In EME it is not possible to nest periodic structures in the way you have in mind. A good alternative in your case would be to combine EME with INTERCONNECT:
- Simulate the Bragg grating with 20 periods and additional length of waveguide in EME and get the S -parameters.
- Use the S-parameters to construct an optical S-parameter object that can be repeated 10 times in the INTERCONNECT simulation.
Both, the EME and INTERCONNECT simulations should be pretty fast. From INTERCONNECT you can get the S parameters for the entire device. To learn more about INTERCONNECT I suggest the following links:
Alternatively, if you want to simulate everything in EME, then you would have to draw the Bragg grating with the 20 periods and then use the periodic group definition. So you will have something like:
[(1, 2, 3, …, 21, 22)^10]
However, for a situation where the number of repeated unit cells in the “inner” structure (the unit cell of the Bragg grating repeated 20 times in your case) is larger than the number of times the inner structure itself is repeated, the EME + INTERCONNECT option is recommended.
Hope this helps!