Due to inherent merits of anti-electromagnetic disturbance, compact size, high sensitivity and low fabrication cost, fiber curvature sensor (FCS) and fiber vibration sensor (FVS) play important roles in optical fiber sensing and optical fiber communication, which can achieve effective structural safety monitoring and be widely used in structural health monitoring basic fields such as machinery manufacturing, bridge transportation, oil and gas pipelines. Moreover, the researches on multi-parameter fiber optic sensors have been driven by miniaturized and multifunctional sensor solutions, as well as the need to meet the measurement of multiple physical quantities in narrow operating environments. While meeting the application requirements of curvature and vibration sensing, it is necessary to further improve sensitivity and response range. In practical sensing applications, solving the cross-sensitivity problem of multiple parameters and applying it in confined spaces and harsh environments also puts forward higher requirements for the compactness, flexibility, and adaptability of sensors. In this paper, a highly sensitive curvature and vibration dual-parameter sensor based on optical reflective coupler probe (ORCP) is proposed and demonstrated. With the advantages of high sensitivity, wide response range, good linearity, high stability, high fidelity, and the probe size is in mm level with compact structure, the dual-parameter sensor based on ORCP would further be widely used in limited space and harsh environment fields, providing good application prospects in oil, coal mine and other structural safety monitoring fields.

The beam propagation method (BPM) was used to simulate the modal field intensity distribution of different ORCPs. In order to fabricate the ORCP, it is necessary to obtain the single mode microfiber coupler (SMC) firstly. Two single mode fibers (SMFs, core/cladding diameter is 8.2/125 μm, NA is 0.14) are aligned with each other before they are fused together using the flame modification method. During the fabrication process, the hydrogen gas flow, stretching speed and length, which determine the performance of the SMC are controlled and optimized. Based on the brittle fracture characteristics of quartz optical fibers, applying axial tension to the fabricated SMC and snapping the waist region with a gem knife to form the Fresnel reflection end face with high quality. The waist diameter and coupling region length of the ORCP are characterized by optical microscope. For curvature sensing, the bending signals applied to the coupling region of the ORCP cause changes in the wavelength and intensity of reflection spectra. A broadband source (BBS, 1250 nm to 1650 nm) is connected to the port 1 and the reflection spectra of the ORCP are recorded by an optical spectrum analyzer (OSA, AQ6370D, resolution of 0.02 nm, 900 nm-1700 nm) real time through port 2 of the ORCP. For vibration sensing, a piezoelectric transducer is connected to the coupling region of the ORCP to apply vibration signals. If the wavelength of narrow linewidth laser was tuned to the reflection spectral wave-nodes of the ORCP, the output intensity will be modulated. A tunable laser source (TLS, line width < 5 kHz) is connected to the port 1 and output signals (port 2) are recorded by potodetector and oscilloscope to realize the detection of continuous single-frequency, damped vibration signal and sound recognition.

In the measurement of curvature sensing based on the absolute symmetric ORCP, as curvature is increased from 0 m^-1 to 9.58 m^-1, the wavelength red shifts and is stable gradually while the intensity changes weakly (Fig. 6). When the curvature increases from 0 m^-1 to 1.92 m^-1, the wavelength red shifts with sensitivities of 0.63 nm/m^-1 (-0.29 dB/m^-1) @ 1510 nm and 0.58 nm/m^-1 (0.29 dB/m^-1) @ 1470 nm, respectively. The linearity (R²) is ~0.99. As curvature is increased from 1.92 m^-1 to 3.75 m^-1, the sensitivities are 2.75 nm/m^-1 (2.16 dB/m^-1) @ 1470 nm and 2.84 nm/m^-1 (-2.01 dB/m^-1) @ 1510 nm, respectively. The curvature continues to increase from 3.75 m^-1 to 9.58 m^-1, the wavelength and intensity are stable. In the measurement of curvature sensing based on the single ORCP, the envelope of the ORCP's reflection spectrum red shifts with increased curvatures (Fig. 7). As curvature is increased from 0.57 m^-1 to 10.49 m^-1, the shift of the spectral envelope is ~56.6 nm. The measured curvature sensitivity is 11.97 nm/m^-1 (-1.88 dB/m^-1) @ 1470 nm ranging from 0.57 m^-1 to 3.72 m^-1 and the R² is 0.98. When curvature increases from 4 m^-1 to 10.49 m^-1, the sensitivity is 2.63 nm/m^-1@1470 nm with R² of 0.94. The experimental results indicate that the proposed ORCP is suitable for monitoring small curvature deformations. For the vibration sensing, the proposed single ORCP can achieve frequency response range from 185 Hz to 20 MHz without data filtering process. The R² of vibration detection is ~1 and the resolution of real-time vibration signal monitoring can reach 1 Hz with good fidelity (Fig. 9). The ORCP achieves a sensitivity of 0.72 mV/V@80 kHz (Fig. 11) and the highest signal-to-noise ratio is ~53.56 dB @ 2 MHz (Fig. 10). The vibration amplitude of the ORCP at different frequencies is tested for many times and the amplitude fluctuation is <0.1 dB. In addition, the sensor can realize the detection of damped vibration signal and sound recognition with high stability.

A highly sensitive curvature and vibration dual-parameter sensor based on ORCP is proposed and demonstrated. The ORCP is fabricated by melting coupling method and vertical cutting technology. The sensing performance is stable and not influenced by packaging methods, achieving high sensitivity for detecting weak curvature and vibration signals. Applying bending or vibration deformation signals to the ORCP cantilever-beam coupling region changes the refractive index and mode field distribution of the interference supermodes, causing a shift in the wavelength or reflection of the spectrum, realizing sensing of curvature and vibration with high sensitivity. For curvature sensing, the sensitivity is up to 11.97 nm/m^-1 ranging from 0 m^-1 to 10.49 m^-1, and the linearity is >0.98. For vibration sensing, the ORCP has a sensitivity of 0.72 mV/V@80 kHz and achieves an ultra-wideband frequency response range from 185 Hz to 20 MHz with high fidelity and linearity, and the signal-to-noise ratio is ~53.56 dB. In addition, the sensor can realize the detection of damped vibration signal and sound recognition with high stability. The proposed curvature and vibration sensor based on ORCP has the advantages of high sensitivity, wide response range, good linearity, high stability, high fidelity, and the probe size is in mm level with compact structure, supporting its potential application prospects in limited space and harsh environment fields such as oil field, coal mine and other structural safety monitoring fields, which is expected to achieve the prediction of potential threats of infrastructure emergencies.