# Source power

#1

Dear Lumerical Team,
I am using FDTD solutions and have some questions about source parameters.
I illuminate the surface using plane wave source and get electric field, that has to be used for further calculations. However I am interested to do simulations for different values source power. I kindly ask you to explain, what sourcepower(f) means, as well as Amplitude parameter on General tab. How can I do simulations using 200mW laser as a source? And what does source geometry means?
Best regards,
Diana

realtion between amplitude and intensity of electric field
How to calculate the source power?
How can I get mode source poynting vector?
#2

Theoretically, a plane wave is infinite in time and space domain. However, since simulation of a real plane wave is impossible, in FDTD we define a time domain pulse and specify a geometry for plane wave.

The other challenge is the energy associated with a plane wave. Since plane wave is infinite in space, the energy that it carries is infinite. Thus, rather than defining the energy of a plane wave, we define the amplitude of electric field which has units of V/m. So, for example, if the amplitude is chosen to be 1 v/m, the power that it carries at the area (100e-9)^2, defined by source geometry or simulation region, is:

0.5*sqrt(eps0/mu0)*(100e-9)^2 * (1 v/m)^2

Based on this discussion, if you want to inject a 200mW pulse, you need to adjust the amplitude and the geometry. However, for linear simulations the easiest approach is to run simulations for an amplitude of 1 V/m, and then use normalized transmission to calculate the associated power. Also, since you have to use plane wave source with periodic BCs in FDTD simulations, you need to make sure that 200 mW is the power shined on one unit cell in the experiment. This means that you might find power intensity (power per area) a better parameter to work rather than total injected power.

https://kb.lumerical.com/en/index.html?ref_sim_obj_cw_norm_free_space.html

I strongly recommend you to download the simulation file on the KB page, and change the amplitude, and compare the results of the commands below to understand how these values change (although for linear simulations, as discussed above, normalized transmission will be enough to extract any value of interest):

?"max power T1: " +num2str(max(T1))+" W/Hz^2";
?"max power T2: " +num2str(max(T2))+" W";
?"max power T3: " +num2str(max(T3))+" (fraction)";