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IEEE Transactions on Applied Superconductivity

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

Hideo Suzuki, Michitaka Maruyama, Y. Hashimoto, K. Fujiwara, M. Hidaka

Published Jun 1, 2009• 10 citations

Abstract

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 (Jc). The simulation suggested that using Jcof 20-40 kA/cm2enables 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.

DOI: 10.1109/tasc.2009.2018034

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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 (Jc). The simulation suggested that using Jcof 20-40 kA/cm2enables 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.

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Investigation of a 5-bit Flash-Type SFQ A/D Converter Using 10

\hbox{kA/cm}^{2}

Niobium Process and Locally Isolated Grounds

We have investigated technologies for high-frequency and stable operation in a 5-bit flash-type single-flux quantum analog-to-digital (A/D) converter. We designed and evaluated several comparators with different designs of the transformer and found that the length of the primary line was the principal factor to determine the bandwidth, which was improved from 7 to 13 GHz by reducing the length from 860 to 290 μm. We have also investigated methods to avoid incorrect operation caused by ground return current of the bias current: one is locally extracting the return current around the component circuits, and the other is to use locally isolated ground for each component circuit. An integrated 5-bit A/D converter with an error correction circuit was designed using the locally isolated ground structure, which can avoid ground return current inflow from other component circuits. The A/D converter was operated without extracting ground return current and 10 GHz operation was carried out by means of the beat frequency method in our cryocooling system. Operation for an optical input signal was also observed. In addition, 100 GHz sampling clock operation with 1 kHz input signal was confirmed.