NIR and visible structured beam generation with a dual-force mechanical long-period fiber grating

Abstract

This work presents tunable generation of vortex, vector, and flat-top 1060-nm NIR beams in a few-mode fiber with a dual-force mechanical long-period fiber grating. By varying the applied forces on the fiber grating, the core mode to higher-order mode excitation can be adjusted. Manipulating the beam transformation is achieved by controlling the polarization of the fiber eigenmodes and the mode-coupling efficiency. In addition, our dual-force scheme enables greater flexibility in the mode selection by introducing the force gradient as an additional degree of freedom. By precisely tuning the intensity ratio between fundamental and doughnut modes, we arrive at the generation of propagation-invariant flat-top beams, for which the transverse intensity distribution exhibits a near-uniform profile over a propagation distance of 5 m in free space. The transverse optical field of 532-nm green light from a frequency-doubled Nd-doped yttrium vanadate laser can also be manipulated and coupled into various intensity distributions in a few-mode fiber by using a mechanically induced long-period fiber grating. We show that a doughnut beam, Mexican-hat beam, and crater-lake beam can be generated from the input Gaussian beam via the coupling of the fundamental core mode to a series of co-propagating higher-order modes with properly applied forces and polarizations. Our experimental results establish dual-force mechanically induced LPGs as a flexible and reconfigurable fiber-based tool for structured beam engineering, offering a unified platform for in-situ and robust control of phase, polarization, and amplitude of optical fields across different spectral bands.