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Reflectivity of a cleaved facet forming a laser cavity in a double heterostructure injection laser is analyzed on the basis of a dielectric waveguide model and the reflectivity of modes is numerically provided for a GaAs-AlGaAs double-heterostructure injection laser. The modes that distinguish field distributions perpendicular to a junction plane are considered up to the sixth order with TE and TM modes. The dependence of threshold conditions on the mode, the relations between the lowest threshold mode and the structures of the injection laser are studied. Provided mode reflectivity is solely responsible for modal selection, the single-mode operation is theoretically shown to be possible by choosing a suitable length of the cavity even though the thickness of the active layer is large.