Design and theoretical analysis of optical waveguides of quantum cascade laser at λ∼ 9.5 µm

We report the design and theoretical analysis of optical waveguides of the InP-based quantum cascade laser (QCL) operating at λ∼ 9.5 µm. The effective refractive index, optical confinement and absorption coefficient are calculated by using the finite element method (FEM). From FEM simulations, the effect of the number of active/injector stages (Ns) on the optical confinement and waveguide loss of QCLs was investigated. As the Ns in QCLs increased from Ns = 30 to Ns = 70 for the 19 µm wide QCLs structure, the optical confinement was improved from 73.6 % to 90.6 % and the absorption coefficient was reduced from 3.91 cm⁻¹ to 1.47 cm⁻¹. However, the Ns should be optimized considering the heat generated within the active region because the increase in Ns causes a large voltage rise. The influence of ridge width on the optical confinement and absorption coefficient in QCLs with SiO2 and Si3N4 insulation layers was also studied.