1. What is a beam splitter

Beam Splitter is used to split incident beam power into multiple beams in different spatial directions, reflection and transmission in most cases, via multilayer dielectric stack coating in most cases. The splitting is by design, for example 50:50 non-polarizing beam splitter would divide incident beam into 50% reflectance and 50% transmittance, and the multilayer stack coating is designed to achieve that specific ratio under design parameters (spectral and angular). A polarizing beam splitter (PBS) would want all the s-polarized light to reflect while all the p-polarized light to transmit through.

2. How to model a beam splitter

Since here we are talking about splitting via multilayer coating, this can be modelled using Transfer Matrix Method (TMM). Please click on the link to go through the step by step derivation of reflectance and transmittance from a multilayer stack.

The coating that needs to be optimized for beam splitting is mostly made of alternating stacks of high and low refractive index dielectric layers (e.g. high refractive index example being TiO2, Ta2O5 etc while low refractive index example being SiO2 etc).