Linear transmission of analogue current-signals via fibre optics using the Optically-Coupled Current-Mirror (OCCM) architecture

Abstract

The Optically-Coupled Current-Mirror (OCCM) is a circuit architecture based on a novel optical feedback loop, that allows mirroring a current flowing in an isolated, or high voltage biased conductor, to ground potential. The OCCM has a simple but powerful structure: its input stage is a passive dipole consisting on an LED and a photodiode connected back-to-back. When the dipole is interposed in series with the conductor where a current signal flows, the signal is mirrored to ground potential while keeping full Galvanic isolation with the conductor itself. The OCCM has been used with success to mirror the quasi-DC signals left by stars in the first two Fluorescence Detector (FD) telescopes of the ultra-highenergy cosmic ray experiment Pierre Auger. In each telescope, star signals were recorded by an array of 440 photomultiplier tubes (PMTs) biased with cathode grounded. After the good results obtained with slow signals we have started an optimization of the OCCM architecture for the linear transmission of analogue signals within a bandwidth of about 5 MHz. In addition, we recently started a preliminary study on the behaviour of LEDs and photodiodes at cryogenic temperature. The results were very interesting and showed that AlGaAs LEDs at 77 K behave extremely linear also at very low signal currents, which is the normal operating condition required by the OCCM. Later on we have studied the behaviour of the OCCM with its input stage cooled to 77 K. We verified the increase of the open-loop gain by a factor x 2, and a loop-gain increase by a factor slightly lower than 2 due to a reduction in the photodiode responsivity. The improved performance of the OCCM at cryogenic temperature may open new opportunities for applications with noble liquid calorimeters and other cryogenic detectors.