Improvement of second-harmonic generation in quantum-cascade lasers with true phase matching

The improvement of second-harmonic generation in quantum-cascade lasers with integrated optical nonlinearity was discussed. It was obtained by including phase-matching considerations in the design of the deep-etched ridge waveguide. To minimize the phase mismatch of the zero-order mode of the fundamental light with the second-order transverse mode of the second-harmonic light, the waveguide layer structure was optimized. By the faster decrease of the modal refractive index of the fundamental light with decreasing ridge width relative to the refractive index of the second-harmonic light, exact phase matching was made possible.