Photonics

Photonics

Multiple sources in FDTD

    • Qian Zhang
      Subscriber

      Hi. I'm trying to simulate the transmission of the waveguide.

      Two mode sources (one is fundamental TE mode, and another one is fundamental TM mode) are enabled simultaneously in 3D FDTD simulation. The amplitude of TE source is always kept at 1, and the amplitude of TM mode souce is changed from 0, 1, 2, 3, etc. A 'Frequency domain field and power' monitor is enabled at the end of the FDTD simulation region to record the data. The transmission of the waveguide is plotted by visualizing 'T' of the monitor. The question is the transmission 'T'  is always normalized within the region '0-1', so how the transmission is normalized or calculated when there are 2 sources and when the amplitude of one source increases?

      Thanks!

    • Guilin Sun
      Ansys Employee

      This is a good question. When there are two sources, intterference will occur. TE and TM most of the time are not 100% pure, eg Ex dominanted mode can have minor Ey when the waveguide is along z.  When there are more than one source, the sourcepower will use their incoherent result, however the simulation is coherent. In this case, if it is not for a circularly-polarized mode, what is the purpose to have TE and TM modes at the same time? do they have the same effective index, or degenerated? if yes, in experiment how do you control them? if not, how do you control their relative initial phase? I would strongly suggest you to think those questions carefully before you do the simulation.

    • Qian Zhang
      Subscriber

      Thanks for your reply.

      The purpose to have TE and TM modes at the same time is that we have a structure that requires 2 orthogonal sources. One polarization modulate the other through the device via changing the amplitude, so the power normalization needs to be understood to make sense of the output. The effective index of TE is larger than TM. 

    • Guilin Sun
      Ansys Employee

      If they are not the same degenetated modes, maybe I am wrong but I suppose they are incohent. However simulation is coherent. This might be contradictory. This will need you to justify, as this is a theoretical issue.

      If you really want to do the simulation,it might be good to do two separate simulations. But seems you only want to do one simulation. If you think they are coherent, you can manually calculate their source power, by adding the E and H fields coherently, calculating the Poynting vector, and integrating it. Please refer 

      Ansys Insight: FDTD 初学者: transmission函数的使用、透射率反射率正负号问题

       

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