Ultrahigh-order guide modes are excited by employing the free-space coupling technique in a symmetrical metal-cladding optical waveguide fabricated with a submillimeter-scale transparent PMN-PT ceramics. The voltage-dependent variation of the reflectivity of the light intensity is obtained according to the shift of the attenuated total reflection (ATR) resonance dip and the quadratic electro-optic coefficient of the transparent PMN-PT ceramics is then calculated.