Based on the modified coupled-wave theory,the emission characteristics,including threshold gain,photon density distribution in the cavity,external differential quantum efficiency,ect.for second-order distributed feedback (DFB) lasers are investigated.Numerical simulation results also show that for given device structure with wavelength of 1.55 μm,grating duty cycle has certain influence on the emission characteristics.Finally,optimal grating duty cycle of 0.43 and optimal results include uniform photon density distribution along the cavity,the side-mode suppress ratio as high as 35 dB and external differential quantum efficiency of 47% are obtained.