We report on the use of a Raman microprobe to investigate the nonlinear optical process of polarization rotation in Ti:LiNbO3optical channel waveguides. Selection rules allow the Raman scattered signal collected in a direction orthogonal to the plane of the waveguide to provide information regarding the polarization state of the light propagating in the waveguide. From this data a threshold value of power necessary to initiate the polarization rotation is obtained and used to calculate a value of &bgr;15, an asymmetric component of the photovoltaic tensor. The value of &bgr;15for an X‐cut, Y‐propagating channel waveguide was found to be 8×10−13A/W and that for a rapid thermally annealed Z‐cut, Y‐propagating sample 4×10−14A/W.