The chance to observe optical damages in mode cut-off modulators used to be rare, because little optical power could be coupled into near-cut-off channel waveguides. The coupling capability, however, was found to be higher in our newly constructed three-section hybrid mode cut-off modulator, in which observable optical damage occured. Therefore, in designing guided-wave devices consisting of Ti-diffused waveguides, not only their electric characteristics but also their optical-power carrying capacity should be taken into consideration.