Abstract:An Er~(3+)/Yb~(3+) co-doped double clad fiber(EYDF) was used as gain medium,up to five laser wavelengths are simulataneously observed at room temperature,by incorporation of a stadard multimode optical fiber into an otherwise single-mode fiber ring cavity as a spatial mode-beating filter.The number of lasing wavelength and wavelength spaces can be controlled by changing the state of polarization controller.Without any stabilization equipment,the stable multiwavelength lasing output is obtained,with an output power of 207 mW,the line width less than 0.15 nm and side-mode-suppression-ratio above 32 dB.Using the multimode fiber that supports only a few waveguide modes and optimizing the fusion splices between single-mode and multimode fibers,the stable laser oscillation with more wavelengths can be achieved.