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Wavelength- and OAM-tunable vortex laser with a reflective volume Bragg grating.

Vortex beams carrying orbital angular momentum (OAM) have been recently investigated intensely in optical communication systems, as using OAM mode multiplexing simultaneously with other conventional multiplexing techniques is the key to further expand data capacity. This article demonstrates a wavelength- and OAM-tunable vortex laser at 1.6 µm in an Er:YAG system. For the first time to the best of our knowledge, a reflective volume Bragg grating (VBG) was theoretically and experimentally proved to be an effective OAM-preserving wavelength selector inside the laser cavity. A z-shaped laser cavity employing a VBG as a folding mirror was constructed for the direct generation of vortex beams, and we finally obtained wavelength-tunable beams of five OAM states (0, ± ħ, and ± 2ħ) with a narrow bandwidth less than 0.04 nm. This laser supplies a new way for optical communication by combining the spatial degree of freedom for multiplexing information channels with the conventionally used wavelength domains in packable and robust resonant cavity.