Page 255 - Fiber Bragg Gratings

232 Chapter 6 Fiber Grating Band-pass Filters

the transmission spectrum of one such grating is shown in Fig. 6.3. In
this grating, a phase step of A/4 was introduced in the middle of the
phase mask and replicated. This type of grating is a simple Fabry-Perot
interferometer that has a band stop inversely proportional to ~0.25L^.
A feature to note is the second peak on the side of the main band pass,
which is due to the birefringence of the fiber estimated from the separation
5
to be ~1 X 1(P . The combination of photoinduced and intrinsic birefrin-
gence becomes apparent with the extremely narrow band-pass structure
of the DFB grating. The finesse of this DFB grating was —67. The trans-
mission peak is very sensitive to losses within the grating structure. Note
that although the band pass has not been fully resolved, there is OH~
absorption loss in this grating because of hydrogenation, and conse-
quently, the transmission peak is diminished.
Phase shifts within a grating can be produced in several ways. Can-
ning and Sceats [7] showed that postprocessing the center of a uniform
grating with UV radiation results in a permanent phase-shifted structure.
This method relies on the fact that the UV radiation changes the refractive
index locally to produce an additional phase shift [53]. The UV-induced
refractive index change required in a 1-mm-long fiber for a A/4 phase shift
4
at a wavelength of 1530 nm is —3.8 X 10~ , which is easily achieved. The
DFB structure in a rare-earth-doped fiber is useful for ensuring single-
frequency operation [9], in much the same way as semiconductor DFB
lasers. An inexpensive high-quality fiber-compatible laser exhibiting ex-
tremely low noise is particularly attractive for telecommunications. Initial

Figure 6.3: Transmission characteristics of an 8-mm-long DFB grating pro-
duced by replication of a phase mask with a A/4 phase shift in the center [6].