Continuous-wave measurement of the fiber nonlinear refractive index

We propose a technique for measuring the nonlinear refractive index of fiber based on transmission-coefficient measurements in a fiber Sagnac interferometer. In contrast with traditional methods, the proposed method uses a single optical source operating in cw mode and direct intensity measurement, enabling one to avoid the errors caused by fiber dispersion and uncertainty of spectral peak difference measurements that occur with pulse-based methods. The nonlinear refractive index in 20-mol. % ${\mathrm{GeO}}_2$ fiber was measured to be $\left(3.1\pm 0.2\right)\times {10}^{-16} {\mathrm{cm}}^2 {\mathrm{W}}^{-1}$ at 1064 nm.