The basic principle of L-band is presented,based on 3 dB wide band coupler,the fiber loop mirror with high reflectivity is obtained,through optimal designs and experiments,the two-stage double-pass of L-band with high power amplified spontaneous emission(ASE) fiber source was developed.The fiber lengths of two-stages are about 7 m (low concentration) and 31 m (high concentration respectively,the output power of L-band,when double-pass forward pumping is used in the configuration,is 21.48 mW (13.32 dBm),the mean wavelength is 1 573.52 nm;When double-pass backward pumping is used in the first stage,the output power of L-band is 22.71 mW (13.56 dBm),and the mean wavelength is 1 574.66 nm.By comparison with the characters of the efficiency of pump and the flatness of spectrum,the two-stage double-pass structure with the double-pass backward pumping in the first state is more perfect for the design of L-band with high power.Also,as high power (higher than 30 mW) of C-band is easy to obtain,so output ASE of C+L band (from 1 520 to 1 610 nm) with power higher than 50 mW could be achieved.