Photodiode bridge-based differential readout circuit for high-sensitivity measurements of energy variations of laser pulses for optoelectronic sensing systems

In this paper we present an optoelectronic architecture based on a current-mode photodiode bridge readout circuit that performs differential measurements of the energy variations of nanosecond laser pulses. This operation is achieved by differentially detecting the currents photogenerated by two Si photodiodes one of which is used as the reference signal and the other measures the signal coming from variations of the laser pulse energy. The circuit has been designed for high-speed operations (i.e., large bandwidth) and its features can be simply optimised as a function of the laser pulse-width (e.g., in the nanosecond and subnanosecond regime) and of its repetition rate from few Hz up to few MHz. Moreover, the electronic circuitry allows to achieve the initial photodiode bridge balancing condition by variable control voltages so zeroing the overall interface offsets due to optical and electronic device mismatches and initial bridge unbalance conditions. The readout circuit has been fully characterised by performing both electrical and optical measurements and by varying its gain, detection sensitivity and energy-per-pulse changes between the signal and the reference employing 10ns laser pulses at a repetition rate of 20Hz. These measurements have been compared with those ones obtained using a standard commercial lock-in amplifier as the conditioning circuit under the same experimental conditions. The proposed optoelectronic architecture is able to reach a maximum detection sensitivity of 7m V/ fJ corresponding to a detection resolution of the laser pulse energy variations equal to 0.14×10 -3 fJ. These results prove an increase of 1225 in the detection sensitivity and resolution obtained by employing a lock-in amplifier in a differential measurement configuration.