# Use MODE to simulate waveguide mode - Larger than 100% fraction

#1

Hi,
I’ve tried to simulate a wedge waveguide in MODE solutions. When I am getting the confinement of the waveguide, I found that the fraction of the power in some specific area sometimes is larger than 100%. But the fraction of the power in the whole simulation area is still 100%. Does this mean some wave are traveling backward direction that caused negative power? If so, how to get the exact confinement under this situation?
Thanks.

(The license I used is belonging to the umassb)

#2

Hi, @d02245001 !

Please, be more specific. What did you mean by the fraction of power in an area ?

#3

Hi,
Like the picture shows, the (power) fraction integrated inside the square area(white line) is 105.078%. The point is: Why the fraction of power integrated can be larger than 100%? (which means some locations have negative fraction of power) How to avoid this or to calculate the correct confinement?

#4

Can you attach your simulation file?

#5

InGaAsold.lsf (3.1 KB)
Please use 3um as wavelength and near n ~ 2 to find the 1st mode (neff~2.18+0.0467).
Then use the Power and Impedance Integration integrate over (x:-1.25~1.25 y:-0.1~0.1) as the photo shows.
Thanks!

#6

#8

MgF2.txt (2.3 KB)
This material file and the script are all it would need.

#9

The power integrated over an area is the integral of the z-component of the Pointing vector, which is a local characteristic of the mode field. In contrast to intensity, this can take both positive and negative values. Therefore, you were write saying

Does this mean some wave are traveling backward direction that caused negative power?

However, notice that the the mode as a whole is propagating in a predefined direction and this is the mode profile that yields varied signs of the Pointing vector across the mode area.

If so, how to get the exact confinement under this situation?

I think that the intensity is more appropriate to define mode confinement.

#10

Thank you @msaygin.

#11

However, I am curious about why it has a negative Poynting factor at some location. Is there any physical explanation for that??

#12

Thanks @msaygin for a detailed response. As he mentioned, at some locations Poynting vector has a negative sign, i.e. there are reflected power and as a result, the value is above 100%. The electric field intensity in this region is 91%, and can not exceed 100% in any region.

I do not have a clear intuition why we have some reflected power in this region, but I think there might be cases that total transmitted power is positive but there are locally reflected power. One thing that you can try is to extract E and H fields and construct Poynting vectors and study to make sure that it is negative in some region. This is being said that you need to do convergence testing to make sure that current results converge. A possible scenario is that if you use finer mesh, or wider FDE span, you see no reflected power.