Temperature-insensitive refractive index sensor based on high-order-resonance short-line long-period fiber grating

Long-period fiber grating based refractometers have attracted great attention for various applications, but they usually suffer from the cross sensitivity to temperature in practice. Here, we demonstrate a temperature-insensitive refractive index sensor based on a short-line long-period fiber grating (SL-LPFG). It is fabricated by using femtosecond laser to periodically inscribe short lines in a standard single-mode fiber (SMF) with a small duty cycle of 0.3, and resonates to a cladding mode with a high diffraction order. This high-order-resonance SL-LPFG responds to ambient refractive index with the sensitivities of 175.84 nm/RIU and 367.63 nm/RIU in the ranges of 1.3381 to 1.3763 and 1.3763 to 1.4008, respectively. Meanwhile, it presents a temperature sensitivity as low as 4.0 pm/℃, which is almost one percent of the temperature sensitivity of the similar period LPFG fabricated by UV exposure method. It is significant for LPFG-based refractometers in normal SMFs to minimize thermal interference without the assistance of an additional sensing unit and matrix interrogation. Therefore, the proposed high-order-resonance SL-LPFG provides a high-performance and low-cost solution for refractive index related applications, especially in medical, chemical, and biological fields.