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The ‘auto shutoff min’ setting for the transmission intensity

    • Altair45
      I currently investigate the Phase-shifted Bragg grating waveguide using FDTD simulation. In my work, the simulated transmission result of the DFT monitor is different from the reference data regarding the transmission intensity as attached images below.
      I suppose that 'auto shutoff min' can influence the difference in transmission intensity. The default value of it is '1e-5', and then it can be converted to '-50dB' in transmission. From the simulation data, the transmitted power is reflected extremely to -45dB at about 1580nm maximum.
      So, I tried to set the 'auto shutoff min' to '1e-10', but there is no noteworthy change in the results. 
      I'd like to ask for it. Are there any limits on the minimum value of the transmitted power in the DFT monitor?
      It would be appreciated if you could advise on this problem.

      First image is the simulated data and the second is reference data. (Han, Changhao, et al. "Slow light silicon modulator beyond 110 GHz bandwidth." arXiv preprint arXiv:2302.03652 (2023).)


    • Guilin Sun
      Ansys Employee

      To have accurate frequency domain result from time domain signal, the signal should be completely decay, which is roughly controled by the autoshutoff min (becuase it is computed in time domain). I believe you have used sufficently small autoshutoff min in this case, as there is no strong resonance. does the simulation terminate before 100%, say before 95%? if not, you will need to significantly increase the simulation time, for example 10 or 100 times more than the current value, and let the autoshutoff min to terminate the simulation.

      The rriples can aslo be originated from PML: too close or not thick enough, or improper type. Please increase the distance between the PML and the device, usually half wavelength or so; increase the number of PML layers; and if there are higher diffraction orders with large angle, in particular over 80 deg, please use Steep Angle PML. 

      If the mesh is very fine in front of PML, you may consider to use custom PML . If you do not know the PML parameters, you can copy them from the current PML, and only increase the max number of pml layers and use it. 

      Please note that the publication used dense wavelength sampling. You can increase the number of frequencies later. However you can choose "use wavelength spacing" either for the specific monitor, or in the "global monitor" settings since the plot is with wavelength.

      I hope those will be helpful. Please try.

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