In this communication, an experimental study of continuous wave laser beam self-focusing in KNbO3 is conducted. A natural low frequency oscillation in the self-phase modulation process is observed when the crystal is in short-circuit. These oscillations are shown to be responsible for a priori non-reproducible results obtained under DC applied electric field. The consequences of this natural oscillation are investigated in terms of self-phase modulation and using a spectral approach under periodic low frequency applied field.