No abstract

Tilted fiber Bragg grating (TFBG) and reflective tilted fiber Bragg grating (R-TFBG) were proposed and demonstrated in the graded-index multimode fiber (GI-MMF). The TFBGs with grating planes tilted at an angle of 2.5° corresponding to the fiber axis were inscribed. The TFBGs in the GI-MMF had the good linear sensitivity to the temperature, strain and curvature. The fiber was then cleaved at the far end of the TFBG to form an R-TFBG using the Fresnel reflection of the fiber end. The reflective spectra of the R-TFBG were given, and the temperature sensing properties were also investigated.

Few-mode fiber Bragg grating (FM-FBG) has wide applications in the field of multi-wavelength fiber lasers and fiber-optic sensing. In this letter, FBG is successfully written in a novel type of FM fiber (FMF) based on 248-nm excimer laser and a uniform phase mask. A low-loss coupling between the FMF to a singlemode fiber is realized by a graded-index multi-mode fiber with a length of about a quarter pitch. The sensing properties of the FMF grating are further investigated. The experimental results indicate that the novel FM-FBG has sensitivities of temperature, strain, and bending of 11.2 pm/• C, 1.3 pm/µε, and 636.9 pm/m −1 , respectively.

Novel fiber Bragg gratings (FBG) based settlement sensors are developed and the sensing performance is evaluated in both the lab and field environments. The settlement is determined by precisely measuring the pressure of the water level difference. The experimental results indicated that the sensor has good linearity, good repeatability and high accuracy in the settlement measurements. The influence of temperature fluctuations on the sensor's performance is eliminated and long-term test results proved the good stability of better than ±0.1% of the sensor. Such a FBG sensing system has been applied to monitor the settlement of high-speed railways in practice.