350 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.13: (a) Two designs of add–drop multiplexers using a single optical circulator in com-
bination with fiber gratings. (After Ref. [94];©c2001 IEEE; reprinted with permission.)
and leave the device from port 6. The channel to be added enters from port 4. Scheme
(b) works in a similar way but uses two identical gratings to reduce the crosstalk level.
Many other variants are possible.
8.2.4 Star Couplers
The role of a star coupler, as seen in Fig. 8.5, is to combine the optical signals entering
from its multiple input ports and divide it equally among its output ports. In contrast
with demultiplexers, star couplers do not contain wavelength-selective elements, as
they do not attempt to separate individual channels. The number of input and output
ports need not be the same. For example, in the case of video distribution, a relatively
small number of video channels (say 100) may be sent to thousands of subscribers.
The number of input and output ports is generally the same for the broadcast-and-select
LANs in which each user wishes to receive all channels (see Fig. 8.5). Such a passive
star coupler is referred to as anN×Nbroadcast star, whereNis the number of input (or
output) ports. A reflection star is sometimes used for LAN applications by reflecting
the combined signal back to its input ports. Such a geometry saves considerable fiber
when users are distributed over a large geographical area.
Figure 8.14:An8×8 star coupler formed by using twelve 2×2 single-mode fiber couplers.