To improve the environmental stability and self-starting ability, researchers have developed the nonlinear optical-loop mirror and nonlinear amplifying-loop mirror (NALM) mode-locking technologies. The developing process of these technologies can be summarized as the following three phases: non-polarization-maintaining (PM) structures with polarization controllers, PM figure-of-8 cavity structures with dual-gain, and PM figure-of-9 cavity structures with a phase shifter. Among these, the newly invented figure-of-9 NALM mode-locked fiber laser is particularly advantageous owing to its easy self-starting ability, long-term stability, concise structure, and cost-effectiveness. Such a mode-locked fiber laser is increasingly employed in applications such as optical frequency combs, generation of THz radiation, and advanced material micromachining.