Observation of a 2-µm flat-top beam carrying orbital angular momentum by intracavity incoherent superposition

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Flat-top beams exhibit symmetrical intensity and steep edge in cross-section with wide application. Especially, orbital angular momentum (OAM) endows it with the potential to achieve uniform angular momentum transfer or phase encoding. In this Letter, we observed a flat-top beam carrying + ℏ OAM by intracavity incoherent superposition in an X-cavity Tm: YAG ceramic laser. The corresponding M 2 factors are 2.24 and 2.08 with a typical output power of 470 mW around ~2011.2 nm. Simultaneously, a radially polarized vortex beam (RPVB) with + ℏ OAM was observed. Further simulations and experiments revealed that a flat-top beam is formed by the incoherent superposition of RPVB and a Gaussian beam. Using a homemade M–Z interferometer, we observed vortex stripes’ chirality reversal before and after the focal plane of the convex lens. To the best of our knowledge, this is the first observation of a flat-top beam with OAM at 2 µm wavelength.