进一步研究二个问题：(1)理论模拟的方法由快速傅里叶束传播法(FFT-BPM)改为有限差分束传播法(FD-BPM)，并比较了两者的优缺点。(2)模拟设计了对象提高为由GeSi合金单模脊形光波导组成的Ｙ级联1×4光分束器。结果表明，有限差分束传播法比快速傅里叶束传播法及显形有限差分束传播法(EFD-BPM)，除了数学比较容易外，计算速度也快一些。尤其是对传播步长Δz的敏感性，有限差分束传播法远没有快速傅里叶束传播法那样严重，便利了束传播法方法的采用。正如束传播法成功地应用于GeSi合金光波导及Y分支器的理论设计，现在用了1×4Y级联分束器同样得到预想的结果。光束在分束器中传播逐步分成二束再分为四束。符合设计要求。

After preparing the large cross section single mode rib waveguide and it′s Y power splitter of GeSi alloy, this paper studies (1) FD BPM is used to simulate the light propagating in the splitter and the comparison between FFT BPM and FD BPM. (2) The simulation of λ =1.3 μm light propagating in the designed 1×4 splitter. Results show that compating to FFT BPM, FD BPM is easier to deal mathmatiocally and computing speed is faster and not very sensitive to the change of Δz . The simulation of 1×4 power splitter is succeddful. When the single-mode Gauss beam incidents into the splitter, the light propagated in the device according to requirement. At mid-way it splites into 2-beam and inot 4-beam finally.