The authors experimentally investigate the phenomena of the pulse-shape distortion due to the drift of cavity length or modulation frequency in a harmonically mode-locked semiconductor fiber ring laser (SFRL). Also, it is experimentally demonstrated that the optical pulse shaping by cross-phase modulation (XPM) can be applied to enhance the robustness of a harmonic mode-locked semiconductor ring laser to the environmental factors, e.g., the modulation-frequency drift and/or the cavity-length drift. The mechanism of pulse-shape distortion due to the modulation-frequency drift or cavity-length drift is explained and experimentally verified. As the locking condition of the SFRL goes to the rational harmonic mode-locking (RHML) regime, the pulse-shape distortion becomes dominant. To validate the explanation, a simple equation for estimating and comparing the amount of cavity-length detuning in terms of frequency is proposed for comparison, which is called the equivalent frequency detuning (EFD). The distortion mostly appeared in the trailing edge of the pulse. By selectively filtering the XPM-induced red-chirped wavelength components in the rising edge of the pulse, the mode-locked pulse train with large pulse-shape distortion can be reshaped to regenerate a mode-locked pulse train with less pulse-shape distortion than the pulse train without the proposed scheme. This method can be used to reduce the pulse-shape distortion effectively, maintaining the pulse repetition rate under the large modulation-frequency detuning, or equivalently, large cavity-length detuning. The proposed scheme is able to generate the pulse-trains robust to the modulation-frequency drift and/or cavity-length drift of the mode-locked SFRL.
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