Page 293 - Fiber Bragg Gratings

270 Chapter 6 Fiber Grating Band-pass Filters

is tunable, it can be detuned, thus directing the channel at that wave-
length to the through port.
Since the device is symmetric, the channel insert function is per-
formed in a similar manner: When injected into the "add" port, the wave-
lengths are routed to the "through" port. Poor peak and high side-lobe
reflectivity cause cross-talk. The use of unapodized (—14 dB side-lobes),
95% peak reflectivity gratings resulted with in poor cross-talk perfor-
mance. With well-apodized, high-reflectivity gratings, low cross-talk per-
formance is possible and the OC-TADM is appropriate for dense-WDM
applications. In principle, piezoelectric stretchers can be used to make a
fast OC-TADM, switchable in < 1 ^s [65].
In a slightly simpler arrangement, Kim et al. [66] proposed the
use of four identical gratings between two three-port circulators. The
gratings are stretch tuned by piezoelectric stretchers, so that up to
four channels can be dropped or inserted in any combination, when the
Bragg wavelengths of the gratings are tuned to the channel wavelengths.
Mechanical leverage designed into the grating mounts with the piezoelec-
tric stretcher allows the Bragg wavelength of each grating to be tuned
by 2.4 nm/120 V applied.

6.5.1 Reconfigurable OADM

These devices are based on optical switches and circulators and overcome
some of the limitations of the OC-TADM and the GMZI-BPF. A schematic
of the reconfigurable OADM (ROADM) is shown in Fig. 6.35. Two fiber

Figure 6.35: The reconfigurable optical add-drop multiplexer (ROADM)
(after Ref. [67]).