RAMAN-NATH DIFFRACTION FIELDS OF SURFACE ACOUSTIC WAVES BY CHIRPED ACOUSTIC GRATING

The diffraction of surface acoustic waves in Raman-Nath regime on acoustic grating with linearly chirped spatial frequency is investigated.Theoretically,the transfer function of the Raman-Nath grating is considered as the distribution function in the finite aperture and by using the Kirchhoff diffraction integral expression,the acoustic field distribution in transmission region can be considered as the diffraction patterns of the distribution function in the finite aperture.It is shown from the analysis that the Raman-Nath diffraction grating with chirped spatial frequency can perform two functions simultaneously,i.e.,firstly,it looks like a Raman-Nath diffraction grating with constant period and secondly,for every diffracted beam,it is a Fresnel lens.Experimentally,an unique IDT with chirped spatial frequency in transversal direction and with the shape of the sine-function for its fingers is used to simulate the diffracted fields of the grating analyzed theoretically.The diffraction characteristics analyzed theoryetically are confirmed by experiments.