Generation of all-fiber third-order orbital angular momentum modes based on femtosecond laser irradiation

In this article, a long period fiber grating (LPFG) based on femtosecond laser irradiation is proposed to generate the third order Orbital Angular Momentum (OAM) mode. With the advantage of lower heat absorption efficiency, lower power and small focused spot size，this method is more reliable ,repeatable and promising compared with the traditional method based on carbon dioxide laser. The experiments results demonstrated that the fundamental mode can be converted to the linear polarization mode LP31 mode even OAM±3 modes with 98% mode conversion efficiency near 1550nm. The proposed method has the potential to generate higher order mode.