Abstract In order to realize a high flattening C + L band amplified spontaneous emission (ASE) light output, a broadband source using just one 976 nm laser diode (LD) and two stages erbium-doped fibers (EDF) with the same concentration is proposed theoretically and demonstrated experimentally, and the theoretical analysis and experimental verification are presented in details. Firstly, based on the energy level of Er3+, the methods of realizing C + L band ASE output is presented. Then, the system configuration is designed. One 976 nm LD is used for pumping, and the pump light, which is divided into two parts by one coupler, is pumped into two EDFs by wavelength division multiplexer (WDM). A fiber loop mirror (FLM) is used to improve working efficiency. Meanwhile, an optical isolator is used to avoid the effect of reflected wavelength from output facet. Finally, based on the mathematical model of the ASE gain, the length of EDF is discussed as well. In the experiment, by optimizing the length of EDF1 and EDF2, a 0.12 m EDF1 and 16 m EDF2 are adopted. In the end, the flatness of the spectra from 1540 nm to 1610 nm is ±0.91 dB, and the flatness is ±1.65 dB during 1520–1610 nm without any filter. The high flattening C + L band ASE output is achieved by just one 976 nm LD, which can optimize system and reduce the cost as well.