
65Optical Fibers: Signal Attenuation and Dispersion
neers
have
to
characterize
all
optical
properties
of
new
materials.
The
refractive
index
n(λ)
can
u
sually
b
e
ex
pressed
i
n
S
ellmeier’s
d
ispersion
f
ormula
a
s
n
G
k
k
k
2
2
2 2
1( )
( )
λ
λ
λ λ
= +
−
∑
(3.23)
where
G
k
are Sellmeier’s constants
k
i
s
a
n
i
nteger
a
nd
n
ormally
k
=
1
,
2
,
a
nd 3
In
the
late
1970s,
several
silica-based
glass
materials
were
manufactured
and
their
proper-
ties
measured.
The
refractive
indices
were
usually
expressed
using
Sellmeier’s
coefcients.
These
c
oefcients
f
or
s
everal
o
ptical
ber
m
aterials
a
re
g
iven
i
n
T
able
3
.1.
By
u
sing
c
urve
tting,
t
he
r
efractive
i
ndex
o
f
p
ure
s
ilica
n(λ) c
an
b
e