Photonics

Photonics

Mode propagation and overlap in Mode

    • noumanzia
      Subscriber

      Hello!


      There is a feature in FDE to calculate the mode overlap between nearfields of two waveguide modes.

      Is this possible to propagate the near-field some distance x and then take the overlap of this diverged beam with the near field of other waveguide mode using FDE or EME to calculate mode-mismatch.

      I am well ware that it's doable via FDTD but it's computational expensive to propagate the mode in FDTD and calculate coupling efficiency. Also in that case I have to take into account the reflections and need to apply anti reflection coatings. FDTD is definitely more accurate but I am not sure what will be the level of accuracy if FDE/EME is used for this purpose.


      Best regards,

      Nouman

    • Guilin Sun
      Ansys Employee
      If you can provide more details about your device and simulation goal, we may provide better answers. For the limited information, yes, you can project mode in the farfield, 1 meter away in FDE; or using farffeildexact to project into finite distance. However the farfieldexact requires the mode profile in uniform material, which is physically incorrect in FDE/EME. If you want to project a waveguide mode into a uniform material such as air, it is better to use FDTD to simulate the end facet, using its monitor data to project to specified location, then do the coupling or overlap calculation using your own script, with farfieldexact:farfieldexact - Script command
      Facet reflection and transmission calculations
      when you say "anti reflection coatings", I guess you mean to coat the end surface of the waveguide. if not, please give more details.



    • noumanzia
      Subscriber

      Thank you for your thoughts! I want to calculate fundamental TE mode coupling between two waveguides when there is a finite gap between them. In other words, I want to see how the divergence of mode along the propagation direction affects the mode overlap. I am already using FDTD and it works fine, but I assume there should be a much simpler way to find mode overlap by using some kind of a Fourier transform which will give the field after some propagation distance, which I can then overlap with the mode in another waveguide

      In the FDTD model, I do need antireflection coatings at the waveguide facets to match the effective indexes between each material, otherwise, there will be a significant reflection loss at each interface, which will make it difficult to calculate the mode mismatch loss due to propagation.
    • Guilin Sun
      Ansys Employee
      If it is so, the FDTD is the best method, whereas EME is second, since you will need many modes in the gap which can be very challenge to converge. FDE is not good for this. the farfieldexact is much better than the simple Fourier transform, as it is theoretically no approximation. However, all the Fourier transform will involve some kind of approximation.
      For such gap, and titled angle between two waveguides, FDTD is the best.

    • AnkitSoni
      Subscriber

      I'm also dealing with exact problem, please let me know if you found something useful.
      Thank you!
    • ouyangxin
      Subscriber
      Hello, I have done same simulation like you mentioned.
      Unfraternally, I cannot find any better solutions in Lumerical and here is my steps;
      run simulation in FDTD and save the Power monitor data in local drive.
      run FDE to get mode filed, the monitor size and position inside FDE and FDTD setup better be the same to overlap.
      overlap saved monitor data and FDE eigen mode data to calculate coupling.
      It is very dumm but I think that is more accurate and work for simulation.

      If EME solver can add a function,
      like directly overlap one propagated mode between wanted selected mode in second wg, that would be efficient, like PhotoDesign.
    • Guilin Sun
      Ansys Employee
      To calculate the overlap, you do not need to use FDE. Instead, you can use mode expansion monitor to get the mode decomposition:
      Using and understanding Mode Expansion Monitors
      Mode expansion monitor - Simulation object - Lumerical Support
      Monitors - Mode Expansion - Setup & Tips - Lumerical Support
      Monitors - Mode Expansion - Results & Applications
      Or the better, simply use port: Ports (FDTD) - Simulation Object
      Port has mode source from FDE, DFT monitor and mode expansion monitor.

      EME has its own overlap inside each cell.

    • ouyangxin
      Subscriber
      Thank you for you answer. I am still confused, for example, I habe two different waveguides, between each other there is gap.
      I want to know, for example, one fundamental TE mode from left WG, after gap, after a certen distance inside 2nd waveguide, how many TE mode left in second WG's TE mode, if that is possible using port in FDTD to directly get the mode power overlap?
      Thanks in advance.
    • Guilin Sun
      Ansys Employee
      If you simply want to know how many modes are excited in the second waveguide, you do NOT need the overlap. Instead, get the s parameters of the modes in the second waveguide using port, which will give you the transmission coefficients of each mode in the second waveguide excited by the mode in the first waveguide. Please try the port, and check the online documents so it will save you a lot of time doing your own analysis using overlap.
    • AnkitSoni
      Subscriber
      Thank You for your response. It was very helpful.
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