| 摘要 |
<p>Arrangements for generating a well defined traveling wave beam substantially unaffected by diffractive spreading. In different embodiments, the beam can be an electromagnetic wave, a longitudinal beam such as an acoustic beam, or any type of beam to which the Helmhotz generalized wave equation is applicable. Pursuant to the teachings herein, a beam is generated having a transverse dependence of a Bessel function, and a longitudinal dependence which is entirely in phaser form, which results in a beam having a substantial depth of field which is substantially unaffected by diffractive spreading. In first and second disclosed embodiments, respectively, an optical and an acoustical beam are generated by placing an annular beam source (30, 40) in a plane conjugate to the focal plane of a focusing means (32, 42), which results in the generation of a well defined beam (38', 44') because the far field intensity pattern of an object is the Fourier transform thereof, and the Fourier transform of such an annular shape is a circular function.</p> |
