Fiber-optic delay lines for broad-band signal processing

The use of optical fibers as tapped delay lines, in particular, transversal filters, for broadband microwave signal processing applications is discussed. Since optical fibers, especially single-mode fibers, have very low attenuation and near zero dispersion, which allows much larger time-bandwidth products than have been achieved with conventional techniques, they can be implemented in many configurations that can perform signal processing with bandwidths of many gigahertz. The design of single-mode optical fiber transversal filters and their construction using a cascade synthesis technique is discussed. Since only very few bidirectional couplers are needed to achieve a large number of taps, and therefore a high quality factor, this synthesis technique is very simple and cost effective, especially when multiport star couplers are used. However, since single-mode stars are not yet commercially available, the devices presented were fabricated using 2×2 bidirectional couplers. Theoretical analysis of the transfer functions of the filters and experimental results are presented. The cascade technique is extended to build more complex filter structures: many cascade filters are cascaded to form multicascade filters. The improved characteristics attained by such filters are discussed