. a laser fusion system, and really need that low tempco because there's no way to control the temperature everywhere, this might be fine. If you're building a fibre interferometer or other sensor, you also have to worry about facet reflections and the piezooptic (stress-optic) effects e.g. bend birefringence. Cheers, Phil Hobbs | ||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Michelot on February 2, 2009, 5:19 am
Please log in for more thread options Bonjour Phil,
> Every kind of common glass I know about has a TC_opl right around there.
> =A0 Some are a bit more, some a bit less. =A0There are some near zero TCN > (i.e. dn/dT) glasses sold for special uses, e.g. Nd:glass lasers, but > nothing that is commonly made into fibre. Thanks for that valuable information. >=A0You might be able to get a
> special run. =A0If you're trying to preserve timing coherence across some > big gizmo e.g. a laser fusion system, and really need that low tempco > because there's no way to control the temperature everywhere, this might > be fine. My study is more ordinary. I just wanted illustrate the impact of diurnal and nocturnal temperature on fibres, in relation to the wander phase noise (slow variations < 10 Hz) With a delay of 30 ps. km-1.K-1 (even 40 or 50 seems more common), 1000 km on fibres, 20 C of variation, we have a wander of 0,6 to 1 ns. It's not negligible when the specification can recommand around 5 ns during a day. Such a wander can only be fitered (at the electrical level) by a dual memory, with a write timing different on the read timing. >>> (2) I am not sure that we have to write "ps/km/K" or "ps.km.K".
You're right Michelot to be not satisfy in what you read. The good unity is rather "ps.km-1.K-1" or, "ps/(km.K)" or "(ps/km)K-1". Best regards, Michelot | ||||||||||||||||||||||||||||||||||||||||||||||
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>
> (1) Would you have please a real value for a temperature parameter of
> a specific fiber?
>
> I took a coefficient a 30 ps/km/K. That is what I found.
>
>
> (2) I am not sure that we have to write "ps/km/K" or "ps.km.K". Thanks
> for your opinion.
>
> Best regards,
> Michelot