Page 288 - Fiber Bragg Gratings

6.5 The optical circulator based OADM 265

Figure 6.31: A two-wavelength cascaded GMZI-BPF with increased isolation
between the add and drop ports (from: Mizuochi T. and Kitayama T., "Interferomet-
ric cross talk-free optical add/drop multiplexer using cascaded Mach-Zehnder fiber
gratings," in Technical Proc. ofOFC '97, pp. 176-177, 1997.) [57].

RHS and is routed to B2. All other wavelengths arriving at Al simply go
through the gratings and also appear at Bl. Similarly, channels arriving
at B2 are routed to A2, except for the channel at Ay, which is dropped at
Cl and reinserted from C2, to be routed to A2. The inclusion of the
"isolating" grating reduces the leakage between Al and Bl from -26 to
— 71 dB. The improvement is evident in the add-drop function, by a
reduction in the power penalty of 1 dB due to the elimination of coherent
beat noise. The use of higher reflectivity gratings in the MZIs should
eliminate the need for the "isolator" grating, although short-wavelength
radiation loss will remain a problem.

6.5 The optical circulator based OADM

The optical circulator has become extremely important for applications
with fiber gratings. While the use of a fiber grating with a circulator is