Special Fibers and Attenuation Feb. 20, 2018
+
Chromatic dispersion away from
+
Chromatic dispersion near
+
:
use optical elements (e.g. dispersion compensated fiber or fiber
grating) to negate the effect of
+
:
the single mode is composed of 2 orthogonal polarizations.
PMD introduces random delays for pulses in different polarizations.
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Fiber Attenuation: nonuniform refractive index
+
Transmission windows:
3rd window (C band)
around
+
Definition of
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Power budgeting:
+
Cut-off wavelength
+
Birefringence:
Birefringence in fiber causes polarization evoluting along the fiber.
A complete cycle is reached at the beat length
:
where
is the zero dispersion slope, i.e.
or
(
) at
.
:
E.g. dispersion shift fibers, we need to consider high order dispersion
(i.e. 3rd term in Taylor series or
term) and use
.
so that on average
chromatic dispersion is zero and even the dispersion slope can be
reduced (see Sec 5.7.3 pp. 314-319 2nd Ed, pp. 320-325 3rd Ed.)
(
)
A slight difference in delay for high speed (>2.5Gb/s) channel widens pulses.
The dispersion parameter . If the delay is
too larger than 10% of a bit period, power in the slow polarization is
considered lost.
(see Sec 5.7.4 pp. 320-323 2nd Ed., pp. 325-328 3rd Ed.)
Rayleigh
scattering, attenuation coef.
.
Imperfection of fiber (e.g. non-circular), .
Absorption from impurities, .
has the lowest absorption (
)
1st window around 800 nm.
2nd window (S band) around .
4th window (L band) > .
5th window (pending on removing impurities) between 1.55 and 1.3 .
:
In terms of power,
.
In terms of field, .
In terms of decibel (dB), .
Conversion - and
.
Also .
Note - is degradation factors from components in dB,
e.g. 1dB loss in connector means 79.4% of power surviving after the connector.
:
In theory for step index fiber,
we can obtain
from
which means
2nd and higher order mode occurring if
.
In experiment, the single (fundamental) mode decreases to less than 0.1
dB
while the 2nd mode attenuated by 19.3dB.
with
.
Application: