Page 267 - Fiber Bragg Gratings

244 Chapter 6 Fiber Grating Band-pass Filters

in a FP filter reported to date are ~0.3 mm long, separated by a similar
distance [26]. The resulting multiple band pass, which was a shallow
ripple of —50% transmission was used as a guiding filter in wavelength
division multiplexed soliton transmission experiments to suppress Gor-
don-Haus jitter [271.
With stronger gratings, multiple band-pass filters with deeper band
stops are easily possible. However, even slight loss in the grating (absorp-
tion due to OH~ ions) can degrade the transmission peaks substantially.
It is therefore advantageous to use deuterated fiber for this type of a
filter. Figure 6.14 demonstrates a 4-mm-long grating with a gap of 5 mm
in the center. This filter shows —30-dB extinction in the center of the
band pass.
Note that all these filters have a similar narrow band-pass response
that plagues the highly reflecting DFB grating filter. Thus, applications
for such a grating are likely to be in areas in which either high extinction
or high finesse, or low extinction and large bandwidth are required. Figure
6.15 shows the measured transmission of a 0.6-mm-long FP filter with a
2.5-mm gap. The pass bands have been measured with a resolution of 0.1
nm and are therefore not fully resolved. The structure should be deeper
and much narrower. Nevertheless, the dips in transfer characteristics
match the theoretical simulation extremely well with the parameters
shown. Typically, the best results for band-pass peaks for this type of FP
filter, using either chirped or unchirped gratings with an extinction of 30
dB, is -70%.

Figure 6.14: A 4-mm-long grating FP filter with a 5-mm gap and a An of 5
4
X 1(T .