Design of a Few-Mode Erbium-Ytterbium Co-Doped Polymer Optical Waveguide Amplifier With Low Differential Modal Gain

Mode-division multiplexing (MDM) is a new method of space-division multiplexing (SDM) that can break through the limitation of single-mode transmission capacity and greatly improve the capacity of optical fiber communication. In the case of a few-mode (FM) system, it is possible that each mode can carry its own data steam and transmit long distances owing to a few-mode erbium-doped fiber amplifier (FM-EDFA). In addition, an Er
³⁺
-Yb
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co-doped waveguide amplifier (EYCDWA) based on polymer materials has numerous advantages of miniaturization and flexibility in planar integrated optics compared with the EDFA. This study explored the theory of a few-mode Er
³⁺
-Yb
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co-doped waveguide amplifier (FM-EYCDWA) and established a theoretical model of an FM-EYCDWA with a novel modified overlap integral factor. The differential modal gain (DMG) of the FM-EYCDWA can be reduced or even eliminated by equalizing the modified overlap integral factor of each signal mode. An unprecedented doping configuration adapted to the fabrication process of an optical waveguide was proposed to fill the gap between modal gains, which can be optimized by a genetic algorithm (GA). We investigated two optimization cases of three-mode and five-mode group FM-EYCDWAs to verify the validity of the theoretical model; both cases were optimized to minimize the DMGs under the same limitations, and excellent performance was presented, which demonstrated that the design method of the few-mode amplifiers we presented can offer a referable guideline.