We study theoretically and experimentally the subharmonic entrainment (SHE) breather soliton in mode-locked lasers for the first time, in which the ratio of the breather period to the round-trip time is an integer. We build a non-Hermitian degeneracy map of breather soliton, and illustrate that SHE arises between the two exceptional points (EPs). We obtain SHE at the ratio of 20, observe the evolution of breather soliton when tuning the gain and/or cavity loss, and prove that this phenomenon can improve the stability of breather soliton. Our research brings insight into the EP physics of ultrafast lasers and makes the mode-locked laser a powerful test bed for non-Hermitian degeneracy, which may open a new course in ultrafast laser research.

A passively mode-locked Erbium-doped fiber laser using glycerin as a saturable absorber (SA) has been proposed and experimentally demonstrated. The glycerin-SA is made of two fiber ferrules matched into a cannula, and the gap between them is filled with glycerin. Such a SA incorporated into the cavity leads to stable mode-locking operation. The generation of both bright pulse and dark pulse is observed. The bright pulse is centered at 1568.92 nm, with a repetition rate of 23.08 MHz, the maximum output power of 33.5 mW, a duration of 3.57 ps, and a signal noise ratio (SNR) of 55.6 dB, which exhibits good performance on self-starting and stability. Moreover, the dark pulse with the same repetition rate is achieved by adjusting the polarization state properly. The effectiveness of glycerin-SA as a mode-locker is validated. Considering its superiority in terms of low cost, easy fabrication and non-volatile feature, we believe that glycerin-SA can be found many practical applications in ultrafast photonics.

We report on the first experimental observation of passively harmonic mode-locking (HML) in a bidirectional domain-wall dark-soliton fiber laser. The domain-wall solitons (DWSs) are characterized, in which, the strong coupling between the two directions takes great responsibility for the generation. By appropriately altering the polarization state in the cavity, the stable fundamental domain-wall solitons and HML states could be obtained and switched with increasing the pump power. In our experiment, the repetition rate can be tunable from the fundamental mode locking of 2.96 MHz up to eighth-order HML of 23.68 MHz when the pump power is increased from 120 to 542.5mW. Improtantly, this HML operation of dark-pulse DWSs confirms that the DWSs are the intrinsic feature of the mode- locked lasers, as well as the conventional bright solitons. This work could provide a new perspective to the understanding of the formation and transformation of dark solitons, which may also find great potential applications in fiber sensing and future optical communication systems.

A passively switchable erbium-doped fiber laser based on alcohol as the saturable absorber (SA) has been demonstrated. The SA is prepared by filling the gap between two optical patch cords with alcohol to form a sandwich structure. The modulation depth of the alcohol–SA is measured to be 6.4%. By appropriately adjusting the pump power and the polarization state in the cavity, three kinds of mode-locked pulse patterns can be achieved and switched, including bright pulse, bright/dark soliton pair, and dark pulse. These different soliton emissions all operate at the fundamental frequency state, with a repetition rate of 20.05 MHz and a central wavelength of ∼1563 nm. To the best of our knowledge, this is the first demonstration of a switchable soliton fiber laser using alcohol as the SA. The experimental results further indicate that organic liquid-like alcohol has great potential for constructing ultrafast lasers.

Laser gyroscopes afford extremely precise measurement of ultraslow angular velocity and play an irrefutable role in engineering seismology, tidal detection, aviation, aerospace, satellite navigation, and other inertial systems. With recent progress of mode-locked fiber lasers, particularly the realization of effective bidirectional generation, their applications in the fields of gyroscopic sensing have attracted tremendous attention. Besides the merits of excellent structure compactness, maintenance-free operation, and rather low cost, remarkably, the mode-locked laser gyroscope presents a promising approach for eliminating the lock-in effect caused by the synchronization of counterpropagating resonant frequencies, which is the most severe sensing limitation of traditional laser gyroscopes. In this paper, recent advancements and perspectives in this research are reviewed. The fundamentals of gyroscopic sensing employing mode-locked pulse lasers are presented. Architectures of novel mode-locked laser gyroscopes inspired by ultrafast optics, including bright-soliton mode-locked laser gyroscopes and dark-soliton mode-locked laser gyroscopes, are described in detail. Bottlenecks and deficiencies of the exhibited mode-locked laser gyroscopes owning to inherent physical mechanisms or measurement methods currently used are further analyzed. Finally, feasible methods of improving the performance of mode-locked laser gyroscopes are broadly expounded upon to build a bridge between their scientific development research and practical applications.