LASER MODES WITH HELICAL WAVE-FRONTS

We report the operation of an argon-ion laser in pure (single-frequenc y) ''doughnut'' modes of order m = 1, 2, and 3. The phase discontinuit y 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 TE M01) displays a forking interference fringe pattern characteristic of a pure single helix. The m = 2 mode shows a pattern with four extra p rongs, 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 patt ern. Each pure doughnut mode is shown to have two possible states corr esponding to output wave fronts of opposite helicity.