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FWHM and Reflectance of Bragg Grating unexpectedly high

    • jasper


      I simulated the following bragg grating:


      (Setup in Lumerical:)

      Since the sections are both single mode, the mode convergence sweep works out fine. I get the following results:

      The spectrum has a typical bragg grating form, however the FWHM and Reflectance are too high compared to measurements and simulations with other programs. For example, for a corrugation depth of 0.075µm, the FWHM is about 2.3nm when it should be about 0.7nm and the reflectivity is approximately 1 when it should be significantly lower (0.7). I have used the same geometry setup and the same # of periods. The grating period was calculated using the approximation via the average neffs of the fundamental TE modes of each cell: (neff=sqrt((neff_l1^2)*(1-duty_cycle)+(neff_l2^2)*duty_cycle). These are my EME settings:

      Could this perhaps be caused by the integrated wavelength sweep feature in EME? What is going on here?

      I'd appreciate some help.



    • Guilin Sun
      Ansys Employee

      It might be one reason. The built-in wavelength sweep does not count the dispersion, eg, it uses the neff of the initially calculated modes at the specified wavelength.

      Other reason might be:

      1: the geometrically averaged effective index is a approximation

      2: need proper number of modes

      In addition, the group span is set to 0.718143um. It may not matter much but I usually suggest to use fewer digits so to be easier to compare with experiment. 1nm length might be difficult to control in manufacturing.

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