Microwave photonics demodulation based on a fiber loop and a virtual Fabry-Perot interferometer for sapphire fiber temperature sensors.

The interferometric optical path difference (OPD) induced by changes in the measurand within the microwave band is comparatively smaller than that observed in the realm of lightwave, thereby leading to diminished sensitivity in sapphire fiber Fabry-Perot interferometers (FPIs) within the microwave band. In order to enhance sensitivity, we propose a microwave photonics sensing system predicated on a fiber loop and a virtual FPI. By employing a constructed fiber loop, the propagation path length of the modulated signal can be extended with an increase in the number of loops. The correlation between the number of loops and sensitivities is examined both theoretically and experimentally. Our findings illustrate a direct relationship between sensitivity and the number of loops, demonstrating an increment sensitivity with each additional loop. Specifically, the FPI frequency domain temperature sensitivity in the 5 loop measures approximately 4652.95 kHz/°C, marking an enhancement of around 9.24 times compared to the 1 loop.