Page 262 - Fiber Bragg Gratings

6.1 Distributed feedback, Fabry-Perot, superstructure, and moire gratings 239

have been successfully demonstrated, with a pass/stop-bandwidth of 0.17
nm/11.3 nm and extinction of ~10 dB. A four-channel filter evenly spaced
over a stop bandwidth of 50 nm has also been reported [15]. Insertion
loss of these types of chirped filters is a problem, since radiation loss on
the blue-wavelength side affects the maximum transmission of the pass
band. As a consequence of large KL and radiation loss, a maximum trans-
mission of —75% was reported for these band-pass filters.
Broader pass-bandwidth filters may be fabricated by the use of
concatenated chirped gratings [16]. The effects of "in-filling" due to the
use of large KL values are diminished by increasing the band-pass
width. The arrangement for such a filter allows better extinction in
the stop band (>30 dB) while permitting the placement of the band-
pass at the required wavelength. Additionally, chirped gratings show
reasonably smooth stop band edges. Concatenating two such gratings
with a nonoverlapping band stop results in a band pass between the
two band-stop regions. While this scheme has been applied to chirped
gratings, Mizrahi et al. [17] have shown that two concatenated highly
reflective gratings with a pass band in between the Bragg wavelengths
can be used as a band-pass filter. Radiation loss within the pass band
are avoided by using a strongly guiding fiber, which further blue-shifts
the radiation loss spectrum from the long-wavelength stop band. The
bandwidth of the pass band was ~1.6 nm with an extinction in excess
of 50 dB and a stop band of ~6 nm.

6.1.2 Superstructure band-pass filter
It has been shown that placing more than a single A/4 phase step within
the grating results in as many band-pass peaks appearing within the band
stop [12]. This principle may be extended to produce the superstructure
grating [18,22], but works in reflection. The reflection spectrum has sev-
eral narrow-bandwidth reflection peaks. The principle has been used in
semiconductor lasers to allow step tuning of lasers. However, a badly
stitched phase mask will produce similar results. Since a phase mask is
generally manufactured by stitching small grating fields together, errors
arise if the fields are not positioned correctly. These random "phase errors"
are like multiple phase shifts within the grating, resulting in multiple
reflection peaks, each with bandwidth inversely proportional to the overall
length of the grating, and spaced at wavelength intervals inversely pro-