Portable Lock-In Amplifier-Based Optoelectronic Readout Circuit for High-Sensitivity Differential Measurements of Laser Pulse Energy Variations

This paper presents a complete integrable optoelectronic architecture based on a current-mode photodiode bridge readout circuit, including an ad-hoc Lock-In Amplifier (LIA), that performs differential measurements of the energy variations of nanosecond laser pulses. The bridge hosts two Si photodiodes: one is used as the reference signal probing the energy of the laser pulse travelling along an unperturbed optical path and the other measures the energy variations eventually occurred in the laser pulses propagating along the sample optical path. The differential measurements are accomplished by detecting the pulsed currents (i.e., the replica of the laser pulses) generated by the two photodiodes obtaining a single output pulsed voltage measured through the LIA. The electronic circuitry allows achieving the initial photodiode bridge balancing condition by variable control voltages. The entire system has been designed for low-voltage, low-power and high-speed operations (i.e., large bandwidth) and its features can be simply optimized as a function of the laser pulse width (e.g., in the nanosecond and sub-nanosecond regime) and repetition rate (e.g., from few Hz up to few MHz). The complete readout circuit was characterized by performing optical measurements employing 10 ns laser pulses at a repetition rate of 20 Hz. The maximum detection sensitivity was 65.9 mV/fJ with the minimum resolution of 1.5 × 10–3 fJ. These results show that the system allows to achieve a maximum detection sensitivity enhancement equal to 11763 with respect to the use of only a commercial LIA under the same experimental conditions.