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


During this paper, we tend to report the optimization of the operating conditions and the maximization of the performances of the section shift detection technique permitting to approach the most effective sensitivity and backbone in terms of sunshine power variations. The system is predicated on the synchronous demodulation technique and uses a Si photodiode operating in photovoltaic mode biased through a tiny amplitude sinusoidal modulating signal. Using a commercial lock-in amplifier for phase measurements and ranging the signal modulating frequency, we tend to prove that it is attainable to obtain the maximum theoretical phase shift variation vary equal to 90° with a resulting sensitivity and resolution equal to 3100°/uW and 3 pW, respectively. We tend to demonstrate that these results are additional than 2 orders of magnitude bigger than those ones achievable by using a Si photodiode in photoconductive mode. As a case-example for chemical applications, we tend to apply the proposed technique to detect the variations of the molar concentration of a methylene blue answer 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 business spectrophotometer and the conventional amplitude detection technique using the lock-in amplifier. Despite the achieved results are limited by the fastened part resolution of the used lock-in amplifier, here, the proposed approach allows to live molar concentration variations with a resolution of seventy nine pM ensuing 1354 times higher than that one obtained using the spectrophotometer and thirty three times higher than the value achieved with the amplitude detection technique.

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