# import source in var-fdtd

Member Posts: 1

I would like to get an E-field profile using 3D fdtd and then use var-fdtd to further propagate the light.

For example, light is coupled to a slab via a grating coupler (simulation using 3D fdtd) and then propagate it in the slab using var-fdtd.

Is it possible to do it in Lumerical? I do not see an option for importing a source in var-fdtd.

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• Posts: 27Ansys Employee

An import source is not available in var-FDTD. If you think it can be very useful and has wide use cases, you might want to submit a feature request through the Idea Exchange (IX). As an alternative, you can consider the following workflow:

i) In the 3D FDTD simulation, calculate how much light is coupled into each of the modes the waveguide supports. If you are considering modes with y-polarization, you only need to calculate the coupling coefficients for these modes only. Assuming propagation in the x-direction, the field can be expressed as a superposition of the individual y-polarized modes:

Ey(y,z) = a1*Ey,1(y,z) + a2*Ey,2(y,z)+ ....

ii) In varFDTD, the 2D field profile ,Ey(y,z), at a waveguide cross-section can be constructed by multiplying the slab mode, E(z), and the field profile, E(y). (See the "Analyze Results" slide of the varFDTD - Solver Physics - Algorithm course)

Ey(y,z) = Ey(y)*M(z) = [a1*Ey,1(y) + a2*Ey,2(y)+ ....]*M(z)

where

• M(z): slab mode in the varFDTD
• Ey,1(y), Ey,2(y), ....: eigenmodes of the waveguide in the varFDTD (based on the effective index profile)

So, you can use the coupling coefficients from the FDTD to obtain the results, Ey(y), in the varFDTD:

Ey(y) = a1*Ey,1(y) + a2*Ey,2(y)+ ....

It should be noted that any fields that are not coupled into the waveguide modes with specific polarization are not accounted for in the varFDTD simulations.