A single-frequency hybrid double-cladding Erbium-Ytterbium co-doped fiber amplifier (EYDFA) with Yb-band amplified spontaneous emission (Yb-ASE) suppression is demonstrated. Two segments of EYDFs, with core di-ameters of 6 mu m and 12 mu m, are fused in sequence to form a hybrid gain fiber. Theoretical analysis indicates that reducing the pump absorption in the initial part of the gain fiber is beneficial to avoid Yb-ASE boosting. A 4.79-W output laser with a linewidth of 1.67 kHz is obtained in the experiment. The Yb-ASE of the hybrid fiber amplifier is suppressed by more than 3 dB when compared to the uniform fiber amplifier with the same gain fiber length, and the slope efficiency is increased by 1.6 %. The loss characteristics of the hybrid gain fiber for backward transmitted light indicate its potential to suppress SBS effects. These results show a compact structure to suppress the Yb-ASE of the EYDF amplifier.
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