High-power fiber lasers have seen astonishingly rapid progress over the last decade.All-fiber laser systems are preferred for simplicity,compactness,robustness and reliability.Characterized by intrinsically low transmission loss,fused components are referred to the optimum choices.Losses can lead to catastrophic component failure due to localized heating at high power levels.The most meaningful benchmark,when considering the power handling of the component,is the capability of the component to handle optical power loss,rather than transmitted power.Design criterions of low transmission loss are described both in single-mode components and multi-mode components.Some advices about package design are proposed.