IV. CONCLUSIONS

　 The performance of a traveling-wave type LiNbO3 optical modulator with superconducting electrodes has been studied theoretically as well as experimentally.　 In the case of velocity matching between signal and optical waves using a shielding plane on top of the coplanar waveguide, numerical calculations of the attenuation constants of both superconducting and normal-conducting striplines indicate that the performance of the optical　 modulator is expected to be much superior to that using normal metals with respect to the figure of merit of bandwidth / driving-voltage.

　We have carried out the experiment on the velocity-matched traveling-wave type LiNbO3 optical modulator whose electrode is a superconductor (NbN), and demonstrated the microwave modulation characteristics as expected from theory.　 From this experiment using low Tc superconductors we can see the dramatic improvement of the optical modulator performance by introducing superconducting electrodes. Studies on the resonant-type optical modulators with high Tc superconducting electrodes are in progress.

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