The optical processors described in the previous sections were analogue. However, in order to compete more efficiently with the digital electronic computers, a very important research effort was directed toward digital optical computing. Again, the field of digital optical computing is extremely broad, and the results obtained are too numerous to be described in the frame of this paper. The interested reader will find several books on the subject, for example, . The proceedings of the numerous conferences dedicated on optical computing are also very instructive. For example, the proceedings of the ICO conference "Optical Computing" held in Edinburgh in 1994 show very well the situation of digital and analog optical computing . Novel optical components such as vertical-cavity surface-emitting lasers (VCELs) or symmetric self-electro-optic-effect devices (SEED) were studied and constructed. Several digital optical computers were proposed, for example, Guilfoyle and Stone constructed a 32-bit, fully programmable digital optical computer (DOC II) designed to operate in a UNIX environment running RISC microcode .
Optical interconnects is a field where optics has a great potential, these interconnects can be guided but also in free space. All the aspects of optical interconnections were studied: components (switches, sources, detectors, etc.), architectures, routing algorithms, and so forth. In 1989, Goodman wrote a complete analysis on optics as an interconnect technology and a brief historical summary of the development of the field of optical interconnect to silicon integrated circuits can be found in a paper written by Miller in 2000 . A very large number of papers were published in the nineties on the subject, for example, on optical perfect shuffle , on hypercube-based optical interconnects , on crossbar networks , on the use of liquid crystal SLMs for optical interconnects , on diffractive optics for optical interconnects , on holographic interconnection networks and their limitations , on board-level interconnects .
Today, optics has no challenger in the domain of telecommunications with the optical fibers and optical cables, the Wavelength-division multiplexing (WDM), the optical amplifiers and the switches based on MEMs.
In 2009, Intel is still studying the possibility of replacing electrical interconnects between chips by optical interconnects with its terahertz bandwidth, low loss, and low cross-talk . Miller published also in 2009 a paper on the device requirements for optical interconnects to silicon chips where he pointed out the need of very-low-energy optoelectronic devices and novel compact optics .