This study proposed an all-fiber Fabry–Perot interferometer (FPI) strain sensor with two miniature bubble cavities. The device was fabricated by writing two axial, mutually close short-line structures via femtosecond laser pulse illumination to induce a refractive index modified area in the core of a single-mode fiber (SMF). Subsequently, the gap between the two short lines was discharged with a fusion splicer, resulting in the formation of two adjacent bubbles simultaneously in a standard SMF. When measured directly, the strain sensitivity of dual air cavities is 2.4 pm/με, the same as that of a single bubble. The measurement range for a single bubble is 802.14 µε, while the measurement range for a double bubble is 1734.15 µε. Analysis of the envelope shows that the device possesses a strain sensitivity of up to 32.3 pm/με, which is 13.5 times higher than that of a single air cavity. Moreover, with a maximum temperature sensitivity of only 0.91 pm/°C, the temperature cross sensitivity could be neglected. As the device is based on the internal structure inside the optical fiber, its robustness could be guarantee. The device is simple to prepare, highly sensitive, and has wide application prospects in the field of strain measurement.
An ultra-sensitive sensor, based on two Fabry–Perot interferometers (FPIs), has been realized for temperature and pressure sensing. A polydimethylsiloxane (PDMS)-based FPI1 was used as a sensing cavity, and a closed capillary-based FPI2 was used as a reference cavity for its insensitivity to both temperature and pressure. The two FPIs were connected in series to obtain a cascaded FPIs sensor, showing a clear spectral envelope. The temperature and pressure sensitivities of the proposed sensor reach up to 16.51 nm/°C and 100.18 nm/MPa, which are 25.4 and 21.6 times, respectively, larger than these of the PDMS-based FPI1, showing a great Vernier effect.
This study fabricated an ultra-high refractive index sensor based on tapered no-core fiber involving a simple inexpensive process. A splice section of no-core fiber in the middle of single mode fiber was tapered to small diameters. The sensor was sensitive to the surrounding refractive index with a large measurement range of 1.3330–1.4437. The refractive index sensitivity differed with varying wavelengths, with a value of 41916 nm/RIU at approximately 1550 nm. It yielded a low temperature sensitivity of 8.1 pm/°C that can be ignored compared with the refractive index sensitivity. The proposed fiber optic refractive index sensor can be used in many fields such as medicine and biochemical applications.
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