Abstract: 3dB wide band coupler is a passive device widely used in optical fiber communication system, optical fiber sensor, optical fiber measurement technology and signal processing system. A fiber fused biconical taper (FBT) 3dB wide band coupler was designed and some samples were manufactured. The bandwidth characteristics of asymmetric FBT coupler were analyzed by coupledmode equation. The bandwidth of the wide band coupler is about 300nm (between 1300nm and 1600nm), while its excess loss is less than 0.5dB.