Settings for Plane and Gaussian Sources

This section describes settings for plane and Gaussian sources.

Select Plane or Gaussian in solver tab and create a plane source or Gaussian source in Composite viewer. Then, set further parameters in the Edit properties interface that automatically pops up.

Settings for Plane Sources

General Settings

Plane Source

The General > Plane sources tab can be used to set the incident axis, amplitude, and other parameters related to a source.

Name	Description
Direction	Incident direction of a plane source, specifically selected as Forward (forward propagation) or Backward (backward propagation).
Incident axis	Select desired incident axis for a plane source from drop-down list.

For settings related to amplitude and phase shift of plane sources, see general settings in Source.

Rotation

For rotation settings, see general settings in Source.

Multi-frequency

Multi-frequency field is an advanced setting that can be disabled for simulation projects where source bandwidth is narrow.

Name	Description
Multi-frequency field	Multi-frequency option allows users to solve propagation characteristics of a source at multiple frequencies in optical devices. It is recommended to use this feature for broadband time-domain simulations and simulations involving injection of sources into dispersive materials, especially at a specific angle.
Frequency points	Specify number of frequencies to be used in calculation of optical field distribution, and set this value to at least 5 in order to ensure accuracy.

Geometry

Geometry tab can be used to set geometric range of a source. See geometry settings in Source.

Polarization

Polarization tab can be used to set polarization of a source. See Polarization below.

Wavelength/Frequency

Wavelength/Frequency tab can be used to set wavelength/frequency of a source. See wavelength/frequency domain settings in Source.

More Information about Plane Sources

If plane source is required to maintain an ideal plane wave, boundary conditions must be set as Periodic or Bloch according to specific project requirements;
For projects involving plane sources with PML boundary conditions at the propagation direction, plane source diffracts at the edges;
In broadband simulations, actual incident angle (non-zero angle) varies with frequency, and thus accurate results require both frequency sweeps and angle sweeps.

Settings for Gaussian Beam

A Gaussian beam in fundamental mode is expressed as:

$E(\rho,z) = \frac{A_0w_0}{w(z)}exp(-\frac{\rho^2}{w^2(z)})exp\{-i\{{k[\frac{\rho^2}{2R(z)}+z]-\Psi(z)}\}\}$

where $\rho=\sqrt{x^2+y^2}$ , the propagation direction is along the z-axis. $|A_0|$ is the maximum amplitude of the electric field, $w(z)$ is the spot radius of the Gaussian beam, and $w_0$ is the beam radius at the waist, which is also the minimum spot radius. $R(z)$ is the radius of curvature of the wavefront, and $\Psi(z)=tan^{-1}(\cfrac{z}{z_0})$ , where $z_0$ is called the collimation distance, which is the distance at which the Gaussian beam radius expands to $\sqrt{2}w_0$ .

General Settings

General tab can be used to set parameters of a Gaussian source, such as phase, amplitude, and beam profile.

Source Injection, Rotation, and Phase Shift

Injection tab can be used to set incident axis and incident direction of a source.

Name	Description
Incident axis	Incident axis of a Gaussian source (normal to incident plane).
Direction	Propagation direction of a Gaussian source, specifically selected as Forward (forward propagation) or Backward (backward propagation).

Rotation tab is related to rotation settings. See general settings in Source.

General tab can be used to set incident amplitude and phase shift of a source. See general settings in Source.

Multi-Frequency

Multi-frequency field is an advanced setting that can be disabled for simulation projects where source bandwidth is narrow.

Name	Description
Multi-frequency field	Multi-frequency option allows users to solve propagation characteristics of a beam at multiple frequencies in optical devices. It is recommended to use this feature for broadband simulations and simulations involving injection of sources into dispersive materials, especially at a specific angle.
Frequency points	Specify number of frequencies to be used in calculation of optical field distribution, and set this value to at least 5 in order to ensure accuracy.

Visualization of Gaussian Beam

Beam profile tab can be used to plot field distribution of a Gaussian Source.

Name	Description
Current index	The (maximum) refractive index of incident surface; a read-only parameter.
Results	Allows users to specify desired type of data to be plotted by selecting either Mode fields or Index from the drop-down list.
Components	When selecting Mode fields under Results, available components are as follows: E Magnitude, Ex, Ey, Ez, H Magnitude, Hx, Hy, and Hz. If Index is selected under Results, available components are Index_x, Index_y, and Index_z.
Scalar	Abs: modulus of selected component; Re: real part of selected component; Im: imaginary part of selected component; Phase: argument of selected component.
Regenerate	Regenerate images.
Show/Hide 3D plot	Show/hide 3D plots.
Visualize data	Open Data visualizer window.
Plot in new window	Plot image in a new window.

Settings for Gaussian Beam

The software provides two types of Gaussian source, one for Gaussian beams using scalar approximation and the other for tightly focused Gaussian beams using thin lenses. The highly focused Gaussian beam is a fully vectorial beam. After the parameter settings are completed, users can view the current field distribution in the visualization of Gaussian beam. When the waist radius is several times larger than the wavelength used, the scalar approximation option should be selected. When the waist radius is the same as the wavelength, the thin lens option should be used.

gauss_source_options

Gaussian beam with scalar approximation

Gaussian options tab can be used to set parameters related to a Gaussian source.

Name	Description
Waist width	Waist width.
Distance from waist	Distance between incident plane and waist.
Offset from center	Transverse offset from the center of waist width.

Note:

A Gaussian beam in fundamental mode is a symmetric distribution field centered around optical axis. Equation $\rho = \sqrt{x^2 + y^2}$ (assuming propagation direction is $z$ ) represents coordinates in incident plane, and parameter Offset from center refers to offset between center of incident optical field and center of Gaussian beam.
A Gaussian beam in fundamental mode is a model that satisfies paraxial approximation conditions and is not suitable for accurately describing non-paraxial situations (a paraxial approximation must typically satisfy $\lambda << w_0$ , which in simulation requires at least $\lambda \approx w_0$ ).

Tightly focused Gaussian beam

The following is a tightly focused Gaussian beam by using a thin lens. The Thin lens tab allows users to set lens related parameters.

Name	Description
NA	The numerical aperture of a thin lens is $n*sinA$ , where $n$ is the refractive index at the position of the source and A is the half angle. The following figure shows the field distribution of Gaussian source at different numerical apertures.
Distance from focus (um)	The distance between the injection plane and the focal point of the lens, where a positive number represents a diverging beam and a negative number represents a converging beam.
Number of plane waves	The number of plane waves used to construct the beam. Increasing the number of plane waves will result in a more accurate beam profile but will also increase the computation time.

gauss_source_na_compare

Geometry

Geometry tab can be used to set geometric range of a source. See geometry settings in Source.

Polarization

Polarization tab can be used to set polarization of a source. See Polarization below.

Wavelength/Frequency

Wavelength/Frequency tab can be used to set the wavelength/frequency of a source. See wavelength/frequency settings in Source.

Polarization

Local and Global Polarization

Polarization is a parameter typically associated with plane sources. FDTD solver allows users to define complex polarization states for a Gaussian source.

The following polarization types are available for Plane and Gaussian sources:

Global polarization: Refers to a situation where polarization characteristics are consistent at each point on incident plane. It can be represented by a single Jone vector. Typical options provided in software include: Linear polarization, Left/Right-handed circularly polarized, Left/Right-handed elliptically polarized, etc.;
Local polarization: Refers to a situation where polarization characteristics are different at each point on incident plane. Typical polarization characteristics provided in software include: Radial polarization, Angular polarization, Vortex polarization, and Arbitrary polarization;
User‘s polarization: To be defined by user.

Polarization Settings

Name	Description
Linearly polarization(θ)	Define polarization angle for linear polarization.
Elliptically orientation angle	Define elliptically orientation angle for left/right-handed elliptical polarization.
Elliptically U	Define length U of first elliptical axis for left/right-handed elliptical polarization.
Elliptically V	Define length V of second elliptical axis for left/right-handed elliptical polarization.
Order 1	Define first radial order for radial/angular/vortex polarization.
Order 2	Define second radial order for radial/angular/vortex polarization.

Case: Define Circular Polarization

Define plane wave for left-handed circular polarization:

Vector is used to plot electric field vector of light in the transmission direction. As shown in figure below, electric field vector exhibits an obvious circular trajectory in transmission direction.