All-fiber magnetic-field sensor based on a device consisting of a microfiber knot resonator and magnetic fluid is proposed for the first time in this Letter. Sensor principles and package technology are introduced in detail. Experimental results show that the resonance wavelength of the proposed sensor regularly varies with changes to the applied magnetic field. When the magnetic field is increased to 600 Oe, the wavelength shift reaches nearly 100 pm. Moreover, the sensor responding to the 50 Hz alternating magnetic field is also experimentally investigated, and a minimal detectable magnetic-field strength of 10 Oe is successfully achieved.

A novel dual parallel structured fiber-optic Fabry-Perot interferometers (FPIs) for ultrasensitive hydrogen ( ${{\text{H}}_{2}}$ ) sensing using the Vernier effect is proposed and demonstrated. Leveraging the high mechanical strength and chemical stability of hexagonal boron nitride (hBN), a nanoscale hydrogen-sensitive thin film of palladium (Pd)-decorated hBN is prepared and integrated with optical fiber to form a probe-type FPI hydrogen sensor. To further enhance the sensitivity, dual FPIs with similar but different cavity lengths are connected in parallel to exploit the Vernier effect, resulting in a significant amplification factor. The sensitivity of dual parallel structured FPIs based on the Vernier effect is $-$ 14.19 nm/%, which is 9.27 times higher than that of the individual FPI within the ${{\text{H}}_{2}}$ concentration range of 0 $ \sim$ 1%. Moreover, as the dual FPIs possess similar temperature coefficients, the Vernier effect reduces the impact of ambient temperature on the sensor, yielding a temperature coefficient as low as $ \sim$ 46 pm/ ${}^\circ \text{C}$ . Additionally, the sensor offers several advantages, including good repeatability, fast response time ( $ \sim$ 31.3 s), compact structure, easy preparation, and low cost, making it a practical solution for detecting trace amounts of hydrogen.

With diameter close to the wavelength of the guided light and high index contrast between the fiber and the surrounding, an optical micro-fiber shows a variety of interesting waveguiding properties, including widely tailorable optical confinement, strong evanescent fields and waveguide dispersion. Among various micro-fiber applications, optical sensing has been attracting increasing research interest due to its possibilities of realizing miniaturized fiber optic sensors with small footprint, high sensitivity, and low optical power consumption. Typical micro-fiber based sensing structures, including Michelson interferometer, Mach-Zenhder interferometer, Fabry-Perot interferometer, micro-fiber ring resonator, have been proposed. The sensitivity of these structures heavily related to the fraction of evanescent field outside micro-fiber. In this paper, we report the first theoretical and experimental study of a new type of refractometric sensor based on micro-fiber three-beam interferometer. Theoretical and experimental analysis reveals that the sensitivity is not only determined by the fraction of evanescent field outside the micro-fiber but also related to the values of interferometric arms. The sensitivity can be enhanced significantly when the effective lengths of the interferometric arms tends to be equal. We argue that this has great potential for increasing the sensitivity of refractive index detection.