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How to find the imaginary part of effective mode index

Posted Dec 16, 2019, 5:50 a.m. EST Low-Frequency Electromagnetics, RF & Microwave Engineering, Wave Optics 4 Replies

YZ Wang
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Recently I was using Comsol to simulate the bending loss of optical fibers, after adding the PML to the fiber, the result of effective mode index is still a real number, to calculate the loss I need to find the imaginary part of Neff, could anybody help with this problem? I will be very appreciated with this!

4 Replies Last Post Feb 6, 2020, 2:51 a.m. EST
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Hello YZ Wang

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Lars Dammann COMSOL Employee

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Posted: 5 years ago Jan 30, 2020, 10:47 a.m. EST

Hi, I assume you have already seen our example model file for bending losses in fibers? If not, you can find it here: https://www.comsol.com/model/step-index-fiber-bend-14189

You can check that your effective mode index indeed has no imaginary part by evaluating imag(Neff).

If there is no loss, I suspect you may be solving a straight wire which actually has no losses. Could you add your model file here or send it to support@comsol.com?

I hope this helps. Best regards, Lars

Hi, I assume you have already seen our example model file for bending losses in fibers? If not, you can find it here: https://www.comsol.com/model/step-index-fiber-bend-14189 You can check that your effective mode index indeed has no imaginary part by evaluating imag(Neff). If there is no loss, I suspect you may be solving a straight wire which actually has no losses. Could you add your model file here or send it to support@comsol.com? I hope this helps. Best regards, Lars
YZ Wang
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Posted: 5 years ago Feb 1, 2020, 5:12 a.m. EST

Hi, I assume you have already seen our example model file for bending losses in fibers? If not, you can find it here: https://www.comsol.com/model/step-index-fiber-bend-14189

You can check that your effective mode index indeed has no imaginary part by evaluating imag(Neff).

If there is no loss, I suspect you may be solving a straight wire which actually has no losses. Could you add your model file here or send it to support@comsol.com?

I hope this helps. Best regards, Lars

Thank you very much for your reply! But even in the example model I can not evaluating the imaginary part with imag(Neff). the result of evaluation is 0. I can see the shift of eletric field distribution, but the neff is always a real number. Yours sincerely, Wang

>Hi, I assume you have already seen our example model file for bending losses in fibers? If not, you can find it here: https://www.comsol.com/model/step-index-fiber-bend-14189 > >You can check that your effective mode index indeed has no imaginary part by evaluating imag(Neff). > >If there is no loss, I suspect you may be solving a straight wire which actually has no losses. Could you add your model file here or send it to support@comsol.com? > >I hope this helps. Best regards, Lars Thank you very much for your reply! But even in the example model I can not evaluating the imaginary part with imag(Neff). the result of evaluation is 0. I can see the shift of eletric field distribution, but the neff is always a real number. Yours sincerely, Wang
Lars Dammann COMSOL Employee

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Posted: 5 years ago Feb 3, 2020, 3:13 a.m. EST

Hi, I tried our example model step_index_fiber_bend and you should get the effective mode index (at radius r0) with the expression "ewfd2.neff/r0". This evaluates to about 1.4435-2.052e-6i, so the imaginary part is pretty small and by default COMSOL is only displaying the real part. You need to activate full precision to get both parts, or explicitly evaluate "imag(ewfd2.neff/r0)". I would also advise to pay attention to the definition of the variables ewfd2.neff and some other variables that derive from the eigenvalue variable lambda (like dampz, dampzdB, beta and others). They are computed as if the problem was 2D and not 2D-axisymmetric, which can give unexpected results. This is the reason we have to devide by r0 in some places. Please see the example models PDF file and the User's Guide to the Wave Optics Module p. 62 ff "Mode Analysis". Best regards, Lars

Hi, I tried our example model step_index_fiber_bend and you should get the effective mode index (at radius r0) with the expression "ewfd2.neff/r0". This evaluates to about 1.4435-2.052e-6i, so the imaginary part is pretty small and by default COMSOL is only displaying the real part. You need to activate full precision to get both parts, or explicitly evaluate "imag(ewfd2.neff/r0)". I would also advise to pay attention to the definition of the variables ewfd2.neff and some other variables that derive from the eigenvalue variable lambda (like dampz, dampzdB, beta and others). They are computed as if the problem was 2D and not 2D-axisymmetric, which can give unexpected results. This is the reason we have to devide by r0 in some places. Please see the example models PDF file and the User's Guide to the Wave Optics Module p. 62 ff "Mode Analysis". Best regards, Lars
YZ Wang
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Posted: 5 years ago Feb 6, 2020, 2:51 a.m. EST

Hi, I tried our example model step_index_fiber_bend and you should get the effective mode index (at radius r0) with the expression "ewfd2.neff/r0". This evaluates to about 1.4435-2.052e-6i, so the imaginary part is pretty small and by default COMSOL is only displaying the real part. You need to activate full precision to get both parts, or explicitly evaluate "imag(ewfd2.neff/r0)". I would also advise to pay attention to the definition of the variables ewfd2.neff and some other variables that derive from the eigenvalue variable lambda (like dampz, dampzdB, beta and others). They are computed as if the problem was 2D and not 2D-axisymmetric, which can give unexpected results. This is the reason we have to devide by r0 in some places. Please see the example models PDF file and the User's Guide to the Wave Optics Module p. 62 ff "Mode Analysis". Best regards, Lars

Lars，Thank you very much！ Yours sincerely, Wang

>Hi, I tried our example model step_index_fiber_bend and you should get the effective mode index (at radius r0) with the expression "ewfd2.neff/r0". This evaluates to about 1.4435-2.052e-6i, so the imaginary part is pretty small and by default COMSOL is only displaying the real part. You need to activate full precision to get both parts, or explicitly evaluate "imag(ewfd2.neff/r0)". >I would also advise to pay attention to the definition of the variables ewfd2.neff and some other variables that derive from the eigenvalue variable lambda (like dampz, dampzdB, beta and others). They are computed as if the problem was 2D and not 2D-axisymmetric, which can give unexpected results. This is the reason we have to devide by r0 in some places. Please see the example models PDF file and the User's Guide to the Wave Optics Module p. 62 ff "Mode Analysis". >Best regards, Lars Lars，Thank you very much！ Yours sincerely, Wang

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