Diffraction properties of opaque disks outside and inside a laser cavity

Diffraction of symmetrical Laguerre-Gauss TEM_p0 beams incident on an opaque disk known as a stop is considered. The near- and far-field patterns are studied. Thanks to zero-field occluding, conversion from TEM₁₀ beam to dark hollow beam can be achieved with better efficiency than from a TEM₀₀ beam. It is shown that the fundamental mode of a laser cavity including a diaphragm and a stop can be TEM₀₀- or TEM₁₀-like in shape depending on their size. This result is interpreted from the new divergence hierarchy, which characterises the diffracted TEM_p0 beams emerging from the stop.