Due to the uncharged nature of photons, optical interconnections potentially offer freedom from mutual coupling effects. This characteristic greatly differentiates optical interconnections from electrical ones based on charged carriers (i.e., electrons). Because the mutual coupling between electrical interconnects increases with increasing signal frequency, the advantage of optical interconnects becomes more important as the need for interconnect bandwidth increases. Although advanced dielectrics can overcome the electrical coupling effects at the lower frequencies, at frequencies approaching the GHz range the frequency dependence of known dielectrics leads to severe limitations for electrical interconnects at the bandwidths that will be required for PetaFLOPS computers.
The uncharged nature of photons leads to another advantage for optical interconnects in PetaFLOPS computers: lower power consumption for the longer interconnects. With electrical interconnects the charge of the electrons leads to a distributed capacitance along the interconnects. Consequently, to accompany the resistivity of metallic conductors, electrical interconnects have an energy requirement that increases with increasing interconnect length. But, for optical interconnects, the energy requirement lies with the optical source and the optical detector rather than distributed along the interconnect. Therefore, beyond some distance (dependent on many link parameters), the optical interconnect becomes more energy efficient. Since this break-even distance can be as small as subcentimeter, optical interconnects can lead to a considerable energy savings for large computing systems such as PetaFLOPS computers, that likely will have hundreds of thousands of interconnects longer than a centimeter (or whatever the break-even distance is).
Optical interconnects have numerous other advantages that may be important to PetaFLOPS computing, such as the ability to frequency multiplex optical signals, electrical isolation between optically interconnected electronic circuits, increased fan-out capability (due to a freedom from capacitive loading effects), and a greater flexibility of routing because optical beams can pass through one another, and the freedom from routing signals in the presence of ground planes. But, the advantages of larger bandwidths and lower energy requirements for the longer interconnects are the most important reasons for optics in PetaFLOPS computing systems. Figure 5.1 illustrates the frequency (bandwidth) and distance limitations of electrical interconnects. It clearly shows that the 10-100 GHz clock speeds projected for PetaFLOPS systems will require these systems to be heavily dependent on optical interconnects.
Optical interconnects may be divided into two classes: guided (fibers and waveguides) and free-space. Since each class has a very different application within PetaFLOPS systems, they will be addressed separately.