Variational - effective index method and finite difference beam propagation method ( fd - bpm ) are both practicable waveguide simulation tools . the variational - effective index method is based on mixing the variational method ( vm ) and the effective index method ( eim ) , it can be used with very accuracy to simulate the field distribution 變分有效折射率法是變分法( vm )和有效折射率法( eim )的結合,它能夠很精確地模擬光場分布情況;有限差分束傳播法則能夠準確地模擬波導的性能。