Page 277 - Fiber Bragg Gratings

254 Chapter 6 Fiber Grating Band-pass Filters

there is little overlap of the reflected spectra. Note that at some Bragg
wavelength detuning, the interference of the reflected light at the coupler
forms moire fringes, and apodization of the band-pass spectrum begins
to occur. In Fig. 6.21 (crosses), note the reduction of the side lobes as the
dissimilar phases in the overlapped spectrum tend to cancel the formation
of the side lobes.
The filter rejection becomes worse when the detuning is one-half of
the FW bandwidth, as shown in Fig. 6.22. In this case, the zero path
difference is well off the optimum for band-pass operation (triangles),
while squares show the band pass response at TT phase difference between
the gratings. Although this spectrum is still not the optimized output, note
the strong apodization in the wings. The nonoverlapped high-reflectivity
(~ 100%) region (within the bandwidth of each grating) averages to approx-
imately 25% of the input power, as in the case of the single-grating Michel-
son device.

Figure 6.22: Two gratings detuned by approximately 0.5 x bandwidth of the
grating. The band-pass characteristics are sensitive to the path-length difference
between the Bragg reflection peaks due to differential phase response of the
gratings. With detuning, the optimum band-pass shifts from the normal zero
phase difference for matched Bragg wavelength case and develops additional
structure, although apodization occurs, reducing the reflection in the wings.