Learning Forum

[jeong hee yun]

As a result of wavelength sweep using the S-parameter of FDTD simulation, the following results were obtained. I have seen other people change their s-parameter results to abs^2 and plot them, and I have done the same. 

1. What is the purpose of changing to abs^2?
2. Also, when plotting abs^2, should I use 10log or 20log?

[Guilin Sun]

1: As you know, S is amplitude parameter. So abs^2 is the intensity/power parameter. For 2X2 device, abs(S11)^2 is power reflection and abs(S21)^2 is the power transmission for the specific mode.

2: if you want to get result in dB, for abs^2, it is 10log10. for abs(S), it is 20log10.

BTW: you mentioned you sweept wavelengh in FDTD? in the port/mode case, once you use broadband, you do not need to sweep since FDTD can give you broadband result from one simulation. 

Mode source in broadband simulations

[jeong hee yun]

When converted to abs^2 value, what exactly should I write as the y-axis name?

[Guilin Sun]

As mentioned previously, abs^2 is power or intensity quantity, and since it is already normalized, you can label it as transmission/reflection of the specific mode. When we say S parameters, they are always associated with modes. In broad definition, plane wave, Gaussian beam and waveguide modes are all "modes".