Measuring corona discharge current directly on the
high-voltage side is essential for the discharge mechanism research
of the test object. The amplitude of the corona discharge current
is in microamperes, and its frequency ranges from dc to
megahertz. An optical microcurrent transducer for measurement
of the current is described in this paper based on an amplitude
modulation principle. A low-pass filter is used to extend the analog
fiber link to recover low-frequency signals. The uncertainty of
the transducer is estimated to be less than 1.3% in ratio, and its
response frequency range is from dc to several megahertz.