I. Introduction
The operation of three-level transition (890–950 nm) of Nd-doped fiber laser is of great interest as it has many applications in water-vapor absorption characterization, frequency doubling, lidar, spectroscopy, etc. Experimental setup (L1, L2: lens; M1: mirror). In these applications, narrow spectral width, high power, and tunable output is highly desirable. However, achieving high-power tunable laser output has long been a great challenge in three-level transition in Nd-doped fiber lasers. Similar to the well-known challenges in three-level transition operation of Yb-doped double cladding fiber lasers [1], operating three-level transition in Nd-doped double cladding fiber lasers also has great difficulty not only due to the existence of undesired extremely serious competition with four-level transition when pumped at 808 nm, but also due to the very poor overlap between pump and signal. Hence, laser operations of the transition in Nd-doped optical fibers have been limited to low power.