Possible Application of Flash-Type SFQ A/D Converter to Optical Communication Systems and Their Measuring Instruments

We propose a possible application of a high-speed flash type single flux quantum (SFQ) analogue-to-digital converter (ADC) to optical communication systems and their measuring instruments. One of the problems in optical networks is the chromatic dispersion over a standard single mode fiber, which has to be recovered by optical dispersion compensation. Recently, there has been great interest in 100-Gbit/s Ethernet technologies as next generation optical networks in which chromatic dispersion is more severe. Among several types of architectures, digital coherent optical communication system is a promising technology. Correction of the dispersion is performed electrically after digital conversion without any optical compensation, which would ease system construction. Digital storage oscilloscopes with high-speed ADCs are crucial for investigating the architecture and for constructing 100-GbE systems. A flash-type SFQ ADC is attractive for its high-speed sampling clock capability, especially as an instrument for a digital coherent receiver. The maximum operating frequency of the comparator in ADCs was studied by simulation for various critical current densities (J_c). The simulation suggested that using J_c of 20-40 kA/cm² enables the comparator to operate at a clock frequency of more than 100 GHz. An O/E converter, a uni-traveling-carrier photodiode, operable at low-temperature, has been developed and 40-GHz operation was confirmed at 5 K. We have also designed a 1-bit slice of a superconducting quantum interference device-(SQUID) wheel-type ADC and its test circuit including a doubler circuit and two driver types.