Gas pressure sensors are essential components in many measurement systems. They hold great value in industrial, medical, environmental monitoring, aerospace, and geological exploration applications, which provide accurate and reliable means of monitoring and controlling gas pressure across various industries. Fiber optical sensors, with their small size, high sensitivity, resistance to electromagnetic interference, and fast response speed, offer marked advantages in measuring physical parameters such as temperature, pressure, and refractive index in sensing applications. The Fabry?Pérot interferometer (FPI), commonly used for gas pressure detection due to its simple fabrication process, has been extensively researched. Previous studies often employed hollow-core fibers (HCFs) as the sensing cavities, leveraging the principle that the gas’s refractive index changes with increasing air pressure. However, a challenge arises in ensuring smooth air entry into the air holes of HCFs. Photonic crystal fibers (PCFs) feature a porous structure that allows gas to smoothly enter the HCF without collapsing or fusing the air holes, even under increased air pressure induced by a pump. This enables the gas inside the PCF to change its refractive index, facilitating accurate gas pressure sensing.

Firstly, the fabrication process of the sensing probe involves only two steps: fusion splicing and cutting, which are accomplished using a fiber optical cutter and a fiber optical fusion splicer. Since there is no specific fusion splicing procedure in the fusion splicing machine, it is necessary to pre-set the fusion splicing parameters for the single-mode fiber and the HCF, as well as the fusion splicing parameters for the HCF and the PCF. This ensures that the HCF does not collapse and minimizes the collapse of the air holes in the PCF, which could otherwise affect the experimental results. Secondly, the length of the PCF has a negligible effect on spectral loss. In this study, the lengths of the HCF and PCF sensing probes are approximately 60 and 370 μm, respectively, which results in a total probe length of about 430 μm. As the ambient gas pressure fluctuates around the sensing probe, the refractive index of the gas within the HCF responds correspondingly to these variations. This change is observable in the reflectance spectra, allowing the sensing probe to detect variations in gas pressure. Lastly, in a similar structure, the single-mode photonic crystal fiber (SM-PCF) is replaced with a large-mode-field photonic crystal fiber (LMA-PCF), and the obtained reflection spectra effectively reflect changes in gas pressure. The sensitivities of the sensing probes using the two different PCFs are compared.

With increasing air pressure, the reflectance spectrum of the sensing probe exhibits a redshift trend. The trough induced by air pressure shifts up to about 3.84 nm/MPa, demonstrating a high linearity of 0.99832 [Fig. 9(a)], which confirms the stability of the gas pressure sensing probe. The sensitivity of the probe aligns consistently with the theoretically calculated gas pressure sensitivity. Theoretically, the probe can detect a maximum gas pressure of 4.76 MPa. However, due to equipment limitations, this study achieves a maximum measured gas pressure of 2.5 MPa. The sensitivity remains nearly unchanged within the 0?2.5 MPa range, highlighting the excellent stability of the sensing probes during gas pressure measurements, as depicted in Figs. 10(a) and 10(b). Table 1 outlines the parameter settings for the fusion splicer used in preparing the sensing probe. Precise control of these parameters is essential to prevent the collapse of air holes in both the HCF and PCF.

In this paper, a highly sensitive all-fiber air pressure sensor with a pressure sensitivity of about 3.84 nm/MPa has been implemented. The sensing probe is fabricated by cascading single-mode fiber, HCF, and PCF. The preparation process is simple and requires only two steps: splicing and cutting. In this study, the HCF serves as the sensing cavity and the PCF as the gas channel. Light beams are reflected from the end face of the single-mode fiber and the two ends of the PCF, which create three types of reflected beams whose interference superposition forms the total output spectrum. The interference spectrum of the cavity formed by the HCF is obtained through fast Fourier transform and Fourier band-pass filtering, which is much simpler than analyzing the entire output spectrum and facilitates subsequent demodulation. Several sensing probes are fabricated by varying splicing parameters, HCF, and PCF lengths. It is observed that their sensitivities vary minimally, demonstrating strong repeatability in probe fabrication. The temperature sensitivity of the fiber optical sensing probe is 12.1 pm/℃. This all-fiber air pressure sensor offers advantages such as high sensitivity, good linearity, compact size, easy preparation, simple operation, and remote monitoring, which indicate broad potential applications across various fields.