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A generalization of the 2 x 2 coupler is the star coupler. This device combines the power from N inputs and then divides it equally on all of the outputs. One popular method of constructing a star coupler is to use the fiber-fused technology. This involves twisting, heating and stretching N fibers together. The resulting device can be used to split evenly an incoming light to N outputs, or it can combine N incoming lights to a single output, or it can combine N incoming lights and distribute evenly to N outputs. However, due to difficulties in controlling the heating and pulling process, fiber-fused technology is limited to a small number of fibers.
An alternative way to construct a star coupler is to combine 3-dB couplers in a Banyan network (see Figure 8.22). Each box in Figure 8.22 represents a 3-dB coupler. The number of 3-dB couplers required for an N x N Banyan network is (N/2) log2 N. Each input is associated with a different wavelength (see Figure 8.22). The star coupler combines all of the wavelengths together and then evenly distributes them on all of the output ports. It can also be used as 1-to-N splitter (i.e., a demultiplexer) or an N-to-1 combiner (i.e., a multiplexer).
8.3.5 Optical Cross-connects (OXCs)
An optical cross-connect (OXC) is an N x N optical switch, with N input fibers and N output fibers. The OXC can switch optically all of the incoming wavelengths of the input fibers to the outgoing wavelengths of the output fibers. For instance, it can switch the
Figure 8.22 A banyan network of 3-dB couplers.
OPTICAL FIBERS AND COMPONENTS
optical signal on incoming wavelength Xt of input fiber k to the outgoing wavelength Xt of output fiber m. If it is equipped with converters, it can also switch the optical signal of the incoming wavelength Xt of input fiber k to another outgoing wavelength Xj of the output fiber m. This happens when the wavelength Xt of the output fiber m is in use. Finally, an OXC can also be used as an optical add/drop multiplexer (OADM). That is, it can terminate the optical signal of a number of incoming wavelengths and insert new optical signals on the same wavelengths in an output port. The remaining incoming wavelengths are switched through as described above.
An OXC consists of amplifiers, multiplexers/demultiplexers, a switch fabric, and a CPU (see Figure 8.23). The CPU is used to control the switch fabric and to run communications-related software, such as routing, signaling, and network management. There are N input and N output optical fibers; each fiber carries W wavelengths X17X2,..., XW. The optical signal from each input fiber is pre-amplified and then it is demultiplexed into the W wavelengths. Each wavelength enters the switch fabric through an input port; the switch fabric then directs each wavelength to an output fiber. The W wavelengths switched to the same output fiber are multiplexed onto the same output fiber, and the multiplexed signal is amplified before it is propagated out onto the link. The switch fabric has NW input ports (one per incoming wavelength) and NW output ports (one per outgoing wavelength). Figure 8.23 gives an unfolded view of the OXC, with traffic flowing from left to right. Each input fiber i and its corresponding output fiber i (where i = 1, 2,... ,N) are associated with the same user. That is, the user transmits to the OXC on input fiber i, and receives information from the OXC on output fiber i.
The incoming wavelengths are switched to the output fibers optically, without having to convert them to the electrical domain. In view of this, such an OXC is often referred to as a transparent switch. This is in contrast to an opaque switch, where switching takes place in the electrical domain. That is, the input optical signals are converted to electrical signals, from where the packets are extracted. Packets are switched using a packet switch, and then they are transmitted out of the switch in the optical domain.
In the example given in Figure 8.23, we see that wavelengths X1 and XW of input fiber
1 are directed to output fiber N. Likewise, wavelengths X1 and XW of input fiber N are directed to output fiber 1. Let us assume that incoming wavelength XW of input fiber k has
Figure 8.23 A logical diagram of an OXC.
to be directed to output fiber N. As can be seen, this cannot happen since XW of output fiber N is in use. (This is known as external conflict.) However, if the OXC is equipped with a wavelength converter, then the incoming wavelength kW can be converted to any other wavelength which happens to be free on output fiber N, so that the optical signal of kW can be directed through to output fiber N. Wavelength converters, which can be made using different types of technologies, are very important in optical networks.
OXCs are expected to handle a large number of ports, with a large number of wavelengths per fiber. They are also expected to have a very low switching time. This is the time required to set up the switch fabric so that an incoming wavelength can be directed to an output fiber. The switching time is not critical for permanent connections, but it is critical for dynamically established connections; it is also critical in OBS (see Chapter 10).