Recently, specialty fibers with various functional material doping have attracted significant attention. In this paper, we
fabricated Nb-doped silica fiber and measured its resonant optical nonlinearity with long-period fiber gratings (LPFG)
interferometer. The Nb-doped fiber was made with a combined MCVD and ALD technology. Then, we fabricated a pair
of LPFGs and cascaded them as a Mach-Zehnder interferometer (MZI).By measuring the wavelength shifts of the
interference fringe with the 532nm laser pump power, the resonant nonlinear refractive index of Nb-doped silica fiber
around 1537 nm was estimated to be 8.12×10<sup>-16</sup>m<sup>2</sup>/W.