A microstrip T-transducer excites forward volume magnetostatic waves (MSWs) in a [BiLu]3Fe5O12film. An optical beam is edge coupled into the film and the transverse MSW–optical interaction is observed at high microwave power levels. Using the coupled mode equations, we demonstrate that a T-transducer causes the intensity of the diffracted beam to be a product of terms involving the magneto-optic coupling coefficient(&kgr;)and the phase mismatch between the optical guided modes and the MSWs. Taylor series expansions for&kgr;and the MSW wave number(&bgr;)are used to obtain an analytic expression for the MSW–optical interaction. The dependence on&bgr;causes a shift in the interaction spectrum with increasing power. After correcting for this shift, the residual dependence on power is attributed to a quadratic correction to the dynamic magnetization which in turn affects&kgr;. We demonstrate how the quadratic dependence plays an important role in accurately determining an expansion for the MSW dispersion relation. ©1997 American Institute of Physics.