Influence of surrounding media refractive index on the thermal and strain sensitivities of long-period gratings.

A detailed study of the thermal and strain sensitivities of a long-period grating when the device is immersed in different external media is presented. The range of refractive indices analyzed are within 1.000 to 1.447, corresponding to samples of air, water, ethanol, naphtha, thinner, turpentine, and kerosene. Within the same range of refractive indices, the strain sensitivity is between (-0.24 +/- 0.03) and (-0.94 +/- 0.11) pm/microepsilon. For the grating immersed in these fluids, the refractive index sensitivity ranges from -3 to -1035.6 nm per refractive index units. The coupling thermo-optic coefficients and the strain-optic coefficients are also measured, resulting in the range from (2.45 +/- 0.04)x10(-5) to (15.89 +/- 0.82)x10(-5) deg C(-1) and (-1.15 +/- 0.04) to (-1.61 +/- 0.04) microepsilon(-1), respectively. A noticeable nonlinear behavior of the thermal sensitivity is found for external media with refractive indices higher than 1.430.

Simultaneous temperature and strain measurements performed by a step-changed arc-induced long-period fiber grating.

A compact sensor based on step-changed arc-induced long-period fiber gratings was implemented to discriminate between temperature and strain. The proposed sensor consists of a single long-period grating with two sections written consecutively in the SMF-28 fiber using the electric arc discharge technique. The two sections have the same period but different fabrication parameters. The operation of the sensor relies on the existence of a difference between the values of temperature and strain sensitivity of two neighboring resonances observed in the spectrum of the step-changed grating. The temperature and strain resolutions obtained for the sensor are 0.2 degrees C and 35 micro epsilon, respectively.

Refractometric sensor based on a phase-shifted long-period fiber grating.

A refractometric sensor based on a phase-shifted long-period fiber grating written by electric-arc discharges is presented. Transmission and reflective configurations for refractive index measurements are studied. It is observed that the reflective topology permits better performance compared with the transmission one, which is the approach normally utilized in the context of long-period fiber sensing. The resolution achieved in the measurement of refractive index enables the application of this sensing head structure in demanding situations, such as the measurement of the level of salinity of water.

Arc-induced long-period gratings in aluminosilicate glass fibers.

Permanent long-period gratings were written using arc discharges in two aluminosilicate fibers, one of which was doped with erbium. Reversible gratings were also mechanically induced in both fibers. The thermal behavior of the arc-induced gratings was investigated at up to 1100 degrees C. It was found that the shift of the resonant wavelengths exhibited a well-defined linear dependence on temperature up to 700 degrees C.