High-Sensitivity High-Resolution Optical Phase Shift Detection Technique Using a Si Photodiode Operating in Photovoltaic Mode

In this paper, we report the optimization of the operating conditions and the maximization of the performances of the phase shift detection technique allowing to approach the best sensitivity and resolution in terms of light power variations. The system is based on the synchronous demodulation technique and uses a Si photodiode operating in photovoltaic mode biased through a small amplitude sinusoidal modulating signal. Employing a commercial lock-in amplifier for phase measurements and varying the signal modulating frequency, we prove that it is possible to obtain the maximum theoretical phase shift variation range equal to 90° with a resulting sensitivity and resolution equal to 3100°/uW and 3 pW, respectively. We demonstrate that these results are more than two orders of magnitude greater than those ones achievable by employing a Si photodiode in photoconductive mode. As a case-example for chemical applications, we apply the proposed technique to detect the variations of the molar concentration of a methylene blue solution by measuring the variations of the transmittance of a laser beam passing through the substance. The experimental findings are compared with those ones achieved by using a commercial spectrophotometer and the conventional amplitude detection technique employing the lock-in amplifier. Despite the achieved results are limited by the fixed phase resolution of the employed lock-in amplifier, here, the proposed approach allows to measure molar concentration variations with a resolution of 79 pM resulting 1354 times higher than that one obtained using the spectrophotometer and 33 times better than the value achieved with the amplitude detection technique.