Abstract

The few-mode erbium-doped fiber amplifier (FM-EDFA) is a key component of a few-mode-fiber-based optical communication system to allow the simultaneous amplification of all signal mode groups. This paper proposes the single-trench ring-core FM-EDFA for gain equalization of five signal mode groups—${\mathrm{LP}}_{01}$, ${\mathrm{LP}}_{11}$, ${\mathrm{LP}}_{21}$, ${\mathrm{LP}}_{31}$, and ${\mathrm{LP}}_{41}$. The proposed fiber with ring-core ${\mathrm{Er}}^{+3}$ doping and fundamental pumping is used to study the gains and differential modal gains (DMGs) of signal mode groups. In this study, we have shown that introduction of a trench increases the modal confinement of higher mode groups and decreases that of lower mode groups in the ring region (or doped region) slightly. This helps in controlling the DMG and also increases the effective index difference $\mathrm{\Delta }{n}_{\text{eff}}$ between the adjacent modes. We show that at 1530 nm signal wavelength, the DMG of five mode groups is reduced from 1.32 dB (without trench) to 0.45 dB (with trench) with gain in excess of 20 dB. Also, the mode spacing $\mathrm{\Delta }{n}_{\text{eff}}$ increases from $4.9\times {10}^{-4}$ to $5.1\times {10}^{-4}$. Over the C-band, we have achieved more than 20 dB amplification and nearly 1 dB gain excursion while maintaining sufficient mode spacing $\mathrm{\Delta }{n}_{\text{eff}}>5.1\times {10}^{-4}$ to avoid mode coupling due to macrobending.

© 2018 Optical Society of America

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