Tunable and switchable harmonic h-shaped pulse generation in a 303  km ultralong mode-locked thulium-doped fiber laser

Abstract

We experimentally demonstrated a type of tunable and switchable harmonic h-shaped pulse generation in a thulium-doped fiber (TDF) laser passively mode-locked by using an ultra-long nonlinear optical loop mirror (NOLM). The total cavity length was ~3.03 km, the longest ever built for a TDF laser to our best knowledge, which resulted in an ultra-large anomalous dispersion over -200 ps 2 around the emission wavelength. The produced hshaped pulse could operate either in a fundamental or in a high-order harmonic mode-locking (HML) state depending on pump power and intra-cavity polarization state (PS). The pulse duration, no matter of the operation state, was tunable with pump power. However, pulse breaking and self-organizing occurred, resulting in high order HML, when the pump power increased above a threshold. At a fixed pump power, the order of HML was switchable from one to another by manipulating the PS. Switching from 8 th up to 48 th order of HML was achieved with a fixed pump power of ~4.15 W. Our results revealed the detailed evolution and switching characteristics of the HML and individual pulse envelope with respect to both the pump power and PS. We have also discussed in detail on the mechanisms of both the h-shaped pulse generation and the switching of its HML. This contribution would be helpful for further in-depth study on the underlain dynamics of long duration, particular-envelope pulses with ultra-large anomalous dispersion and ultra-long roundtrip time.

Switchable Generation of Multipulse Patterns of Noise-Like Pulse and Dissipative Soliton Resonance in a Fiber Laser

“…When PLD2 enlarges from 5.18 to 7.18 W, both the leading and trailing intensities increase. It seems that high pump power further induces asymmetry of the intensity distribution inside the pulse, just like the phenomenon reported in ref [10], [27]. Fig.…”

“…This new type of pulse is experimentally observed and theoretically confirmed to share similar features as square pulse and achieve high pulse energy [9], [10]. The reported splitting pulses of the h-shaped pulse remain h-shaped and still increase with pumping power [27]. One possible mechanism for the formation of h-shaped pulse is that the pulse envelope contains an unclamped portion which is different from typical NLP pulses that are com- pletely clamped [10].…”

Generation of h–Shaped pulse in a mode–Locked erbium-doped fiber laser

“…So, the properties of trailing portion are just caused by the peak power clamping (PPC) effect rather ASE. The PPC effect contributes to the formation of h-shaped pulses and inhibits pulse splitting by flattening the lengthy trailing section [15]. Meanwhile, the PPC effect is associated with the nonlinearity in the laser resonant cavity, which implies the intensive nonlinearity can reduce the switching threshold [29].…”

“…And the complex dynamic processes occur in fiber lasers under the action of nonlinear effects. Therefore, various pulse shapes will be generated from passively MLFLs, for example bunched multi-solitons [6], rectangularwave pulses [7][8][9], step-like pulses [10,11], chair-like pulses [12,13], and h-shaped pulses [14,15].…”

“…In 2019, Zheng demonstrated that nanoscale h-shaped pulses with μJlevel pulse energy could be generated in the figure-of-9 fiber laser [21]. The generation of h-shaped pulse was realized by Zhao by employing an ultralong fiber laser resonator, revealing that by increasing pump power or varying intra-cavity polarization state, the h-shaped pulse could be generated with different orders of harmonic mode-locking state [15]. Compared to lowdimensional material SAs [22], nonlinear multimode interference (NLMMI) [23,24] mode-locked technology, the mode-locked principle of NPR is based on the accumulation of different nonlinear phase shifts in the cavity.…”