The primary objective of this study is to develop a high-power, all-fiber oscillator than can deliver exceptional output power and beam quality, which is crucial for advanced applications in material processing, laser manufacturing, and defense technology. The oscillator is intended to address the sustained demand for more robust and simpler laser systems that can operate at higher power levels without compromising stability and reliability. Nonlinear effects (NLEs) and transverse-mode instability (TMI) in fiber lasers, which are significant barriers to achieving high-power, high-beam-quality laser outputs, are addressed in this study.

In this study, a simple design of an all-fiber oscillator [Fig. 1(a)] using 30 μm /600 μm large-mode-area (LMA) ytterbium-doped fibers and matched fiber gratings inscribed by femtosecond laser phase-mask techniques is employed to form a resonant cavity. The system utilizes a low numerical aperture fiber to increase the loss of higher-order modes via a dual-wavelength pump source (969 nm and 982 nm) to reduce the thermal load and increase the gain saturation. TMI and NLEs are suppressed by appropriately designing the fiber grating bandwidth and optimizing the fiber coiling to ensure that multiple modes resonate adequately within the cavity while minimizing the intermodal coupling. Additionally, the angled cutting of the pigtail of a cladding light stripper can prevent backward Raman light from oscillating within the cavity.

The oscillator achieves an unprecedented output power of 10.07 kW. The system demonstrates a slope efficiency of 72.3% and an optical-to-optical efficiency of approximately 72% [Fig.1 (b)]. The laser output spectrum features a 3-dB bandwidth of approximately 6.6 nm, with a Raman suppression ratio of approximately 16 dB [Fig. 1 (c)]. Notably, the laser maintains a stable operation with no significant TMI in the frequency domain [Fig. 1 (e)]. However, the beam quality degrades slightly with increasing power [Fig. 1 (f)]. This is attributed to the increasing contribution of higher-order modes, which is typically encountered in high-power fiber lasers. The results underscore the effectiveness of the implemented strategies in managing nonlinearities and TMI, thus providing a basis for further improving fiber-laser performance.

In this study, the development of a high-power, all-fiber oscillator that affords an unprecedented laser output exceeding 10 kW is successfully demonstrated. The innovative use of low numerical aperture LMA fibers and optimized fiber gratings as well as the effective management of nonlinear effects and TMI are pivotal in achieving these results. The findings not only validate the feasibility of scaling fiber laser power while maintaining operational stability but also highlight the potential for further improving beam quality by continuously refining the manufacturing technologies of optical fibers and fiber gratings. The advancements resulting from this study are critical for the further development of high-power laser applications, particularly in the demanding industrial and defense sectors.