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Twentieth-order rational harmonic mode-locking (RHML) semiconductor optical amplifier fiber laser (SOAFL) pulses are demonstrated by using 1-GHz backward dark-optical comb injection. Maximum frequency detuning range of plusmn 300 Hz, broadened pulsewidth of 35 ps, and highest average power of 0.4 muW for twentieth-order RHML-SOAFL pulse are characterized. The extraordinary phenomena on the red-shifted wavelength from 1535.5 to 1541.5 nm and the corresponding spectral linewidth reduced from 12 to 3 nm are observed with the RHML order increasing to 8 or higher. Such a less pronounced RHML mechanism at higher orders occurred in the optically injection mode-locked SOAFL is mainly attributed to the weak mode-locking strength at high RHML orders as compared to continuous-wave lasing mechanism.