20 10 0 - 10 - 20
20
2.88× 10-6
147
40
60
80
100
120
140
20
10
0
- 10
- 20
MODELING
Multiphysics software models
multimaterial optical fibers
fabricated via HPCVD
SUBHASIS CHAUDHURI and JOHN BADDING
Optical fibers have revolutionized communications technology, and in addition
are being used in a variety of other areas such as the medical arena, sensing,
cutting and drilling, defense applications, and light sources in the form of
fiber lasers and amplifiers. 1
The drawing technique conventionally used to manufacture optical fibers
limits the materials that can be incorporated in them. Another approach,
templated deposition inside hollow-core
optical fibers using high-pressure chem-
ical vapor deposition (HPCVD), has
emerged as an important technique to
incorporate different materials such as
metals, semiconductors, and insulators
in fibers, enabling the exploration of
various properties of these materials. 2
HPCVD to obtain
step-index optical fibers
Using HPCVD, semiconductors such as
silicon and germanium have been depos-
ited inside hollow-core optical fibers to
form step-index fibers that guide light
by total internal reflection. Compared
to silica glass, silicon and germani-
um have a much wider transparen-
cy window that extends well into the
mid-infrared, high linear and nonlinear
refractive indices, and a
high damage threshold.
These properties enable
the use of these fibers for
the delivery of high-pow-
er infrared (IR) light, nonlinear opti-
cal processes, and IR countermeasures.
The size of the core and the refractive indices of the core and clad-
ding determine the number of optical
modes that can propagate in these fi-
bers. Multiphysics simulation software
has been used to model various step-index fibers to understand the light-guid-
ing properties of these fibers at various
wavelengths and how it is affected by
altering the core size.
HPCVD to obtain Bragg
optical filters
Apart from total internal reflection,
light can propagate in a fiber by sev-
eral other mechanisms such as the for-
mation of a photonic bandgap, 3
antiresonant reflection. 4 Bragg fi-
ber is a special type of fiber that
guides light via the formation of
a photonic bandgap. It consists of
a hollow core bound by concen-
tric alternating layers of high- and
low-refractive-index materials, which
gives rise to the photonic bandgap. 5
Using a Bragg fiber, light can be guid-
ed in a hollow core. This has certain ad-
vantages over light guidance in a step-in-
dex fiber, such as reduced latency, low
absorption loss, and negligible optical
nonlinearities. Bragg fiber was conceptu-
alized in 1978, but because of the struc-
tural complexity of drawing such fiber
by traditional drawing techniques, very
few Bragg fibers have been fabricated. 6, 7
It is even more difficult to incorpo-
rate robust refractory materials as layers
for the Bragg fiber via the drawing tech-
nique. This limitation has been over-
come by using the HPCVD technique to
deposit materials inside a hollow silica
capillary to fabricate a fiber, rather than
drawing it. The light-guidance proper-
ty of the fiber depends on a number of
factors, predominantly the diameter of
the hollow core, refractive indices of the
Computer modeling of semiconductor-
core optical fibers made using high-
pressure chemical vapor deposition
reveal the fibers’ useful properties.
