Contents I. Introduction
In several decades, the bright soliton has been intensively investigated and used in the optical communication systems. As well known, the dark soliton (DS) is also a solution of the nonlinear Schrödinger equation [1] and it has advantage over the bright soliton in the optical communication systems. Under consideration of the loss, the broadening rate of the DS is less than the bright one. In addition, the broadening degree of the DS is about half of the bright one [2]. The robustness of DS is much stronger than bright one [3]. So the DS has the prospection to further explore in the field of the nonlinear optics and future optical telecommunication systems. In 2009, Tang et al. observed the dark pulse emission firstly in an all-normal dispersion erbium-doped fiber laser [4]. In the same year, Xu et al. reported the DS pulse spectral sideband in an all-fiber erbium-doped ring laser with a net-negative dispersion cavity [5]. Li et al. have explored several operational states in an all-normal-dispersion ytterbium-doped fiber (YDF) ring laser, In the experiment, dark-bright pulse pairs harmonic mode locking (HML) counterparts and dark pulses HML counterparts have been obtained [6]. In 2014, Song et al.obtained high repetition rate of 280 GHz DS formation in a net normal dispersion cavity fiber laser by using the cavity self-induced modulation instability [7].
