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With the intensity distribution of interference fringes formed by a capillary filled with transparency liquid, the variations in refractive index (RI) of nonlinear optical materials, corresponding to change of the intensity at the centre of fringes for a period, are deduced. Using this conclusion, we measure variations in RI of water solution of methyl orange (MO) excited by laser at 441.6nm. As an experimental result, the change of RI is 0.00216 and the corresponding effective index of refraction is -1.65×10-10 m2W-1. So, it can be used to measure nonlinear optical materials as a novel and simple method.

The reflection spectrum of Methyl Red (MR) doped PMMA film is measured and modified. The refractive index of the sample is gained by calculation of Kramers- Kronig relation with the data of the reflection spectrum. The refractive index change of the sample is gained with the absorption specta. The MR doped PMMA sample has a refractive index change under 532nm laser excitation, in which the valley value happens around 400 nm and peak value around 600 nm. This result shows that the MR doped PMMA sample has an obvious photo induced refractive index change, especially at 400nm and 600nm.