Fig. 1. Schematic of long period fiber grating.

Fig. 2. Transmission spectrum of the fabricated grating measured with un-polarized light. Inset shows the photograph of the grating, where the grating length is indicated by the red light scattered out of the fiber, and the interval of the ruler is millimeters (mm).

Fig. 3. Transmission spectra of the fabricated grating with two orthogonal input polarizations (P1 and P2).

Fig. 4. Schematic of the experimental setup for twist sensing. SC, super-continuum source; PC, polarization controller.

Fig. 5. Evolution of (a) transmission spectra (peak positions are denoted by black circles), (b) peak intensity, (c) peak wavelengths with increasing twist angle, and (d) wavelength-interrogated twist sensitivity calculated from the fitted function at each measurement point.

Fig. 6. Evolution of (a) peak intensity and (b) peak wavelength with increased strain, when the grating was pre-twisted with an angle of 10 deg (blue circles), 30 deg (black rectangles), and 50 deg (magenta triangles). The linear fitting of the evolution is depicted by red lines.

Fig. 7. Evolution of (a) peak intensity and (b) peak wavelength with increased temperature, when the grating was pre-twisted with an angle of 10 deg (blue circles), 30 deg (black rectangles), and 50 deg (magenta triangles). The linear fitting of the evolution is depicted by red lines.