Laguerre–Gaussian beams with a nonzero azimuthal index are known to carry orbital angular momentum (OAM), and are routinely created external to laser cavities. The few reports of obtaining such beams from laser cavities suffer from inconclusive evidence of the real electromagnetic field. In this Letter we revisit this question and show that an observed doughnut beam from a laser cavity may not be a pure Laguerre–Gaussian azimuthal mode but can be an incoherent sum of petal modes, which do not carry OAM. We point out the requirements for future analysis of such fields from laser resonators.
© 2014 Optical Society of America
Lasers and Laser Optics
Original Manuscript: October 2, 2013
Revised Manuscript: December 5, 2013
Manuscript Accepted: December 8, 2013
Published: January 30, 2014
Igor A. Litvin, Sandile Ngcobo, Darry Naidoo, Kamel Ait-Ameur, and Andrew Forbes, "Doughnut laser beam as an incoherent superposition of two petal beams," Opt. Lett. 39, 704-707 (2014)