338 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.6: Passive photonic loop for local-loop applications. (After Ref. [31];©c1988 IEE;
reprinted with permission.)
of available wavelengths. Moreover, each node requires many receivers (equal to the
number of nodes), resulting in a considerable investment in hardware costs.
A tunable receiver can reduce the cost and complexity of the Lambdanet. This is
the approach adopted for theRainbow network[29]. This network can support up to
32 nodes, each of which can transmit 1-Gb/s signals over 10–20 km. It makes use of a
central passive star (see Fig. 8.5) together with the high-performance parallel interface
for connecting multiple computers. A tunable optical filter is used to select the unique
wavelength associated with each node. The main shortcoming of the Rainbow network
is that tuning of receivers is a relatively slow process, making it difficult to use packet
switching. An example of the WDM network that uses packet switching is provided by
theStarnet. It can transmit data at bit rates of up to 1.25 Gb/s per node over a 10-km
diameter while maintaining a signal-to-noise ratio (SNR) close to 24 dB [30].
WDM networks making use of a passive star coupler are often calledpassive op-
tical networks(PONs) because they avoid active switching. PONs have the potential
for bringing optical fibers to the home (or at least to the curb). In one scheme, called
apassive photonic loop[31], multiple wavelengths are used for routing signals in the
local loop. Figure 8.6 shows a block diagram of such a network. The central office
containsNtransmitters emitting at wavelengthsλ 1 ,λ 2 ,...,λNandNreceivers operat-
ing at wavelengthsλN+ 1 ,...,λ 2 Nfor a network ofNsubscribers. The signals to each
subscriber are carried on separate wavelengths in each direction. A remote node mul-
tiplexes signals from the subscribers to send the combined signal to the central office.
It also demultiplexes signals for individual subscribers. The remote node is passive
and requires little maintenance if passive WDM components are used. A switch at the
central office routes signals depending on their wavelengths.
The design of access networks for telecommunication applications was still evolv-
ing in 2001 [26]. The goal is to provide broadband access to each user and to deliver
audio, video, and data channels on demand, while keeping the cost down. Indeed,
many low-cost WDM components are being developed for this purpose. A technique
known asspectral slicinguses the broad emission spectrum of an LED to provide mul-
tiple WDM channels inexpensively. Awaveguide-grating router(WGR) can be used
for wavelength routing. Spectral slicing and WGR devices are discussed in the next
section devoted to WDM components.