Abstract: A quantum cascade laser has an input waveguide that propagates a transverse optical mode of electromagnetic radiation generated by the QCL, a multimode interference (MMI) coupler, and an output waveguide. The coupler has (a) an input end that receives the transverse optical mode from the input waveguide, (b) a tapered section that modifies a transverse width of the transverse optical mode, and (c) an output end that outputs a modified-width transverse optical mode to the output waveguide. Modifying the transverse width of the transverse optical mode permits the input waveguide to have a different width than the output waveguide.

Abstract: A quantum cascade laser (QCL) may include multiple branch waveguide regions having one or more laser cores providing optical gain at one or more output wavelengths, a stem waveguide region, and multiple couplers arranged to couple light from the plurality of branch waveguide regions to the stem waveguide region. Each of the couplers may include two or more curved waveguide regions having a continuously-varying radius of curvature providing that a fundamental transverse mode at the output wavelengths is dominant, and a coupler to combine light from the two or more curved waveguides and maintain dominance of the fundamental transverse mode at the output wavelengths. The fundamental transverse mode at the output wavelengths may be dominant in output light from the stem waveguide.