The optically coupled current mirror (OCCM) is a novel optoelectronic architecture that replicates at ground potential a current flowing in an isolated conductor. The mirroring action is done without need to bring any power to the potential of the conductor where the current flows, as its input stage is completely passive. This feature is particularly important when the conductor is at high voltage (HV), a typical case being the photomultiplier tubes (PMTs) biased with positive HV, or when the number of channels is very large as it is the case of many particle physics experiments. The OCCM has been used to equip the two prototype telescopes, each one comprising 440 PMTs, of the ultra-high-energy cosmic-ray experiment Pierre Auger. Despite that the PMT's anodes were at HV, the DC or slowly varying component of the anode current was measured with high resolution, clearly revealing the presence of faint stars, planets, and other similar objects which determine the sky background light. In this paper we describe the principle of operation of the OCCM and its application to the measurement of the anode current of PMTs biased with cathode grounded, as well as the first results obtained with the OCCM on linear transmission of 2-μs-width pulses, via fiber optics.