Optical Amplifiers COMPONENTS

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 200 OPTICAL FIBERS AND COMPONENTS a 2D MEMS cross-bar b Micro-mirror Down Actuator Mirror Up … … … … … … … … … … … … … … … … i j Input ports Output ports Figure 8.24 2D MEMS switching fabric. down position. In this way, the incoming light will be reflected on the i, jth micro-mirror and redirected to the jth output port. The micro-mirrors are positioned so that they are at 45 ◦ angle to the path of the incoming wavelengths. The incoming wavelengths have to be collimated i.e., they travel exactly in the same direction. Micro-mirror control is straightforward, since it is either up or down. The number of micro-mirrors increases with the square of the number of the input and output ports. Therefore, 2D architectures are limited to 32 × 32 ports or 1024 micro-mirrors. The main limiting factors being the chip size and the power loss due to the distance that the light has to travel through the switch. The 2D MEMS architecture can be used to construct an optical adddrop multiplexer OADM . This device is connected to a WDM optical link and it can drop i.e., terminate a number of incoming wavelengths and insert new optical signals on these wavelengths. The remaining wavelengths of the WDM link are allowed to pass through. The specific wavelengths that it addsdrops can be either statically or dynamically configured. An OADM can also adddrop wavelengths from a number of WDM links. A logical diagram of an OADM is shown in Figure 8.25. The optical signal on the WDM link is demultiplexed, and each wavelength is directed to the upper input port of a 2 × 2 optical switch. The wavelength is switched to the lower output port of the 2 × 2 … Add wavelengths Terminate wavelengths l 1 ,l 2 ..,l W l 1 ,l 2 ..,l W Figure 8.25 A logical design of an OADM. COMPONENTS 201 switch if it is to be dropped, or to the upper output port if it is allowed to pass through. The lower input port of the 2 × 2 optical switch is used for the wavelength to be added in; it is always switched to the upper output port of the switch. There is one 2 × 2 optical switch for each wavelength. For example, assume that wavelength λ i is to be dropped. Then its 2 × 2 optical switch is instructed to direct the wavelength coming into its upper input port to the lower output port. At the same time, a new data stream is modulated onto the same wavelength λ i , which is directed to the lower input port of the same 2 × 2 optical switch. This new added wavelength is switched to the upper output port. All of the wavelengths that exit from the upper output ports of the 2 × 2 optical switches are multiplexed and propagated out onto the link. This OADM can be easily implemented using the 2D MEMS architecture. As shown in Figure 8.26, each micro-mirror is a pair of micro-mirrors, which operate simultane- ously. That is, they both go up or down at the same time. Assume that the ith incoming wavelength has to be dropped. Then the i, ith micro-mirror pair is activated; that is, it goes up. All of the other micro-mirrors on the ith row and ith column are down. The incoming light from the demultiplexer is reflected on one of the two micro-mirrors and is directed into the ith drop port. At the same time, the new wavelength from the ith add port is reflected off of the other micro-mirror and is directed to the output port. If the ith wavelength is not to be dropped, then all of the micro-mirror pairs on the ith row are not activated. That is, they are all down, and the ith wavelength simply passes through the OADM uninterrupted. In the 2D MEMS architecture, all of the light beams reside on the same plane – thus the name of the architecture. In the 3D MEMS architecture, the light beams travel in a three-dimensional space, which allows scaling far beyond 32 ports. In the 3D MEMS architecture, each micro-mirror is gimbaled in two dimensions see Figure 8.27. The micro-mirror is attached to an inside ring so that it can rotate over the x axis. The inside ring is attached to an outside ring so that it can rotate on the y axis. Using this gimbaled mechanism, the micro-mirror can rotate freely about two axis. The 3D MEMS architecture is shown in Figure 8.28. There is a MEMS array of micro- mirrors associated with the input fibers, and each micro-mirror is dedicated to an input fiber. Likewise, there is another MEMS array of micro-mirrors associated with the output Add wavelengths Drop wavelengths l 1 ,l 2 ..,l W l 1 ,l 2 ..,l W ·· ·· ··· ··· ··· ··· ··· ··· ··· ··· ·· ·· ··· ··· i Figure 8.26 A 2D MEMS OADM.