A large communications network can be pictured as having two main parts: a transmission plant and switching facilities. The first transports traffic between network nodes, while the second routes traffic over the transmission plant to get it from its source to its destination. In recent years, optical transmission technology has progressed much faster than optical switching, with interesting consequences.
The equipment available for switching optical signals today is almost all of the hybrid optical-electronic-optical (O-E-O) type, which is expensive to build, integrate, and maintain. As a result, these switches have not been widely deployed. So, while telephone companies can carry tremendous amounts of information between fixed points, they have little ability to accommodate changes in traffic patterns in real time.
O-E-O switches separate incoming optical signals into individual wavelengths (optical demultiplexing), convert each wavelength into a single high-speed electronic data stream, and demultiplex the high-speed data streams into many low-speed channels. They then route each channel path digitally, combining (multiplexing) groups of low-speed channels into high-speed streams and modulating each high-speed stream onto an optical wavelength. Finally, through optical (wavelength-division) multiplexing, they place many of the optical wavelengths onto an optical fiber. Since there is no optical path from input to output, these switches are called "opaque."
The advantages of this approach are powerful. Since each data stream has been converted to electronic form, each stream can be monitored and dynamically routed independent of all the others. But the drawbacks are equally formidable. Not only are O-E-O switches expensive, they are also incapable of handling signals that do not conform to standard data rates and formats. They consume kilowatts of power. And, although an O-E-O switch can route individual packets, it requires a variable amount of time to read and interpret a received packet's header information, and then to deliver the packet to the correct output channel.
The result is a delay, or latency, that can range from microseconds to hundreds of milliseconds. This variable delay may constitute a fatal shortcoming in the future, when even real-time traffic like voice and video will be carried over packet-switched networks. Certainly it can be devastating to streaming multimedia communications.
What the telecommunications industry is crying out for are all-optical (also called photonic, or transparent) switches in which optical signals are routed without intermediate conversion into electronic form. Of course, those switches should be cheap and capable of dealing with the thousands of inputs and outputs that traditional electronic switches handle so well.
Several approaches are being explored for making the necessary devices. These include arrays of tiny movable mirrors, known as microelectromechanical systems, or MEMS, and units based on holographic crystals, liquid crystals, total internal reflection, and polarization-dependent materials. The problem is to figure out which all-optical switching technology to use in what application.
Unlike O-E-O switches, present all-optical switches (often referred to as O-O-O switches) are not by themselves capable of separately routing each of the low-speed data streams carried by a single input wavelength. Fortunately, though, that capability is not an immediate requirement for many applications. Today, O-O-O switches can direct individual wavelengths (and, of course, multiplexed groups of wavelengths) and are therefore best suited for fault recovery--that is, automatically switching in a good fiber link to replace one that has been cut or otherwise rendered inoperable--and switching fibers and wavelengths from one link to another as traffic patterns vary with the time of day or season of the year.
As for the future of O-O-O switches, experiments under way at Southampton Photonics in the UK, among other places, show that it is possible to recognize individual packet headers while signals are in the optical domain. So all-optical routing is right around the corner. Once the missing link of optical memory (or buffering) is provided, it should not be long before transparent switches have most--if not all--of the capability of their opaque brethren while greatly exceeding them in performance.
Many possible technologies are being applied to create optical switching systems. In fact, any physical process that will affect some property of light without causing too much loss can be used. Affected properties can include propagation speed, polarization, and direction. Any changes in them are exploited to redirect light beams as desired from an input to an output.
The most mature approach available is precision bulk optics, which creates robust connections. The technology takes many forms--for example, having a motor move a precision mirror surface to direct an input light beam from one output to another. Examples are Lucent Technologies' original direct beam-steering technology (implemented in various forms by Astarte Fiber Networks and Creo Products), DiCon Fiberoptics' moving prisms, and Lightpath Technologies' rotary switches.
These switches can have exceptional optical performance (low loss, reflection, and crosstalk) because they rely on highly mature manufacturing techniques. Yet there are three pronounced limitations in bulk optics that prevent the technology from sweeping the all-optical infrastructure. They are too expensive, too large, and too slow.