Page 105 - Fiber Bragg Gratings

84 Chapter 3 Fabrication of Bragg Gratings

Figure 3.20: The scheme of writing gratings in a fiber while it is being drawn
from a preform: 20-ns pulses at a wavelength of 248 nm were used to inscribe a
sequential array of 50 gratings stepped at 0.1 nm. Also demonstrated were 120
gratings written at the same wavelength. The wavelength stability was good, but
the peak reflectivity was —3%. Gratings with a wavelength spacing of 5 nm were
also reported [122].

coating may well be restricted to the use of CW UV sources, since high-
power pulsed sources are likely to damage the polymer coating unless
tight focusing is used to reduce the intensity at the surface [119].

3.1.11 Long-period grating fabrication
Long-period gratings have found applications as lossy filters. These grat-
ings couple the guided and the radiation modes in the copropagating
direction, and typically require periods in the region of 100 to 500 /mi.
LPGs are therefore most conveniently fabricated by UV exposure through
a shadow mask [72]. These gratings tend to be longer than reflective
Bragg gratings (tens of millimeters) since the periods are a few hundred