Page 299 - Fiber Bragg Gratings

276 Chapter 6 Fiber Grating Band-pass Filters

A PBS-OADM has been demonstrated for a single channel using seven
wavelengths spaced at 0.8-nm intervals. The center channel was dropped
with a cross-talk penalty of 0.3 dB when the same wavelength was added
at a transmission rate of 2.5 Gb/sec. Heating part of one arm of the PBS-
OADM by 65°C induced a change of 0.3 dB at the output [73]. However, it
remains to be seen how this device will function under full environmental
testing. Figure 6.40 shows a schematic of the PBS-OADM [73].

6.7 In-coupler Bragg grating filters

Co- and contradirectional wavelength selective couplers have been known
for a long time [74,75]. There are a number of ways that gratings in-
couplers may be used to form band-pass filters. Figure 6.41 shows three
different types of couplers, which include gratings to assist (grating-as-
sisted coupler, GAG), to frustrate (grating-frustrated coupler, GFC) and
to reflect (Bragg reflection coupler, BRC) light of a particular wavelength
that meets the phase-matching requirements.
The period of the refractive index-perturbation for codirectional grat-
ing-assisted coupling [GAG, Fig. 6.41 (j)] between two dissimilar fibers
is determined by the difference in the propagation constants of the two
guides. This is generally small, and therefore the period is long. For weak
overlap of the fields, the coupled-mode equations (see Chapter 4 on long-
period gratings), describe the interaction between the modes. The coupling
between the guides is sinusoidally periodic with length of the grating-
assisted region. The coupling has a relatively broad bandwidth (tens of
nanometers) and therefore poor wavelength selectivity, unless the device
can be made very long.
A normally 100% coupler is strongly detuned by the dispersion of the
grating and so fails to behave as a coupler near the Bragg wavelength,
and is called a grating-frustrated coupler [GFC, Fig. 6.41 (ii)]. It works
on the following principle: Two fibers with identical propagation constants
will exchange power at all except the "grating-frustrated" wavelength.
The in-fiber grating is a Bragg reflector at the frustrated-wavelength and
is present in only one of the fibers. The far end of the input fiber becomes
the "drop" port and is the one that does not contain the grating.
The Bragg reflecting coupler [BRC, Fig. 6.41 (Hi) and (iv)] requires
a perturbation with a short period, as is the case for Bragg reflection,
being dependent on the sum of the magnitudes of propagation constants of