Very high-sensitivity tunable phase detection of light power variations using electrical modulation of Si-photodiode in photovoltaic regime

The use of a silicon (Si)-photodiode (Si-PD) operating in photovoltaic mode as a tunable very high-sensitivity phase detector of light power variations is demonstrated. This result is reached by applying to the Si-PD an electrical modulation through an AC excitation signal without a DC reverse bias. This allows employing the synchronous demodulation technique for phase detection that shows much better performances with respect to the conventional amplitude measurements, mainly overcoming the issues related to the electrical noise and the resolution limitation due to the selected full scale. The phase variations to be detected occur between the modulating signal and the one provided by the Si-PD as a function of the light power variations on its sensitive area. The proposed approach notably simplifies both the electronic circuitry and the optical setup, minimising the detection system complexity and size. Experimental results demonstrate that it is possible to achieve phase detection sensitivity, with respect to light power variations, of up to 410°/μW as a function of the Si-PD settable operating conditions. Using a standard commercial lock-in amplifier with a 0.01° phase resolution is possible to obtain a light power resolution of up to 25 pW.