Since the building blocks, such as all-optical logic gates, optical switches, optical memory, and optical interconnections are all available, why then don't we have an optical computing system? There are several factors impeding the technology, the most important of which are: cascadability, material development, and funding. Cascadability to integrate a large number of all-optical gates is a highly complex problem and a major obstacle in the way of building a complete optical computing system. Binary half adders data processors, which combine several optical logic gates, are the basic building blocks of binary operations. Several of them have been successfully demonstrated . An all-optical half adder at a rate of 10Gbps was recently demonstrated by Kim et al. . Nonlinear optical mechanisms play important roles in ultra-fast, all-optical logic gates and optical switches. Most of the nonlinear mechanisms in these switches require pumping high optical power into the system for these devices to function. However, the high optical power in the system has the disadvantage of generating undesirable signals such as stimulated Brillouin scattering (SBS) and self-phase modulations (SPM), which might affect the system's reliability. Material scientists and chemists, therefore, have the challenging problem of finding materials with adequate response at low power and at the same time demonstrate reliability, speed, and optical efficiency.
Furthermore, development of an optical computer is an interdisciplinary enterprise requiring coordination and funding of optical engineers, material scientists, chemists, physicists, computer architects, and representatives of other disciplines. Government funding incentives, to fill a gap created by industry's return-on-investment requirements, can encourage formation of such teams and expedite development of optical computer systems. It is projected that the development of a bulky prototype optical computer, capable of duplicating the performance of a conventional desktop, could occur within the next 1015 years. With the current level of progress in the area of optical computing research, it may easily take up to 1520 years before optical computers commonly appear on our desktops.