According to the first aspect of the present invention, there is provided to a method for forming a plurality of fiber Bragg gratings in a plurality of optical fibers. The second aspect of the present invention relates to a fiber-based system. The fiber-based system is for adjustable mode conversion of an electromagnetic signal that passes therethrough. The fiber-based system comprises a plurality of few mode fibers, and a plurality of fiber Bragg gratings formed in the few mode fibers. The third aspect of the present invention relates to the use of the fiber-based system for providing adjustable mode conversion of an electromagnetic signal for mode division multiplexing (MDM) applications. The present invention is also applicable in other fields such as Space Division Multiplexing applications and spatial mode fiber laser generation applications, telecommunications, fiber optics, information technology (IT), computer science, among others.