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Plenary Highlights from the LEOS 2002 Annual Meeting |
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Introduction
Photonic
crystal fibers (PCFs—sometimes also known as “holey”
or “microstructured” fibers) have been the focus of increasing
scientific and technological interest since the first working example
was produced in late 1995 (reported at the Optical Fiber Communications
Conference in March 1996 [1,2]). Examples of different types of fibres
are shown in Fig. 1. Although superficially similar to a conventional
optical fiber, PCF has a unique microstructure, consisting of an array
of microscopic holes (or channels) that run along the entire length
of the fiber. These holes act as optical barriers or scatters, which
suitably arranged can “corral” light within a central core
(either hollow or made of solid glass). The holes can range in diameter
from ~25nm to ~50mm. Although most PCF
is formed in pure silica glass, it has also recently been made using
polymers [3] and non-silica glasses [4], where it is difficult to
find compatible core and cladding materials suitable for conventional
total internal reflection guidance.
PCF supports two guidance mechanisms: total
internal reflection, in which case the core must have a higher average
refractive index than the holey cladding; and a two-dimensional photonic
bandgap, when the index of the core is uncritical—it can be hollow
or filled with material [5,6]. The propagation diagram for a typical
PCF fibre is shown in Fig. 2. Light can be controlled and transformed
in these fibers with unprecendented freedom, allowing for example
the guiding of light in a hollow core, the creation of highly nonlinear
solid cores with anomalous dispersion in the visible and the design
of fibers that support only one transverse spatial mode at all wavelengths.
Applications are emerging in many diverse areas of science and technology.
For example, as first shown by Ranka et al.
[7], an ultra-small core fibre made from solid glass and surrounded
by very large air-holes can be arranged to have a zero chromatic dispersion
wavelength in the 800nm Ti:sapphire band. This fibre produces spectacular
spectral broadening of high repetition rate 100 fsec pulses, with
a brightness some 10,000x brighter than the sun and a similar bandwidth,
as shown in Fig. 3. This source is transforming the fields of optical
coherence tomography, spectroscopy, and frequency metrology [8], as
illustrated in Fig. 4.
In its hollow core form [9], PCF also solves
a key long-standing challenge in photonics, for which there is no
good conventional solution: How to force light to interact—strongly,
reproducibly, and over long pathlengths—with low-density materials
such as gases, vapours, and liquids. This is an exciting development
with major implications for numerous gas-based nonlinear optical and
laser devices. Recently a hydrogen Raman cell was demonstrated [10]
with a threshold energy of 800nJ—some 100x lower than previously
reported. In September 2002, breakthrough losses of 13dB/km were reported
in hollow-core photonic bandgap fibre by a team from Corning [11].
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