GPON architectures and devices are on the first line for solving current and future access systems where multiple services are offered. Optical couplers are key components if a high splitting ratio is required. Beam propagation method is analyzed as a simulating tool for light propagation through some optical couplers. It is used paraxial Fresnel or wide angle Pade approximation to solve the Helmholtz equations in two and three dimensions. Waveguide, substrate parameters and approximation method are varied for method evaluation. An optical star coupler is used for method evaluation. It is compared the transmissive and reflective configuration with different propagation conditions.