A novel spectrum differential integration (SDI) method has been proposed and verified in an in-line fiber Mach-Zehnder (MZ) refractive index (RI) sensor using salt solutions. In SDI method, the difference between two interference spectra is determined by pointwise subtraction at each wavelength, followed by integration of the absolute differences along the scan range. Compared with the widely used peak wavelength shift method, the SDI method is more reliable over a wide wavelength range (on the order of 400 nm) and results in higher sensitivity as well as reduced device-dependence. The SDI method can also be utilized with other kinds of modal interferometric sensors.
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