Thanks to the advantage of balancing nonlinear effects and transverse mode instability, vary core diameter active fiber (VCAF) has been widely used in high power ytterbium-doped fiber lasers in recent years. Up to now, VCAF has developed from the basic form of the original tapered fiber to the spindle-shaped and saddle-shaped fiber with different characteristics and has been applied in conventional fiber lasers, oscillating–amplifying integrated fiber lasers, and quasi-continuous wave fiber lasers and successfully improved the performance of these lasers. In the present study, a 6110 W fiber laser amplifier is realized based on a tapered fiber. The maximum output power of a fiber laser amplifier based on spindle-shaped fibers is 6020 W with a beam quality of M²~1.86. In this paper, we first introduce the basic concept of VCAF and summarize its main fabrication methods and advantages in high-power fiber laser applications. Then, we will present the recent research results of high-power fiber laser employing VCAF in our group and clarify the outstanding advantages of VCAF compared with the constant core diameter active fiber (CCAF).

With increasing output power, stimulated Raman scattering (SRS) becomes one of the main limitations for high power fiber laser oscillators. In this work, the SRS threshold dependence on the bandwidths of fiber Bragg gratings (FBGs) has been experimentally investigated in co-pumped and counter-pumped ytterbium-doped all-fiber laser oscillators, respectively. In both co-pumped and counter-pumped laser oscillators, FBGs with respective bandwidths of 0.24 nm, 0.5 nm, 1 nm and 2 nm are used as the output coupler. It is found that with the increasing bandwidth of the output-coupler FBG (OC-FBG), the SRS threshold of the laser oscillator also increases and when the bandwidth of the OC-FBG reaches over ∼1 nm, the SRS threshold increases slowly. Compared with co-pumping configuration, a counter-pumped laser oscillator holds a higher SRS threshold when the same OC-FBG is applied. By employing a counter-pumping configuration and increasing the bandwidth of the OC-FBG to 2 nm, the output laser power is further scaled to 3 kW-level with a slope efficiency of 70%. The M ² factor is measured to be ∼1.3 at the maximum output power.

Fiber lasers are increasingly employed in radiative environments, such as outer space and nuclear facilities. However, their performance is significantly compromised under irradiation due to the accumulation of defects in fiber components. Despite various radiation-hardening strategies, including optimized fiber designs and post-treatment techniques, these approaches have been unable to fully mitigate performance degradation. In this study, we demonstrate near 100% power recovery in a 150 W fiber laser exposed to radiation, achieved through an in-situ photobleaching technique utilizing low-power blue light. This all-fiber photobleaching strategy not only restores laser performance but also provides a robust, compact, and scalable solution for developing high-performance radiation-resistant fiber lasers. The proposed approach holds significant potential for advancing laser applications in challenging radiative environments, such as space exploration, nuclear power plants, and medical radiotherapy.

Tapered Yb-doped fiber (T-YDF) can balance the suppression of transverse mode instability (TMI) and stimulated Raman scattering (SRS) in high power fiber lasers. In this work, we have constructed an all-fiberized master oscillator power amplifier emitting at the central wavelength of 1050 nm based on a T-YDF and investigated the laser performance in respective co-pumped and counter-pumped configurations. The results show that the T-YDF can effectively mitigate SRS effect in short-wavelength fiber lasers. A nearly single mode beam quality (M2∼1.20) is obtained at output power of 3 kW-level. In particular, the SRS suppression ratio in the co-pump and counter-pump scheme is about ∼33.2 dB at ∼3039 W and ∼46.6 dB at ∼2818 W, respectively. By further optimizing the structure of the tapered active fiber, it is promising to further improve the output power and beam quality in short-wavelength fiber lasers.

Traditional ytterbium-doped high-power fiber lasers generally use a unidirectional output structure. To reduce the cost and improve the efficiency of the fiber laser, we propose a bidirectional output fiber laser (BOFL). The BOFL has many advantages over that of the traditional unidirectional output fiber laser (UOFL) and has a wide application in the industrial field. In theory, the model of the BOFL is established, and a comparison of the nonlinear effect in the traditional UOFL and the BOFL is studied. Experimentally, high-power continuous wave (CW) and quasi-continuous wave (QCW) BOFLs are demonstrated. In the continuous laser, we first combine the BOFL with the oscillating amplifying integrated structure, and a near-single-mode bidirectional 2 × 4 kW output with a total power of above 8 kW is demonstrated. Then, with the simple BOFL, a CW bidirectional 2 × 5 kW output with a total power of above 10 kW is demonstrated. By means of pump source modulation, a QCW BOFL is developed, and the output of a near-single mode QCW laser with a peak output of 2 × 4.5 kW with a total peak power of more than 9 kW is realized. Both CW and QCW output BOFL are the highest powers reported at present.