Photonics

Photonics

Single mode coupling of dipole source

    • noumanzia
      Subscriber

      I am using a mode expansion monitor to calculate the coupling efficiency in a single-mode waveguide for a light emitted from a dipole source. I understand that T_forward in mode expansion monitor will calculate the fundamental mode power in waveguide normalized to the source power and that way I get coupling efficiency.

      Is this the right way because I am getting incorrect results (couple efficiency > 100%).

    • noumanzia
      Subscriber
      Related to the same simulation: I have enclosed a dipole source in a 3D monitor but after running the simulation I do not find the power results in the monitor output (image below)
      Monitor settings
      Simulation type: All, Monitor type: 3D, only output power is selected in "Data to record"


    • Taylor Robertson
      Ansys Employee
      Hey For a dipole source the power is normalized by a dipole in an infinite homogenous media. For dipoles in a more complicated environment, you can actually radiate more power.
      When you right click on the power monitor and visualize T, thetransmissionis normalized tosourcepowercommand by default. However, the sourcepower is not necessarily the power radiated by the dipole source when it is in an inhomogeneous media (in order words, thepurcel factoris not equal to 1). If this is the case, you will need to re-normalize the transmission to thedipolepowercommand, i.e. transmission(ÔÇ£monitorÔÇØ)*sourcepower(f)/dipolepower(f);
      Also maybe check that the simulation is running to completion: https://forum.ansys.com/discussion/24200/ansys-insight-transmission-results-greater-than-one.
      Regarding the power from a 3D monitor; I don't think this is well defined. The power is calculated by integrating the pointing vector flux through the plane. Over what plane would you integrate would you integrate the flux for a 3D monitor. If you want to know what the power leaving a box is, then I would suggest the Transmission box, or Directivity from the object library.

      For power absorbed in the box use a power absorbed monitor from the object library.
      Best
    • sagar
      Subscriber
      are you able to get coupling efficiency? I am also trying to do mostly the same thing like I have placed the dipole in the middle of the waveguide and chose the diameter in such a way that only fundamental mode will propagate through it and trying to get the transmission value of the fundamental HE11 mode through the power monitor placed on top of the waveguide and selected the fundamental mode by mode expansion monitor, but getting a significantly different valve of T_total and T_forward.
      could you please explain the reason why such differences are visible? Is it like dipole can excite all the modes in the waveguide and T_total corresponds to all the modes? or T_total is not related to mode at all, it is just calculating total power radiated by the power. Even if this case as I am looking at the same monitor placed at the top of the waveguide and whatever power is propagating is happening through the waveguide will be captured by the power monitor and it should be equal to the fundamental modes because other modes can not travel through it. Please explain this point. Thank you.
    • Taylor Robertson
      Ansys Employee

      The T_total will include any radiation through it like scattered light, and guided modes; so even though it is a single mode waveguide T_forward and T_total are not guaranteed to be the same. Also if you have a symmetric waveguide (like a cylinder or square) then there will be polarization degeneracy ie modes with the same index and orthogonal polarizations, so you should include both contributions.
      https://support.lumerical.com/hc/en-us/articles/360034902433-Using-and-understanding-Mode-Expansion-Monitors

    • sagar
      Subscriber

      Thank you for the reply.
      I have another doubt. In the mode selection option of "mode expansion monitor", there are four options, i.e Fundamental mode, Fundamental TE mode, Fundamental TM mode, and user select. In the user select option, I can see only two modes are forming as the diameter of the antenna is less leading to a lower "V-number". Are these two modes corresponding to degenerate fundamental modes as there is structural symmetry in my antenna (cylindrical)?
      I did the simulation by taking the contributions for all four options but getting different T_forward for each case. So to select the Fundamental mode should I go with the option of "Fundamental mode" or do I have to calculate the contribution of the first two degenerates modes forming in the "user select" option?

      And for dielectric waveguide there is no fundamental TE or TM, so far I have understood the fundamental mode will be a hybrid mode as I can see in the simulation also the possible modes it is forming in the "user select" option all have almost the same TE/TM fraction(%).
    • Taylor Robertson
      Ansys Employee

      These are essentially the same mode, they are just degenerate in terms of polarization. It is sufficient to use the two solutions with orthogonal polarizations. I would suggest user select option to get these as fundamental TE and TM is not necessarily well defined here. You can find the actual polarization state, for a given input, by considering T_forward as the coefficients of two orthogonal basis states. The presence of a rectangular mesh does break the symmetry slightly, so you will likely see modes that are predominantly X, or Y polarized. Also you will often see the polarization of the mode being rotated as it propagates if these degenerate modes are not perfectly orthogonal. Then power in one mode will couple to its complement and back. You can look at this by doing overlap analysis.

      Best

    • sagar
      Subscriber
      Thank you very much for your answer.
      Degeneracy will come because of the symmetry in the structure. So the first two modes are forming in the "user select" option are degenerate modes. But I calculated T_forward for both modes; mode_1 and mode_2 and got different results and transmission values are far away from T_total.
      What you mentioned in the answer "You can find the actual polarization state, for a given input, by considering T_forward as the coefficients of two orthogonal basis states." Could you please explain this point in detail? Are the contribution of these two modes will give the transmission correspond to the fundamental HE11 mode?
      In the simulation, I also calculated the T_forward value for Fundamental mode, Fundamental TE mode, Fundamental TM mode by mode expansion monitor.
      You can see here I am getting maximum T_forward corresponds to fundamental TE mode where the contribution of fundamental TM mode is nearly zero, and the red plot corresponds to fundamental modes; I don't understand how it is being calculated.
      My doubt is should I go with these options like fundamental, FTE, or FTM or should I go with the first two modes generated in the "user select" option.
      How do I calculate the contribution of the "fundamental HE11" mode? Is it possible to calculate the value?


    • Taylor Robertson
      Ansys Employee
      Hey The HE11 is mode is the name given to the fundamental mode of the optical fiber, which can be calculated analytically in cylindrical coordinates. The mode solver calculates the modes numerically, and so should find a mode equal to HE11 mode. Note the mode solver works in cartesian coordinates, and suffers from numerical discretization errors, so some discrepancies will be inevitable. You are in a fortunate position having an analytic solution for convergence testing; this is typically not the case since the vast majority of structures do not permit analytic solutions.
      https://support.lumerical.com/hc/en-us/articles/360045466233-Analyzing-Simulation-Results-Convergence-Testing
      In this sort of situation I would suggest using the user select option and compare the calculated modes, with the solution for HE11. Check the mode profiles and effective indices to make sure you are using the correct modes. It seems that you are sweeping over the core diameter? I would ensure that the modes are the same for each case. If there are errors perform convergence testing as above. I will note that the analytic solution for HE11 also permits two orthogonal polarizations.
      https://www.globalspec.com/reference/28741/203279/3-5-fundamental-he11-mode
      You may get want to place a few mode expansion monitors at increasing distance along the propagation axis. The MEM should allow you to find the power contained in a specified guided mode. There may be unguided scattered light through the monitor, so placing the monitors further away you should see the T_total values reduce, but the T_forward values should be consistent.
      If you are not really interested in the polarization of the mode at that plane perhaps you can average the power of the two polarizations of the HE11 mode. There may be some ambiguity for the mode solver about the mode labelling and ignoring the polarization may make it more intuitive. The T_total envelope is determined by the simulation set-up.

      Best
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