Abstract
In this work we described an optical fibre sensing system for detecting low levels of methane. The properties of hollow-core photonic crystal fibres are explored to have a sensing head with favourable characteristics for gas sensing, particularly in what concerns intrinsic readout sensitivity and gas diffusion time in the sensing structure. The sensor interrogation was performed applying the Wavelength Modulation Spectroscopy technique, and a portable measurement unit was developed with performance suitable for remote detection of low levels of methane. This portable system has the capacity to simultaneously interrogate four remote photonic crystal fibre sensing heads. Copyright © 2009 J. P. Carvalho et al.

Abstract
In this letter, interferometric sensors based on antiresonance reflecting optical waveguide (ARROW) fibers were developed, and used to sense strain and temperature. Two types of solid-ore ARROW fibers were considered and signal demodulation was achieved by using the white light interferometric technique. The ARROW fibers have two rings of high index rods arranged in a hexagonal structure with a lattice constant of 6 m. The different sizes of the rods cause different measurand sensitivities for the two fibers. Resolutions of +/- 1.1 mu epsilon and +/- 0.07 degrees C were achieved for strain and temperature, respectively.

Abstract
In this work a modal interferometer based on arc-induced long-period gratings (LPGs) in a Mach-Zehnder configuration is evaluated as a sensing structure for environmental refractive index measurement. To interrogate this sensing device, coherence addressing and pseudo-heterodyne processing were used. The influence of geometric effects such as stretching, bending and twisting the interferometer on the sensitivity to refractive index changes was studied. It is shown that due to the antisymmetric nature of cladding modes in arc-induced LPGs, it is possible to tune the system sensitivity to external refractive index by simple mechanical action. The experimental results show that it is possible to tune the sensitivity to external refractive index by more than 50% by control of the curvature in the Mach-Zehnder interferometer.

Abstract
A novel sensing configuration for measuring humidity based on a long-period fibre grating coated with a thin film of silica nanospheres is proposed. The polymeric overlay is deposited on the grating using the electrostatic self-assembly technique. This thin film changes its optical properties when exposed to different humidity levels that translate into a shift of the resonance wavelength of the fibre grating. Wavelength shifts up to 12 nm in a relative humidity range from 20% to 80% are reported, and it is further demonstrated that such humidity sensitivity has negligible thermal dependence.

Abstract
Fiber Bragg Gratings (FBG) are widely used in various fields, including optical fiber sensors. In this work, the temperature and strain response of C-band FBG in pure silica four-leaf clover shaped suspended-core fibers was analyzed. These FBGs were fabricated by femtosecond laser exposure, which enabled the refractive index modulation of the pure-silica-core of the fibers. We compared the Bragg wavelength variation with strain and temperature for two different suspended-core fibers (256b2 and 256b5). The 256b2 fiber has a core diameter of 4,9 µm and a hollow hole inside the core with 1,4 µm; the 256b5 fiber has a solid silica core with a 7,2 µm diameter. For strain and temperature characterization, the sensing head was attached to a translation stage with a resolution of 1 µm and was placed in a tubular oven, which permits a temperature reading to be set with an error smaller than 0,1 °C. Both have shown the same sensitivity to strain (1,2 pm/µe) but different sensitivity to temperature variation (8,4 pm/°C and 10 pm/°C respectively). The relative difference between the thermal coefficients of the two selected Bragg signatures is 16%. The results obtained indicate that these gratings can be used in optical fiber sensing, for example in the context of the important problem of simultaneous strain and temperature measurement. © 2009 SPIE.

Abstract
An ultra-narrow transmission filtering scheme that combines the response of a phase-shift fiber Bragg grating and a broadband rejection micro-optical filter is proposed. The full implementation process is disclosed starting from the simulated spectral responses, grating fabrication and final assembling and test. This scheme intends to be an alternative and feasible engineering solution for one-end optical filtering interface for signal processing and analysis. © 2009 SPIE.