Fig.1 (a) illustrates the schematic diagram of the proposed MBEFL. Two feedback branches and a ring cavity are connected by a four-port circulator. Two feedback branches consist of 9.4 km
and 25.2 km single mode fiber (SMF), a four-port circulator (cir 1) and the loop-back three-port optical circulator(cir 2 and cir 3), respectively. Erbium doped optical fiber amplifier (EDFA) is composed of a 980 nm pump laser, 10 m EDF, an isolator (ISO) and two wavelength selective couplers (WSCs). The 980 nm pump laser is divided into two parts by a 3 dB optical coupler to improve the performance of the EDFA. Two WSCs are used to couple the pump and the generated Stokes lines. Cir 1 can ensure the unidirectional oscillation of the optical laser to in crease the OSNR and decrease the reverse Rayleigh scattering.