The introduction of hydrogen into single crystals of barium sodium niobate, which occurs during the usual poling procedure, causes refractive‐index changes that permit the propagation of optical guided waves. Propagation of TM modes can be achieved even when the boundary between the hydrogenated and unhydrogenated region of the crystal is remote from the crystal faces. For thin hydrogenated layers, both TE and TM modes can be propagated. Phase‐matched frequency doubling of the cw output of a Nd&sngbnd;YAG laser was demonstrated in such a layer.