A dual-wavelength Er-doped fiber laser based on polarization hole burning(PHB) of multi-mode fiber is proposed.The structure of single mode fiber(SMF)-multi-mode fiber(MMF)-SMF makes reflection modes in different polarization directions of MMF split.Thus,dual-wavelength laser output is realized through PHB.Moreover,the wavelength spacing can alter through changing the length of MMF.In this paper,the output characteristics are compared when a MMF with length of 1.6 m and 3 m are introduced,respec...