It is crucial to set PMLs refractive index to be the same as adjoining dielectric medium, since any discontinuity in refractive indices (any interface) introduces reflections. Near the inner boundary PML is purely dielectric, and slowly is becoming more and more absorbing. It is usually just fine to use default PML in COMSOL. However, it is important to use swept mesh for PML meshing. Five layers are good starting point, and each meshing element should be of the same size as nearest meshing element in the modeling space.