Mid-infrared spectroscopic detection of refractive index in CO2 laser-written long-period fibre grating

A method using Fourier-transform infrared, reflection, microscopy for measuring the refractive index modulation in a CO₂ laser-written long-period fibre grating is suggested. A mid-infrared spectrum was observed to detect the peak position around 1120 cm-¹, which was correlated to the refractive index at the visible to near-IR frequency region in silica glass. The observed reflection peak was clearly modulated to have the same periodicity as that of the recorded grating. This method presents attractive features such as the obviation of complicated data processing, rapid measurement and no need to use matching oil.