Page 33 - Fiber Bragg Gratings

14 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

by optical means and require sophisticated methods. The task is not made
easy by the various nomenclature used in labeling, so that unraveling
defects is made inaccessible to the layman. A simple overview of the
important defects is given and we point to the literature for a detailed
discussion [3,4]. Section 2.3 looks at the evidence of photoexcitation of
electrons and, in conjunction with Section 2.2, the methods for the detec-
tion of defects. The routes used to photosensitize and fabricate fibers are
presented in the last section.

2.1 Photorefractivity and photosensitivity

It is useful to distinguish the term photorefractivity from photosensitivity
and photochromic effect. Photorefractivity refers to a phenomenon usually
ascribed to crystalline materials that exhibit a second-order nonlinearity
by which light radiation can change the refractive index by creating an
internal electric field [5]. Photosensitivity invariably refers to a permanent
change in refractive index or opacity induced by exposure to light radiation
with the internal field playing an insignificant role. The term traditionally
applies to the color change in certain glasses with exposure to ultraviolet
radiation and heat. Photochromic glass does not depend on the application
of heat to change opacity, and the action is reversible. However, a combina-
tion of these properties is possible in glasses and is a novel phenomenon,
which is currently being studied, not least because it is poorly understood.
Considering the normal polarization response of materials to applied elec-
tric fields may provide a physical insight into the phenomenon of photore-
fractivity and poling of glass.
The induced polarization, P, in a medium can be described by the
relationship

where D is the displacement, E is the applied field, e 0 is the free space
permittivity, and P is the induced polarization. In a material in which
the polarization is nonlinear, the polarization may be expanded in powers
of the applied field as