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The past decade has witnessed tremendous achievements of ultrafast fiber laser technologies due to rapid developments of saturable absorbers (SAs) based on, in particular, nanomaterials such as 0D quantum dot, 1D carbon nanotubes, 2D layered materials, and 3D nanostructures. However, most of those nanomaterials-based SAs are inevitably absence of the high damage threshold and all-fiber integration, therefore challenging their applications on highly integrated and high-energy pulse generations. Recently, the real all-fiber SAs based on the nonlinear multimodal interference (NLMMI) technique using multimode fibers are demonstrated to overcome the above limitations. In this review, a detailed summary of the recent advances in NLMMI-based all-fiber SAs is provided, including the fundamental theory, implementation scenarios, and ultrafast fiber lasers of the all-fiber SAs, covering wide wavelength range of 1, 1.55, and 2 μm. In addition to the state-of-the-art overview, optical rogue waves in the all-fiber SA-based ultrafast fiber laser are extensively analyzed, which reveals the laser physics behind the dynamics from low-energy to high-energy pulses and directs the design of high-energy ultrafast fiber lasers. The conclusions and perspectives of the all-fiber SAs are also discussed at the end.

By means of the emerging dispersive Fourier transform technique, we captured the pulse-resolved spectral evolution dynamics of double-pulse mode-locking in a single-walled carbon nanotube based Er-doped fiber laser from the initial fluctuations, monitoring the evolution process up to 10 s (corresponding to ∼260 million roundtrips) discontinuously. Two distinctly different evolutionary types of double-pulse mode-locking have been investigated in detail: splitting from one pulse and forming simultaneously. Relaxation oscillations, beating dynamics, transient bound state, spectral broadening, and variation of pulse interval have been observed in the evolution processes of the double-pulse mode-locking. Our study will be helpful for the further research of double-pulse mode-locking.

We demonstrate a passively Er-doped fiber lasers in normal dispersion region using nonlinear multimodal interference technique-based modulator, which generate the transition from dark pulse to single bright pulse and harmonic bright pulse. With the pump power set as 210 mW, the dark pulse possessing 40 MHz repetition frequency occurs. When the polarization states are suitable varied in the cavity, single-bright and second harmonic bright pulses can be achieved. The simulation method based on Ginzburg-Landau equation is provided to illustrate the laser physics and reveal the underlying principles of the transition from dark to bright pulses and the generation of harmonic pulses. Polarization modal dispersion resulting from multimode fibers plays a vital role for these phenomena. This proposed fiber laser possessing cost-efficient, versatile, all-fiber structure, provides a promising solution for large integrated networks and photonics.

Chirped fiber Bragg gratings (CFBGs) have been extensively employed as dispersion compensation element in mode-locked fiber lasers. However, multiple functions of CFBG in fiber lasers have not been studied adequately. Here, triple functions of CFBG, dispersion compensation, distributed reflection performance, and stabilizing mode-locked operation, are experimentally investigated. Firstly, the generation of dissipative solitons (DSs) in an all-normal-dispersion Yb-doped fiber laser is reported, while the mode-locked operation is proved to be insufficiently stable. In order to improve performance of the laser, a conventional solitons (CSs) fiber laser is obtained by inserting a CFBG with large anomalous dispersion into the laser cavity. Besides, the distributed reflection performance of CFBG is researched and a wavelength-tunable fiber laser with tuning range of center wavelength from 1033.5 to 1035.5 nm is achieved. It is no doubt that our work opens up venues to create wavelength tunable Yb-doped fiber laser systems with extremely high stability.

We have proposed a switchable mode-locked fiber laser by means of carbon nanotube saturable absorber and fiber Bragg gratings (FBGs). The single-wavelength mode-locking operation can be switched between 1549.5 and 1559.5 nm, respectively, which correspond to the central wavelengths of two FBGs. With the appropriate setting of polarization controller, the stable dual-wavelength operation can be achieved due to the high stability of saturable absorber based on carbon nanotubes. Our method provides a simple, stable, low-cost, dual-wavelength ultrafast-pulsed source.

We demonstrate a passively mode-locked all-fiber laser incorporating a piece of graded-index multimode fiber as a mode-locking modulator based on a nonlinear multimodal interference technique, which generates two types of coexisting high-energy ultrashort pulses [i.e., the conventional soliton (CS) and the stretched pulse (SP)]. The CS with pulse energy as high as 0.38 nJ is obtained at the pump level of 130 mW. When the pump increases to 175 mW, the high-energy SP occurs at a suitable nonlinear phase bias and its pulse energy can reach 4 nJ at a 610 mW pump. The pulse durations of the generated CS and SP are 2.3 ps and 387 fs, respectively. The theory of nonlinear fiber optics, single-shot spectral measurement by the dispersive Fourier-transform technique, and simulation methods based on the Ginzburg–Landau equation are provided to characterize the laser physics and reveal the underlying principles of the generated CS and SP. A rogue wave, observed between the CS and SP regions, mirrors the laser physics behind the dynamics of generating a high-energy SP from a CS. The proposed all-fiber laser is versatile, cost-effective and easy to integrate, which provides a promising solution for high-energy pulse generation.

Taking advantage of the dispersive Fourier transformation technique, the decaying evolution processes of double-pulse mode-locking in a single-walled carbon-nanotube-based Er-doped fiber laser are observed in detail for the first time to our knowledge. The decaying dynamics of the double-pulse mode-locking state is analyzed in the spectral and temporal domains. We reveal that the two pulses in one cluster disappear either simultaneously or one by one during the decaying processes of double-pulse mode-locking states. In addition, the spectral evolution patterns of the special double-pulse states (i.e., bound states) are extremely distinct at different decline rates of the pump power.

We propose a quadruple-wavelength tunable and dual-wavelength switchable ultrafast fiber laser by incorporating four fiber Bragg gratings. The tunable operations depend on the losses induced by intracavity polarization controllers. By appropriately setting the states of PCs, quadruple-wavelength picosecond solitons can be realized at 1530, 1540, 1550 and 1560 nm, respectively. When the losses are suitable, solitons with two different wavelengths can reach the gain balance, and the dual-wavelength operations are switched among the four wavelengths. These results demonstrate that the proposed fiber laser offers potential opportunities in exploiting flexibly switchable multi-wavelength pulse sources.

We have proposed a nanotube-mode-locking fiber laser that can generate conventional solitons and stretched pulses in the same cavity. The net cavity dispersion of laser is slightly negative. The central wavelength and bandwidth of the conventional solitons are 1584.7 and 9.2 nm, respectively. The stretched pulse locates at 1572 nm with a bandwidth of 18.8 nm. The switchable mode-locking operation is mainly attributable to the bandwidth-dependent stretching factor that is tunable due to the spectral filtering effect induced by nonlinear polarization rotation.

We have experimentally observed the multiple bound states in a linear-cavity fiber laser mode-locked by a mixture of graphene and single-walled carbon nanotubes. The proposed laser can deliver the fundamental frequency soliton as well as the two and three bound-state solitons at suitable conditions. The numerical simulations confirm the experimental observations. Both the theoretical predictions and experimental results reveal that the spectral filtering effect plays a key role on the lasers.

We have proposed a compact dual-wavelength all-fiber pulse laser based on a single-walled carbon nanotube and chirped fiber Bragg gratings (CFBGs). A transmission filter is composed of a circulator and two CFBGs and is capable of controlling the operation of the proposed fiber laser. Mode-locking operations can be switched between 1551.2 and 1548.6 nm with the appropriate adjustment of polarization controller. Our laser delivers the pulses with the spectral bandwidth of about 0.6 nm and the pulse duration of about 7 ps. This work provides a low-cost, stable, and dual-wavelength ultrafast-pulsed laser source suitable for practical applications.