Sub-100 fε resolution strain sensing based on an optical fiber frequency comb

In this paper, we report a high-resolution optical fiber strain sensing system based on optical frequency comb (OFC) beat-frequency demodulation. This system enables both static and dynamic strain detection, featuring a large dynamic strain measurement range and a large spectral range. The sensor employed a fiber Fabry-Perot interferometer (FFPI) as the strain sensing element, which was probed by a narrow-linewidth laser stabilized to it. By leveraging the concept of beat frequency demodulation, the OFC, translated the optical frequency shifts within the strain sensor into the radio frequency (RF) for detection. Our findings revealed a static strain resolution of 247 pε, a dynamic strain resolution of 87 fε/Hz^1/2 at 8 kHz, and a large spectral range of 10 kHz, which enables it to be far away from low-frequency noise. Additionally, we investigated the potential of a free-running OFC with a stable repetition rate for high-resolution static strain sensing. By actively sweeping the OFC's offset frequency in conjunction with the FFPI, we achieved a static strain resolution of 630 pε. This sensor system offers high resolution, moderate cost, and a degree of portability, rendering it suitable for a range of geophysical research applications.