Torsion Sensing Characteristics of Mechanically Induced Long-Period Holey Fiber Gratings

We present the spectral transmission response of a long-period holey fiber grating mechanically induced in two different twisted holey fibers. We have found that each resonance wavelength of the grating splits when the holey fiber is twisted prior pressure application. The new upper and lower resonance wavelengths show an opposite symmetrical shift that increases proportionally with the induced twist ratio, and it does not depend on tuning and twist direction. The shift observed in the splitting of the peaks in the mechanically induced long-period holey fiber grating is higher than that obtained in standard single-mode fibers and its sensitivity depends of the transverse structure of the holey fiber. High sensitivity is observed in the fiber with asymmetrical cladding structure. These results are important in the design of new torsion sensors based in long-period holey fiber gratings.