Page 294 - Fiber Bragg Gratings

6.5 The optical circulator based OADM 271

circulators sandwich a set of optical cross-connect switches connected in
series. Fiber gratings at the optical channel wavelengths are connected
between one of the output ports of the cross-connect switches. This way,
the incoming signals may be switched to the grating or bypass it. When
switched to the grating, the channel at the grating wavelength Aj is
dropped and routed to the drop port. All other channels proceed to the next

switch, where the choice is repeated for the other channels. If, however, A l
bypasses the first grating, it goes on to the through port. The same applies
to the other channels. At each two-by-two optical cross-point switch, a
channel (or more, depending on the number of gratings between the
switches) may be dropped. The ROADM is extremely flexible, allowing
the node to be programmed relatively fast (<50 /us). The insertion loss of
the optical switches is low (0.7 dB), and the device has been demonstrated
as a two-channel ROADM [67], with a total insertion loss of 3 dB. The
equalizing filters shown in Fig. 6.35 are intended to compensate for the
loss of the switches and the gratings so that all channels suffer the same
insertion loss.
A modification of the ROADM shown in Fig. 6.35 results in the simul-
taneous add-drop function, including automatic dispersion compensation
of all channels, using dispersion compensating gratings (DCG) [68]. The
channel dropping gratings are replaced by chirped dispersion compensat-
ing gratings. Uniform unchirped gratings can be used from either direc-
tion, whereas the dispersion of chirped gratings is reversed if it is turned
around. Therefore, the chirped gratings cannot simply replace the un-
chirped gratings in the ROADM. Channels to be dropped are compensated
for dispersion in the ROADM with DCGs. In order to dispersion compen-
sate the "through" channels, the optical circuit has to be altered. The
modified reconfigurable dispersion compensating ADM (RDC-ADM) is
shown Fig. 6.36. The "through" channels are routed via the "through path"
to the output circulator and reinjected into the cross-point switches. Each
"through" channel is routed to the appropriate grating via the switches
and dispersion compensated by reflection, to retrace the path to the circu-
lator, and finally to the output. Channels are also inserted without reflec-
tion; they simply bypass the DCG allocated to their channel, so as to avoid
the additional dispersion. The four-channel RDC-ADM was demonstrated
with a 4 X 10 Gb/sec WDM system and used apodized 100-mm-long
gratings for each channel designed to compensate for the dispersion of 8
km of standard fiber (1312 psec/nm). An important feature of the configu-
ration was the arrangement of gratings and amplifiers to equalize the