Downconversion Circuit in Photonic Integrated Sensing System Based on an Optoelectronic Oscillator

Today, optical sensors and their interrogation systems have received significant development. The frequency interrogation method showed its advances in different sensing applications. However, its structure based on an optoelectronic oscillator needs to improve with easy and non-expensive microwave frequency measurement methods. This paper considers a photonic integrated frequency interrogation scheme supplemented with a frequency converter for a liquid refractive index sensing system with a phase-shifting waveguide Bragg grating as a sensing element. This element performs as a notch filter in the optoelectronic oscillator feedback circuit. External influence (liquid refractive index changing) on the grating causes a Bragg wavelength shift. It, in turn, causes a microwave oscillations frequency change. The frequency downconverter reduces the measured frequency value, simplifying the sensing system's complexity and reducing costs. We performed the frequency interrogation system simulation for the case of liquid refractive index variation from 1.390 to 1.391. The microwave frequency at the optoelectronic oscillator output was 3.01 GHz to 13.49 GHz. The interrogation system speed was 12.5 MHz. We also briefly reviewed commercially available frequency downconverter integrated circuits and other components, and showed the possibility of the described scheme of practical implementation. One can apply the proposed scheme to various fluids analysis, including detecting and evaluating pathologies in blood and other biological materials.