Page 35 - Fiber Bragg Gratings

16 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

The first term in Eq. (2.1.9) indicates a permanent index change, whereas
the third term is the usual quadratic nonlinear effect known as the dc-
Kerr effect. We have used a prime on the n' 2, to distinguish it from the
optical Kerr constant n 2. The interesting relationship is described by the
remaining term,

This relationship is analogous to the linear electro-optic effect, in which
the applied field operates on an enhanced nonlinearity, 2n' 2 E dc, due to the
frozen internal field. If the internal field is large, then a useful nonlinearity
is possible. This effect is believed to be partly the basis of poled glass [2].
In crystalline media with a large photorefractive response, the nonlin-
2)
earity ^ is several orders of magnitude larger than the next higher order
3)
coefficient, ;^ (and hence n 2) in glass. From the first term in Eq. (2.1.9)
we can calculate the required field for a change in the refractive index of
3 3) 22 2 2
10~ . With a measured value of ^ ~ 10~ m~ V~ for silica, a large inter-
1
9
nal field of ~10 V/m would be necessary, equivalent to n 2 of ~1 pm V" .
These values have been exceeded in UV photoelectrically poled fiber,
with the highest reported result of ~6 pm/V [6]! Combined with the low
dielectric constant of silica, it has a potentially large bandwidth for electro-
optic modulation. Just how such a large field may develop has been de-
bated. However, it has been suggested by Myers et al. [7,8] that the poling
voltage is dropped across a thin layer (~5 /ttm) within the glass, causing
huge fields to appear.
The electro-optic nature of UV photoinduced refractive index in Bragg
gratings has not been reported, although the presence charges related to
defects could indeed develop an internal field, as in the case of second-
harmonic generation in glass [9]. In the next section, we consider some
of the important defects, which are of interest in unraveling the mystery
of photosensitivity of glass.

2.2 Defects in glass

The nature of fabrication of glass is ideally suited to promoting defects.
The chemical reactions that take place in a modified chemical vapor
deposition (MCVD) [10] process are based on hot gases reacting to form
a soot deposit on the inside of a silica support tube or on the outside in
outside vapor phase deposition (OVD). The process allows the ratio of
reactive gases such as silicon/germanium tetrachloride and oxygen to be