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IEEE Transactions on Applied Superconductivity

Optically and thermally tunable superconducting transmission lines

Shinho Cho, Chang‐Sik Son, Jonghun Lyou

Published Jun 1, 2003• 0 citations
Publisher

Abstract

Using the method of time-domain optoelectronic sampling spectroscopy, we have tuned the ultrafast electrical pulse propagating through YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) transmission lines as a function of temperature and optical pulse energy. Electrical pulses are generated by photoconductively shorting charged semiconducting optical switches with 70 psec optical pulses at 532 nm. The optically generated and detected electrical pulses have frequencies extending up to 100 GHz. The superconducting transmission lines are microstrip spiral lines on the LaAlO/sub 3/ substrates. By varying the optical pulse energy from 0 to 23 pJ, the delay time through the 66-mm-long superconducting line is tuned by 54 psec at 55 K and shows a quadratic dependence on the optical pulse energy. As for the 250 mm-long transmission line, the delay of 69 psec is observed as the temperature increases from 26 K to 63 K.

DOI
10.1109/tasc.2003.814141

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Optical modulation is investigated for superconducting microstrip delay lines on 2-in-diameter, 10-mil-thick LaAlO/sub 3/ substrates. The linewidth of 80 mu m yields a characteristic impedance of 50 Omega . The linepitch of 100 mil is chosen to minimize the coupling between adjacent lines. The total meander line length is 64 cm. The measured delay is 8 ns with operation from DC to 20 GHz. The design was implemented using niobium, niobium nitride, and YBCO. The measurements were performed in a dewar and in a closed-cycle refrigerator, with both RF and optical access. Optical illumination of the lines resulted in a phase shift of the transmitted signal. Using a 10-mW He-Ne laser, phase shifts of up to 360 degrees at 20 GHz were measured with NbN and YBCO. The phase shifts increased linearly with frequency and with optically intensity. The effect can be attributed to a small, bolometric modulation of the line inductance.>
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The ballistic transport of picosecond digital signals along terminated microstrip lines is a key feature of single flux quantum (SFQ) logic. In this work, we have used fast photoconductors driven by a femtosecond laser to measure directly for the first time the propagation and termination of picosecond pulses along Nb microstrip lines. Electrical pulses as short as 0.8 ps full-width-at-half-maximum (FWHM) have been measured. After propagating distances of 200 /spl mu/m, 1 mm, and 6.5 mm, the pulses broadened to 1.0, 1.2, and 1.8 ps (FWHM), respectively. In the frequency domain, attenuation is found to be negligible up to the gap frequency of Nb(0.7 THz), beyond which attenuation increases rapidly. We have also measured the reflections of the picosecond pulses off different terminations: open circuit, short circuit, and various resistors. The effective suppression of reflections with matched resistive loads is demonstrated up to frequencies approaching 1 THz.>