We discuss a self-imaging phenomenon in a multimode interference (MMI) coupler. From experiment, different self-images, which are undefined in MMI theory, are observed. These undefined self-images are named "extraneous self-images" (Ex_SIs) for convenience. To estimate the applicability of the Ex_SIs, the characteristics of both the single self-image (0 dB self-image, 0 dB SI), which is defined in MMI theory, and the Ex_SI are compared and analyzed through simulation and experiment. The results show that the Ex_SI has an imaging period that is the same as the 0 dB SI and that the excess loss and the extinction ratio of the Ex_SI improve more than that of the 0 dB SI as the imaging period increases.
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机译：我们讨论了多模干扰（MMI）耦合器中的自成像现象。通过实验，观察到了MMI理论中未定义的不同自我图像。为了方便起见，这些未定义的自映像被称为“外部自映像”（Ex_SI）。为了评估Ex_SI的适用性，通过仿真和实验比较和分析了MMI理论中定义的单个自镜像（0 dB自镜像，0 dB SI）和Ex_SI的特性。结果表明，Ex_SI的成像周期与0 dB SI相同，并且随着成像周期的增加，Ex_SI的过量损耗和消光比比0 dB SI的改善更大。
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