Highly Sensitive Refractometer Using a Fiber Bragg Grating Fabricated by Femtosecond Laser

A highly localized fiber Bragg grating is inscribed using point-by-point femtosecond laser direct writing technique. By monitoring the location of the cut off wavelength, we demonstrate that the FBG has a highly-linear sensitivity for refractive index (RI) measurement. By monitoring the “peak-to-dip” intensity change near the cut-off wavelength, an ultra-high refractive index sensitivity of 2924.3dB/RIU and a RI resolution of 3.4×10^-7 are achieved. As different surrounding refractive index corresponds to different location of the cut off wavelength, this FBG based refractometer can measure RI with wide range measurement range together with high sensitivity.