Heavy metal ions probe with relative measurement of fiber Bragg grating

Nickel ions (Ni2+) probe with enhanced sensitivity is experimentally demonstrated by using a normal fiber Bragg grating (FBG) cascaded with functionalized fiber tip. Bilayers of chitosan/poly acrylic acid is functionalized directly onto the cleaved fiber tip via layer by layer assembly. When the Ni2+ is absorbed onto the bilayers, the Fresnel reflectivity of the fiber tip, which forms the noise in the background of FBG's reflection spectrum, will be modulated due to Ni2+-caused refractive index variation of the bilayers. By demodulating the optical reflectivity ratio between FBG peak and background noise of the reflection spectrum, Ni2+ concentration can be measured successfully with a maximum sensitivity of 40.52 dB/mM. The relative method not only eliminates the influence caused by power fluctuation of the light source on measurement accuracy, but also allows independent temperature measurement through the shift of Bragg wavelength.