Page 21 - Fiber Bragg Gratings

2 Chapter 1 Introduction

sion-multiplexed (WDM) systems, channel selection, and deployment of
transmitters in the upstream path in a network, and should make routing
viable. The fascinating technology of photosensitive fiber is based on the
principle of a simple in-line all-fiber optical filter, with a vast number of
applications to its credit.

1.1 Historical perspective

Photosensitivity of optical fiber was discovered at the Canadian Communi-
cation Research Center in 1978 by Ken Hill et al. [1] during experiments
using germania-doped silica fiber and visible argon ion laser radiation.
It was noted that as a function of time, light launched into the fiber was
increasingly reflected. This was recognized to be due to a refractive index
grating written into the core of the optical fiber as a result of a standing
wave intensity pattern formed by the 4% back reflection from the far end
of the fiber and forward-propagating light. The refractive index grating
grew in concert with the increase in reflection, which in turn increased
the intensity of the standing wave pattern. The periodic refractive index
variation in a meter or so of fiber was a Bragg grating with a bandwidth
of around 200 MHz. But the importance of the discovery in future applica-
tions was recognized even at that time. This curious phenomenon re-
mained the preserve of a few researchers for nearly a decade [2,3]. The
primary reason for this is believed to be the difficulty in setting up the
original experiments, and also because it was thought that the observa-
tions were confined to the one "magic" fiber at CRC. Further, the writing
wavelength determined the spectral region of the reflection grating, lim-
ited to the visible part of the spectrum.
Researchers were already experimenting and studying the even more
bizarre phenomenon of second-harmonic generation in optical fibers made
of germania-doped silica, a material that has a zero second-order nonlin-
ear coefficient responsible for second-harmonic generation. The observa-
tion was quite distinct from another nonlinear phenomenon of sum-
frequency generation reported earlier by Ohmori and Sasaki [4] and Hill
et al. [51, which were also curious. Ulf Osterberg and Walter Margulis [6]
found that ML-QS infrared radiation could "condition" a germania doped-
silica fiber after long exposure such that second-harmonic radiation grew
(as did Ken Hill's reflection grating) to nearly 5% efficiency and was soon
identified to be a grating formed by a nonlinear process [7,8]. Julian