The Optical Communications Reference

3.4

Chapter 3.4

Photonic bandgap–guided

Bragg fibers

Pal

3.4.1 Introduction

Consequent to the mind-boggling progress in highspeed

optical telecommunications witnessed in recent times, it

appeared tha t it would only be a matter of time before

the huge theoretical bandwidth of 53 THz offered by

low-loss transmission windows (extending from 1280 nm

[235 THz] to 1650 nm [182 THz]) in OH



-free high-

silica optical fibers would be tap ped for telecommuni-

cation through dense wavelength division multiplexing

techniques! In spite of this possibility, there has been

a considerable resurgence of interest among researchers

to develop specialty fibers, that is, fibers in which

transmission loss of the material would not be a limiting

factor and in which nonlinearity or dispersion properties

could be conveniently tailored to achieve transmission

characteristics that are otherwise almost impossible to

realize in conventional high-silica fibers. Research

targeted at such fiber designs gave rise to a new class of

fibers, known as microstructured optical fibers (see

Chapter 3). One category of such microstructured fibers

is known as photonic bandgap fibers (PBGFs). In a con-

ventional optical fiber, light is guided by total internal

reflection because of the refractive index contrast that

exists between a finite-sized cylindrical core and the

cladding of lower refractive index that surrounds it. On

the other hand, in a PBGF, light of certain frequencies

cannot propagate along directions perpendicular to the

fiber axis but instead are free to propagate along its length

confined to the fiber core. This phenomenon that forbids

the propagation of photons (of certain frequencies)

transverse to the axis of microstructured fibers, led to the

christening of these specialty fibers as pho tonic bandgap-

guided optical fibers, in analogy with the electronic band-

gaps encountered by electrons in semiconduc tors. In

contrast to the electronic bandgap, which is the conse-

quence of periodi c arrangement of atoms/molecules in

a semiconductor crystal lattice, photonic bandgap arises

due to a periodic distribution of refractive index in certain

dielectric structures, generically referred to as photonic

crystals (reminiscent of semiconductor crystals in solid-

state physics) [1]. If the frequency of incident light hap-

pens to fall within the photonic bandgap, which is char-

acteristic of the photonic crystal, then light propagation is

forbidden in it. Depending on the number of dimensions

in which periodicity in refractive index exists, photonic

crystals are classified as one-, two-, or three-dimensional

photonic crystals (Fig. 3.4.1) [2]. Opals are naturally

occurring photonic crystals consisting of a three-

dimensional lattice of dielectric spheres, and opalescence

is a direct consequence of their photonic crystal nature.

The iridescent wings of some butterflies and beetles are

also essentially the result of naturally occurring photonic

crystal effects that evolved on their surface with time. In

1987, Yablonovitch first proposed the po ssibility of con-

trolling properties of light through the photonic bandgap

effect in man-made photonic crystals.

PBGFs essentially consist of a core surrounded by

a periodic cladding having a photonic crystal-like struc-

ture (i.e., a periodi c refractive index distributi on). As

explained above, the inherent periodicity of the cladding

results in a photonic bandgap, which forbids propagation

Guided Wave Optical Components and Devices; ISBN: 9780120884810

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