Multimode fibers and the number of modes, how to identify different modes


#1

I have a file where I’m just trying to just find the visualize the modes of a few mode fiber in MODE. However, I don’t understand what the results appear to tell me. I’m expecting to see 7 propagating modes from the based on http://www.photonics.byu.edu/fibercalculator.phtml. Where wavelength is 1.55um, core radius is 10um, n1 = 1.4435 and n2 = 1.44.

In the eigensolver analysis window, when I calculate modes, I end up seeing 12 mode? I just don’t get it. What am I doing wrong? multimode.lms (274.7 KB)


#2

Hi, @jkoehl !

My guess is that the number of guided modes obtained in the web-site is based on an approximate expression, which are usually quite accurate when the number is very high. Anyway, it is better to see what formula they use to estimate the number of modes.

Also, please notice that you better fill all the cladding space around the waveguide core homogeneously, so that the simulation area does not include the outer radius. However, that in your simulation this does not much better, since the outer radius is quite large and a mode tail does not even touch it.


#3

Hi @msaygin,

Thank you responding! I inspected the website and found that they calculate the V number, then use if/else statements to determine the number of modes:
Nmodes.value = 1;
}
if (Vnumber.value >2.405 && Vnumber.value <3.7){
Nmodes.value = 4;
}
if (Vnumber.value >=3.7 && Vnumber.value <5.1){
Nmodes.value = 7;
}
if (Vnumber.value >=5.1 && Vnumber.value <5.5){
Nmodes.value = 9;
}
if (Vnumber.value >=5.5 && Vnumber.value <6.5){
Nmodes.value = 12;
}

…is this accurate-ish?

Also, what you’re saying is that M = (V^2)/2 works best for large M’s, hence I’ll have optimize a little if I want a small M, like M=6?

Finally, when I’m calculating modes in MODE, I’m expecting to see the LP modes (similar to https://www.rp-photonics.com/lp_modes.html) But it look me that LP01 and LP21 are present, but LP11 isn’t. Am I looking at the results incorrectly?

Thanks
jkoehl


TE0 has one index and not two
The design of leaky mode planer waveguide
#4

Dear @jkoehl

Sorry for the long wait.

I am not an expert in how engineers name the fiber modes, but I think for every LP mode, you can have different polarizations. For example, for LP01, you have TE and TM mode (the first two mode obtained in FDE solver). Modes from #3-6 (four modes called TE01, TM01, and two HE21) are called LP11 modes as is explained in the link below:

Again, I am not expert in how they name the modes, but mode 11 and 12 might be two different polarization for LP02 and #17 and #18 might be LP03.

Based on the links that you provided, the number of modes calculated in the links seems reasonable. This is being said that I am not too familiar with how they obtained the theoretical calculations for the number of modes.

Please let me know of your thoughts and I will be glad to be of a help.