Hey Yannis, I’ve never seen anything like that before. My inclination is that it’s something to do with pixel size and spacing, but in order to help fully I’d need to see the full ZAR file. Can you post it here?

Hi Mark,

yes I figured it might have to do with that. Please find the ZAR file attached. It would be good to know if this issue can be alleviated to get the correct near-field wavefront.

Hi Yannis,

I’ve just looked at your system, and the user-defined aperture does impose a grid of lines on the beam. But the values in the file you posted are not zeros in the gaps. You can look at them close up visually with a cross-section graph in the POP output or in the text window to see the numbers. 

If we’re still not seeing the same things, I’d suggest opening up a formal case so we can dig a little deeper.

Hi Kevin, thank you for your reply,

the POP irradiance values are zero at the pixel grid surface (e.g. surface 10), see attached. Going further away from the pixel grid (i.e. backwards propagating), the values indeed become positive. Though I understand how diffraction would broaden the shape of the pixel squares at longer distances, I am still unsure about why the irradiance would be set to zero (similar to applying a mask) at the pixel grid. This is rather problematic as I am interested in the near field, close to the pixels, and I am unsure whether the drops in the irradiance is physical or not. If I think of a planar wave incoming onto the pixel gaps (slits), I don’t think that the observer would see zero intensity at the slits, or close to them.

Thoughts are welcome,
Yannis

Hi Yannis,

Thanks for clarifying. Yes, on surface 10 you do see the intensity go to zero in the gap regions. The surface aperture acts as a mask in this case, and for a given pixel there is either normal intensity or none. This is a limitation of a pixel-based field calculation if you have structures with a size comparable to the pixel size, as is the case here. If you made very slight changes in the gaps it is likely you would not see any change because it is below the resolution. Depending on the fraction of the pixel filled or blocked, you could also possibly see Moiré patterns and similar non-optical effects that are artifacts of the resolution.

The solution for using these fine structures is to either determine that the output is within your specifications or to go with a much higher resolution that samples across the smallest structure you have.

Hi Kevin,

so, to clarify, you think that if the user-defined aperture (pixel grid) was more finely sampled, the artifacts could be mitigated? It is true that something looks off with how the pixel grid is sampled when using a 1024x1024 sampling in the POP tool. The pixel gaps should have the same size everywhere, but in some places they seem wider (see attached plot). So, indeed this looks like a mismatch of resolution between the user-defined, hard-coded aperture, and the POP sampling. This makes sense.

Yannis