A dual-wavelength and dual-soliton operation in a passive mode-locked Er-doped fiber laser is demonstrated by using nonlinear polarization rotation(NPR) technique and the formation mechanisms of this phenomenon are experimentally investigated.It is found that a high linear birefringence in the laser cavity is the key to obtain the dual-wavelength and dual-soliton mode-locked output.In order to get stable output,a broad gain bandwidth and a high gain are preferred.By introducing high linear birefringence in ...