The mode source can be used to inject the mode of a waveguide or fiber.
The mode source includes an integrated mode solver with uses the finite-difference eigenmode
solving algorithm to calculate the field profile of the supported modes of a given structure
If you are interested in learning more about the mode solving algorithm, see the reference
paper listed below.
The mode solver assumes that the cross section of the structure constant along the propagation
direction, so it cannot be used to solve for Bloch modes which are the 3D modes of a waveguide
that has periodicity along the propagation direction, such as a photonic crystal waveguide
like the one shown here.
To set up the mode source, the source should be positioned over the cross section of the
waveguide or fiber.
The simulation mesh of the FDTD solver region is used as the grid over which the supported
modal fields are calculated, so to increase the spatial resolution of the calculated mode
profile, the simulation mesh can be made finer.
The mode source settings allow you to specify whether you want to find the mode for a straight
or bent waveguide with a given bending radius, and also specify rotation angles to match
the angle of the waveguide if the waveguide is not aligned with the x, y, or z-axis.
You can choose to inject the fundamental mode which is the mode with highest effective index,
or fundamental TE or fundamental TM mode which correspond to the mode with the highest effective
index which has the desired TE-like or TM-like polarization.
In FDTD Solutions, TE-like modes correspond to modes where the electric field polarization
is primarily along the y-direction for sources injected along the x-axis, and primarily along
the x-direction for sources injected in the y or z-axis directions.
You can also choose the user select option which allows you to click on the Select Mode
button to open up a new window where you can specify advanced mode solving settings such
as a number of trial modes, and the effective index to search near, as well as custom boundary
conditions for the mode solver region.
After calculating the modes using the specified mode solver settings, modes will appear in
the model list at the top of the window, and you can select a mode from the mode list to
plot the field profile of the mode.
You can also see details about the calculated mode in the mode list table such as the effective
index of the mode, loss, and polarization fraction.
The formulae for the polarization fraction calculation and waveguide TE/TM fraction are
listed on this slide, but additional information about the calculations can also be found on
the Knowledge Base page linked below.
Some applications where the mode source can be used is in fiber optics simulations, and
simulations of integrated optics components such as waveguide couplers, ring resonators,
The main application where you would want to inject the supported mode of a waveguide
is for extracting the S-parameters of an integrated optics component.
Instead of using the mode source, port objects can be used.
Port objects are used especially for S-parameter extraction, and ports act as both mode sources
as well as monitors to calculate and return S-parameters results, simplifying the process
of S-parameter extraction.
Port objects will be introduced in detail in the Monitors section of the course.
In the next unit, we'll demonstrate the setup of a mode source for an asymmetric directional