Controlled generation, manipulation and frequency conversion of broadband orbital angular momentum spectrum of asymmetric optical vortex beams

TL;DR

This paper demonstrates the generation and control of broadband orbital angular momentum spectra in asymmetric optical vortex beams using spiral phase plates, and their frequency doubling with increased efficiency linked to asymmetry.

Contribution

It introduces a method to generate tunable, broad OAM spectra in asymmetric vortices and transfers these spectra through nonlinear frequency doubling, expanding control over vortex beam properties.

Findings

Achieved >95% conversion efficiency in generating vortex beams of orders 1-6.

Demonstrated transfer of broad OAM spectra through frequency doubling.

Showed increased conversion efficiency with higher asymmetry in pump vortices.

Abstract

We report on generation and control of tunable, broad orbital angular momentum (OAM) spectrum of a vortex beam. Using two spiral phase plates (SPPs), we have converted the Gaussian beam of Yb-fiber femtosecond laser at 1064 nm into optical vortices of orders, l=1-6, with conversion efficiency >95%. By adjusting the transverse shift of the SPPs with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order, l, into an asymmetric vortex beam of broad OAM spectrum of orders, l, l-1, l-2,..,0. While the position of the SPPs determines the weightage of the OAM modes, however, the weightage of the higher OAM modes transfer to lower OAM modes with the shift of SPPs and finally resulting a Gaussian beam (l=0). Using single pass frequency doubling of asymmetric vortices in a 5-mm-long bismuth triborate crystal, we have transferred the pump OAM spectra, l, l-1, l-2, ..., 0, into the broad spectra of higher order OAM modes, 2l, 2l-1, 2l-2, ...,0 at green wavelength, owing to OAM conservation in nonlinear processes. We have also observed that the conversion efficiency of the frequency-doubled vortices of all orders increases with the increase in the asymmetry of the pump vortices. Using pump power of 4.6 W, we have generated symmetric vortices at green wavelength of output powers, 1.2, 0.76, 0.47, 0.4, 0.32, and 0.3 W and orders of ls=2, 4, 6, 8, 10, and 12, respectively.