Page 22 - Fiber Bragg Gratings

1.1 Historical perspective 3

Stone's [9] observation that virtually any germania-doped silica fiber dem-
onstrated a sensitivity to argon laser radiation reopened activity in the
field of fiber gratings [10,11] and for determining possible links between
the two photosensitive effects. Bures et al. [12] had pointed out the two-
photon absorption nature of the phenomenon from the fundamental radia-
tion at 488 nm.
The major breakthrough came with the report on holographic writing
of gratings using single-photon absorption at 244 nm by Gerry Meltz et
al. [13]. They demonstrated reflection gratings in the visible part of the
spectrum (571-600 nm) using two interfering beams external to the fiber.
The scheme provided the much-needed degree of freedom to shift the
Bragg condition to longer and more useful wavelengths, predominantly
dependent on the angle between the interfering beams. This principle
was extended to fabricate reflection gratings at 1530 nm, a wavelength
of interest in telecommunications, also allowing the demonstration of the
first fiber laser operating from the reflection of the photosensitive fiber
grating [14]. The UV-induced index change in untreated optical fibers
4
was ~10~ . Since then, several developments have taken place that have
pushed the index change in optical fibers up a hundredfold, making it
possible to create efficient reflectors only a hundred wavelengths long.
Lemaire and coworkers [15] showed that the loading of optical fiber with
molecular hydrogen photosensitized even standard telecommunication
fiber to the extent that gratings with very large refractive index modula-
tion could be written.
Pure fused silica has shown yet another facet of its curious properties.
It was reported by Brueck et al. [16] that at 350°C, a voltage of about 5 kV
applied across a sheet of silica, a millimeter thick, for 30 minutes resulted
in a permanently induced second-order nonlinearity of ~1 pm/V.
Although poling of optical fibers had been reported earlier using electric
fields and blue-light and UV radiation [17-19], Wong et al. [20] demon-
strated that poling a fiber while writing a grating with UV light resulted
in an enhanced electro-optic coefficient. The strength of the UV-written
grating could be subsequently modulated by the application of an electric
field. More recently, Fujiwarae£ al. reported a similar photoassisted poling
of bulk germanium-doped silica glass [21]. The silica-germanium system
will no doubt produce further surprises.
All these photosensitive processes are linked in some ways but can
also differ dramatically in their microscopic detail. The physics of the
effect continues to be debated, although the presence of defects plays a