Generation of non-Gaussian Optical Vortex in a Degenerate Optical Resonator with an Intra-Cavity
YuanYao Lin1*
1Department of Photonics, National Sun Yat-sen University, Kaohsiung City, Taiwan
* Presenter:YuanYao Lin
Optical vortex (OV) is more than a beam of annular-shaped intensity profile. It carries well-defined 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 a wide range of promising applications.

We propose a novel approach to generate non-Gaussian optical vortex beams using a degenerate optical resonator with an intra-cavity spiral phase plate (SPP)[1]. The vortex resonator has a near semi-spherical configuration of 2-fold degeneracy perturbed by SPP supporting only the multipass transverse (MPT) modes from the off-axis rays retracing skewed V-shaped paths in the resonator by the aid of linear and nonlinear coupling effects. According to the off-axis beam coupling model we proposed, the OV beam created in the vortex resonator is the superposition of phase-locked off-axis MPT modes instead of Laguerre-Gaussian modes belonging to single-pass transverse category of the same optical axis. In the laser crystal, laser beams in the radiation cone following the closed skewed V-shaped orbitals overlap with the neighboring beams and strong couplings occur. The mutual coherence among the laser beams is established and the laser beams become in-phase to achieve the minimum system energy analogous to the coupled oscillator system described by Kuramoto model. The
OAM of the radiation field as a whole directly emulates the topological charge of the SPP in the resonator. Notably the phase singularity induced by the SPP hinders the on-axis (z-axis) rays to resonate and the radius of the radiation cone in this vortex resonator is mostly determined by the aberrations of the resonator system.

The OV beam was realized in DPSS laser system emitting OV beam at the wavelength of 1064 nm. When it was pumped by diode laser at the wavelength of 808 nm, the resonator with Nd:YAG crystal and intra-cavity SPP of a unit topological charge placed near the output coupler 10% transmission at the resonating wavelength of 1064 nm lased OV beam of a positive unit topological charge. High quality annular-shaped intensity profile and spiral phase structure was measured. The laser threshold and slope efficiency were characterized to be 1.52 W and 0.19 under the 2% power sbatility and 2 μm rad beam pointing stability. The phase-locking scenario was confirmed by measuring the coherence of the lasers beam with specially designed profile. Moreover when placing a Cr:YAG crystal as a saturable absorber into the vortex cavity, Q-switched pulses were generated at a repetitive frequency of 10.5 kHz, yielding a pulse duration around 56ns and a pulse energy of 26 μJ.

Using the vortex resonator, high quality OV beam of various topological charge are expected to be generated to benefit scientific and industrial applications. Moreover this system also serves an useful platform to shape laser beams structurally, to study the laser dynamics and to combine radiations coherently.

[1]Y.-Y. Lin, C.-C. Yeh, H.C. Lee, ”Generation of optical vortices in a degenerate optical resonator with an intra-cavity
spiral phase plate,” arXiv:1707.00467 (2017).

Keywords: optical vortex, laser resonator, diode pumped solid state laser, orbital angular momentum