PURPOSE: To perform effective optical heterodyne modulation by offsetting the bias frequency to be applied to crystal bodies consisting of an optical ultrasonic material in the case of providing point-symmetrically two sets of double refracting prisms and said crystal bodies and obtaining the diffracted light to be modulated from incident light.

CONSTITUTION: Incident light f0 holds an orthogonal polarizing component and is made incident on a wedge-shaped double refracting prism 1. The light is separated to the luminous flux having different refracting angles by the polarizing component and is then made incident on a crystal body (AOM)2 consisting of an optical ultrasonic material. The AOM12 is excited by adding a modulating frequency Asinωm to, for example, 140MHz ultrasonic bias frequency. The parallel output of the AOM2 is made incident on the AOM3 of the constitution provided in point symmetry and is emitted therefrom after said output is condensed on a wedge-shaped double refracting prism 4. The AOM3 is made only the driving frequency fM2=140MHz and the direction is made opposite to fM1. The output light fout is thus thoroughly removed of 140MHz bias frequency and the effective heterodyne modulation is accomplished.