Page 122 - Fiber Bragg Gratings

3.3 Type IIA gratings 101

correct lengths. After a single grating is scanned into the fiber, the fiber
is translated along accurately, and the next phase mask grating moved
vertically into position for writing the succeeding grating. Any small inac-
curacy in the placement (stitching) of the phase mask grating can be
"trimmed" [110], using a single UV beam exposure to adjust the phase
between adjacent sections [112].
Using this method, gratings 2 meters long have been written in a
single contiguous piece of fiber, with a chirp bandwidth of —15 nm [111],
and a 1.3-meter long grating with a bandwidth of 10 nm has been used
for multichannel dispersion compensation [75,112].
The detailed characteristics of step-chirped and super-step-chirped
gratings are discussed in Chapter 4.

3.2 Type II gratings

Fiber gratings formed at low intensities are generally referred to as Type
I. Another type of grating is a damage grating formed when the energy of
the writing beam is increased above approximately 30 mJ [119]. Physical
damage is caused in the fiber core on the side of the writing beams. The
definite threshold is accompanied by a large change in the refractive index
modulation. It is therefore possible to write high-reflectivity gratings with
a single laser pulse. Above 40-60 mJ, the refractive index modulation satu-
3
rates at around 3 X 10~ . Energy of the order of 50-60 mJ can destroy the
optical fiber. The sudden growth of the refractive index is accompanied by
a large short-wavelength loss due to the coupling of the guided mode to the
radiation field. The gratings generally tend to have an irregular reflection
spectrum due to "hot spots" in the laser beam profile. By spatially filtering
the beams, gratings with better reflection profiles have been generated but
with a much reduced reflectivity [113]. These gratings decay at much higher
temperatures than Type I, being stable up to ~ 700°C. Some of the properties
of these gratings are outlined in Chapter 9.

3.3 Type IIA gratings

Yet another type of grating is formed in /lora-hydrogen-loaded fibers. These
may form at low power densities or with pulsed lasers after long exposure
[114]. The characteristics of a Type IIA grating are the growth of a zero-
order (N = 1) grating, and its erasure during which a second-order grating