Abstract: Ultrasonic grating is a dynamic phase grating, and it is different from ordinary static phase grating. When a laser beam vertically passes through the ultrasonic standing waves, the diffraction field contains new frequency components, so its space spectrum is also changed. Based on the principle of Fourier optics, the optical field of a laser beam vertically passing through the ultrasonic standing waves in Raman-Nath diffraction is derived rigorously, and then the space spectrum of diffraction field of ultrasonic standing wave is obtained. Thus grating constant of ultrasonic grating is deduced strictly.

Key words: Fourier optics, ultrasonic grating, standing wave, grating constant