Generation of nearly nondiffracting Bessel beams with a Fabry–Perot interferometer

An ideal zero-order Bessel beam consists of a superposition of monochromatic plane waves with wave vectors lying on a conical surface having the same magnitude. Several experiments have been reported that achieve such a superposition of plane waves. For example, this type of angular spectrum can be obtained by applying an annular slit in the focal plane of a lens1 or by the use of an axicon,2 a holographic process,3,4 a Fabry–Perot interferometer,5,6 or a special type of laser cavity.7,8 A novel concept for the generation of nearly nondiffracting Bessel beams as applied to microlithography was described in Ref. 9. The experimental arrangement is shown in Fig. 1. A pointlike source was formed by focusing a He–Ne laser beam (l 5 632.8 nm). Such a point source illuminated a scanning Fabry–Perot interferometer, which produced a concentric ring system in front of a lens. The image produced by the lens was magnified by two microscope objectives, and the intensity distribution was observed with a CCD camera. The lens aperture was adjusted so that it transmitted only the first Fabry– Perot ring and blocked all other rings. The measured intensity distribution in planes perpendicular to the optical axis is given by the J0 function. This result was expected because a century ago10–12 it was recognized that the diffraction pattern of a narrow annular aperture can be described by the zero-order Bessel function. Because of the annular illumination of the lens, the depth of focus increased and the transverse resolution could be improved by a factor of 1.6. This paper reports on an analytical wave optics description of experimental results obtained in Ref. 9.