The Optical Communications Reference

of certain frequencies of light into the cladding. Conse-

quently, if light of these frequencies is launched into the

core of the micro-structured fiber, it could be confined

within this region and be guided along the fiber length.

PBGFs can be broadly classified as either Bragg fibers [4]

or photonic crystal fibers [5, 6]. The former consists of

a one-dimensional cladding that is periodic along the radial

direction, whereas the latter consists of a two-

dimensional periodic cladding extending along the

directions radial as well as azimuthal. In contrast to

conventional fibers, a PBGF can guide light through the

low-index core (air) and hence offers several advantages as

compared with light transmission through conventional

fibers. In air-core PBGFs, because light is guided in air,

two ma jor benefits gained are much lower transmission

loss and reduced sensitivity to nonlinear optical impair-

ments such as four-wave mixing, cross-phase modulation,

and so on. Following the same reasoning, the power-

handling capability of these fibers would be much better

and material dispersion effects would be much reduced.

In addition, because of a multitu de of physical parameters

that can be altered independently, their propagation

characteristics can be tuned with ease to control and

maneuver light guidance for a variety of applications,

ranging from telecommunications to sensors [7]. In this

chapter our focus will remain only on Bragg fibers.

3.4.2 Bragg fibers

Bragg fibers consist of a low-index core surrounded by

periodic multilayer cladding (concentric layers) of

alternate high and low refractive index materials (each of

which has a refractive index higher than that of the core).

As mentioned above, the inherent periodicity in the

cladding forms a photonic bandgap does not allow light of

certain frequencies from leaking through the cladding

and thereby confines it within the core region. Fig. 3.4.2

shows the cross-sectional view of a conventional optical

fiber (a) and a Bragg fiber (b) (air core). The Bragg fiber

was proposed as early as 1978 [4]. However, because of

the lack of advanced fabrication techniques and doubts

related to thei r applicability [8], there was hardly any

further research in this field for almost a decade. Interest

revived again in the late 1990s in the context of a flurry of

research on photonic crystals, and several articles

reported successful fabrication of broadband, low-loss,

hollow-core fiber waveguides suitable for various ranges

of wavelengths [9,10].

3.4.2.1 Bandgap in one-dimensional

periodic medium

The multilayer cladding of the Bragg fiber is periodic

along the radial direction and results in a one-dimensional

photonic bandgap. It is functionally similar to a multi-

layer planar stack that consists of thin films of alternate

refractive indices, n

1

and n

2

, having thickness l

1

and l

2

,

respectively, such that L ¼ l

1

þ l

2

(Fig. 3.4.3). Accord-

ingly, wave guidance in a Bragg fiber can be convenien tly

understood in terms of the physics that underlies the

formation of the bandgap and the concept of decaying

Bloch waves in a multilayer planar stack. Following the

1 dimensional 2 dimensional 3 dimensional

Fig. 3.4.1 Schematic representations of photonic crystals in one, two, and three dimensions. (After [2].)

High index

core

Low index

core (air)

Low index

cladding

Periodic

cladding

(a) (b)

Fig. 3.4.2 (a) Cross-sectional view of a conventional fiber. (b) Cross-sectional view of a Bragg fiber.

154

SECTION THREE Optical Fibers