High-Sensitivity Differential Interface for the Detection of Energy Variations of Nanosecond Laser Pulses for Spectroscopic Applications

This paper reports on a photodiode bridge interface circuit that is able to perform differential measurements of the energy variations of 10-ns laser pulses obtained by differentially detecting the photocurrents generated by a reference and a signal photodiode. A complete characterization of the interface circuit is reported by varying its gain and the energy-per-pulse difference between the signal and the reference laser pulses at a 20-Hz operating frequency (i.e., the laser repetition rate). Under these conditions, a maximum detection sensitivity of about 7 mV/fJ has been achieved. The functionality of the interface circuit has been tested by detecting the Rhodamine-6G photoluminescence emission after pumping the sample with 10-ns laser pulses at a 20-Hz repetition rate. In particular, the Rhodamine-6G was diluted in ethanol, and the interface circuit has been used to measure the photoluminescence emission as a function of the variations of its molar concentration. The experimental findings demonstrate the possibility to achieve interface circuit sensitivities up to 1225 mV/
$\mu \text{M}$
corresponding to a minimum detection resolution of 0.8 pM with a measured rms output noise level of about
$1~\mu \text{V}$
. It is also reported that these results enhance more than three orders of magnitude, the sensitivity achieved by using a standard/commercial lock-in amplifier in a differential measurement configuration.