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We demonstrate the fabrication of the interlaced tilted long-period fiber grating (ITLPFG) using focused CO 2 laser. Due to the asymmetric refractive index distribution, the grating has very weak sensitivity in the two opposite directions to reduce the influence on the other two bending directions with higher bending sensitivities of −9.16 and 5.14 nm/m−1, respectively. The temperature and refractive index sensing characteristics of the ITLPFGs are also investigated experimentally. The ITLPFGs could be adopted as temperature-insensitive directional bending sensor. [ABSTRACT FROM AUTHOR]

We demonstrate the inscription of long-period gratings (LPGs) in a fluorine-doped double-cladding fiber (DCF) with a low refractive index trench. The grating has a single resonance dip in the wide wavelength range from 1200 nm to 2400 nm. The transmission spectra and cladding mode evolution of the DCF-LPG during the cladding etching process are investigated theoretically and experimentally. The mode order of the cladding modes decreases with one order of every 20 μ m diameter etching, while the resonance wavelength of the DCF-LPG barely shifts when the fiber diameter decreases gradually. Only LP 11 mode can be excited in the etched DCF-LPG when the fiber diameter is less than 40 μ m. The characteristics of the DCF-LPGs are investigated experimentally. The torsion sensitivity was measured to be −0.154 nm/(rad/m) and −0.271 nm/(rad/m) at 1. 55 μ m and 2. 0 μ m wavebands, respectively, which are one order of magnitude higher than that of the conventional LPG. The proposed DCF-LPG has potential application as a tunable band-rejection filter in broad wavebands. • Only one resonance dip of the DCF-LPG can be observed in the wavelength range from 1200 nm to 2400 nm. • The mode order of the cladding modes decrease with diameter etching, while the resonance wavelength barely shifts. • The twist sensitivity of the DCF-LPGs at 1. 55 μ m and 2 μ m wavebands was one order of magnitude higher than that of the conventional LPG. [ABSTRACT FROM AUTHOR]

The thermal stability of uniform fiber Bragg gratings (FBGs) inscribed in single mode double cladding fiber (DCF), standard single mode fiber (SMF-28) and dispersion shifted fiber (DSF) by 193 nm excimer laser was experimentally investigated. When the temperature rises from 30 °C to 800 °C, the grating contrast of FBGs with the saturated refractive index modulation drops slowly, and the corresponding 3 dB bandwidth gradually decrease After annealing at 400 °C for 2000 h, it is found that the resonance wavelength shift of FBGs mainly occur during the first 200 h. It is also observed that the FBG inscribed in DCF requires a longer time to stabilize wavelength than the FBG written in SMF-28, about 1500 h, which suggests that long-term pre-annealing is required to ensure accurate wavelength measurement. After 2000 h of annealing treatment, the FBGs can still have excellent sensing performance in the temperature range of 0–400 °C. • The thermal stability of FBGs fabricated in three types of optical fibers by 193 nm excimer laser are investigated. • The characteristics of the transmission contrast and bandwidth of FBGs during the annealing process are analyzed. • The wavelength shift rate of FBGs are investigated after the annealing at 400 °C for 2000 h. [ABSTRACT FROM AUTHOR]

We propose a fiber torsion sensor based on double-helix long-period grating (DH-LPG) written in a tapered polarization-maintaining fiber (PMF). A series of DH-LPGs were fabricated in the tapered PMFs with waist diameters of 85 to 67 μ m by a CO 2 -laser heating and twisting system. The torsion, temperature and surrounding refractive index sensing characteristics were investigated experimentally. Thanks to the special structures combined DH-LPGs with tapered PMFs, the proposed sensor can realize directional torsion measurement. The maximum torsion sensitivity of the proposed grating could be up to -2.28 nm/(rad/m) when the tapered waist diameter was 67 μ m. The DH-LPGs have potential application in field of all-fiber high sensitivity directional torsion sensing. • Novel and repeatable fabrication methods for double-helix long-period gratings. • Detailed analysis in the sensing characteristics with various waist diameters. • Available for vector torsion measurements. • High torsion sensitivities and wide measurable twist rate ranges. [ABSTRACT FROM AUTHOR]