Exploring vortex structures in orbital-angular-momentum beams generated from planar geometric modes with a mode converter

Abstract

It is theoretically demonstrated that the planar geometric mode with a π/2 mode converter, so called the circularly geometric mode, can be solved from the inhomogeneous Helmholtz equation by considering the pump distribution on the lasing mode. Theoretical analysis clearly reveal that the vortex structures of circularly geometric modes are determined by the minimum order of transverse lasing modes, the total number of transverse lasing modes and the degenerate condition in the cavity. Moreover, we experimentally manifest that the circularly geometric mode can be generated from the selective pumped solid-state laser with an external π/2 mode converter. To explore the vortex structures of the generated geometric modes, the interference patterns are performed by an experimental apparatus consisting of a Mach-Zehnder interferometer. The good agreement between experimental observations and numerical calculations confirms the analysis of vortex structures is reliable.

Truncated triangular diffraction lattices and orbital-angular-momentum detection of vortex SU(2) geometric modes

“…The whole field shows singularities with number of n 0 + QM leading to a large fractional OAM. However, the classical interferometry cannot identify the actual phase and topological charges of SU(2) vortices, because the exotic intensity fields forbid the overall emergence of interference fringes [18]. We hereinafter introduce an effective method to detect the OAM in both center and partial regions for SU(2) vortices.…”

“…The exotic modes in the frequency degenerate cavity under * shenyj15@mails.tsinghua.edu.cn off-axis pumping are always called geometric modes [14][15][16][17][18][19]. The nonplanar geometric modes can be obtained by astigmatic transformation of planar geometric modes [14,18] and can also be directly emitted from frequency degenerate cavity [19,22]. The SU(2) geometric modes has been used to explore novel high-pulse-energy vortices [14,19] and structured polarized beams [22,23].…”

“…However, it has never been settled on how to experimentally measure the actual OAM of an SU(2) wave-packet. Although the interferometer experiment of SU(2) vortices was illustrated for verifying the existence of OAM and topological charge [18], yet the interferograms cannot be used to count the actual value of topological charges as the case of a classical doughnut-shape beam. The key issue leading to this difficulty is that the SU(2) vortices have complicated multi-singularity and fractional OAM structure.…”

“…The key issue leading to this difficulty is that the SU(2) vortices have complicated multi-singularity and fractional OAM structure. The exotic singularities distribution gives rise to not only the center dark region but also several partial dark regions, which forbid the emergence of overall strips in conventional interference technique [18,19]. The topological charges exist not only in the center dark region but also in the partial dark regions.…”

Hybrid topological evolution of multi-singularity vortex beams: generalized nature for helical-Ince–Gaussian and Hermite–Laguerre–Gaussian modes

“…As distinctive structured light fields with phase singularities, optical vortices carrying orbital angular momentum (OAM) have hatched plenty of modern scientific applications in optical tweezers [1][2][3], optical communications [4,5], quantum entanglement [6][7][8] and nonlinear optics [9][10][11]. Besides the classical Laguerre-Gaussian (LG) beams carrying integer OAM with a single phase singularity, the multi-singularity vortex beams carrying fractional OAM were also reported [12][13][14][15][16][17]. The unique characteristics of multi-singularity beams and fractional OAM can be utilized to significantly increase capacity in optical communication systems [5,18,19], scale multiparticle manipulation technologies in optical tweezers [2,3,20,21], drive advanced micro-opto-mechanics [22], flexibly shape light beam in 3-dimensional [23][24][25], and explore novel optical phenomena such as optical vortex knots [26][27][28] and spin-to-orbital conversion [29].…”