When a piezoelectric transducer is bonded to an acousto-optic (A-O) crystal, it will generate an acoustic traveling wave or standing wave in the medium as illustrated in Fig. 35. The elasto-optic interaction then sets up a periodic variation in refractive index characterized by acoustic angular frequency W and wave vector K of magnitude 2Л/Λwhere sound velocity V =Ω/K. The incident optical wave (ki) will be diffracted by K into a wave kd = ki K. However, only a fraction of the incident optical or acoustic beam will be effective in the process.

When an electric field (E) is applied to an electro-optic (E-O) crystal, the refractive index of E-O crystal will change linearly to electric field. The phenomenon is called linear electro-optic effect.