Design Parameters for Multimode Fluoride Fibers: Effects of Coating and Bending on Loss
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DESIGN PARAMETERS FOR MULTIMODE FLUORIDE FIBERS: COATING AND BENDING ON LOSS
EFFECTS OF
L. E. BUSSE AND I. D. AGGARWAL Naval Research Laboratory, Code 6505,
Washington, DC
20375-5000
ABSTRACT Numerical
calculations
of
losses
due
to
polymer
coatings
and
macrobending have been made for step index multimode fluoride fibers. avoid
such
losses,
fiber
core
and
cladding
diameters
and
To
numerical
aperture must be chosen carefully.
Introduction Fluoride
single mode optical fibers provide the potential
for very
long length communications networks with their predicted minimum loss of
0.01 dB/km at 2.5 pm.
Many fibers currently being made are step index
multimode,
to their relative ease
however,
due
of fabrication.
To the
authors' knowledge, there are no published criteria for parameters such as core and clad diameters and numerical aperture. In this paper it will be
shown
that
substantial
such
induced
parameters losses
due
must to
be
chosen
typical
carefully
polymer
to
avoid
coatings
and
macrobending.
Coating Losses The theory applied [1]
involves numerically calculating the induced
loss due to an outer jacket, core and cladding.
assuming no mode coupling and a lossless
The absorption loss of a typical polymer coating was
determined to be 107 dB/km at 2.5 pm. Table I lists core diameters (2a),
numerical apertures (N.A.)
resultant values for V, the normalized frequency (at A-2.5 pm),
and for step
index multimode fluoride fibers and a typical silica fiber (at A-1.3 pm).
Mat. Res. Soc. Symp. Proc. Vol. 172. e1990 Materials Research Society
178
TABLE I:
Step index multimode fiber parameters
179
Fig. 1 shows,
for various values of V, the dependence of the fraction
of modes with calculated induced losses > 0.01 dB/km on the ratio of clad to core diameter for each value is
(b/a).
In Table
I the values of b/a and 2b are given
of V such that only 10% of the modes have such loss.
apparent that such parameters
of the silica fiber. of 56 pm and an N.A.
For example, of 0.2 (V=14)
preclude coating induced
losses,
a fluoride fiber with a core diameter needs an overall diameter of 181 jim to compared to the 50 um/125 pm structure
suitable for silica fiber.
801 0
660ov=10 0 -
= 0U
40
0
20
"AV=14 1-)
n
It
are significantly different from those
* V = 20 0V=24 L
b/a (Clad dia./Core dia.) Fig. 1: Modes with loss > 0.01 dB/km
180
The which
is
total
induced
loss
depends
strongly
often assumed to be Gaussian
cases b/a - 2 and 3)
on
[2,3].
the mode Fig.
distribution
2a shows
(for
the
the calculated losses for a fluoride fiber with V-14
assuming the Gaussian distributions shown in
Fig. 2b characterized by the
parameter G, i.e.:
P(9)/9 - exp[-(O/GOc) where 8c = the critical
angle.
There
is
a
2
]
(1)
strong dependence on the mode
distribution, at this point unknown for fluoride fiber. the calculated loss with G =o
By comparison,
for the silica fiber was only 0.007 dB/km
at 1.3 jm.
m %0
02 101
"W 0 00 •j 10 1 -(., 10_2
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