Finite-element Maxwell’s equations modeling of etched air/dielectric Bragg mirrors

The importance of diffraction losses in etched air/GaAs Bragg mirrors in waveguides is theoretically evaluated. A complete three-dimensional resolution of the Maxwell equations by a finite element method is performed, and shows the great importance of diffraction into the substrate. Two cases are distinguished: the weak guiding case, where the index contrast between waveguide and substrate is low (example: GaAs/AlAs), and the strong guiding case where this contrast is high, thanks to the use of lower refractive index substrates, such as oxidized AlAs. The latter solution is found to be very promising since the diffraction losses into the substrate are practically eliminated.