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(shè)計y分支波導(dǎo)結(jié)構(gòu)時,考慮常規(guī)工藝誤差的影響,引入多模波導(dǎo)作為y分支波導(dǎo)結(jié)構(gòu)的過渡區(qū),利用參數(shù)空間搜索法來設(shè)計結(jié)構(gòu)參數(shù),實現(xiàn)低損耗的y分支結(jié)構(gòu)。