Measurement of the index of refraction of an liquid by a cladding depleted fiber Bragg grating

We demonstrated a method of index of refraction measurement for unknown liquids by using a fiber Bragg grating (FBG) which the cladding layer is depleted to enhance the measurement of Bragg wavelength shift due to the variation of the index of the liquid samples. Process of the etching of the cladding layer of the FBG is described, and the procedure to calibrate the wavelength shift of the FBG in variation of standard index oil is discussed. Measurement of index of a sample liquid as high as 1.45 and within accuracy of 0.01 can be achieved, and a FBG signal interrogation system has been integrated to measure the index of refraction of water with different sugar contents.