Learning Forum

[Md. Atiqur Rahman]

I'm a new user of lumerical fdtd and trying to simulate a directional coupler in SOI platform. One waveguide tapered down from 840 nm to 835 nm and another waveguide is 400 nm wide. Gap between them is 200 nm. During simulation at 1550 nm wavelength Neff for TE1 is 2.21929 but when I set the source wavelength range from 1530-1565 nm the Neff for TE1 at center wavelength ~1550 nm changes to 2.26563. Please can anyone tell me why this happening?

2. Can I do wavelength sweep simply by setting min wavelength and max wavelength in source settings? or I have to use optimization and sweep window to do wavelength sweep?

[Amrita Pati]

Hello,

1. The slighly different values of effective index in both the cases could be most likely due to material fitting. I would recommend you to check the material explorer for both the simulations and see if the FDTD model uses the same permittivity/index values for 1550 nm.

2. If you use the min wavelength and max wavelength in source settings, the source is injected at the central frequency but the results are calculated based on the number of frequency points you have provided in your monitor. These recorded results are over the entire bandwidth. Is the source profile highly dependent on the frequency?

Regards,
Amrita

[Md. Atiqur Rahman]

 

Thank you for your kind reply.

 

1. By using mesh override I fixed this problem. Now refractive index isn’t changing anymore.
2. Yes, I chose 36 points on monitor to see results between 1530 -1565 nm. Is it valid to be considered as wavelength sweep result? For your ques : I'm using mode source for excitation, waveguide material is silicon in SOI platform. I think source profile isn't highly dependent on frequency.

[Amrita Pati]

Great to know that the first problem is solved!

Thank you for providing more information. Like I mentioned before, the mode is injected only at the central frequency. So, if the profile is not highly dependent on the frequency, then you can inject this one mode to sweep over the entire wavelength range using the min wavelength and max wavelength in source settings. If you require a smoother curve over the bandwidth, I would suggest increasing the number of frequency points.


Regards,
Amrita

[Md. Atiqur Rahman]

Thank you very much for your fast response.