Optical clock distribution in electronic systems

Techniques for distribution of optical signals, both free space and guided, within electronic systems has been extensively investigated over more than a decade. Particularly at the lower levels of packaging (intra-chip and chip-to-chip), miniaturized optical elements including diffractive optics and micro-refractive optics have received considerable attention. In the case of optical distribution of data, there is the need for a source of optical power and a need for a means of modulating the optical beam to achieve data communications. As the number of optical data interconnections increases, the technical challenges of providing an efficient realization of the optical data interconnections also increases. Among the system signals which might be transmitted optically, clock distribution represents a substantially simplified problem from the perspective of the optical sources required. In particular, a single optical source, modulated to provide the clock signal, replaces the multitude of optical sources/modulators which would be needed for extensive optical data interconnections. Using this single optical clock source, the technical problem reduces largely to splitting of the optical clock beam into a multiplicity of optical clock beams and distribution of the individual clacks to the several portions of the system requiring synchronized clocks. The distribution problem allows exploitation of a wide variety of passive, miniaturized optical elements (with diffractive optics playing a substantial role). This article reviews many of the approaches which have been explored for optical clock distribution, ranging from optical clock distribution within lower levels of the system packaging hierarchy through optical clock distribution among separate boards of a complex system. Although optical clock distribution has not yet seen significant practical application, it is evident that the technical foundation for such clock distribution is well established. As clock rates increase to 1 GHz and higher, the practical advantages of optical clock distribution will also increase, limited primarily by the cost of the optical components used and the manufacturability of an overall electronic system in which optical clock distribution has been selectively inserted.