An Introduction to Photonic Switching Fabrics

As our information-hungry society moves toward ubiquitous broadband services there will be the need for telecommunications switching systems able to switch and control large numbers of users sending and receiving this high-bit-rate information. Aggregate capacities of these future systems could exceed 1 Tb/s by the turn of the century. Some of the new services that will require these large capacities include the transport and switching of NTSC video, enhanced-quality television (EQTV), high-definition television (HDTV), switched video, high-data-rate file transfers and information retrieval, animated graphics, in addition to the need for an interconnect for diskless workstations and local area networks/metropolitan area networks (LAN/MAN). These new services are the future of telecommunications companies and thus the driving force to bring photonics into switching systems.

The purpose of this chapter is to introduce the photonic switching systems designers to some of the optically transparent or relational devices that can be used as building blocks in constructing larger photonic switching systems. By understanding the basic properties and attributes of these devices, the systems designer can determine the limitations that will constrain the systems he or she designs. Finally, it should be understood that the material in this chapter has been selected to teach the basic properties and attributes of several optically transparent devices from a systems perspective rather than from a device physics viewpoint. The design of these devices is beyond the scope of this book.

This chapter covers systems that use optically transparent devices for space-division switching, time-division switching, and spectral-division switching. Some systems that we will discuss, especially those classified as using spectral-division switching, may contain both optically transparent and optical logic components, but the optical logic components are usually at the edge of the network and the information is distributed principally through optically transparent devices. Since optical logic devices are not covered until Chapter 4, some devices are introduced in a rudimentary way when their characteristics are important to the system under consideration. More rigorous device descriptions can be found in Chapters 2 and 4.

Free-space digital optics relies on optical devices which may be required to play the same role as a transistor in digital electronics. In addition to being an optical port, i.e., a modulator or emitter and a detector, they may also be required to act as a thresholding device. Such devices are called optically bistable (OB). OB devices are optical elements which, over some range of light input powers, have two possible output states. The range of powers over which they are bistable corresponds to a region of hysteresis and is bounded by two discontinuities at which switching between the two states can occur. Such switching can be induced by holding close to one of these discontinuities and making an incremental change in the total light input. This power increment can derive from an independent (signal) input. It is possible to obtain a change in output larger than the signal input and hence achieve digital gain.

Free-space digital optics is a topic based on many disciplines: nonlinear optics, computer and switching network architectural design, semiconductor physics, mechanical design, and, of course, optical system design. Initial work in this area concentrated on the discovery and development of nonlinear optical effects with which to form optical switching devices or logic gates. Progress on the device front stimulated research on switching and computing architectures to capitalize on the potential advantages of free-space digital optics. However, without arrays of practical devices, realistic demonstrations of these architectures were not possible. With the development of batch-fabricated symmetric SEEDs, nonlinear interference filters, and liquid-crystal and magneto-optic spatial light modulators, more complex system experiments became possible.(1–5) The demonstration of these experiments required careful attention to the optical and opto-mechanical system design, in addition to significant device and architectural research.

Photonic switching architectures that are based on logic devices have been under study since the early 1980s. Logic-based systems can be subdivided into two broad categories: (1) guided wave systems based on logic devices and (2) free-space systems based on logic devices.