about two slit simulation at the foci

[tyjisinap]

I want simulate the situation of two slit at the foci, and to get the spatial coherence, but I found that after the slit, the coordinate will be enlarge to 1e6 m. and can not get right result. I think this may be caused by the accuracy of calculation. how can I get the right result?

[rafaelcelestre]

It is hard to pinpoint what is wrong with your simulation without seeing it (e.g. propagation parameters and so on). The question you pose at the end is too loose and hard be be addressed. Nonetheless, I would suggest you:

a) check that at the slit position you have enough sampling points
b) propagate the beam to the observation position (this without the slits) and check the range
c) try using special Free-Space Propagators (parm. # 3 on the wavefront propagation parameters)

There are other ways to calculate the coherence length. You could calculate the mutual intensity using the following function:

<<srwl_wfr_emit_prop_multi_e>>

where:
:param _char: radiation characteristic to calculate:
0- Total Intensity (s0);
1- Four Stokes components;
2- Mutual Intensity Cut vs X;
3- Mutual Intensity Cut vs Y;
4- Mutual Intensity Cuts vs X & Y;
"""
Here you can check the results from the mutual intensity vs. the double pinhole experiment (p. 16): https://www.synchrotron-soleil.fr/en/file/8902/

Another way to calculate the coherence length is proposed by H. Westfahl Jr et. al. (The coherent radiation fraction of low-emittance synchrotrons, J. Synchrotron Rad. (2017). 24, 566-575).