Strain and temperature characteristics of a long-period grating written in a photonic crystal fiber and its application as a temperature-insensitive strain sensor

Strain and temperature characteristics of a long-period grating (LPG) written in an endless-single-mode photonic crystal fiber (ESM-PCF) are investigated theoretically and experimentally. By use of a dispersion factor γ, a deeper understanding of the behavior of LPG in the ESM-PCF is achieved. The negative strain sensitivity of the LPG is explained by the negative value of the dispersion factor γ. Our analysis clearly reveals the significant effect of the waveguide dispersive characteristics of the cladding modes on the strain and temperature characteristics of the LPG in the ESM-PCF. By selecting an appropriate grating period, a simple, low-cost LPG sensor with approximately zero temperature sensitivity but large strain sensitivity is realized.