{"id":288023,"date":"2023-06-09T22:02:28","date_gmt":"2023-06-09T22:02:28","guid":{"rendered":"\/forum\/forums\/topic\/difference-between-fdtd-and-eme-results-for-evanescent-waveguide-coupler\/"},"modified":"2023-06-09T22:06:55","modified_gmt":"2023-06-09T22:06:55","slug":"difference-between-fdtd-and-eme-results-for-evanescent-waveguide-coupler","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/difference-between-fdtd-and-eme-results-for-evanescent-waveguide-coupler\/","title":{"rendered":"Difference between FDTD and EME results for evanescent waveguide coupler"},"content":{"rendered":"<p>&nbsp;<\/p>\n<p>Hi there,<\/p>\n<p>I am trying to design an evanescent coupler consisting of a linearly tapered GaAs waveguide resting on top of a high-aspect ratio SiN waveguide all buried in SiO2. I would like to use the EME solver so that I can efficiently vary the taper length and save lots of computation time. To verify my EME results, I ran the same setup in FDTD and receive very different transmission results (transmission from GaAs fundamental TE mode to SiN fundamental TE mode). In FDTD, I measure this using the expansion in the output port and in EME I measure this using the S21 scattering matrix element. I will list the various things I have checked in both my simulations below. Please help me figure out why these methods give me such drastically different results, thanks!!<\/p>\n<p>Thanks in advance,<\/p>\n<p>Austin<\/p>\n<p>FDTD:<\/p>\n<ul>\n<li>Doubled simulation time and increased mesh accuracy from 2 to 3 and get same results.<\/li>\n<li>Mesh overrides for both waveguides in the Y and Z directions were applied to ensure accuracy.<\/li>\n<li>No modes are cutoff by PML boundary when looking at mode profiles via ports<\/li>\n<\/ul>\n<p>EME:&nbsp;<\/p>\n<ul>\n<li>Ran script to verify number of cells in tapered region is sufficient.<\/li>\n<li>Mode convergence sweep looks normal.<\/li>\n<li>Checked all diagnostics Lumerical suggests and found no issues apparent to me.<\/li>\n<\/ul>\n<p>Simulation parameters of interest:<\/p>\n<p>Target wavelength: 950nm<\/p>\n<p>GaAs waveguide dimensions: 250nm wide tapered down to 100nm over 40um, 150nm thick. Refractive index 3.6. Mesh override: dy = 0.04um, dz = 0.02um<\/p>\n<p>SiN waveguide dimensions: 2um wide, 40nm thick. Refractive index 1.943. Mesh override: dz = 0.005um<\/p>\n<p>SiO2 refractive index: 1.4511<\/p>\n<p>&nbsp;<\/p>\n<p><a class=\"wp-colorbox-image cboxElement\" href=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347519-EME_sim_setup_updated.PNG\"><img decoding=\"async\" src=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347519-EME_sim_setup_updated.PNG\" alt=\"\"><\/a><\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p><a class=\"wp-colorbox-image cboxElement\" href=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347453-FDTD_sim_setup.PNG\"><img decoding=\"async\" src=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347453-FDTD_sim_setup.PNG\" alt=\"\"><\/a><\/p>\n<p><a class=\"wp-colorbox-image cboxElement\" href=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347469-EME_transmission_50modes_20cells.png\"><img decoding=\"async\" src=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347469-EME_transmission_50modes_20cells.png\" alt=\"\"><\/a><\/p>\n<p>&nbsp;<\/p>\n<p><a class=\"wp-colorbox-image cboxElement\" href=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347486-FDTD_transmission_40um-linear-taper_1300fs_mesh-acc-3.png\"><img decoding=\"async\" src=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/06\/09-06-2023-1686347486-FDTD_transmission_40um-linear-taper_1300fs_mesh-acc-3.png\" alt=\"\"><\/a><\/p>\n","protected":false},"template":"","class_list":["post-288023","topic","type-topic","status-closed","hentry"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["2846","3002"],"_bbp_author_ip":["96.7.218.223"]," _bbp_last_reply_id":["0"]," _bbp_likes_count":["0"],"_btv_view_count":["541"],"_edit_lock":["1686348215:220720"],"_bbp_topic_status":["unanswered"],"_bbp_status":["publish"],"_bbp_topic_id":["288023"],"_bbp_forum_id":["27833"],"_bbp_engagement":["2846","3002"],"_bbp_voice_count":["2"],"_bbp_reply_count":["1"],"_bbp_last_reply_id":["290657"],"_bbp_last_active_id":["290657"],"_bbp_last_active_time":["2023-06-28 01:10:33"]},"test":"austin-granmoecolorado-edu"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/288023","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\/288023\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=288023"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}