Fig. 1. Configuration of the proposed fiber-optic sensor that consists of two mutually coupled and geometrically identical FP resonators, with one experiencing a gain and the other an equal amount of loss. The two FP resonators (FP1 and FP2) are implemented based on three uniform FBGs (FBG1, FBG2 and FBG3). FBG, fiber Bragg grating; EYDF, erbium-ytterbium co-doped fiber. The black arrow denotes the pump propagation direction.

Fig. 2. (a) Numerical results for the relationship between eigenfrequency splitting and resonance detuning induced by bending. (b) The results from (a) in a logarithmic scale. (c) The relationship between the sensitivity enhancement factor and perturbation. The inset is a zoom-in view, in which the enhancement factors of this work and other EP sensors are noted. The orange line represents a sensor with a linear relationship such as a conventional FBG sensor. EF: enhancement factor.

Fig. 3. Schematic diagram of the experimental setup. OVA, optical vector analyzer; ISO, isolator; VOA, variable optical attenuator; FBG, fiber Bragg grating. PC, polarization controller; WDM, wavelength division multiplexer.

Fig. 4. Measured transmission spectra of the sensor operating at the EP (a) without perturbation and (b) with perturbation. The sensing mode to be monitored is indicated in the dashed box.

Fig. 5. Interrogation of the sensor by translating the wavelength splitting in the optical domain to a microwave frequency separation in the microwave domain. FBG, fiber Bragg grating; TLS, tunable laser source; PC, polarization controller; PM, phase modulator; WDM, wavelength division multiplexer; OC, optical circulator; PD, photodetector; VNA, vector network analyzer.

Fig. 6. The spectral responses of the MPF when the curvature is increased from (a) 0.28, (b) 0.56, (c) 0.90 to (d) 1.24 m⁻¹.

Fig. 7. (a) Microwave frequency splitting as a function of the optical fiber curvature. Inset shows the results in a logarithmic scale. (b) Measured enhancement factor as a function of the optical fiber curvature. Inset shows the measured resonance frequency detuning between FP1 and FP2 as a function of the optical fiber curvature.