Page 268 - Fiber Bragg Gratings

6.2 The Fabry-Perot and moire band-pass filters 245

Figure 6.15: Measured transmission characteristics of a fiber FP filter. The
length of each grating was 0.3 mm, a 2.5 mm gap and with a peak-to-peak refractive
3
index modulation of 5 x 1CT . A theoretical fit to the data shows excellent
agreement, although the peak transmission has not been fully resolved in the
measurement [29]. A maximum extinction of >30 dB was measured.

Wide-bandwidth (140-nm) fiber grating Fabry-Perot filters fabricated
in boron-germanium codoped fibers have been demonstrated with a fi-
nesse of between 3 and 7 [28]. Two identically chirped, 4-mm-long gratings
with a bandwidth of ~150 nm and reflectivity of >50% were written in
the fiber, displaced from each other by 8 mm. The resulting FP interference
had a bandwidth of 0.03 nm and a free-spectral range of 0.09 nm. A larger
free-spectral range was obtained by overlapping the gratings with a linear
displacement of 0.5 mm. These gratings had a bandwidth of 175 nm in
the 1450-1650 nm wavelength window. A finesse of 1.6 with an FSR of
1.5 nm was demonstrated. These fiber-grating FP-like devices may find
applications in fiber laser and WDM transmission systems.
A further possibility of opening up a gap within the stop band is to
write two gratings of slightly different Bragg wavelengths at the same
location in the fiber [30] to form a moire fringe pattern. The physical
reason why a band pass results may be understood by noticing that the
phase responses of the gratings are not identical. Thus, at some wave-
length, the phases can be out by TT radians. If the wavelength difference
is made larger, it is possible to create more than one band pass. The
mechanics of producing such a band pass have been demonstrated by