Optical vortex lasers by the coherent superposition of off-axis multiple-pass transverse modes in an azimuthal symmetry breaking laser resonator

Abstract

Optical vortex (OV) laser is generated by the superposition of coherently phase-locked off-axis laser beamlets in a nearly semi-spherical optical resonator with intra-cavity azimuthal symmetry breaking (ASB) optics. Due to the ASB, this laser resonator rejects radiation to resonate on-axis but supports off-axis beamlets belonging to the multiple-pass transverse (MPT) modes. These independent off-axis MPT modes, which are experimentally proven to retrace V-shaped paths in the resonator span ring-shaped intensity distribution and are coherently phase-locked to form an OV with a wavefront dislocation mirroring the topological charge (TC) of the ASB optics. Pumped by a conventional diode laser without critical beam shaping, we successfully generate randomly polarized OV having an annular intensity distribution and a positive unit TC of more than 99.9% at a central wavelength of 1064 nm when the power of pump laser goes above 1.52 W. The build-up of the deterministic phase relation among the independent MPT modes is demonstrated by spatial coherence measurement of the laser from a perturbed ASB resonator. This ASB resonator not only provides an easy approach to generate intrinsic vortex laser with power scalability but also serves as an useful platform to shape laser beams structurally, to study the laser dynamics and to combine radiations coherently.

Spiral-Phase-Defect Resonator and Its Application in Vortex Laser of Controllable Topological Charges

“…The clear spiral interference patterns in Fig. 5(e), (f) and (g) not only quantifies the topological charge of the +1, -1 and +2 in the optical vortices, which imitate the topological charge of the SPD encoded by the SLM in the laser resonator, but also manifests the high spatial coherence of the vortex laser beam from ASB resonators [26,32].…”

“…2(a) in Ref. [26]. In that interferometer, the two beams divided from a single vortex laser are displaced deliberately after the beam combiner (the second beam splitter) and one of the beams is focused onto the ring region of another beam which expands freely and becomes a background field with a flat wavefront.…”

“…3(a) and (d). By fitting the interference fringe with that of the coherent superposition of multiple vortices [26], observation of the vortex carrying topological charge as a multiple of topological charges in the SPD shown in Fig. 6(a) and (b) indicates that the generation of a high-order vortex from the cascade coupling process, such as the vortex of topological charges −2 and −3 in Fig.…”

“…To overcome the difficulty, a cylindrical focusing lens was required in the Oron's proposal in 1999 and 2000 [17,18], which restored the broken azimuthal symmetry and swapped back the topological charges. Moreover, a laser cavity with intracavity spiral phase element resonating only off-axis modes was introduced to overcome the singularity [26,32]. In a SPD of finite radius encoded by SLM, broken azimuthal symmetry was introduced into the laser cavity as well.…”

“…It rejects the resonance of on-axis modes, such as the LG family. An optical vortex laser formed by the coherent superposition of off-axis multiple-pass transverse modes that are mutually nonorthogonal was demonstrated, and a vortex purity greater than 99.99% was achieved in a continuous-wave regime [26]. Laser beams with controllable OAM are important tools for the development of OAM-related science and technology.…”