Hello YJ,

Just a small question. Your file is a zmx file and doesn’t contain the POP settings, so could you either send us an archive file (File...save as...zar) or check the following?

I can see that the Object Space NA is set to 0.14. Is it given at 1/e^2?

Then there are 2 wavelengths in the system. Which one are you using for POP?

If I use wavelength 1 = 1.55um and NA(1/e^2)=0.14, that gives a waist of 3.5um. Is this what you are modelling? Is the distribution of the beam uniform or gaussian? What is the definition of the receiving fiber?

We always recommend those articles to check POP propagation: 

• Using Physical Optics Propagation (POP), Part 1: Inspecting the beams

• Using Physical Optics Propagation (POP), Part 2: Inspecting the beam intensities

• How to model laser beam propagation in OpticStudio: Part 3 - Using Physical Optics Propagation to model Gaussian beams

The Prop Report is a first step, but then we recommend checking the phase of the beam at each surface. The risk with POP is not to sample properly a surface and then get an incorrect result.

Waiting for your answer.

Hello Sandrine,

Thank you for your recommendation!

the Object Space NA 0.14 is given at 1% of the peak density, corresponding to an NA of 0.09 at 1/e^2, so the waist is calculated to be 5.2 um, according to the formula MFD=4λf/πd, i.e. 1/2(MFD) The wavelength I used for POP is 1550 nm. The distribution of the beam is assumed to be Gaussian.

*.zar is not supported by the zemax community. Please access the .zar under the link: https://cloud.tugraz.at/index.php/s/CEX3nNXTTngCL2C

Best regards,

YJ

​ Hi YJ,

Thank you for sharing your file. zar is unfortunately not supported by our platform so we usually send zip files. 

Just looking at the editor, we know that the geometric rays are likely to be different from POP as the beam is propagated over a long distance where the beam will diffract with POP but will stay unchanged with rays. So I agree that POP is a good choice in that case.

I then looked at the POP ZBF. One important thing to note since your beam is polarized is that when we use Polarization, 2 beam arrays will be propagated, one each for the X and Y polarizations of the beam. 
The phase on surface 2 is clearly under sampled:

As a first step, I can use rays to go through the lenses. I have ticked “Use Rays to Propagate to Next Surface” on all the lenses but not on the long distance.

• “Use Rays to Propagate to Next Surface” is selected for surfaces 2,3,4,7,8,9. It is quite a useful option here since each individual lens adds aberrations where the phase will be hard to sample, but are then compensated by the following lens.
• On surface 6, I increased the sampling because it was under sampled on surface 10:

  

   

• On surface 11, I also increased the sampling:

  

   

It gives me an enough sampled beam. The diffraction is only considered from surface 1 to before the 1st lens, on the long distance and from after the last lens to the image plane.

So this result doesn’t consider the diffraction that happens at the lenses but considers the main diffraction which is on the long distance. I have attached my file.
The process is quite tedious since I am checking the phase at each surface and then adjusting the sampling manually until I see that it is visually properly sampled. Let me know your thoughts.

Just as an extra note here, we can also tell that how much diffraction happens in the long distance. POP tells me that the beam size changes from a radius of 5mm to a radius of 15mm (I checked visually). The ray approach is not the right one here since the beam stays constant with rays.

Dear Sandrine,

Thank you very much for the detailed explanation! Sorry for the late reply. I can’t open your .zar file because of the older version of OpticStudio.

I modified the procedure following your reply. The images look great! Most obviously, those “spooky” parts at the edge of the images are gone. I can’t reproduce your result, but they are quite similar:

Fiber Efficiency: System 0.662671, Receiver 0.408930, Coupling 0.270986 (-5.67 dB).

May I ask how you configured the starting X/Y-Sampling and the starting grid width for POP?

Another question, i.e. the phase on Surface 3 (I didn’t tick “use rays to propagate to next surface on it) still looks weird, however I adjusted those parameters, while the phase seems to be perfect on the subsequent surfaces. Is this what you said “aberrations resulting from each individual lens compensated by the following lens”?

Besides, I measured a coupling efficiency of 0.36 (-4.44 dB) for 60 m when the weather was good, which is by far the largest value I have measured. It is ~33% higher than the theoretical value, i.e. 0.27 (-5.67 dB). Why is that, in your opinion?

Attached are the procedure and results. Are they similar to yours?

Thank you once again!

Best regards,

YJ