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We have developed 1.3 μm n-type modulation-doped strained-layer quantum-well lasers. Modulation-doped lasers with long cavities (low threshold gain) exhibit much lower threshold current densities than conventional lasers with undoped barrier layers. The lowest threshold current density we obtained was 250 A/cm2 for 1500 μm long lasers with five quantum wells. The estimated threshold current density for an infinite cavity length was 38 A/m2/well. This is the lowest value for InGaAsP-InGaAsP and InGaAs-InGaAsP quantum well lasers to our knowledge.