Page 291 - Fiber Bragg Gratings

268 Chapter 6 Fiber Grating Band-pass Filters

reflecting at a wavelength shorter than X n. However, since the loss in
traversing the grating is always on the short-wavelength side of the Bragg
wavelength, each wavelength is reflected without incurring radiation loss.
The situation is different if the order of reflection is reversed, as is
the case when the light is injected into the "insert" port. The longest
wavelength is reflected first and so does not incur loss. The next wave-
length (in this example, at ~1550 nm) to be reflected lies on the short-
wavelength side of the reflection shown Fig. 6.32 and so suffers twice the
radiation loss a of approximately 6-8 dB. Again, each wavelength A n has
to traverse n - I gratings reflecting at longer wavelengths and suffers a
loss of (n — 1) X a dB. This loss causes severe skew in the wavelength
response of the OADM. Although the radiation loss spectrum shown in
Fig. 6.33 is very high and has a large bandwidth, the insertion loss argu-
ment applies to all such OADM devices. The insertion loss varies across
the bandwidth but is always cumulative in one direction and can be more
important for wavelengths that have not been dropped but lie on the
short-wavelength side of the OADM. There are methods that can be used
to reduce the radiation loss of the gratings, for example, by design of the
fiber [60,61] to suppress radiation mode coupling, or by choosing the
channel spacing such that none lie in the radiation loss region [17].
Intrachannel cross-talk occurs if the added channel is derived from
the same source as the dropped channel and leaks through from the
"insert" to the "drop" port. If a strong signal is added at the OADM, a
small fraction leaks through to the drop port if the grating reflectivity is
not very high. This small "breakthrough" can be of the same order of
magnitude as the signal, which has been attenuated by the link loss and
dropped at the OADM. If the added and dropped wavelengths are the
same, signal beat noise occurs and degrades the bit error rate (BER).
Alternatively, the breakthrough signal can cause simple cross-talk be-
tween adjacent channels that are dropped, in the same way it can be
transmitted to the "add" port. The associated power penalty has been
calculated to be

for a ratio r of the leaked "add" and the dropped signal powers for coherent
signals. This is much higher than for incoherent signals [62], for which

Dispersion due to the gratings is an issue, which depends on the
signal and grating bandwidths. Gratings show strong dispersion at the