Continuous wave optical vortices by coherent superposition of off-axis multiple-pass geometric mode in optical resonator with an intra-cavity spiral phase plate

• Main Authors: Jyun-bo Tang, 湯鈞博
• Other Authors: Yuan-Yao Lin
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/kg55fg

碩士 === 國立中山大學 === 光電工程學系研究所 === 106 === Optical vortex (OV) is more than a beam of donut-shaped intensity profile. With phase singularities in the center, OVs thus carry orbital angular momentum (OAM) in the photons within which distinguishes OV beams from vector beams of radail or azimuthal polarizations. The unique property of OV beam have attracted growing attentions due to the wide range of promising applications including biomedical Science, microscopy, particle manipulation, astronomy and cosmology and optical communications. Although a number of techniques are devised to generate OV with helical wavefront by beam conversion, such as π ⁄ 2 conversion, holograms, anisotropic media (q-plate), spiral phase plate, but not one of these approaches achieve a satisfactory conversion efficiency and beam quality. This work proposes to use a near hemi-spherical resonator of 2-fold degeneracy for the direct generation of high order OV without and with intracavity spiral phase element, for example SPP(charges), for direct high azimuthal order OV excitation. Without intra-cavity SPP, OV in the form of Laguerre Gaussian mode of topological charge (TC) with undetermined sign can be generated as the pumping beam is deliberately off-set from the cavity axis. Moreover, by shaping the pump beam with an axicon lens, degenerate high order LG beams are experimentally demonstrated with unstable phase structure. When an intra-cavity SPP is introduced, single pass transverse verse mode such as LG modes are forbiden. In such a cavity any one of the beamlets inside retraces itself twice to complete a closed orbital following a V-shaped trajectories, and beamlet is modulated in by the SPP. This cavity therefore support the oscillation of multi-pass transverse mode (MPT mode). The OV beam created in the proposed vortex resonator is the superposition of phase-locked MPT modes instead of LG modes and will thus mimic the topological charge (TC) of the SPP. For the MPT modes following the closed v-shaped orbitals, they overlap and are strongly coupled in the laser crystal as a result of stimulated emission process. When the intra-cavity SPP is of a positive unit TC, the generated vortex beam carries the positive unit TC in the forward direction while zero TC in the backward direction. Replacing the SPP of TC number of 2, the laser emits vortex beam of a positive unit TC in the forward direction and a negative unit TC in the backward direction. It is believed the phase structure of the OV beam follows the Kuramoto model with the minimum energy criterion.