Ultra-high sensitivity strain sensor based on biconical fiber with a bulge air-bubble

All-solid highly sensitive fiber-tip magnetic field sensor based on a Fabry-Perot interferometer with a breakpoint structure

An all-solid fiber-tip Fabry-Perot interferometer (FPI) coated with a nickel film is proposed and experimentally verified for magnetic field sensing with high sensitivity. It is fabricated by splicing a segment of a thin-wall capillary tube to a standard single-mode fiber (SMF), then inserting a tiny segment of fiber with a smaller diameter into the capillary tube, and creating an ultra-narrow air-gap at the SMF end to form an FPI. When the device is exposed to magnetic field, the capillary tube is strained due to the magnetostrictive effect of the nickel film coated on its outer surface. In addition, owing to the unique breakpoint sensitivity-enhancement structure of the air-gap FPI, the elongation of the capillary tube whose length is over 100 times longer than the air-gap width is entirely transferred to the cavity length change of the FPI, and the sensor is extremely sensitive to the magnetic field as proved by our experiments, achieving a high sensitivity of up to 2.236 nm/mT for a linear magnetic field range from 40 to 60 mT, as well as a low-temperature cross-sensitivity of 56 µT/°C. The all-solid stable structure, compact size (total length of ∼3.0 mm), and reflective working mode with high magnetic field sensitivity indicate that this sensor has good application prospects.

Hollow microsphere probes formed by hollow core optical fiber discharging for monitoring gas pressure and temperature

A hollow core fiber (HCF) is spliced with a single-mode fiber, and then, the end face of the HCF is etched to form a microsphere interferometer for measuring gas pressure and environmental temperature. The total length of each microsphere is less than 200 μm. We fabricated two such structures and used femtosecond laser pulses to drill micro-holes on the HCF walls of both structures. One of the structures is directly used to measure air pressure, achieving a sensitivity of up to 2.857 nm/MPa while being almost insensitive to temperature. This structure is capable of assessing pressure down to 3.4 kPa within the range of 0-0.5 MPa. Another structure is filled with thermally sensitive material dimethyl silicone oil through a micro-hole, and then, it is sealed with AB adhesive to form a harmonic Vernier effect temperature sensor, with a sensitivity of up to -5.16 nm/°C. This structure is capable of assessing temperature down to 0.38 °C within the range of 30-60 °C. Additionally, the sensors have good repeatability and stability and compact structure and simple manufacturing and can be used as a sensing probe for monitoring gas pressure and temperature under extreme environments.

Highly sensitive strain sensor based on tapered few-mode fiber

A high sensitivity strain sensor using a sandwich structure of "single mode fiber (SMF)-few mode fiber (FMF)-single mode fiber (SMF)" was proposed and experimentally validated. The designed sensor is achieved by splicing a segment of FMF between two segments of SMFs, and then using a fiber optic fusion tapering machine to double the length of FMF. Introducing tapered optical fibers into the structure to excite more evanescent waves improves the sensitivity of the sensor to the surrounding environment. In addition, due to the fact that the FMF is tapered into a very fine shape, the tensile stress applied to the FMF will increase. Therefore, conical FMF has excellent stress concentration ability, which is easily deformed under stress, thus achieving a high strain sensitivity of -23.9 pm/με. Finally, a cascaded FBG was used to compensate for the temperature cross-sensitivity of the sensor. This strain sensor with an extremely simple structure and high sensitivity has wide application value in the industry.

Microbubble-probe WGM resonators enable displacement measurements with high spatial resolution

A microbubble-probe whispering gallery mode resonator with high displacement resolution and spatial resolution for displacement sensing is proposed. The resonator consists of an air bubble and a probe. The probe has a diameter of ∼5 µm that grants micron-level spatial resolution. Fabricated by a CO₂ laser machining platform, a universal quality factor of over 10⁶ is achieved. In displacement sensing, the sensor exhibits a displacement resolution of 74.83 pm and an estimated measurement span of 29.44 µm. As the first microbubble probe resonator for displacement measurement, the component shows advantages in performance, and exhibits a potential in sensing with high precision.