Ultrabroadband Long-Period Fiber Gratings for Wavelength-Tunable Doughnut Beam Generation

Abstract

An ultrabroadband long-period fiber grating with 10-dB bandwidth of 155 nm and 3-dB bandwidth of 415 nm is demonstrated around an optical wavelength of 1060 nm in a few-mode fiber. Combined with a wavelength-tunable ytterbium-doped fiber laser, the application of the force-gradient method achieves the generation of optical doughnuts in the spectral range of 1030 nm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~1099$ </tex-math></inline-formula> nm. This long-period fiber grating has also been used with an amplified-spontaneous-emission source to obtain a broadband optical doughnut. Instead of varying the grating period, the ultrabroad bandwidth is obtained by adjusting the effective index difference between the core mode and the higher-order modes, which is realized by the variation of an applied force in the few-mode fiber, and the force gradient along the grating is quantitatively determined to be 2.11 N/cm.