I. INTRODUCTION
In the high-speed optical communications of 10 Gbit/s, external
optical modulators using an electro-optic material LiNbO3 (referred to
as LN hereafter) [1]-[3] and those using semiconductor devices[4] have
already been put to practical use in place of the direct modulation of
semiconductor lasers, owing to their low chirping and high-speed modulation
characteristics. There have still been great needs for realizing larger
bandwidths and smaller driving voltages for these optical modulators due
to increasing demands for multimedia communications.
Among various optical modulators with LiNbO3, an optical modulator
with a transmission line for a traveling-wave signal (a traveling-wave
type optical modulator) is widely used. The factors limiting the performance
of this modulator are known to be i) phase-velocity difference between
an optical wave and a signal wave (velocity mismatch), and ii) attenuation
of a signal wave in the transmission line. The first problem has been
solved by devising a novel electrode shape [2] or introducing a shielding
plane and a buffer layer [3], [10]. In order to overcome the latter problem,
we have proposed to introduce a superconducting electrode which has extremely
low loss and low dispersion instead of a normal-conducting electrode. It
has been shown theoretically [5] that the performance of the traveling-wave
type optical modulator with a superconductor electrode is far superior
to that with a normal-conductor electrode, and preliminary experiments
in the case of velocity mismatch have been reported earlier [5]- [8].
In this paper we have fabricated a superconducting shielding plane
for velocity matching and studied the microwave characteristics of the
velocity-matched traveling-wave type LiNbO3 optical modulator with superconductor
electrodes (NbN) in the frequency range between dc and 26.5 [GHz], and
demonstrated the theoretical prediction.
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