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A gain medium and an interband cascade laser, an interband cascade amplifier, and an external cavity laser having the gain medium are presented. The gain medium can include any one or more of the following features: (1) the active quantum well region includes a thick and In-rich GaInSb hole well; (2) the hole injector includes two or more GaSb hole wells having thicknesses in a specified range; (3) the electron and hole injectors are separated by a thick AlSb barrier to suppress interband absorption; (4) a first electron barrier of the hole injector region separating the hole injector region from an adjacent active quantum well region has a thickness sufficient to lower a square of a wavefunction overlap between a zone-center active electron quantum well and injector hole states to not more than 5%; (5) the thickness of the first InAs electron well in the electron injector, as well as the total thickness of the electron injector, is reduced; (6) the number of cascaded stages is reduced; (7) transition regions are inserted at the interfaces between the various regions of the gain medium so as to smooth out abrupt shifts of the conduction-band minimum; (8) thick separate confinement layers comprising Ga(InAlAs)Sb are disposed between the active gain region and the cladding to confine the optical mode and increase its overlap with the active stages; and (9) the doping profile of the cladding layers is optimized to minimize the overlap of the optical mode with the most heavily-doped portion of the InAs/AlSb SL cladding layers. An interband cascade laser, an interband cascade amplifier, or an external cavity laser employing a gain medium having these features can emit at a wavelength of about 2.5 μm to about 8 μm at high temperatures.