Learning Forum

[minusunny]

Hi,

I am trying to simulate Subwavelength grated MMI. Should i go with EME or FDTD for initial design?Should i go with PML boundary conditions or Bloch ?

 

[Guilin Sun]

If the device is relatively short and you want broadband result, you can use FDTD. With mode source or port, you should use PML, except the structure has infinite periodicity. In such case you can use "periodic BCs". Bloch BCs are for periodic structure with angled plane wave incidence only.

In most cases EME is better, as it can reuse the periodicity: you only need to draw one period then you can set the number of periods before running "eme propagate".

As I did not see exactly the configuration of your device, those are my suggestions. please try. If you have new questions please write a new post.

Please note, in FDTD, when the periodic type of BC is used, it assumes infinite periodicity; Whereas in EME, except that you set the boundary conditions, the number of periods is alway finite.

[minusunny]

Thank you. Do i need to enable CVCS in EME?Also , do i need to increase the number of cells?

[Guilin Sun]

Yes you can.

Depending o your machine, you can use more numbers of modes,and you can also use the built-in mode convergence weep.

[minusunny]

I tried to simulate the same device in FDTD. I am illuminating only one port of the 2 X 4 MMI and after simulation, i got an image like this. What are the possible reason? I used the port size same as input waveguide

[Guilin Sun]

The excitation port should be as large as possible to not truncate the source mode;

it seems the PML mightnot be effective as there are interference patterns in the input/output straight waveguides. Please check the PML;

please extend the device outside of the PMLs, but not let them end inside PML;

You may also need to run long simulation time, with smaller autoshutoff min, say 1e-6.

Please add at least one point time monitor at the output waveguides and make sure the signal is completely decayed. You should observe a series of pulses with smaller and smaller amplitude due to the MMI cavity.

This post can be focused on using FDTD.

 

[minusunny]

I still have issues. I have attached the field profile after simulation, the input port mode,PML settings and advance settings.

[Guilin Sun]

The ripples on the left: it is due to reflection that creates interference patterns.

the cavity: it has standing waves

the right side: it seems either higher order modes are excited, or PML reflection. It might also be due to the iterfence from the leaked beams as the cavity cannot hold all the fields into proper channel.

[minusunny]

So, is it a design flaw or a simulation window issue?

[Guilin Sun]

None of them. It is the designed device behavior. You may need to improve the design, and do a convergence test: Convergence testing process for FDTD simulations 

[minusunny]

It worked for me when i did in EME.