Objective In recent years, there has been a rapid progress in the development of high-power fiber lasers, which are widely used in the fields of laser marking and material processing as well as numerous industrial applications. The main factors that limit the output power of fiber lasers are the mode instability and nonlinear effects, including stimulated Raman scattering and stimulated Brillouin scattering. To suppress these nonlinearities, the core size of large mode area active fibers should be increased. However, this may lead to the degradation of beam quality. The core diameter of tapered fibers gradually increases as the length increases, therefore suppressing the nonlinear effects. Moreover, the tapered area, which satisfies the adiabatic taper principle, facilitates in achieving excellent beam quality. Tapered active fibers have been used in various applications such as continuous-wave fiber laser oscillators or amplifiers, ultrafast laser systems, and single-frequency fiber amplifiers. In early 2020, researchers from National University of Defense Technology proposed the Yb-doped double-tapered double-cladding fiber (DT-DCF), which consists of a thin-core section at both ends and a large-core section in the middle. In the present study, all-fiber high-power laser amplification is performed based on the self-fabricated Yb-doped DT-DCF. The laser system achieves a single-mode laser output with a maximum power of 4 kW and a mass factor M² of 1.33.

Methods We construct a master oscillator power amplifier system based on the homemade DT-DCF. This system is 21-m long and has small-core and large-core sections with core/cladding diameters of 22/413 and 32/600mm, respectively. Fusion splices connect all the components. The seed has a center wavelength of 1080 nm and a output power of 103 W. After passing through the cladding light striper (CLS), the seed light is injected into the amplifier. Subsequently, bidirectional pumping is applied to laser amplification. Seven laser diode (LD) modules with a center wavelength of 976 nm are divided into two groups comprising two and five modules to pump the DT-DCF through the forward and backward couplers, respectively. After the amplification stage, the CLS is utilized to strip out the residual pump power and the laser is finally output to free space through the endcap for the measurement of power, spectrum, and beam quality.

Results and Discussion The output power increases linearly with the pumping power. When the pumping power is 4.75kW, the output power reaches 4kW. The corresponding optical efficiency and slope efficiency are 82% and 83%, respectively. The M² at the highest power is 1.33 [Fig. 3(a)], exhibiting the single-mode output characteristic of the system. Mode instability limits further power scaling of the single-mode output. If the pump power increases to over 4.75kW, time domain fluctuation of kHz can be observed, indicating the initiation of mode instability. The output laser has a center wavelength of 1080nm, and its spectrum broadens as the output power increases. For the spectrum under the highest output power, the Raman suppression ratio reaches up to 44dB [Fig. 3(b)], demonstrating the ability of the DT-DCF to inhibit the nonlinear effects.

Conclusions In summary, we have established a 1080-nm all-fiber amplifier based on DT-DCF. This amplifier can achieve a 4-kW single-mode output laser with a slope efficiency of 83% and M² of 1.33. Our results indicate that the DT-DCF can simultaneously suppress the nonlinear effects and transverse mode instability, thus providing a beneficial reference for further power scaling of single-mode fiber lasers.