Contents 1. Introduction
Mode division multiplexing (MDM) transmission is a candidate for realizing transmission capacities above 100 Tb/s/fiber. In an MDM system, differential modal gain (DMG) among transmission modes in a few-mode optical amplifier severely degrades system performance [1]. There are three major techniques for controlling DMG in a few-mode amplifier. First technique controls the mode content of the pump light [2]. Second technique manages the electric field distribution of each transmission mode by means of the refractive index profile (e.g. ring core Erbium doped fiber (EDF) [3]). Final one tailors the spatial distribution of Erbium dopant [4]. DMG values of less than 1 dB have been achieved with these techniques. However, the first approach needs additional optical components and the other two methods require complex design and precise fabrication of the EDF. Femto-second laser inscription is now attracting much attention as a technique for realizing a smooth refractive index change [5], birefringent refractive index modification [6], and empty void [7] within glass. It has been also reported that laser inscription enabled to attenuate a particular core in a multi-core fiber [8].
