It follows that , which was derived from the power absorbed in the active region, is a function of the placement of the active region in the optical field. When detectors with thick active layers which span accross several periods of the standing wave are considered, the SWE can be neglected. For very thin active layers, which are necessary for strained layer absorbers, the SWE must be considered. The effective absorption coefficient is the normalized integral of the product of and the field intensity across the absorption region.
Several assumptions simplify the analysis.
Finally, reflections from the active region interfaces are also neglected. This approximation is valid when heterostructures having subtle permittivity changes are considered. An exact numerical method for the more general case is described in the following section. It increases rapidly with representing the resonant cavity field enhancement. The position dependence represents the influence of the standing wave.