Twist Sensitivity of Cladding-Mode Resonances and Its Cross-Sensitivity to Strain and Temperature in a Mechanically Induced Long-Period Fiber Grating

Abstract

Twist sensitivity of cladding-mode resonances in a mechanically induced long-period fiber grating formed over a single-mode fiber is experimentally demonstrated and theoretically analyzed. Of the two usual cladding-mode resonances corresponding to LP₁₁ and LP₁₂, higher-order mode LP₁₂ is more sensitive to twist in comparison with the lower-order mode LP₁₁. The extent of down-shifting of resonant wavelengths depends on twist-induced circular birefringence and the modal field distribution of the cladding-modes inside the fiber. When the fiber is severely twisted to 3.5 rad/cm, a shift sensitivity is observed of 1 nm/(rad/cm) for the LP₁₁ mode and 4.23 nm/(rad/cm) for the LP₁₂ mode. The fiber breaks when the twist rate exceeds 3.5 rad/cm. In comparison with LP₁₂, the LP₁₁ resonance is almost independent of the axial strain variation with an ultra-low sensitivity of 0.18 pm/με, and it is also almost insensitive to the temperature variation with a coefficient of 35 pm/°C. Forming the LP₁₁ resonance far away from its cut-off wavelength, a widely tunable band-pass filter is also demonstrated with a very high twist sensitivity of 8.75 nm/(rad/cm) and negligible cross-sensitivity to strain and temperature. The experimental and theoretical results are very useful in selecting sensitive and stable cladding-mode resonances in the design of new mechanically induced long-period fiber gratings based torsion sensors and tunable band-pass filters.