Review of High Average Power and High Beam Quality LD-Pumped Ytterbium-Doped Fiber Laser Oscillators and Amplifiers

Significance High power fiber lasers with high beam quality have been widely adopted as laser sources in many applications, such as laser cutting, laser welding, manufacturing, and military defense. The advantages of high conversion efficiency, good beam quality, compact structure, and reliable stability, are favored by the consumers. There has been a remarkable increase in the output power of fiber lasers, and the business consumption of high power fiber lasers in the market shows a remarkable increase in recent years. High power and high beam quality are essential aspects of many manufacturing and defense applications. There are two strategies to achieve high power laser beams, which are multi-laser beam combinations and power scaling of single fiber laser. The single fiber laser with high power can be directly employed as a laser source in the manufacture or serve as an element in multi-laser beam combinations. However, the power scaling of high brightness monolithic fiber laser oscillators is limited by the manufacturing techniques of fiber laser components, stimulated Raman scattering (SRS) effect, and transverse mode instability (TMI) effect. In order to achieve a monolithic fiber laser with high power and high beam quality, the SRS and TMI effects in the fiber lasers have to be effectively mitigated based on the current power handling ability of the laser components. In this paper, three strategies to achieve high power fiber lasers are respectively reviewed and discussed, which correspond to the fiber laser oscillators, the fiber laser amplifiers, and the oscillating-amplifying integrated fiber lasers. Progress First, the progress of kW-level fiber laser oscillators is reviewed. The published reports on high power fiber laser oscillators are reviewed in respective spatial configurations and all-fiber configurations. In the spatial configurations, reflection mirrors serve as cavities mirrors, and the coupling of pump and signal lasers are achieved by the lens. In the all-fiber configuration, the fiber Bragg gratings serve as cavity mirrors, and the pump/signal fiber combiners are packaged in all-fiber format. The all-fiber configurations have the advantages of compact structure and reliable stability. The research on high power fiber laser oscillators in recent years is outlined, and descriptions of the typical results are respectively reviewed. With the optimization of the pump scheme, fiber Bragg grating (FBG) parameters, and gain fiber parameters, the all-fiber laser oscillators are scaled to over 8 kW, and further power scaling is mainly limited by the fiber nonlinear effect and TMI effect. The development of femtosecond laser inscription of FBGs also enables the high quality FBGs for the high power fiber laser oscillators. Second, the progress of high power fiber laser amplifiers is reviewed. Without the limitation components of FBGs, the fiber laser amplifiers can be scaled to kW-level with enough pump power and appropriate large mode area ytterbium-doped fiber. There have been many reports on kW-level fiber laser amplifiers in the last decade around the world. However, open reports on high power fiber laser amplifiers over 5 kW-level are still in a small quantity. We focus on reviewing the typical results of over 5 kW with good beam quality, which are outlined and described in detail. Output laser power of 13 kW with beam quality M-2 of 2.9 is reported in the few-mode region. While in the nearly single mode region, an output laser power of 6 kW with M-2 of about 1.3 is reported. Further power scaling is also mainly limited by the fiber nonlinear effect and TMI effect. Similarly, ytterbium-doped fibers with large mode areas and specially designed mode discrimination techniques are promising in further power scaling of the high power fiber laser amplifiers. Third, a new conception of the combination of advantages of fiber laser oscillators and amplifiers is proposed, which is called oscillating-amplifying integrated fiber laser. Physically, it is a master oscillation power amplification (MOPA) structure with no cladding light stripper (CLS) or isolator between the seed and power amplifier. Technically, it enjoys the advantages of high anti-reflection ability and simple control logic, similar to typical fiber laser oscillators. Many researchers have investigated the new structure, and the results are diverse, especially in the application of narrow linewidth laser amplification. Reports show that the new structure also has encountered the limitations of SRS and TMI in the power scaling, which have to be mitigated in future studies. Conclusions and Prospects In this work, three strategies for achieving high power fiber lasers are reviewed. The recent progress and typical results are outlined and described. In the section on fiber laser oscillators, the results are described in respective spatial and all-fiber configurations. In the section on fiber laser amplifiers, results with over 5 kW are outlined and discussed. In the section on oscillating-amplifying integrated fiber laser, recent results are described. No matter which strategy is chosen, the high power scaling encounters the limitations of fiber nonlinear effect and TMI effect. The most promising techniques to settle the limitations lie in the new design and manufacture of large-mode area ytterbium-doped fibers.