Power scaling of fiber lasers is challenged by several factors, such as the brightness of the pump source, the nonlinear effect, modal instability, and so on. Pumping active fibers with high-brightness fiber lasers instead of laser diodes is a promising solution for the brightness limitation and modal instability. In this paper, for the first time to our knowledge, we present a general review on the achievements of various kinds of high-power fiber lasers based on the tandem pumping scheme in the past few years. The requirements for tandem pumping ytterbium (Yb), erbium (Er), thulium (Tm), and holmium (Ho)-doped fibers are analyzed, and corresponding achievements are summarized. Hundreds of watts of fiber lasers at $\sim 1020$, $\sim 1500$, and 1900 nm and hundred-watt-level fiber lasers at $\sim 1150$ and $\sim 1180\mathrm{nm}$ have been successfully achieved. Then, these powerful fiber lasers with high brightness can be employed as pump sources for Yb-, Er-, Tm- and Ho-doped fibers. Moreover, a recent experimental result of a 3.5 kW Yb-doped fiber amplifier in an all-fiber format is reported in addition to previous typical achievements. The underlying challenges for further power scaling, including the nonlinear effect suppression and special fiber design, are briefly discussed. Exploring the tandem pumping scheme in novel application fields is discussed as well.

In this work, a confined-doped fiber with the core/inner-cladding diameter of 40/250 μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate gas deposition technique, and subsequently applied in a tandem-pumped fiber amplifier for high-power operation and transverse mode instability (TMI) mitigation. Notably, the impacts of the seed laser power and mode purity are preliminarily investigated through comparative experiments. It is found that the TMI threshold could be significantly affected by the seed laser mode purity. The possible mechanism behind this phenomenon is proposed and revealed through comprehensive comparative experiments and theoretical analysis. Finally, a maximum output power of 7.49 kW is obtained with the beam quality factor of approximately 1.83, which is the highest output power ever reported in a forward tandem-pumped confined-doped fiber amplifier. This work could provide a good reference and practical solution to improve the TMI threshold and realize high-power high-brightness fiber lasers.

Raman fiber lasers (RFLs) are currently promising and versatile light sources for a variety of applications. So far, operations of high power and brightness-enhanced RFLs have absorbed enormous interests along with rapid progress. Nevertheless, the stable Raman lasing at high power levels remains challenged by the thermal effects. In an effort to realize more effective thermal management in high power RFLs, here we demonstrate for the first time, an all-fiberized Raman fiber amplifier (RFA) based on metal-coated passive fiber enabling high power lasing. By employing aluminum to the coating of graded-index (GRIN) passive fiber, the thermal dissipation of the fiber is more sufficient to support low-temperature but high-power operation. The maximum output power reaches 3.083 kW at 1130 nm with a conversion efficiency of 78.7%. To the best of our knowledge, this is the highest power attained in the fields of Raman lasers based on merely nonlinear gain. Stokes emission in the GRIN fiber enables brightness enhancement (BE) of output laser. Accordingly, the beam quality parameter ${M^{2}}$ and BE value are 3.12 and ~9 when the signal power is 2.364 kW, which reach 5.72 and ~2.9 at maximum power, respectively. As far as we know, we have achieved the highest power record among all the brightness-enhanced Raman lasers in any kind of configurations.

Thanks to the unique properties such as spatially coherent, broadband emission spectrum, and high temporal stability, superfluorescent fiber source (SFS) has shown tremendous potential in wide applications of sensing, imaging, spectroscopy, and material processing. The fast development of active fiber and pump diode provides unprecedented opportunity for the performance scaling of SFS. In this paper, the new horizon opened by the recently developed SFS will be reviewed. First, the output power scaling of SFS with different architectures will be summarized, and a kilowatt-level high power SFS based on a tandem pumping technique will be demonstrated for the first time. Second, spectrum manipulation of SFS, including coverage extending and spectrum shaping, will be introduced in detail. What is more, the spectrum evolution of narrowband SFS in power scaling will be numerically modeled and evaluated. Third, several novel applications of SFS, including midinfrared laser generation, nonlinear effect suppression, sensing, and imaging, will be given, indicating the versatile performance of SFS compared with traditional fiber laser oscillators. Based on the new developed SFS, we will present the first hundred-watt level linearly polarized random fiber laser. In the last section, future endeavors on SFS will be presented.

In this study, we investigated the influence of fiber parameters on stimulated Raman scattering (SRS) and identified a unique pattern of SRS evolution in the counter tandem pumping configuration. Our findings revealed that the SRS threshold in counter-pumping is predominantly determined by the length of the output delivery fiber rather than the gain fiber. By employing the counter tandem pumping scheme and optimizing the fiber parameters, a 10 kW fiber laser was achieved with beam quality M² of 1.92. No mode instability or severe SRS limitation was observed. To our knowledge, this study achieved the highest beam quality in over 10 kW fiber lasers based on conventional double-clad Yb-doped fiber.