I have a confusion about using Mode Expansion Monitor in FDTD. I am trying to simulate the interleaved PN junction based optical modulator from the knowledgebase of lumerical.com (https://kb.lumerical.com/en/pic_modulators_interleaved_junction_microring.html). I used the “np density” grid attribute object to import the carrier density and used the Soref and Bennett. However, the example in the knowledgebase use (n,k) import option and uses an script to calculate the phase change due to the PN shifter.
I am having some problem understanding the Mode Expansion Monitor. The situation of my simulation is depicted in the following diagram for simplicity. I am launching the fundamental TE mode using the mode source as depicted. After some distance (say 50 nm) is located a frequency domain field and power monitor to act as the “input field” for the expansion monitor. 5 to 10 nm after this frequency monitor is located the PN junction phase shifter. Again, after 5-10nm distance after the PN junction shifter is located the Mode expansion monitor. Both the monitors have same span (in x and z directions). I have setup the expansion monitor to calculate the power coupled in the fundamental TE mode after the mode has propagated through the phase shifter.
My understanding was that the neff and mode profiles contained in the mode expansion monitor would reflect the phase change accumulated by the injected mode after it has traveled through the phase shifter and hence the neff would be different than that of the neff in the mode source, However, in the simulation both have same value. Could you please explain a little as to where am I going wrong and how can I setup my simulation to record the phase change and loss due to the phase shifter? Can I get complex value of the neff from any monitor which represents the accumulated phase and loss of the injected mode.
I hope I explained my problem clearly but please let me know if there is any ambiguity in my post.
Eagerly waiting for a prompt response.