Wave Optics Module Updates

For users of the Wave Optics Module, COMSOL Multiphysics® version 5.3a brings automatic physics-controlled meshing, a Helmholtz-compliant implementation for Gaussian background fields, new postprocessing variables, and more. Browse all of the Wave Optics Module updates below.

Physics-Controlled Meshing

Both of the interfaces Electromagnetic Waves, Frequency Domain and Electromagnetic Waves, Beam Envelopes now have physics-controlled meshing enabled by default. The physics-controlled meshing algorithm has been updated to handle general frequency-dependent material properties as well as the Drude-Lorentz, Debye, and Sellmeier dispersion models. The physics-controlled meshing functionality can also be used for mode analyses.

For the Electromagnetic Waves, Beam Envelopes interface, the default setting is to create a swept mesh in 3D and a mapped mesh in 2D, with the additional option of creating a tetrahedral or triangular mesh, respectively.

A screenshot of the physics-controlled mesh settings for Electromagnetic Waves, Beam Envelopes.

The new Physics-Controlled Mesh section of the Electromagnetic Waves, Beam Envelopes interface settings, as exemplified for the Directional Coupler tutorial model.

The new Physics-Controlled Mesh section of the Electromagnetic Waves, Beam Envelopes interface settings, as exemplified for the Directional Coupler tutorial model.


Application Library paths for examples using the new physics-controlled mesh:
Wave_Optics_Module/Gratings_and_Metamaterials/hexagonal_grating
Wave_Optics_Module/Gratings_and_Metamaterials/negative_refractive_index
Wave_Optics_Module/Gratings_and_Metamaterials/plasmonic_wire_grating
Wave_Optics_Module/Optical_Scattering/beam_splitter
Wave_Optics_Module/Optical_Scattering/brewster_interface
Wave_Optics_Module/Optical_Scattering/nanorods
Wave_Optics_Module/Optical_Scattering/scattering_nanosphere
Wave_Optics_Module/Verification_Example/dielectric_slab_waveguide
Wave_Optics_Module/Verification_Example/fresnel_equations
Wave_Optics_Module/Verification_Example/symmetric_laser_cavity
Wave_Optics_Module/Waveguides_and_Couplers/directional_coupler
Wave_Optics_Module/Waveguides_and_Couplers/optical_ring_resonator
Wave_Optics_Module/Waveguides_and_Couplers/photonic_crystal
Wave_Optics_Module/Waveguides_and_Couplers/step_index_fiber_bend

Default Wavelength for Study Steps

When you add a frequency- or wavelength-dependent study step, for instance the Wavelength Domain or the Frequency Domain study step, together with either of the Electromagnetic Waves, Frequency Domain or Electromagnetic Waves, Beam Envelopes physics interfaces, a default wavelength of 1 μm will be used.

Helmholtz-Compliant Gaussian Beam Background Field

A new Gaussian beam background field implementation is available, where the beam focal plane is approximated using a summation of plane waves that are propagating with wave vectors pointing in a distribution around the main propagation direction. The advantage of this implementation as compared to the paraxial approximation implementation is that the plane-wave expansion implementation is a true solution to the Helmholtz equation, as each plane wave is a solution to the Helmholtz equation. As the name suggests, the paraxial approximation is only an approximate solution to the Helmholtz equation that should not be used to represent tightly focused Gaussian beams.


Application Library path for an example using the Helmholtz-compliant Gaussian beam background field:
Wave_Optics_Module/Optical_Scattering/nanorods

New Postprocessing and Visualization Tools for Reflectance and Transmittance

New postprocessing variables have been introduced to simplify the evaluation of reflectance, transmittance, and absorptance expressions. For instance, instead of writing abs(ewfd.S11)^2 for the reflectance at port 1, you can now simply write ewfd.Rport_1. The variables named ewfd.Rport_<x> and ewfd.Tport_<x> are based on the port name <x>. Similarly, when periodic ports and diffraction order ports are used, there are new variables based on the mode number and the mode polarization. For instance, the variable ewfd.Rorder_p1_op represents the reflectance, for a 2D mode, with order 1 and out-of-plane polarization. In addition, there are summation variables for the total reflectance, total transmittance, total reflectance and transmittance, and the absorptance.

For frequency or wavelength sweeps, parametric sweeps, and auxiliary sweeps, new default plots of the reflectances and transmittances are generated. When the study does not perform any sweeps, instead of a default plot, a global evaluation is evaluated in a table for the reflectance and transmittance variables.


A plot from the Hexagonal Grating tutorial.

A default plot of the diffraction efficiencies for the modes in the Hexagonal Grating tutorial model.

A default plot of the diffraction efficiencies for the modes in the Hexagonal Grating tutorial model.
A screenshot from the Dielectric Slab Waveguide tutorial model.

The default Global Evaluation node, the settings, and the associated table (bottom right) from the Dielectric Slab Waveguide tutorial model.

The default Global Evaluation node, the settings, and the associated table (bottom right) from the Dielectric Slab Waveguide tutorial model.

New Effective Index Variable for Boundary Mode Analysis

When a Boundary mode analysis is performed, new variables for the effective indices of the modes are created. The name for those variables follows the pattern <phys>.neff_<x>, where <phys> is the physics interface tag and <x> is the port name. As an example, Port 1 in the Electromagnetic Waves, Frequency Domain interface would have a variable name emw.neff_1.

New Tutorial Model: Slot Waveguide

This model analyzes the mode propagation within a nano-slot waveguide. In a slot waveguide configuration, two high-refractive-index slabs are placed adjacent to the low-refractive-index slot. Contrary to the behavior for regular dielectric waveguides, the mode in the slot waveguide is confined to the low-refractive-index slot material. This is shown by running a mode analysis on a 2D cross section of the slot waveguide. Further analysis is carried out to optimize the width of the slot to deliver maximum optical power and optical intensity through the slot area.

A plot from the Slot Waveguide tutorial model, new with COMSOL Multiphysics version 5.3a.

This surface plot shows the confinement of the light to the central low-index slot area. The plot variable is the x-component of the electric field.

This surface plot shows the confinement of the light to the central low-index slot area. The plot variable is the x-component of the electric field.

Application Library path:
Wave_Optics_Module/Waveguides_and_Couplers/slot_waveguide

Updated Tutorial Model: Fresnel Lens

This model has been updated to include a comparison between the Electromagnetic Waves, Frequency Domain and Electromagnetic Waves, Beam Envelopes interfaces. The analytical results match closely, while the computation time is much faster for the Electromagnetic Waves, Beam Envelopes interface. Furthermore, the step-by-step instructions now demonstrate how to add a model method. The model method can be used for automatically rebuilding the complex geometry if a geometry parameter has been changed. Finally, the model now uses a perfectly matched layer to more efficiently absorb the scattered light from the diffractive structure, improving the comparison to the analytical result.

A plot from the Fresnel Lens tutorial model, updated for COMSOL Multiphysics version 5.3a.

An updated comparison plot that includes the results from a simulation with the Electromagnetic Waves, Beam Envelopes interface.

An updated comparison plot that includes the results from a simulation with the Electromagnetic Waves, Beam Envelopes interface.

Application Library path:
Wave_Optics_Module/Verification_examples/fresnel_lens

Adaptive Frequency Sweep

The new Adaptive Frequency Sweep study type can be used to run models faster and with a fine frequency resolution by using a reduced-order model in the frequency domain. For example, you could compute the response of a linear or linearized model subjected to harmonic excitation for several frequencies. The asymptotic waveform evaluation (AWE) model reduction is performed by a moment matching technique where Padé approximation or a Taylor series expansion is used for the transfer function in a specified frequency interval. The AWE expressions are automatically chosen based on the port settings, but can optionally be specified by user-defined expressions. A user-defined expression can be entered for the error estimation as calculated by the AWE algorithm. When the expression used for the AWE method represents a sufficiently slowly varying quantity vs. frequency, then the simulation can be run using a very fine frequency resolution without much impact on performance. The AWE method has been available in previous versions, but not in an easy-to-access dedicated study type.

An illustration of the Adaptive Frequency Sweep study type, new with COMSOL Multiphysics 5.3a.

For the waveguide iris filter model, an S-parameter comparison between adaptive frequency sweep and regular sweep is shown. The simulation can be run with a 10 times finer frequency resolution in a similar amount of time as with the discrete sweep simulation.

For the waveguide iris filter model, an S-parameter comparison between adaptive frequency sweep and regular sweep is shown. The simulation can be run with a 10 times finer frequency resolution in a similar amount of time as with the discrete sweep simulation.

Slit Port Visualization: More Intuitive Arrow Direction

Interior ports with an active slit condition now show the direction of the power flow with an arrow symbol. You can easily switch the direction of the power flow by clicking the Toggle Power Flow Direction button.

A demonstration of the Toggle Power Flow Direction button in COMSOL Multiphysics 5.3a.

By clicking the Toggle Power Flow Direction button, you can change the direction of the power flow on an interior slit port.

By clicking the Toggle Power Flow Direction button, you can change the direction of the power flow on an interior slit port.

Data Refinement Using a Combine Solutions Study Step

The Combine Solutions study step can be used to filter out and remove unwanted solutions. This functionality can be used, for example, to filter out the first and last % of the frequency spectrum for a Time to Frequency FFT study step. Parts of the solution can be excluded based on a user-defined expression.