{"id":395036,"date":"2024-11-23T00:36:24","date_gmt":"2024-11-23T00:36:24","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/query-related-to-rough-waveguide-3d-fdtd-simulation-time-negative-transmission\/"},"modified":"2024-11-23T00:36:24","modified_gmt":"2024-11-23T00:36:24","slug":"query-related-to-rough-waveguide-3d-fdtd-simulation-time-negative-transmission","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/query-related-to-rough-waveguide-3d-fdtd-simulation-time-negative-transmission\/","title":{"rendered":"Query related to Rough waveguide 3D FDTD, Simulation time, Negative Transmission"},"content":{"rendered":"<p>&lt;p&gt;Hi,&lt;br&gt;&lt;br&gt;I am simulating a Silicon waveguide with surface roughness (wrapped by SiO2). width=500nm (x-axis), height=220nm (y-axis) and length=2um\/10um (z-axis). I am using 3D FDTD for the simulations. Source details are attached below. My 3D FDTD region is 12nm in X-axis, 8nm in Y-axis and whole region in Z-axis. I have checked that the mode profile is showing 1e-10 (color bar) around 2.5nm in x and y direction. The mesh accuracy is 3 and the autoshutoff is default 1e-5. I am also using the input, transmission DFT monitor and mode expansion at the given locations.&lt;br&gt;&lt;br&gt;Zmin=0;&lt;br&gt;&lt;br&gt;addmode;&lt;br&gt;#?set;&lt;br&gt;set(&#8216;injection axis&#8217;, &#8216;z-axis&#8217;);&lt;br&gt;set(&#8216;direction&#8217;, &#8216;forward&#8217;); &lt;br&gt;set(&#8216;z&#8217;, Zmin+0.02e-6);&lt;br&gt;set(&#8216;x span&#8217;, X_span);&lt;br&gt;set(&#8216;y min&#8217;, Ymin); set(&#8216;y max&#8217;, Ymax);&lt;br&gt;set(&#8216;set wavelength&#8217;,1);&lt;br&gt;set(&#8216;wavelength start&#8217;, wavelength); &lt;br&gt;set(&#8216;wavelength stop&#8217;, wavelength); &lt;br&gt;set(&#8216;mode selection&#8217;, Mode_Selection);&lt;br&gt;set(&#8216;optimize for short pulse&#8217;,0);&lt;br&gt;updatesourcemode; &lt;br&gt;&lt;br&gt;addpower; # Power monitor, input&lt;br&gt;set(&#8216;name&#8217;, &#8216;input&#8217;);&lt;br&gt;set(&#8216;monitor type&#8217;, &#8216;2D Z-normal&#8217;);&lt;br&gt;set(&#8216;x span&#8217;, X_span);&lt;br&gt;set(&#8216;y min&#8217;, Ymin); set(&#8216;y max&#8217;, Ymax);&lt;br&gt;set(&#8216;z&#8217;, Zmin+0.02e-6);&lt;br&gt;set(&#8220;override global monitor settings&#8221;,1);&lt;br&gt;set(&#8220;partial spectral average&#8221;,1);&lt;br&gt;set(&#8220;total spectral average&#8221;,1);&lt;br&gt;&lt;br&gt;addpower; # Power monitor, output&lt;br&gt;set(&#8216;name&#8217;, &#8216;transmission&#8217;);&lt;br&gt;set(&#8216;monitor type&#8217;, &#8216;2D Z-normal&#8217;);&lt;br&gt;set(&#8216;x span&#8217;, X_span);&lt;br&gt;set(&#8216;y min&#8217;, Ymin); set(&#8216;y max&#8217;, Ymax);&lt;br&gt;set(&#8216;z&#8217;, Zmax-0.1e-6);&lt;br&gt;<a href=\"https:\/\/innovationspace.ansys.com\/community\/?hashtag\/set\/\" target=\"_blank\" rel=\"noopener\">&lt;span class=&#8221;ps-stream-hashtag&#8221;&gt;#set&lt;\/span&gt;<\/a>(&#8216;frequency points&#8217;,10);&lt;br&gt;set(&#8220;override global monitor settings&#8221;,1);&lt;br&gt;set(&#8220;partial spectral average&#8221;,1);&lt;br&gt;set(&#8220;total spectral average&#8221;,1);&lt;br&gt;set(&#8220;output Px&#8221;,1);&lt;br&gt;set(&#8220;output Py&#8221;,1);&lt;br&gt;set(&#8220;output Pz&#8221;,1);&lt;br&gt;&lt;br&gt;addmodeexpansion;&lt;br&gt;set(&#8216;name&#8217;, &#8216;expansion&#8217;);&lt;br&gt;set(&#8216;monitor type&#8217;, &#8216;2D Z-normal&#8217;);&lt;br&gt;set(&#8216;x span&#8217;, X_span);&lt;br&gt;set(&#8216;y min&#8217;, Ymin); set(&#8216;y max&#8217;, Ymax);&lt;br&gt;set(&#8216;z&#8217;, Zmax-0.1e-6);&lt;br&gt;set(&#8216;frequency points&#8217;,10);&lt;br&gt;set(&#8216;mode selection&#8217;, Mode_Selection);&lt;br&gt;setexpansion(&#8216;T&#8217;,&#8217;transmission&#8217;);&lt;br&gt;&lt;br&gt;&lt;br&gt;I am facing below mentioned issues. &lt;br&gt;&lt;br&gt;a) My input DFT monitor, transmission (T) shows negative values. why?? If the waveguide length is 2um, I am negative negative values for all the surface roughness irrespective of the meshing, FDTD region or number of PML layers. For waveguide length 10um, I am getting negative values for roughness (sigma_rms) more than 6nm. &lt;br&gt;&lt;br&gt;b) How to calculate the propagation loss in such a cases? Is there a reliable way to calculate the propagation loss, scattering loss, radiative loss, absorption, etc. Any formula or any monitor that can provide me these values.&lt;br&gt;&lt;br&gt;c) To simulate the FDTD, I have defined fine mesh regions around the roughness and coarse mesh at other regions. The simulations are taking 40-70hours for a single simulations on a 32 thread CPU even if all the CPUs are assigned to one simulation. Is there a way to reduce the simulation time? I have already tried with mesh override but I lose significant accuracy for higher surface roughness. &lt;br&gt;&lt;br&gt;d) It is possible to simulate a short rough waveguide and then use the output of DFT monitor at the end of first waveguide to use as a source to simulate the similar short waveguide in a different simulation? If yes, kindly guide me through it.&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-395036","topic","type-topic","status-publish","hentry","topic-tag-waveguide-2","topic-tag-3DFDTD-1","topic-tag-computation-time-1"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["441143","33648","4274"],"_bbp_author_ip":["2a01:4b00:d126:1d00:2440:e3:408b:4080"],"_btv_view_count":["387"],"_bbp_topic_status":["unanswered"],"_bbp_topic_id":["395036"],"_bbp_forum_id":["27833"],"_bbp_engagement":["441143","33648","4274"],"_bbp_voice_count":["3"],"_bbp_reply_count":["5"],"_bbp_notification_enabled":["33648"],"_bbp_last_reply_id":["397002"],"_bbp_last_active_id":["397002"],"_bbp_last_active_time":["2024-12-10 22:25:52"]},"test":"naveen-kumarglasgow-ac-uk"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/395036","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\/395036\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=395036"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}