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The lasing characteristics of separate-confinement-heterostructure (SCH-structure) distributed-feedback (DFB) diode lasers are examined theoretically and experimentally. Wave propagation in five-layer SCH waveguides is analyzed to estimate such parameters as the lasing wavelength, coupling constant, and external quantum efficiency. Spectral and modal behavior are studied in the experiment and compared with the theoretical predictions. Diodes are shown to lase in a single longitudinal mode with a definite polarization. Spectral width is about 300 MHz just above the threshold, and becomes wider with increased excitation level. An output power of 40 mW with an external quantum efficiency of 5 percent is obtained under CW operation.