Photo-detectors and Optical Receivers

COMPONENTS 199 to be directed to output fiber N . As can be seen, this cannot happen since λ W 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 λ W can be converted to any other wavelength which happens to be free on output fiber N , so that the optical signal of λ W 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 wave- lengths 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. An OXC should have a low insertion loss and low crosstalk. Insertion loss is the power lost because of the presence of the switch in the optical network. Crosstalk occurs within the switch fabric, when power leaks from one output to the other outputs. Crosstalk is defined as the ratio of the power at an output from an input to the power from all other inputs. Finally, we note that an OXC should have low polarization-dependent loss. There are several different technologies for building a switch fabric of an OXC, such as multi-stage interconnection networks of directional couplers, digital micro electronic mechanical systems MEMS, and semiconductor optical amplifiers SOA. Other technolo- gies used are micro-bubbles, and holograms. Large OXC switch fabrics can be constructed using 2 × 2 switches arranged in a multi-stage interconnection network, such as a Banyan network and a Clos network. A 2 × 2 switch is a 2 × 2 directional coupler which can direct the optical signal on any input to any output j . There are various types of 2 × 2 switches, such as the electro-optic switch, the thermo-optic switch, and the Mach-Zehnder interferometer . MEMS and SOA are promising technologies for constructing all optical switches, and are described below. MEMS optical switch fabrics Micro electronic mechanical systems MEMS are miniature electro-mechanical devices that range in dimension from a few hundred microns to millimeters. They are fabricated on silicon substrates using standard semiconductor processing techniques. Starting with a silicon wafer, one deposits and patterns materials in a sequence of steps in order to produce a three-dimensional electro-mechanical structure. MEMS are complex devices, but they are robust, long-lived, and inexpensive to produce. Optical MEMS is a promising technology for constructing all optical switches. Below, we describe a 2D MEMS, 3D MEMS , and 1D. The 2D MEMS optical switch fabric consists of a square array of N × N micro-mirrors arranged in a crossbar see Figure 8.24a. Each row of micro-mirrors corresponds to an input port, and each column of micro-mirrors corresponds to an output port. Also, each input and output port of the crossbar is associated with a single wavelength. A micro-mirror is indicated by its row number and column number. A micro-mirror see Figure 8.24b consists of an actuator and a mirror, and it can be either in the down or up position. For an incoming wavelength on input port i to be switched to output port j , all of the micro-mirrors along the ith row, from column 1 to port j − 1 have to be in the down position, the micro-mirror in the i, j position has to be up, and the micro-mirrors on the jth column from rows i + 1 to N have to be in the