LASER MODES WITH HELICAL WAVE-FRONTS

We report the operation of an argon-ion laser in pure (single-frequency) ''doughnut'' modes of order m = 1, 2, and 3. The phase discontinuity at the center of these modes leads to striking two-beam interference patterns that clearly demonstrate the existence of a helical cophasal surface (wave front). The doughnut mode with m = 1 (usually called TEM01*) displays a forking interference fringe pattern characteristic of a pure single helix. The m = 2 mode shows a pattern with four extra prongs, establishing that the cophasal surface is a two-start or double helix; the m = 3 mode is a triple helix with a six-extra-pronged pattern. Each pure doughnut mode is shown to have two possible states corresponding to output wave fronts of opposite helicity.