A femtosecond laser-machined few-mode fiber laser sensor for refractive index and temperature measurement in 1.56 μm and 2 μm bands

A femtosecond laser-machined few-mode fiber (FMF) laser sensor has been proposed and demonstrated for refractive index and temperature measurement in the 1.56 μm and 2 μm bands. The sensing structure consists of a slotted FMF sandwiched between two single-mode fibers (SMFs), and the multimode interference effect works as the sensing mechanism. The slotting treatment increases the evanescent field of the FMF, making it more sensitive to environmental changes, and the slot depth is optimized to obtain the superior sensing performance. Respectively in 1.56 μm and 2 μm bands, the sensor performance in terms of refractive index sensing and temperature sensing is evaluated using self-constructed fiber ring lasers, and the repeatability and stability have been validated. The slotted FMF sensor can respond to temperature and refractive index changes, which improves the application range of the sensor. Furthermore, the sensor can operate independently in two different communication bands, allowing the sensor to be optimized for specific wavelength applications and improving wavelength flexibility and selectivity. The fiber laser sensing device also possesses high signal-to-noise ratio, exceptional spectral resolution, and low operating threshold. To our knowledge, we propose a pioneering scheme that the sensor realizes parameter detection at two bands. It holds broad application prospects in the fields of biomass detection, food safety, and medical engineering in the near-infrared wavelength range.