The realization of optical amplifiers delivering a well-controlled gain level for the different modes used within the frame of spatially multiplexed transmissions is a crucial issue for the development of such systems. An alternative to controlling the modal gain during the amplification process itself is to adjust the differential losses of the modes. Ideally, this should be done in the amplifier, while preserving an all-fiber configuration, a simple fiber design and a simple pumping scheme. This is what the authors are doing here by creating an empty void in the core of a two-mode erbium-doped fiber, the void being photo-induced in the core with a femtosecond laser with well-controlled position and dimension. The differential modal gain at the amplifier output is reduced from 2.1 to 0.5 dB by adding a 5.6 µm diameter void sphere in the center of the core and located in the middle section of the amplifier. This is achieved together with a gain larger than 22 dB, without degradation of the noise figure and without significant wavelength dependence over the C-band. This work opens the way to more complex devices allowing to balance the gain of fibers supporting more modes.
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