Page 706 - Introduction to Information Optics

690 12. Networking with Optics

Site A SiteB

Fiber Fiber Fiber

r^

Drop Add Drop Add

Fig. 12.17. A point-to-point optical link with OADMs.

in a simple form at the same wavelength as the one just dropped, into the
express channels. All channels exit at the output port. The application of this
technology in a point-to-point network is demonstrated in Fig. 12.17. Multiple
channels are transmitted between major locations A and B. In between there
are several small network nodes that have some traffic demands. OADMs oifer
these intermediate nodes access to a portion of the network traffic while
maintaining the integrity of the other channels. Without OADM, all channels
have to be terminated at the intermediate node even for a small portion of
traffic exchange, which results in a much higher cost.
Cascaded OADMs with architecture as shown in Fig. 12.16 can be used to
add or drop multiple channels in one intermediate node, but this comes with
high overall insertion loss. Since the wavelengths of WDM filters are predeter-
mined, the wavelengths to be added/dropped at each intermediate node have
to be carefully preplanned, so this type of OADM is also called fixed OADM
(FOADM).
Another type of OADM architecture, as shown in Fig. 12.18, is being
aggressively pursued. A pair of DWDM demultiplexer and multiplexers are
interconnected by an array of optical switches. The demultiplexer separates

Fig. 12.18. Schematic of a reconfigurable OADM.