What ultimately limits capacity and connectivity in optical interconnects?

We revisit some fundamental laws of physics governing light propagation in linear media in order to have a better understanding of the roles of optics for parallel interconnections of digital processors. The degree of freedom obtainable using optical techniques can be derived from the domains of space, spatial frequency, time, and temporal frequency. We pinpoint various fundamental barriers and practical limits of optics for handling point-to-point communications. Possible schemes of using optical techniques to maximize the information transmission capacity while minimizing the channel complexity are also discussed.