8.2. WDM COMPONENTS 347
Figure 8.11: Schematic of a waveguide-grating demultiplexer consisting of an array of wave-
guides between two free-propagation regions (FPR). (After Ref. [74];©c1996 IEEE; reprinted
with permission.)
that acts as a grating. Such gratings are calledarrayed waveguide gratings(AWGs) and
have attracted considerable attention because they can be fabricated using the silicon,
InP, or LiNbO 3 technology [74]–[81]. In the case of silica-on-silicon technology, they
are useful for making planar lightwave circuits [79]. AWGs can be used for a variety
of WDM applications and are discussed later in the context of WDM routers.
Figure 8.11 shows the design of a waveguide-grating demultiplexer, also known
as a phased-array demultiplexer [74]. The incoming WDM signal is coupled into an
array of planar waveguides after passing through a free-propagation region in the form
of a lens. In each waveguide, the WDM signal experiences a different phase shift
because of different lengths of waveguides. Moreover, the phase shifts are wavelength
dependent because of the frequency dependence of the mode-propagation constant.
As a result, different channels focus to different output waveguides when the light
exiting from the array diffracts in another free-propagation region. The net result is
that the WDM signal is demultiplexed into individual channels. Such demultiplexers
were developed during the 1990s and became available commercially by 1999. They
are able to resolve up to 256 channels with spacing as small as 0.2 nm. A combination
of several suitably designed AWGs can increase the number of channels to more than
1000 while maintaining a 10-GHz resolution [82].
The performance of multiplexers is judged mainly by the amount of insertion loss
for each channel. The performance criterion for demultiplexers is more stringent. First,
the performance of a demultiplexer should be insensitive to the polarization of the
incident WDM signal. Second, a demultiplexer should separate each channel without
any leakage from the neighboring channels. In practice, some power leakage is likely to
occur, especially in the case of dense WDM systems with small interchannel spacing.
Such power leakage is referred to as crosstalk and should be quite small (<−20 dB)
for a satisfactory system performance. The issue of interchannel crosstalk is discussed
in Section 8.3.