Dark and bright pulses in a SOA-Based fiber ring laser

• Dark pulse emission is obtained in a SOA-Based fiber ring laser using NPR technique.
• By adjusting two PCs, bright pulse and dark pulse could be switched each other.
• The extinction ratio of PCs is measured in two cases of bright and dark pulses.

We demonstrate the dark pulse emission in a semiconductor optical amplifier (SOA)-Based fiber ring laser with net abnormal dispersion. By means of the polarization-dependent property of SOA, the laser is mode-locked by using nonlinear polarization rotation (NPR) technique. Stable bright pulses and dark pulses are observed through changing the current of the SOA and altering the polarization states. The dependence of both the polarization states and the current of SOA on the formation of dark pulses in the cavity have been carefully investigated, revealing that the output average power of the measured dark pulse is far greater than that of the bright pulse under the same current. Moreover, the evolution process from bright pulse to dark pulse has been studied, showing the role of the extinction ratio of PCs in the transition process. The polarization dependent and bistability properties of SOA and the extinction ratio of PCs are dominant for the dark pulses generation, giving a further evidence of the generation of dark pulses in different laser cavity.