Download (direct link):
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 technologies used are micro-bubbles, and holograms. Large OXC switch fabrics can be constructed using 2 x 2 switches arranged in a multi-stage interconnection network, such as a Banyan network and a Clos network. A 2 x 2 switch is a 2 x 2 directional coupler which can direct the optical signal on any input to any output j. There are various types of 2 x 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 x N micro-mirrors arranged in a crossbar (see Figure 8.24(a)). 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 8.24(b)) 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
OPTICAL FIBERS AND COMPONENTS
?2 ... ?2 Ó ¦ Ó ¦ ¦ Uoor ¡ j ¡
(a) 2D MEMS cross-bar
Figure 8.24 2D MEMS switching fabric.
down position. In this way, the incoming light will be reflected on the (i,j)th 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 x 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 add/drop 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 adds/drops can be either statically or dynamically configured. An OADM can also add/drop 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 x 2 optical switch. The wavelength is switched to the lower output port of the 2 x 2
Figure 8.25 A logical design of an OADM.
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 x 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 x 2 optical switch for each wavelength. For example, assume that wavelength is to be dropped. Then its 2 x 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 , which is directed to the lower input port of the same 2 x 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 x 2 optical switches are multiplexed and propagated out onto the link.