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Design and Analysis Guides
A practical guide to working across Lumerical and Zemax OpticStudio: ZBF file exchange, FDE mode analysis, overlap analysis and improving fibre coupling efficiency, with the settings that matter at each step.
This article explains how to transfer information from Zemax into the Lumerical MODE finite-difference eigenmode (FDE) solver. It is useful for multi-stage systems where part of the system is bulk optics and another part is a waveguide. In this example we look at coupling from a focusing lens into a small silica fibre. First, the polarised beam is exported to the Lumerical eigenmode solver as a Zemax Beam File (.zbf). We then calculate the overlap and power coupling between the modes created in the Lumerical eigenmode solver and the exported Zemax beam. The overlap analysis between beams in Lumerical suggests a better mode, which is then exported back as a Zemax Beam File from Lumerical to OpticStudio .
This article explains how to transfer information from Zemax into the Lumerical Eigenmode solver software. It is useful when part of a system can be simulated efficiently in OpticStudio but another part, such as a waveguide or a photonic crystal, needs an electromagnetic propagation tool. The Lumerical finite-difference eigenmode (FDE) solver can be used to determine the physical properties of the optical modes supported by an arbitrary waveguide geometry.
This example uses the Lumerical eigenmode solver software to look at coupling from a focusing lens into a small silica fibre. The tutorial assumes some familiarity with the Lumerical software.
Start OpticStudio. Choosing an analysis feature such as the 2D layout lets you see how the rays converge to a point. The image plane acts as the input facet on the receiving side of the fibre, which uses a material of refractive index 1.43 and the AR coating COAT I.99, which reflects 1% and transmits 99%.
Choose Analyze -> Physical Optics to confirm that a Gaussian beam with a 6 micron waist focuses at the image plane of the lens system.
The beam waist calculated at the image plane is 5.8787 um and the Rayleigh range is 0.1 mm. Fibre coupling calculated with a 6 um input waist is 95% on the receiving side.
From here you can have OpticStudio write out a beam file, which you will need to import into Lumerical later. Click the Settings option at the top of the Physical Optics Propagation window, select the Display tab and click the option to save the output beam. Then set the file name to Fiber_output.zbf and press OK. Select the “use polarization” checkbox to define a vector beam. Without polarisation the beam is scalar, and reading the zbf file in Lumerical then requires a script command.
These files are normally saved in the {Zemax}\POP\BEAMFILES directory, so you can find the file there.
In this section we build a step-index fibre. Open the Lumerical launcher and select the finite-difference eigenmode (FDE) solver. The step_index_fiber.lms file can also be downloaded from the download section of this Ansys Lumerical article.
The physical structure of the step-index fibre is built using the Structures tab in the layout editor. Click the arrow on the Structures button and select Circle from the pull-down menu.
Next, edit the object properties.
Set the properties of the circular cladding and core according to the table below.
From the Structures section of the Design tab, select the Circle to add to the object tree. Select the circle in the object tree, click the Edit Properties button, and edit the properties of the circle according to the table below.
| ab | Property | Value |
|---|---|---|
| name | Core | |
| x (μm) | 0 | |
| y (μm) | 0 | |
| z (μm) / z span (μm) | 1 | |
| radius (μm) | 9 | |
| Material | Object defined dielectric | |
| Refractive index | 1.44 | |
| Mesh order | 2 |
From the Structures section of the Design tab, select another Circle to add to the object tree. Select the circle in the object tree, click the Edit Properties button, and edit the properties of the circle according to the table below.
Note: the mesh order is set to 5 (higher than the default of 2) so that in meshing, the “clad” structure does not fill the region where it overlaps the “core” structure. The z extent of the “clad” structure is also made slightly smaller than that of the “core” structure, so that the “core” structure is not hidden by the “clad” structure in the viewport. Alternatively, you can make the “clad” structure semi-transparent by setting an alpha value below 1 on the graphical rendering tab of the “clad” properties.
| ab | Property | Value |
|---|---|---|
| name | cladding | |
| x (μm) | 0 | |
| y (μm) | 0 | |
| z (μm) / z span (μm) | 0 / 1 | |
| radius (μm) | 26.389 | |
| Material | Object defined dielectric | |
| Refractive index | 1.4 | |
| Override mesh order | ☑ | |
| Mesh order | 5 |
Click the FDE object, click the Edit Properties button to the left of the object tree, and edit the properties according to the table below. Size the simulation region so that the cladding cylinder fits entirely inside it, and set all simulation region boundaries to open. The aim is to assign the simulation boundary conditions at the outer surface of the cladding circle (see boundary conditions).
| Tab | Property | Value |
|---|---|---|
| Geometry | X | 0 |
| X span | 35 | |
| Y | 0 | |
| Y span | 35 | |
| Z | 0 |
You can display the refractive index profile as in the figure below. To display it, click the mesh structure.
With the physical structure and simulation region defined, we turn to the finite-difference eigenmode solver (FDE) analysis component in MODE. Now that the structure is built, we move on to setting the field to excite. The Zemax Beam File has already been exported as single mode coupler.ZBF. Here we import that single mode coupler.ZBF file into the deck in the eigensolver analysis window.
Load the ZBF file from Zemax LLC\Documents\Zemax\Samples\POP\BEAMFILES.
To display the field in Lumerical you must calculate the modes from the Zemax beam file. The wavelength must be set to 1.55 um, the same as in OpticStudio. Click Calculate Modes and the mode area is displayed. See the figure below.
Overlap is defined as the fraction of the field that overlaps between two field profiles, and is calculated with the expression below, where E1 and H1 are the fields of mode 1 and E2 and H2 are the fields of mode 2.
Power coupling expresses the fraction of power that couples from one mode into another. For an input mode (power Pin) and the i-th mode (power Pi), power coupling is given by the expression below. Power coupling gives the total input coupling, accounting for both the mode overlap and the effective index mismatch between the modes.
Input: E_input and H_input
ith Mode: E_i and H_i
To determine the spot size that couples best to the fundamental mode generated from the Zemax Beam File, we calculate the overlap with an ideal Gaussian beam. Selecting the Overlap tab shows the screenshot below. Clicking the Calculate button computes the overlap and power coupling between the currently selected mode and the currently selected D-CARD.
As seen earlier, mode 2 has the greatest power coupling and overlap with the Zemax beam file. On the Beam tab you need to check the Gaussian parameters so that the effective areas match. The effective area of the Gaussian beam exported from the Zemax Beam File is pi*w0^2, with w0 = sqrt(122.557/pi) um = 6.3 um. Mode 2 gives w0 = sqrt(122.557/pi) um = 6.7 um. Matching the mode area of 144.713 um2 requires a waist radius of w0 = sqrt(1.2/pi) um = 6.7 um. Export mode 2 to OpticStudio as a Zemax Beam File and check the fibre coupling efficiency.
Export the mode as a D-CARD in the deck. A high-efficiency D-CARD can be saved as a Zemax Beam File. Import this beam into OpticStudio and check the coupling efficiency.
Import the new Zemax Beam File created in the Lumerical deck into the OpticStudio Beam File Viewer. You can see that the waist radius of 6.544 um suggested by Lumerical mode 2 has been imported into the Zemax Beam File Viewer. Changing the waist size in the POP beam definition and the fibre data improved the fibre coupling efficiency to 96.02%. Previously the beam waist was 6 um and the fibre coupling efficiency was 95.47%.
This article has explained how to design an optical system by linking Lumerical and OpticStudio. The approach is useful for multi-stage systems where part of the system is bulk optics and another part is a waveguide.
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