Electrical self-modulation of optical sensors for light power measurement in chemical applications by phase detection technique

Abstract

In this paper we demonstrate how a novel amplitude-independent electrical self-modulation technique based on phase detection, shows the same performances of conventional experimental apparatus employed for the amplitude measurement, making use of a lock-in amplifier and mechanical or electro/acousto-optical amplitude modulators to periodically vary the power of continuous-wave light sources. The new approach allows to detect phase variations, between a reference and an input voltage signal generated by a photodiode, as a function of the changes of the continuous-wave light power impinging on it. In particular, the proposed electrical self-modulation mechanism acts directly on the DC reverse bias voltage applied to the photodiode by superimposing on it a small AC voltage waveform. As a consequence, the resulting optoelectronic apparatus is notably simplified since it is composed of only a continuous light source, so making the system suitable for portable micro-devices in chemical applications. In the paper we present a comprehensive analysis of the sensing device electrical behaviours by studying and comparing the performances of the photodiode equivalent circuits in both the cases where the optical modulation of the continuous light source is performed by using a mechanical chopper and where the electrical self-modulation is applied. Based on this analysis, we report experimental findings of how the measured phase variations are related to the light power changes. Finally, as a case-example for chemical applications, the proposed approach has been also utilised to detect and quantify variations of the molar concentration of methylene blue diluted in distilled water.