Abstract: The invention pertains to a non-linear optically active (NLO) side chain polycarbonate and to NLO active waveguides containing said polycarbonate. Waveguides containing this NLO polycarbonate have a low loss of signal. Polycarbonates based on bischloroformates of hexafluorobisphenol A, hexafluorotetrabromobisphenol A and bisphenol S or deuterated bisphenol A or sulphonylbiphenol are especially preferred within the framework of the invention: they were found to give an exceptionally low loss of signal. The invention further pertains to cross-linkable and already cross-linked polycarbonates. Especially preferred polycarbonates according to the invention are those where the diol comprises a rigid donor group. These polycarbonates have a high Tg, which renders them thermally stable.