Learning Forum

[yuqizhu]

I'm trying to understand the far field projection of a gaussian source with NA=0.1. I calculated the far field projection from fields measured on a 2d monitor that is normal to the propagation direction. With the 'thin lens' option, there is a hole at the center of the beam. Is this expected/correct? if so, why? Given that my wavelength~1 um and beam waist ~3 um, I thought I should use 'thin lens'. I also tried applying the 'scalar approximation'. The resulting beam cross section is qualitatively different, and more like what I expected. Does it mean I should use salar approx for this beam? Previously I also did some scattering/force calculations using the E- and H-fields from monitors; those results are at lease qualitatively similar regardless of how I configured the gaussian source.

[Guilin Sun]

Not sure if your plot is near field or far field. Here is my test:

Above is the farfield, and below is the near field:

Please check if the Gaussian beam injection plane is large eough.

Since NA=0.1 is relatively small, you can use scalar Gaussian beam. Of course you can also use NA thin lens beam which is more accurate.

 

 

[yuqizhu]

 

Attached is the field plot of the source and config. Currently my source span is roughly 6 beam waist radius. I tried increase the source span to 10 beam radius, and had the same problem. The plots I showed earlier was from a 2D freq monitor ~10 um above the injection plane. I also attached a screenshot of part of the layout and the monitor's visualiztion in the gui

Thanks,

Yuqi

[Guilin Sun]

The source beam profile is correct. The monitor data is after some propagation. It is hard to say what causes this shallow center. You can place a xz plane monitor and check the field profile. You may also check Born&Wolf's book for fields near focus plane. It is not a solid cone as expected intuitively.