In the simulation of optical waveguides, the methods FDTD and BPM are complimentary. Problems that BPM simulates well are not practical by FDTD. Problems that BPM cannot simulate are often possible in FDTD. BPM is for weakly guiding waveguides having mostly small refractive index contrasts. The problems are within the paraxial approximation. Waveguide splitting, tapering, and bending generally occur over distances too great for practical analysis by FDTD. Waveguides having high refractive index contrast can be made much shorter because of strongly guiding properties. Those problems are generally not paraxial and outside of the paraxial approximation used by BPM. On the other hand, those waveguides tend to be short, making analysis by FDTD practical.
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