Page 290 - Fiber Bragg Gratings

6.5 The optical circulator based OADM 267

tivity and extremely high return loss of the circulators are a distinct
advantage for this type of function, despite their insertion loss of ~1 dB.
Since the circulator-fiber grating band-pass filter is not interferometric,
it is intrinsically stable in its operation, but remains an expensive solution
for some application. However, for amplified long-haul fiber communica-
tion routes with large capacity (e.g., submarine systems), the cost of a
few of circulators is unlikely to be an overriding factor.
The issues that need to be addressed with the OC-ADM are the
channel-dependent insertion loss, intra- and cross-channel cross-talk, and
the dispersion penalty due to the bandwidth of the gratings. Channel-
dependent loss is primarily due to the "blue-wavelength" radiation loss
exhibited by all fiber Bragg gratings. Thus, reflected light from the
shorter-wavelength gratings may experience loss either in the drop or in
the add function, unless the gratings are fabricated with care. Figure 6.32
shows a sequence of gratings reflecting at A l5 A 2 . . . X n. The gratings are
arranged so that the first grating reflects at the shortest wavelength
and the last, A n, at the longest. A highly reflective grating transmission
spectrum is shown in Fig. 6.33. The loss on the blue-wavelength side
extends over a wide bandwidth. If each grating has similar transmission
characteristics, spaced, say, 10 nm apart, the blue-wavelength radiation
loss will increase with each additional grating. Light injected from the
short wavelength side (e.g., from the "input" port) is reflected in sequence,
so that the shortest wavelength is reflected first, and then the second
shortest, and so on. Each wavelength A n has to traverse n — 1 gratings

Figure 6.33: The measured transmission spectrum of a 7-nm full-bandwidth,
highly reflecting chirped grating with the associated "blue"-wavelength loss ex-
tending over almost the entire gain bandwidth of an erbium amplifier.