Conclusion and Outlook
Fiber lasers that emit monochromatic light have been intensively investigated owing to its high purity in optical spectrum and the resulting significant width of the scope of applications that are currently accessing and could access this type of laser source. The main properties that render the single-frequency fiber laser popular are its inherent low noise and narrow linewidth, as well as the all-fiber format that makes the system flexible and reliable. Based on a generally fiber laser, single-frequency lasing can be realized given that the employed filtering element is narrow enough, while the optical gain mechanism can be fundamental transition in rare earth ions or fiber nonlinear effects or an association of both. The narrowband filtering mechanism can be various on the basis of optical fibers, leading to various kind of laser cavity configurations. The laser cavity can be also manipulated to achieve desired performances such as linear polarization operation, lower noise, narrower linewidth, wider tunability, frequency modulation, Q-switched pulsing output, etc. In addition, the invention of highly rare earth ion-doped multicomponent soft glass fiber enables the realization of high-power operation in a compact short-cavity laser cavity, while maintaining or even improving other performing indexes of the single-frequency laser. Finally, higher laser power can be obtained by implementing the well-developed master oscillator fiber amplifier and subsequent laser coherent beam combining.