RESUMO
In this letter, a highly sensitive bending sensor based on an embedded multimode D-shaped long period fiber grating (EMD-LPFG) is proposed. The novel sensor is applied to carry out vector bending measurement. The proposed LPFG is fabricated by polishing on the prepared structure which is formed by periodically splicing between single mode fiber (SMF) and multimode fiber (MMF). Since the cross section of the embedded MMF is D-shaped, we named it EMD-LPFG. Due to the asymmetric modulation of the refractive index on the fiber by the CO2 laser, the sensor has the ability to distinguish the bending directions, and the MMFs provide higher bending response. The experimental transmission spectrum can match the simulation results well. The experimental results show that the average bending sensitivities in three orthogonal directions are 70.21 nm/m-1 (0°), 9.75 nm/m-1 (90°), -12.04 nm/m-1 (180°) and 9.98â nm/m-1 (270°), respectively. Meanwhile, the temperature sensitivity is 30 pm/°C in the range of 25 °C to 75 °C. According to the ultra-compact structure with the total length of 2.5 mm, high bending sensitivity and ability to distinguish the bending direction, the novel sensor has potential in bending measurement.
RESUMO
A novel embedded ultra-long period fiber grating (EULPFG) based on a graded index multimode fiber (GI-MMF) is proposed for temperature measurement. Due to the small RI difference of the modes near the GI-MMF self-imaging point, the resonant peak of transmission spectrum is wavelength-insensitive to refractive index (RI), strain and bending. However, the sensor is sensitive to temperature. The experimental results show that the temperature sensitivity of the EULPFG is 90.77 pm/°C. The sensitivities of other physical parameters are suppressed, and the suppressed sensitivities are at least one order of magnitude less than those of similar sensors. The EULPFG with anti-interference from other parameters is expected to be used in ocean monitoring systems to measure the temperature of the seawater.
