The fiber-optic cascaded Fabry-Perot (FP) cavity structure is a significant interferometric sensor configuration, offering superior spectral resolution compared to Mach-Zehnder type sensors. In response to the comb-spectrum characteristics of fiber-optic cascaded FP sensors, we propose a novel optical vernier processing method. This method calculates the peak positions of individual resonance peaks without the need to track the drift of envelope modulation curve peaks, as required by traditional optical vernier scale methods. Utilizing this approach, we have significantly enhanced the sensitivity of cascaded FP cavity sensors. The strain and temperature sensing tests achieved sensitivities of 4.76 pm/μϵ and 271.61 pm/°C, respectively, representing an increase of over 20 times compared to a single-stage cascaded FP cavity. More importantly, the Figure of Merit (FOM) for the cascaded FP cavity sensors can be improved by more than two orders of magnitude under the same sensitivity amplification factor, compared to traditional optical vernier technologies based on envelope curve recognition. This also effectively avoids potential errors that may arise during the fitting process of the envelope modulation curve. These advantages endow the cascaded FP cavity sensors with significant potential for application in the field of high-performance fiber-optic sensing.

Multi-core fibers have been widely used for vector-bending sensing due to their off-axis distributed cores. In contrast to vector-bending sensors based on Bragg gratings, fiber Fabry–Perot (F–P) interferometers are more advantageous due to their ease of fabrication and potential for introducing the Vernier effect to further improve sensitivity. We propose and experimentally demonstrate a cascaded Fabry–Perot (F–P) cavity vector bending sensor. From the experimental results, the sensor has a strong bending dependence with a maximum sensitivity of 123.12 pm/m⁻¹, and the curvature magnitude and direction can be reconstructed from the tilted wavelength shift of the asymmetric fiber-core F–P cavities.

In this work, an envelope-fitting-free, peak-recognition-based optical vernier method is proposed to demodulate fiber optic vector bend sensor for high-sensitivity bend measurement. The fiber bend sensor was made by inscribing a double-cascade Fabry-Perot (FP) interferometer in two edge cores of a seven-core fiber using femtosecond laser machining. Utilizing the proposed peak-recognition optical vernier method, a maximum bending sensitivity of 3270.2 pm/m⁻¹ has been achieved, which is approximately 26 times that of a single fiber cascaded FP cavity sensor. Since the peak-recognition optical vernier method does not require fitting of the envelope modulation curve, the limit of detection is independent to figure of merit (FOM), which is usually inversely proportion to the sensor sensitivity, thus offering higher measurement accuracy and reliability.