We report the fabrication of buried optical channel waveguides in strontium barium niobate nonlinear ferroelectric crystals by direct ultrafast laser inscription. These waveguides are strongly polarized and can be reversibly switched on and off by changing the temperature of the crystal, a characteristic we attribute to the bistable enhancement of the electro-optic coefficients at the ferro to paraelectric phase transition. (C) 2010 American Institute of Physics. [doi:10.1063/1.3429584]