Suppression of pulse shape distortion caused by frequency drift in a harmonic mode-locked semiconductor ring laser

We propose and demonstrate a novel method to generate an optical pulse train in a harmonic mode-locked semiconductor ring laser that is not prone to the frequency drifts of locking elements. This technique is based on wavelength conversion by cross-phase modulation in a semiconductor optical amplifier inside the ring laser and external filtering of the induced red-chirped frequency components. Though the pulsewidth increased from 45 to 65 ps at a repetition rate of 2.5 GHz due to the characteristics of the external optical filter, the frequency locking range was increased by a factor of 2.9 compared with that of a conventional harmonic mode-locked semiconductor ring laser. In addition, the pulse train over the frequency detuning range of -250 to +250 kHz did not show any distortion in pulse shape.