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Sampled grating has been used to enlarge the allowed range of coupling coefficient for high spatial hole-burning corrected single-frequency yield in conventional index-coupled DFB lasers with 1%-80% mirror combination. Theoretical calculation using an effective-index transfer matrix method shows that the allowed range of coupling coefficient increases approximately by 20% in the proposed sampled grating distributed-feedback lasers (DFB) lasers compared with conventional ones. Thus, the tolerance to the coupling coefficient variation improves to the level typically achieved in grating fabrication. The improvement is attributed to the flattened axial photon distribution of gap modes, and therefore stability in other lasing characteristics can be expected.