We present nonlinear optical spectroscopy of the model conjugated polymer MEH-PPV, a derivative of poly(paraphenylenevinylene) (PPV) and compare it with PPV. We perform multiphoton excited fluorescence spectroscopy and z-scan studies of solutions compared with third-harmonic generation, linear and nonlinear waveguide spectroscopy with intensity dependent prism coupling of thin films in the NIR region. Spectra of the nonlinear absorption coefficient and nonlinear refractive index are used to identify the figures of merit (FOM). We observe a spectral window at 1100 - 1200 nm were the application demands for all-optical waveguide switching are fulfilled. We demonstrate all-optical refractive index changes in the order of 0.001. Control of the molecular weight of MEH-PPVs enables improved film forming properties, reduced birefringence and ultimately low waveguide propagation losses < 1 dB/cm.