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Effective mode index!!!!!

Posted 24 apr 2018, 13:19 GMT-4 Wave Optics, Results & Visualization, Studies & Solvers Version 5.3 3 Replies

Hong Ikjun
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Hey guys... I need a help about effective mode index..

When I simulate the mode analysis, the result shows like below.

1.4498(1) 1.4498(2) 1.4499(1) 1.4499(2)

"(1),(2)" <==== What does this means???????????????? Please answers my question.... I can't get the proper explanation....

3 Replies Last Post 26 apr 2018, 08:05 GMT-4
Sergei Yushanov Certified Consultant
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Posted: 7 years ago 24 apr 2018, 16:01 GMT-4

Hong,

It means that you are getting degenerated modes, i.e. different mode shapes with the same (or extremely close) effective mode index. Plot modes 1.4498(1) and 1.4498(2) and you will see that field distributions for these modes are different. Within 5-6 digits, degenerated modes have the same propagation constant and effective mode index. Use full precision under global evaluation and you will see that there is difference in 7th -9th digits for these modes, as shown in the attached image.

Regards,

Sergei

Hong, It means that you are getting degenerated modes, i.e. different mode shapes with the same (or extremely close) effective mode index. Plot modes 1.4498(1) and 1.4498(2) and you will see that field distributions for these modes are different. Within 5-6 digits, degenerated modes have the same propagation constant and effective mode index. Use full precision under global evaluation and you will see that there is difference in 7th -9th digits for these modes, as shown in the attached image. Regards, Sergei

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Hong Ikjun
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Posted: 7 years ago 25 apr 2018, 08:40 GMT-4

Sergei,

Thanks for your clear explanation..! I understand what you mean. I have one more question about effective index.. When I tried to solve planar waveguide using numerical analysis.

The height of n2 = 18um n1 = 1.44915530 n2 = 1.44977969 n3 = 1.44915530

Planar waveguide is like n1 | n2 | n3 structure, and I find the effective index by solving wave equation. In theoretically, Neff = 1.44955601, but I can't find the matched effective index in the comsol effective mode index's result.

In other words, in the comsol's effective mode index list does not have my theoretical result (Neff = 1.44955601)..

Please refer to the following attatched file, and help me -

Sergei, Thanks for your clear explanation..! I understand what you mean. I have one more question about effective index.. When I tried to solve planar waveguide using numerical analysis. The height of n2 = 18um n1 = 1.44915530 n2 = 1.44977969 n3 = 1.44915530 Planar waveguide is like n1 | n2 | n3 structure, and I find the effective index by solving wave equation. In theoretically, Neff = 1.44955601, but I can't find the matched effective index in the comsol effective mode index's result. In other words, in the comsol's effective mode index list does not have my theoretical result (Neff = 1.44955601).. Please refer to the following attatched file, and help me *-*

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Sergei Yushanov Certified Consultant
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Posted: 7 years ago 26 apr 2018, 08:05 GMT-4

Hong,

Keep in mind that dispersion curves usually have several branches, meaning that mode with the same effective index can exist at different frequencies. So, if you are looking for the specific mode and don't see it in you model, try to change Mode analysis frequency.

Regards,

Sergei

Hong, Keep in mind that dispersion curves usually have several branches, meaning that mode with the same effective index can exist at different frequencies. So, if you are looking for the specific mode and don't see it in you model, try to change Mode analysis frequency. Regards, Sergei

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