First , we simply introduced the optical wave - guide theory : the fluctuation equations of planar wave - guide and the preparation of embranchment wave - guide . then based on maxwell equations and in the paraxial limit we deduced the formula needed in calculating fd - bpm . the refractive index section was n ( x , y ) 然后我們?cè)趍axwell方程的基礎(chǔ)上,根據(jù)慢包絡(luò)近似理論,在折射率截面為n ( x , z )的條件下,利用有限差分近似來代替偏微分方程,推出了有限差分光束傳播法計(jì)算所需要的公式。