352 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.16: (a) Schematic illustration of a wavelength router and (b) its implementation using
an AWG. (After Ref. [79];©c1999 IEEE; reprinted with permission.)
that the WDM signal at each port is composed of channels entering at different input
ports. This operation results in a cyclic form of demultiplexing. Such a device is an
example of a passive router since its use does not involve any active element requir-
ing electrical power. It is also called astatic routersince the routing topology is not
dynamically reconfigurable. Despite its static nature, such a WDM device has many
potential applications in WDM networks.
The most common design of a wavelength router uses a AWG demultiplexer shown
in Fig. 8.11 modified to provide multiple input ports. Such a device, called thewave-
guide-grating router(WGR), is shown schematically in Fig. 8.16(b). It consists of two
N×Mstar couplers such thatMoutput ports of one star coupler are connected with
Minput ports of another star coupler through an array ofMwaveguides that acts as
an AWG [74]. Such a device is a generalization of the MZ interferometer in the sense
that a single input is divided coherently intoMparts (rather than two), which acquire
different phase shifts and interfere in the second free-propagation region to come out of
Ndifferent ports depending on their wavelengths. The symmetric nature of the WGR
permits to launchNWDM signals containingNdifferent wavelengths simultaneously,
and each WDM signal is demultiplexed toNoutput ports in a periodic fashion.
The physics behind the operation of a WGR requires a careful consideration of
the phase changes as different wavelength signals diffract through the free-propagation
region inside star couplers and propagate through the waveguide array [74]–[81]. The
most important part of a WGR is the waveguide array designed such that the length