# import source in var-fdtd

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.

## Answers

27Ansys EmployeeAn 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

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.