Hi Giuseppe,

It is possible this task is more complicated than it first appears.

To begin with, I suggest that this should be done in sequential mode, not nonsequential. There are several reasons for this. In general, optimizations for image quality are best done in sequential because the optimization methods are better at choosing rays and are more stable and robust. It is also easier to specify constraints. Also, in nonsequential, the calculation of the spot radius requires that the spot cover a reasonable group of pixels, and the method cannot determine the radius of a spot smaller than a pixel. This creates difficulties when a very small spot is anticipated.

And there is another very significant reason to use sequential. In most laser designs, the goal is to achieve diffraction limited performance. In the focusing case, the geometric spot radius determined by ray tracing will be significantly smaller than the actual spot radius achieved, which is limited by diffraction effects. Sequential mode has an analysis method called Physical Optics Propagation (POP) which simulates diffraction and can be used to optimize for the diffraction limited spot.

The general method in sequential mode is to first optimize the design for minimum spot radius using ray tracing, and then refine the design by setting up a POP analysis and the using the POPD operand instead of ray tracing to optimize for minimum spot radius. (This can resut in a design a slightly different from the one that minimized the geometric radius.)

Using POP is complicated and requires study. There are several good resources on the Zemax site. I especially recommend all three parts of “Using Physical Optics Propagation (POP).”

Kind regards,

David

https://support.zemax.com/hc/en-us/articles/1500005488601-Using-Physical-Optics-Propagation-POP-Part-1-Inspecting-the-beams

https://support.zemax.com/hc/en-us/articles/1500005490261-How-to-model-laser-beam-propagation-in-OpticStudio-Part-1-Gaussian-beam-theory-and-ray-based-approach

https://support.zemax.com/hc/en-us/articles/1500005490541-Simulating-lasers-webinar

https://support.zemax.com/hc/en-us/articles/1500005489641-Laser-applications-webinar

https://support.zemax.com/hc/en-us/articles/1500005488421-Exploring-Physical-Optics-Propagation-in-OpticStudio