{"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>Hi,<br><br>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.<br><br>Zmin=0;<br><br>addmode;<br>#?set;<br>set(‘injection axis’, ‘z-axis’);<br>set(‘direction’, ‘forward’); <br>set(‘z’, Zmin+0.02e-6);<br>set(‘x span’, X_span);<br>set(‘y min’, Ymin); set(‘y max’, Ymax);<br>set(‘set wavelength’,1);<br>set(‘wavelength start’, wavelength); <br>set(‘wavelength stop’, wavelength); <br>set(‘mode selection’, Mode_Selection);<br>set(‘optimize for short pulse’,0);<br>updatesourcemode; <br><br>addpower; # Power monitor, input<br>set(‘name’, ‘input’);<br>set(‘monitor type’, ‘2D Z-normal’);<br>set(‘x span’, X_span);<br>set(‘y min’, Ymin); set(‘y max’, Ymax);<br>set(‘z’, Zmin+0.02e-6);<br>set(“override global monitor settings”,1);<br>set(“partial spectral average”,1);<br>set(“total spectral average”,1);<br><br>addpower; # Power monitor, output<br>set(‘name’, ‘transmission’);<br>set(‘monitor type’, ‘2D Z-normal’);<br>set(‘x span’, X_span);<br>set(‘y min’, Ymin); set(‘y max’, Ymax);<br>set(‘z’, Zmax-0.1e-6);<br><span class=”ps-stream-hashtag”>#set<\/span><\/a>(‘frequency points’,10);<br>set(“override global monitor settings”,1);<br>set(“partial spectral average”,1);<br>set(“total spectral average”,1);<br>set(“output Px”,1);<br>set(“output Py”,1);<br>set(“output Pz”,1);<br><br>addmodeexpansion;<br>set(‘name’, ‘expansion’);<br>set(‘monitor type’, ‘2D Z-normal’);<br>set(‘x span’, X_span);<br>set(‘y min’, Ymin); set(‘y max’, Ymax);<br>set(‘z’, Zmax-0.1e-6);<br>set(‘frequency points’,10);<br>set(‘mode selection’, Mode_Selection);<br>setexpansion(‘T’,’transmission’);<br><br><br>I am facing below mentioned issues. <br><br>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. <br><br>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.<br><br>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. <br><br>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.<\/p><\/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":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t