{"id":414121,"date":"2025-02-11T20:42:05","date_gmt":"2025-02-11T20:42:05","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/debugging-wierd-divergence-issue-in-fdtd-for-a-bend-waveguide\/"},"modified":"2025-02-11T20:42:05","modified_gmt":"2025-02-11T20:42:05","slug":"debugging-wierd-divergence-issue-in-fdtd-for-a-bend-waveguide","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/debugging-wierd-divergence-issue-in-fdtd-for-a-bend-waveguide\/","title":{"rendered":"Debugging Wierd Divergence Issue in FDTD for a bend waveguide"},"content":{"rendered":"

<p>I am simulating an 180 degree bent waveguide with 3D FDTD, the version is 2025 R1 on GPU. Mesh is 3, only using Si and SiO2 in the material library. PML on all direction except z max is symmetric. The setup is shown in the following pic:<\/p><p> <\/p><p><\/p><p><\/p><p>I am running a sweep from small to big bend, the size is from around 50x50x1 um to 170x170x1 um. I noticed the simulation is stable for smaller bends, and get unstable for larger bends and significantly diverged for biggest bends (I see the autoshutoff value rise much more than 1 after runing a few hours with the biggest rings), see the following summery.<\/p><p><\/p><p>Also I noticed, for all those diverged bend simulation, there seem to be a “hot spot” on the E field plot at some part of the waveguide<\/p><p><\/p><p>I tried to debug based on the page: https:\/\/optics.ansys.com\/hc\/en-us\/articles\/11277217507603-Troubleshooting-diverging-simulations-in-FDTD#:~:text=Lumerical%20software%20has%20an%20automatic,a%20diverging%20simulation%20is%20detected.<\/p><p>On a 50x50um bend (small one, cuz the big ones takes too long), First I tried changing all boundaries to Metal, and there seems no divergence (the autoshufoff value is always very close to 1 even after a long simution time), thus I concluded that mine is PML type of divergence.<\/p><p>I first tried to change the PML profile to “stable”, but interestingly, this seems lead to 0.03 percent of reflection after each time the pulse hit the PML boundary. I see the autoshutoff value go from almost 1 to ~0.03 then to ~0.0009 in a long time simulation. And the autoshufoff value is constant when the simulation was running between these three points. This is strange, cuz if I change the profile back to “standard”, the there seems to be minimal reflection. the autoshutoff min reached and it triggered the early shutoff while running. I assume “stable” in theory should have much more complete absorption…<\/p><p>I also tried “stable” PML profile on big simulations, autoshutoff level still runs high<\/p><p><\/p><p>I also tried to disable “extend structure through the pml”, but there is no effect on the result.<\/p><p>Distance between structure and boundary and simulation regions size are also varies, but no improvement observed.<\/p><p>I also thought about the possibility of dispersion material, but it seems 25R1 on my computer gas issue with Visulizer. But I checked the system check, the material “Error RMS” is ~0.02 and I am just using silicon and silicon dioxide.<\/p><p>Could anyone comment on what might be my issue? And what else I can do to get rid of divergence? Thanks.<\/p><p> <\/p><p> <\/p><\/p>\n","protected":false},"template":"","class_list":["post-414121","topic","type-topic","status-publish","hentry","topic-tag-3FDTD-1","topic-tag-bending","topic-tag-divergence","topic-tag-divergence-detected","topic-tag-fdtd","topic-tag-pml-boundary-1"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27833"],"_bbp_topic_id":["414121"],"_bbp_subscription":["45962","20121"],"_bbp_author_ip":["206.87.146.99"],"_bbp_last_reply_id":["423281"],"_bbp_last_active_id":["423281"],"_bbp_last_active_time":["2025-02-26 02:17:04"],"_bbp_reply_count":["5"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["2"],"_bbp_engagement":["45962","20121"],"_btv_view_count":["359"],"_bbp_topic_status":["unanswered"],"_edit_lock":["1739395463:473055"]},"test":"mathewyuece-ubc-ca"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/414121","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":0,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/414121\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=414121"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}