When the spot size of Gaussian laser beam is smaller than altrasonic wavelength and their ratio is suitably selected, the intensity of focused laser beam can be nearly flat in the central region.

• 当光斑尺寸小于超声波长,而且二者比率选择适宜时,聚焦激光束的强度轮廓在中心区域即可展平,以致得到空间均匀的辐照。