FDTD and modal properties

[engmfarhat]

Could any one know how to obtain the modal properties of photonic crystal fiber like effective index, dispersion, .... from the FDTD method.
I know that we must take first FFT then what?

 

[shenhongj2004_05]

i have the same problem as you have now, have you solve it already ? can you share it with me?

 

[engmfarhat]

I didn't solve it yet but i will try
Do you calculate the transmission using fdtd?

 

[shenhongj2004_05]

yeah,but my fdtd code don't work successfully.
i can't get correct pbg of the ez

 

[engmfarhat]

Could you calculate the transmission through simple line defect 2D PCF?
If so could you expalin to me how?
What is the proplem you face? If i can i will help you ? We can discuss this together?
So:
Could you say to me what is the feature of the program? What it can calculate?

 

[shenhongj2004_05]

my code is about PhC waveguide(2D).
with pml boundary , the total time step is 10000 steps ,
i want get defect mode e,g, the band gap of defect mode
but i can't get the peak in the frequency graph , i always get a line (w=0);
i know this is because there are some problem when i fft the ez field

Added after 2 minutes:

i don't have idea of phc fiber , but i read you talking about fft, so i want you help me

 

[engmfarhat]

Did you use point or line detector?

Did you get correct Ez field before you transform it using fft?

did you get correct propagation through the waveguide?

 

[engmfarhat]

where are you?
Could you answer my previous quesions

 

[xiangyuanjiang]

can give me the code?

 

[a_shamsafar]

hi i think optiFDTDsolve your problem

 

[rrumpf]

If you are just looking for information like dispersion and effective indices, FDTD is NOT an efficient solution. FDTD is great for modeling propagation through waveguide bends and discontinuities. Otherwise, look for a frequency-domain model to compute the modes. A finite element package is probably best, but it should be easy for you to code your own frequency-domain finite-difference method.

I could be wrong here, but it sounds like OptiFDTD has a frequency-domain solver built into it to calculate the modes. If so, this could be a solution for you depending on how much it costs.

-Tip

 

[zhh309sx]

FDTD is not a good method because it is a scalar method not a vector method. A photonic crystal fiber shoule be computed by full vector method for example full vector FEM method.

 

[rrumpf]

While it is certainly possible to formulate essentially any numerical technique as a scalar method, FDTD is by its nature a fully vectorial and fully rigorous method; aside from the approximation of fitting the fields to a discrete grid. In fact, I am not sure I have ever seen it implemented as a scalar method.

In my mind, FDTD has some important benefits and drawbacks you should consider. Here are only a few:

Benefits
1. FDTD is a fully vectorial and fully rigorous method!!!!
2. FDTD is a time-domain method so it is possible to characterize a device over an incredibly huge range of frequencies in a single simulation. Frequency-domain methods must repeat their computations for every frequency of interest.
3. FDTD is not based on linear algebra so if the problem size doubles, the number of computations only doubles. Methods based on linear algebra operations like matrix division or eigen-system computations scale exponentially.

Drawbacks
1. It is very difficult, but not impossible, to implement source waves at oblique angles of incidence.
2. FDTD must iterate for incredibly long periods of time when there are sharp or abrupt features in their spectral response.
3. It is very difficult in many situations to exploit longitudinal periodicity. For example, if you want to model transmission through a photonic crystal slab that is 100 periods thick, FDTD must store all of these periods in memory at once. Other methods based on scattering matrices (or similar approaches) can compute on unit cell, then stack that on itself 100 times to dramatically improve efficiency.

-Tip