Learning Forum

[Shashank Gupta]

Hello Sir,

I am following the curved waveguide taper example " https://optics.ansys.com/hc/en-us/articles/360042799713-Curved-waveguide-taper-varFDTD-and-FDTD- " for taper design for my material. The gaussian source defined in the exampler uses  'use thin lens' method in the beam option setting. I am unable to understand how the beam option parameters (NA, lens diameter, beam diameter, etc) is defined ?

I think it depends on the effective refractive index. If so then how to decide these values for different refractive index (neff for my material is 1.91).

Also, boundary condition in varFDTD uses 'uniaxial anisotropic PML' setting. How to define these parameters?

 

Thanks

 

 

[Guilin Sun]

For this example, the diameter of the taper is very large, so it uses a Gaussian beam. Instead of the common scalar Gaussian, it uses vectorial beam, similar to the case when the scalar gaussian beam passes an microscope object lens. Thus it has NA, the Numerical Aperture. As for other parameters,such as lens diameter, beam diameter, I would rather not change the default, since they may change the effective NA.

NA is n*sin(theta) where "n" the refractive index of the material where the beam source is located, and theta is the geometrical angle which allows the the ray enters the lens. 

Numerical aperture - Wikipedia

Please note that "n" is not "neff" which is the mode property.

As for  'uniaxial anisotropic PML', it is a legacy PML.  Please refer 

https://optics.ansys.com/hc/en-us/articles/360034382674-PML-boundary-conditions-in-FDTD-and-MODE

Usually, if you do not know how to modify, simply use the default settings. You only need to modify when the default settings do not work, or violate physics. 

 

 

 

[Shashank Gupta]

Thank you for your reply. The parameters defined in the curved waveguide taper is different from the general p I still have few doubts.

1. Can common scaler approximation gaussian beam work for larger diameter? Will it be able to generate accurate results or not?
2. If not, then how do I define NA and other values for my case?  My waveguide dimension and material is different, thus 'n' will be different (n=2.19).
3. Is theta in the NA =n*sin(theta), is taper angle?