Photonics

Photonics

how to excite a mim cylinderical waveguide in lumerical fdtd?

    • zahra mansouri
      Subscriber

      i am simulationg a mim cylindercical waveguide in lumerical fdtd. and i used a mode expansion monitor in middle of the propagation lenght of my waveguide. i get a t backward much bigger  than t forward. for example t backward is 0.2 and  t forward is -0.09. what is the reasons of this results? i think its from my source. i use a mode source and the geometry of this source is a rectangle tangent to the waveguide. i think the metal layer caused a huge reflection and become a weak coupling to structure. how can i optimised the excitation this waveguide ?

    • Guilin Sun
      Ansys Employee

      You place a monitor inside the cavity, what is the purpose? you cannot measure it in experiment. What is the propagation direction, forward along positive axis or backward?

      You should use a proper mode source for the exitation, set longger simulaiton time and smaller autoshutoff min to get accurate result. The simulation should be terminated by fulfilling the autoshutoff min, not the set simulation time.

    • zahra mansouri
      Subscriber

      i placed a mode expansion monitor in the cross section of waveguide along the propagation direction for finding the T forward and T backward. the propagation direction is along the +z direction. my purpose is to find the how power tansmitted along waveguide lenght. i send my designed structure picture in fdtd. the biggest monitor is mode expansion monitor and the others are power monitors. the soucre is at the bottom of the waveguide. i think that the proper coupling doesnt occure . how can i know that my excitation is true?is it normal that t backward be larger than t forward?

    • Guilin Sun
      Ansys Employee

      This is a section of waveguide in air, which is a cavity. What source you are using? how many sources?  In your experiment how do you exite it? how do you mean the transmission?  When a source is inside a cavity, the transmission or refleciton will not be accurate as the reflection will modify the source power.

       

       

    • zahra mansouri
      Subscriber

      i used a mode source for exciting this waveguide and i set it in the bottom of the waveguide in +z direction that is along the propagation direction. i used only one source. i wanted to analyze the propagation and power of a special mode along this waveguide. i want use a 4micron lenght of this waveguide for a device so i wanted to full analyze this in fdtd solution. for simulation in fdtd i set the waveguide lenght to 9 micron and put some cros sectional and longitudinal power monitor and two expansion monitor along the waveguide. i didnt investigate how to excite it in experiment yet. now i am only studing some features of this waveguide. i meant the ration of p2/p1 for transmittion. with using the data of the mode expansion monitor i get t forward much smaller than t backward. what u mean from cavity? is your meaning that i should continue the waveguide out of the fdtd region and i put the source in the out of the simulation boundary?

    • Guilin Sun
      Ansys Employee

      any material interface forms reflection, and two such interfaces will form a cavity. I am not sure exactly what do you want to measure, but p2/p1 will change with distance, because inside the cavity it has standing wave. You need to develop a math formula for the fields or power transmission inside the cavity at different location.

      If you want to know the mode property of such waveguide, a better way is to use FDE in MODE. You can also use mode source with "custom" selection to calcualte mode properties.

      Or, you use long waveguide, and use the PML to truncate the waveguide to avoid refleciton. But I do not know the meaning doing this, as for straight waveguide, the mode property is sufficient.

       

       

    • zahra mansouri
      Subscriber
      I also used FDE in mode solution and i get the mode profile. Now i should get the how power propagated in this waveguide. I need the power in the end of the 4 micron waveguide. I used a long waveguide and i continue the end of waveguide outside the fdtd region with pml boundary for avoiding reflection but I think from the excitation point(beginning of the waveguide that I used mode source) i have reflection and proper coupling didn't occur.
    • Guilin Sun
      Ansys Employee

      If I am correct, you want to know the power transmission at 4um away from the mode source for an infinitely long waveguide. If so, it is absolutely no need to simulate it with FDTD. Instead, you can get mode property from FDE and get the loss constant. https://optics.ansys.com/hc/en-us/articles/360034917493

      I am not sure what kind of coupling you are refering to.  For such waveguide, there is no any change of geometry or surrounding index, is there any other mechanism for the coupling? Maybe you did not give all the details? Please note, when you calculate the mode source, it uses all the materials composing this waveguide. Without geometry or index change, for such a single waveguide, I do not see any physics that creates some kind of coupling. Please think it over and elabote more.

      You may refer to the mode analysis here:

      https://optics.ansys.com/hc/en-us/articles/360034917233-MODE-Finite-Difference-Eigenmode-FDE-solver-introduction

      https://courses.ansys.com/index.php/courses/lumerical-fde-solver-physics/

      https://optics.ansys.com/hc/en-us/sections/360007871773

       

       

    • Guilin Sun
      Ansys Employee

      Please note that for infinitely long waveguide, both ends much be outside of PML.

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