This paper, on the basis of coupled-mode equations, analyzes the bandwidth characteristic of asymmetric fused biconical taper (FBT) coupler, and proposes and proves a solution of all-wave coupler (O+E+S+C+L band).We also succeed in making all-wave couplers with a bandwidth of 390nm, while the excess loss is only 0.1 dB and the coupling ratio is 1∶99