Reduction of differential modal gain in 2-modeamplifier using void-inscribed EDF

We propose a technique for reducing the differential modal gain (DMG) that occurs in a two-mode erbium-doped fiber (2M-EDF) by inscribing voids in the core center of a 2M-EDF with a femtosecond laser. We show that an empty void inscribed at the core center can attenuate the linearly polarized (LP01) mode selectively while suppressing excess loss for the LP11 mode. We also reveal that DMG can be controlled by means of void diameter. The longitudinal position dependence of the void in a 2M-EDF was also investigated considering its influence on gain and noise figure (NF) characteristics. Finally, we realize a sufficiently low DMG of less than 0.5 dB in the full C-band as well as a sufficient gain and NF by using the proposed technique.