Applying an exact modal propagation analysis method and self-imaging theory, i analyzed the principle of mmi coupler . then, the self-imaging effect in graded-index waveguide has been analyzed and simulated using three-dimensional bpm . considering normal technology error, a low loss y-branch structure ( symmetric or asymmetric ) with a multimode waveguide transition section is introduced and designed 在設計y分支波導結構時,考慮常規(guī)工藝誤差的影響,引入多模波導作為y分支波導結構的過渡區(qū),利用參數(shù)空間搜索法來設計結構參數(shù),實現(xiàn)低損耗的y分支結構。