Page 264 - Fiber Bragg Gratings

6.1 Distributed feedback, Fabry-Perot, superstructure, and moire gratings 241

Figure 6.11: (a) The reflectivity spectrum of a superstructure grating with
9 X 222 micron grating sections separated by 1-mm gaps. Refractive index modula-
3
tion amplitude is 10~ . (b) The transmission spectrum of the grating shown in
(a). Also shown is the transmission spectrum of a single section of the grating of
0.181 microns long. The envelope has been normalized to fit the superstructure
spectra.

coupler once again. The reflected spectrum is therefore -6 dB relative to
the input signal. A fiber circulator overcomes this loss penalty [21]. The
insertion loss of a circulator is approximately 1 dB, so that an efficient
multiple band-pass filter can be fabricated.
In an interesting demonstration, a chirped superstructure grating
has been used for multiple-channel dispersion compensation, since the
repeat band stops have a near-identical chirp [22]. The advantage of such
a scheme is that it requires only a single temperature-stabilized grating
to equalize several channels simultaneously, although the reflection coeffi-
cient varies for each reflection.