Formulation of design tasks for a fiber-optic reference air sensor for qualitative and quantitative monitoring of air parameters

Introduction. In various applications, such as qualitative and quantitative real-time air monitoring for managing greenhouse gas concentrations and assessing the efficiency of solar panels—both of which are influenced by ambient temperature and relative humidity—there is a significant demand for groups of combined fiber-optic sensors (CFOS). These sensors utilize Fabry-Perot resonators (FPR) and fiber Bragg gratings (FBG) arranged in a quasi-distributed manner, including configurations that are remotely spaced from the data collection and processing points. Each CFOS group requires a reference sensor (RCFOS) dedicated solely to monitoring air parameters, ensuring the reliability and calibration of measurements.

The aim of this work is to formulate design challenges and evaluate the feasibility of implementing a multi-parameter RCFOS for monitoring air parameters, including temperature, pressure, and relative humidity. This implementation aims to enhance metrological and functional characteristics based on the Vernier effect and Edlen equations, while also providing cost-effective address multiplexing and interrogation within multi-sensor networks based on address fiber Bragg structures (AFBS) and microwave photonic technologies.

Methods. A critical metric used to characterize the optical Vernier effect is the sensitivity enhancement factor, or M-factor. This factor compares the Vernier envelope with the interference signal from the measuring FPR. A high M-factor can be achieved through two-parameter parallel measurements, such as air pressure and temperature, or by sequentially activating independent interferometers to monitor each parameter separately, for example, air temperature and relative humidity. The AFBS is a fiber Bragg structure with an optical frequency response that includes two narrow-band FBG components separated by a unique address frequency in the radio frequency range. This address frequency remains constant, even when the AFBS undergoes deformation or temperature changes. The microwave photonic principle of AFBS interrogation allows the inclusion of several structures with the same central wavelength but different address frequencies in a single measurement network. This setup facilitates the parallel interrogation of the RCFOS while minimizing the impact of external physical parameters on the information transmitted through the fiber, making it narrow-band for each sensor.

Conclusion. The research proposes a concept for developing an RCFOS for multi-sensor quasi-distributed networks that enable qualitative and quantitative real-time monitoring of air parameters. Key elements of the concept include a new technology for manufacturing the FPR section within the single fiber structure using catastrophic melting effects for improved reliability in field conditions and microwave photonic technology to simplify interrogation subsystems. The last one employs radiation generated in the AFBS section of the sensor, which was previously used only for reference temperature control. Estimates indicated that the RCFOS designed based on this concept can measure ambient temperatures ranging from -60°C to +300°C with a sensitivity of approximately 13 pm/°C using AFBS. Additionally, it can measure temperatures from +10°C to +60°C with a sensitivity of approximately -500 pm/°C using a sequential FPR scheme. Relative humidity can be measured with a sensitivity of roughly 400 pm/%RH within the 20% to 90% RH range, while air pressure can be measured with a sensitivity of approximately 400 pm/MPa up to 0.5 MPa using the parallel FPR scheme.