Learning Forum

[Frederic Wu]

Hello,

I would like to know if the interference between waves affect the DFT monitor's measurement. I am interested in transmitted power and I wonder if destructive interference will affect and thus decrease the reading.

Thanks for your time

[Guilin Sun]

FDTD solves the Maxwell Equations discretely. It involves all the physics, including interference, whether it is constructive or destructive.  You cannot isolate them as this is physics.  The DFT monitor will give you "net" transmitted power (ratio to source power).

[Frederic Wu]

So when waves pass through a DFT monitor and destructively interfere with one another, the "net power" will be lower than if the waves constructively interfere?

[Guilin Sun]

I am not sure if you can control the interference, because it is the device behavior.  AS mentioned previously, you cannot sepatate them. I am not sure for your device what exactly the quesiton is. What do you concern here? what determines the constructive/destructive interference and how do you interven this? could you please give draft about the device , the source and the monitor and what the final result do you want?

 

[Frederic Wu]

I simply wonder how the DFT monitor works. The structure I build reflects light in different layers (it's called DBR reflector), so when the waves reach the monitor, the waves will be out of phase. Does the phase difference and interference reflect on the measurement of the DFT monitor?

[Guilin Sun]

Yes, the DFT monitor actually uses Fourier transform of the signal to the frequency domain result. Please refer 

Ansys Insight: Fdtd中是如何计算对不同波长的响应的

https://optics.ansys.com/hc/en-us/articles/360034394234-Understanding-frequency-domain-CW-normalization

 

[Frederic Wu]

2 questions

1. Just to confirm my understanding, the DFT monitor takes phase shift into account. So if the reflected waves are out of phase, interference will affect the reading of the reflected power?
2. Does waves with different frequencies interfere with one another? So for example, there are a 3000nm and a 5000nm wave. Does the two waves interfere or does fourier transform ignore the interference across waves with different frequencies?

[Guilin Sun]

A1: this is definitely correct.

A2: In simulation, no, waves with different frequencies DO NOT interfere with one another by Maxwell Equations. This is physics, as FDTD solves the Equations through the first principle, with discretization.