Rational harmonic mode-locking (RHML) fiber lasers generating picoseconds pulsewidth at high-repetition-rate have emerged as a key component for the high-bit-rate optical time-division multiplexing (OTDM) communication system. In this research, we have discovered higher order RHML semiconductor optical amplifier fiber laser (SOAFL) has the degradation on mode-locking capacity, and an output pulse-train with un-equalized peak amplitudes. Therefore, the main focus of the dissertation is focused on the pulse quality analysis and improvement of RHML-SOAFL via optical pulse injection. First, we observed the degradation on mode-locked mechanism of the dark-optical-comb injection mode-locked semiconductor optical amplifier fiber laser (SOAFL) at RHML order increases to >8. Such a less pronounced RHML mechanism at higher orders is mainly attributed to the weak mode-locking strength at high RHML orders as compared to continuous-wave (CW) lasing mechanism, which has been quantified by reduction of spectral linewidth and pulse-shortening force, and the ratio of DC/pulse amplitude enhancement for discriminating 1st to 20th-order RHML capability. To overcome the un-equalized RHML peak intensity, optical injection induced gain modulation of a SOA are demonstrated to equalize the peak intensity of 5-GHz and 40-GHz RHML-SOAFL by using 1-GHz inverse-optical-pulse and a reshaped 10-GHz gain-switching FPLD pulse injection, respectively. The optical injection mode-locking models are constructed to simulate the compensation of uneven amplitudes between adjacent RHML pulse peaks before and after pulse-amplitude equalization. The optimized RHML pulse exhibits a signal-to-noise suppression ratio of 45-dB, and the clock amplitude jitter below the threshold limitation of 10%. On the other hand, to avoid the mode-locked degradation on RHML, a 2nd-order fractional Talbot effect induced frequency-doubling of 10-GHz optical pulse-train is demonstrated to backward inject a SOAFL for 40-GHz RHML. In comparison with the SOAFL pulse-train repeated at 40-GHz generated by the 4th-order purely RHML process, the optimized 2nd-order fractional Talbot effect in combination with the 2nd-order RHML mechanism significantly enhances the modulation-depth of RHML, thus improving the on/off extinction ratio of the 40-GHz SOAFL pulse-train. Such a new scheme also provides a more stable 40-GHz RHML pulse-train from the SOAFL with its timing jitter reduce. Finally, we established a SHG-FROG to distinguish linear and nonlinear chirp of 10-GHz soliton HML-SOAFL, and further extracted intra-cavity linear dispersion via simulation of Schrodinger equation. After the procedure, the linear chirp almost dominates chirp characteristics for optical pulse injection HML-SOAFL system.