# Different result for Bloch and Periodic Boundary Condition at normal incidence?

#1

I am trying out a simulation with laser gain medium for which CW normalization is disabled. I am getting different power transmission(not normalized) and field intensity for Bloch BC and Periodic BC for normal incidence. As I understand the results should be exactly same for normal incidence. However, for my case it isn’t.

In this simple 3D simulation file for normal incidence of light, Bloch BC along x and y direction produces E field of 4.32e-6 whereas, Periodic BC produces 2.16e-6. Can anyone explain these results?

Query_Bloch_Periodic.fsp (275.8 KB)

#2

Bloch BCs require complex field whereas periodic BCs only need the real-part field. The transmission is the same if you can compare.To check the difference, please add a time monitor and check the E field again.

#3

Thanks.

However, I have noticed that Bloch BCs use complex fields and similar fields can be obtained by taking real parts of the field monitors and normalized transmissions are also same. But, as I am doing simulations with gain mediums I cannot use CW normalization. So to find total transmitted power along a direction, I need to integrate Poynting Vector normal to the monitor along the spatial dimensions.

I find that Poynting vector along z (P_z) from time monitor shows different results for Periodic BC and Bloch BC. Note that I have taken real parts of P_z for both cases in the following figure

The value of P_z at 800nm from DFT monitor (CW disabled) produces different results for Periodic BC and Bloch BC as shown in the following image

.fsp file: Bl and PBC No normalization.fsp (234.0 KB)

#4

They are different because the injection is different. you need to calculate the injection power you will know they are difference. It is about 4x in power because the double excitation for the complex field.