Pressure sensors are indispensable in numerous applications, including biomedical engineering, industrial monitoring, and environmental safety. Optical fiber pressure sensors offer distinct advantages over traditional sensors, such as compact size, lightweight design, corrosion resistance, and the ability to operate reliably in extreme environments. Among these, Fabry-Perot interferometric (FPI) sensors are particularly favored for their exceptional sensitivity, compact structure, and ease of single-ended operation. However, most conventional FPI pressure sensors rely on a diaphragm bonded to the end of an optical fiber to form a sealed cavity, which limits their mechanical robustness and environmental adaptability due to the fragility of the thin diaphragm. To address these challenges, we propose a diaphragm-free fiber-optic FPI pressure sensor based on dual capillary optical fibers. This design leverages the interfaces between capillaries of different diameters and the liquid surface within a capillary column to create “natural” optical reflection surfaces, enabling a diaphragm-free FPI structure within microfluidic channels. This innovation facilitates independent measurement of gas and liquid pressures across diverse working environments.

The proposed FPI pressure sensor is fabricated by fusing a single-mode fiber (SMF) with two capillary optical fibers (COF1 and COF2) of different inner diameters. The manufacturing process involves only two steps: welding and cutting. To minimize interference from Fresnel reflection at the distal end of COF2, the end face of COF2 is polished at a specific angle. During fabrication, optimized discharge power and duration are employed to prevent capillary collapse while ensuring structural stability. When environmental pressure around the sensor probe changes, the refractive index of the air cavity or the position of the liquid surface inside the sensor adjusts accordingly, resulting in observable shifts in the reflection spectrum. These spectral changes enable precise measurement of gas or liquid pressure.

As gas pressure increases, the reflection spectrum exhibits a redshift caused by pressure-induced changes in the refractive index of the air cavity. This redshift demonstrates rapid wavelength shifts with excellent linearity (R2>0.99) and no hysteresis. Pressure cycling tests confirms the sensor’s good repeatability, achieving a sensitivity of 4.09 nm/MPa at approximately 1565 nm. During hydraulic testing, incident light transmitted by the SMF undergoes three reflections: at the SMF-COF1 interface (M1), the COF1-COF2 interface (M2), and the gas-liquid interface of the capillary liquid column (M3). These three reflecting surfaces form a composite Fabry-Perot cavity, comprising two sub-cavities: the first FPI and the second FPI, with the latter serving as the pressure-sensitive chamber. Under varying hydraulic pressures, the sensing cavity length (L_x) is determined from the free spectral range (FSR) of the output spectrum using the formula R_FSR=λ2/(2nL_x). Within the pressure range of 1?10 kPa, the sensor exhibits a cavity length sensitivity of 13.649 μm/kPa, demonstrating excellent repeatability.

This study introduces a diaphragm-free fiber Fabry-Perot interference pressure sensor based on dual-capillary optical fibers, capable of independently measuring gas and liquid pressures. The sensor’s unique dual-capillary structure forms a composite Fabry-Perot cavity composed of a conventional air cavity and an expanded air cavity, with an introduced amplification factor that significantly enhances sensitivity. Compared to other FPI pressure sensors, the diaphragm-free design offers advantages in measurement range, mechanical strength, and long-term stability. Experimental results show that the sensor achieves a sensitivity of 4.09 nm/MPa for gas pressure measurements (0?3 MPa) and 13.649 μm/kPa for liquid pressure measurements (1?10 kPa). With its simple manufacturing process, stable structure, compact size, and adaptability to diverse working environments, this sensor holds significant potential for applications in microfluidic systems and pressure measurement across various fields.