A novel D-shaped fiber structure combined with a fiber Bragg grating for refractive index (RI) and temperature sensing is proposed and experimentally investigated. Thanks to the etched cladding of D-shaped fiber, the resulting interference is strongly influenced by the surrounding refractive index. By monitoring the dip wavelength of D-shaped fiber structure, the refractive index can be measured. Theoretical analysis and simulations were conducted to investigate the cladding mode propagating behavior influenced by the surrounding RI. Combined with the temperature sensing property of fiber Bragg grating (FBG), the possibility of simultaneously measuring the RI and temperature relies on monitoring the resonance dip of the D-shaped fiber modal interferometer and the Bragg wavelength of the FBG. Experimental results showed that RI sensitivity of -31.79 nm/RIU within the range of 1.333 ~ 1.428, and temperature sensitivity of 28.7 pm/°C can be obtained. The proposed sensor with simple configuration, low cost and easy fabrication is suitable for refractive index and temperature sensing applications.

A novel coreless-D-shape-coreless(CDC) fiber structure for liquid level sensing is proposed and experimentally investigated. The D-shape fiber with cladding partially etched is sensitive to the ambient refractive index change, the CDC structure transmission dip wavelength shifts related to the length of D-shape fiber immersed in the liquid, so liquid level is measured by monitoring the dip wavelength. The sensitivities for three different liquids with the RIs of 1.333, 1.355 and 1.377 are 191.89pm/mm, 208.11pm/mm and 213.80pm/mm respectively. The temperature influence is also studied. The sensor exhibits a low temperature cross-sensitivity of -0.128mm/℃. The proposed modal interferometer based on D-shape fiber with high sensitivity, low cost and easy fabrication is suitable to liquid level sensing applications.

We propose a novel M-shaped single mode fiber (M-SMF) sensor based on stimulated Brillouin scattering of higher-order acoustic modes for simultaneous temperature and strain measurement. Numerical investigation of longitudinal acoustic modes in the M-SMF shows that multi-peak Brillouin spectrum in M-SMF is contributed by the couplings between the LP ₀₁ optical mode and multiple acoustic modes. The Brillouin characteristics in the M-SMF are compared with those of standard single mode fiber. Utilizing the multi-peak feature of Brillouin gain spectrum, sensing characteristics of the M-SMF under different temperature and strain conditions were studied. Thanks to the widely spaced L ₀₁ and L ₀₂ acoustic mode peaks in the Brillouin spectrum of M-SMF, the proposed fiber enables discriminative sensing of temperature and strain at an accuracy of 0.47 °C and 12.3 με, respectively.