Page 39 - Fiber Bragg Gratings

20 Chapter 2 Photosensitivity and Photosensitization of Optical Fibers

both indirectly, e.g., by etching glass exposed to radiation or using second-
harmonic generation [9,26,27] as a probe, and directly, e.g., by measure-
ment of photocurrent in germania-doped planar waveguides [28] and
across thin films of bulk glass [29].
It has been concluded that the photocurrent is influenced by the
fluence of the exciting UV radiation; the photocurrent (probably by tunnel-
ing [29]) is a linear function of the power density for CW excitation [28],
while for pulsed, high-intensity radiation, it takes on a two-photon excita-
tion characteristic [29].
The paramagnetic defects of the Ge(/i) type including the E' center
are detected by ESR. The GeE' has an associated optical absorption at
4.6 eV [30].

2.4 Photosensitization techniques

A question often asked is: Which is the best fiber to use for the fabrication
of most gratings? Undoubtedly, the preferred answer to this question
should be standard telecommunications fiber. Although techniques have
been found to write strong gratings in this type of fiber, there are several
reasons why standard fiber is not the best choice for a number of applica-
tions. Ideally, a compatibility with standard fiber is desirable, but the
design of different devices requires a variety of fibers. This does open the
possibility of exploiting various techniques for fabrication and sensitiza-
tion. Here we look in some detail at the behavior of commonly used species
in optical fiber and present their properties, which may influence the type
of application. For example, the time or intensity of UV exposure required
for the writing of gratings affects the transmission and reliability proper-
ties. This results in either damage (Type II gratings) [31] or the formation
of Type I, at low fluence, and Type IIA gratings [32], each of which have
different characteristics (see Section 2.4.1).
The use of boron and tin as a codopant in germanosilicate fibers, hot
hydrogenation and cold, high-pressure hydrogenation, and flame-assisted
low-pressure hydrogenation ("flame-brushing") are well-established pho-
tosensitization methods. The type of the fiber often dictates what type of
grating may be fabricated, since the outcome depends on the dopants.
The literature available on the subjects of photosensitivity, the com-
plex nature of defects, and the dynamics of growth of gratings is vast