We present the generation and optimization of square-wave noise-like pulses (NLPs) in a mode-locked Tm-doped fiber laser. Mode-locking operation around the 2-μm band is achieved by a nonlinear amplifying loop mirror. To optimize the output performance, the figure-eight cavity is modified by employing a polarization-dependent isolator in a unidirectional loop, and the cavity length is only 17.2 m. First, by employing a cavity with pure anomalous dispersion, a conventional soliton can evolve into a square-wave NLP by properly setting the pump power and polarization controllers. The pulse energy of the fundamental-frequency operation can be varied from 2.29 to 3.4 nJ. Using an ultrahigh-numerical-aperture fiber to reduce the net dispersion to −1.033 ps2, the 3-dB bandwidth of the spectrum is broadened to 14.78 nm, and the duration of the autocorrelation spike is only 421 fs. The maximum single-pulse energy can increase up to 4.97 nJ. Due to dispersion management mechanism, the threshold and output power are also significantly improved.
We have experimentally obtained dispersion-managed solitons with sidebands in a passively mode-locked thulium-doped fiber laser. The stable single soliton with sidebands can be converted into two soliton pulses at the pump power of 867 mW with appropriate settings of the polarization controllers (PCs). By increasing the pump power and cautiously adjusting the PCs, the three, four, and five soliton pulses with nonuniform intensity operate with stability in the cavity due to the global soliton interaction caused by unstable continuous waves. Furthermore, the soliton bunch can be observed at the pump power of 1 W. The position of solitons in the soliton bunch is random with a fixed separation that is controllable by changing the linear phase delay. Our work gives insight into the dynamics of multipulse dispersion-managed solitons in a 2-μm mode-locked fiber laser.
We report the experimental results of various soliton molecules in a passively mode-locked thulium-doped fiber laser based on nonlinear polarization rotation (NPR) technology. Not only stable single solitons but also soliton molecules can be observed in the same cavity. With the increase of the pump power, soliton triplets composed of three solitons can also be observed. It is found that the ratio of the pulse separation to the pulse width is less than 5, which indicates that the direct soliton interaction leads to the formation of bound states. On the other hand, the effect of the dispersive wave is to suppress the random relative phase change between the solitons and leads to phase-locking. Meanwhile, we analyzed the complex nonlinear interactions that form soliton molecules and enriched the nonlinear dynamics of soliton molecules at 2-μm.
The stretched pulse has narrower pulse duration and high pulse energy than conventional soliton, so that the stretched pulse can improve the potential application values of 2 μm mode-locked fiber laser in remote sensing, mid-infrared source, material processing and other applications. In this paper, the dispersion management method is adopted to accurately manage the dispersion in the cavity using a commercial ultra-high numerical aperture fiber and a common single-mode fiber. Based on the nonlinear polarization rotation technology to achieve mode-locking, when the pump power is set to 645 mW, the polarization controller is adjusted to achieve stretched pulse output with a de-chirped pulse duration of 581 fs. The center wavelength is 1939.26 nm and the 3-dB bandwidth is 21.8 nm. The repetition rate is 28.9 MHz and the signal to noise ratio is 54.21 dB. At the maximum pump power, the direct output power of the resonant cavity is 9.98 mW, and the corresponding single pulse energy is 0.34 nJ.
In this paper, by shortening the cavity length, we have realized all-fiber Tm-doped fiber laser with pulse width less than 600 fs based on nonlinear polarization rotation effect. Under the condition of pump power of 0.281 W, the traditional soliton limit pulse is realized by adjusting the polarization controllers. The pulse width is 560fs and consistent with the theoretical calculation of the limit pulse width 569fs, repetition frequency is 55.25 MHz, the central wavelength is 1978.34 nm and the 3dB spectral width is 7.25 nm. The traditional soliton index is helpful to improve the application of ultrafast Tm-doped fiber laser in mid-infrared light source and medical operation.