Page 283 - Fiber Bragg Gratings

260 Chapter 6 Fiber Grating Band-pass Filters

of the gratings. The first case is for 10- and 5-mm-long gratings, each
with a chirped bandwidth of 10 nm and with a detuning of 0.344 mm
(Fig. 6.28). The gap opens up close to one long-wavelength end of the
grating, while with a larger AZy of 3.44 mm in the second case, the gap
shifts to the short wavelength end (Fig. 6.29). In either case, moving
away from the phase-matching point [Eq. (6.3.12], the oscillations in the
transmission spectrum become more rapid.
There are important issues relating to the asymmetric grating Michel-
son interferometer band-pass filter. First of all, any interferometer is
sensitive to differential temperature and strain. The asymmetric interfer-
ometer is especially so; however, with fiber leads to the gratings kept in
close proximity, the only region, which needs stabilization, is the differen-
tial path. For sensing applications, the broad bandwidth of the chirped
grating is a distinct advantage. Stabilization of the paths may be done
in a number of ways, for example by the application of a special polymer
coating [46,47], or use of a substrate to compensate for the thermal expan-
sion [48]. The filter has very high extinction, but the stability is polariza-
tion sensitive, and the transfer characteristics are sinusoidal with
wavelength. The period and chirp are easily designed into the filter. Appli-
cations may be found in signal processing, sensors, multiplexing, and
spectral slicing within a well-defined bandwidth. Ideally, it would be
suited to fabrication in fused fibers, close to the coupler, or in planar form.

6.4 The Mach-Zehnder interferometer
band-pass filter

The dual-grating Mach-Zehnder interferometer band-pass filter (GMZI-
BPF) overcomes the severe limitation of the Michelson interferometer
filter — the loss of 50% of the through transmitted light — by recombin-
ing the output at a coupler, as shown in Fig. 6.30. The scheme was proposed
by Johnson et al. [49], and using etched gratings in a semiconductor
waveguide Mach-Zehnder interferometer, Ragdale et al. [50] were able to
show a device operation. A major drawback of a device fabricated in this
way is the high intrinsic losses due to scatter, absorption, and input/
output coupling, although large bandwidths are possible due to the use
of short gratings resulting from the large modulation index of the grating
(air and semiconductor). Additionally, once the device has been fabricated,