The validity of weak-coupling approach for a transmission phase hologram

Weak-coupling methods are convenient tools to analyze the diffraction properties of volume holograms for low diffraction cases, such as holographic data storage. Although the range of "low diffraction" is very important in practice, it is never defined because the parameter of modulation depth is excluded in the weak-coupling methods. In the paper we propose a simple method to estimate the validity of weak-coupling methods. Based on the weak-coupling solution from the coupled wave theory, we can quantitatively calculate the selectivity error of weak-coupling methods. The acceptable range of diffraction efficiency for weak-coupling methods can be determined accordingly.