This tutorial illustrates how to convert information from OpticStudio into Lumerical’ s finite-difference eigenmode (FDE) solver. This can be useful for multi-stage cases in which one part of the system is a bulk optical system and a waveguide in others. In this example, we will look at the coupling from a focusing lens into a small silica fiber. We will export our polarized beam as a Zemax Beam File (.zbf) to Lumerical Eigen Mode Solver and calculate the overlap and Power coupling between modes created in Lumerical Eigen Mode Solver and exported Zemax beam. Overlap analysis between the beams will suggest a better mode from Lumerical modal analysis. This Mode will be exported to OpticStudio back from Lumerical as Zemax Beam File.    

Authored by Sahil Rajan

Introduction

This tutorial illustrates how to convert information from OpticStudio into Lumerical Eigenmode solver software. This can be useful for multi-stage cases in which one part of the system is efficiently simulated in OpticStudio, and other parts (like waveguides, photonic crystals, etc.) require electromagnetic propagation tools. Lumerical’s finite-difference eigenmode (FDE) solver can be used to determine the physical properties of the optical modes supported by an arbitrary waveguide geometry.

In this example, we will look at the coupling from a focusing lens into a small silica fiber via the Lumerical Eigenmode solver software. This tutorial assumes some familiarity with the Lumerical software.

Getting the data from OpticStudio

In this section, we begin by opening up OpticStudio. You can select any of the analysis features, including the 2D layout to see how the rays focus on a point. The Image plane act as the receiver input end of Fiber has material with a refractive index of 1.43 and  AR coating COAT I.99 which reflects 1% and transmits 99%.

 

 

Select Analysis...Physical Optics to see a 6-micron input waist Gaussian being focused on the image plane of the lens system:

 

 

The Beam waist calculated at image plane is 5.8787 micron with a Rayleigh range of 0.1mm. Fiber coupling calculated with input waist of 6 micron is 95 percent at the receiver end

 

From here we can tell OpticStudio to output the beam files. Later we will need to import in Lumerical. Press the Settings option on the top of the Physical Optics Propagation window and then select the Display tab and click on the "Save Output Beam To:" option. Next, set the name of the file to be Fiber_output.zbf. Press OK. Please select the Use Polarization check box to define the vectorial beam. Without the Polarization, beam is scalar then needs to use script command to load the zbf file in Lumerical .

 

 

These files are typically saved in the {Zemax}\POP\BEAMFILES directory so you can find the file there.

 

 

Create the fiber structure for mode calculation

In this section, we will create the step-index fiber Open  Lumerical launcher and select a finite-difference eigenmode (FDE) solver. You can also download the step_index_fiber.lms file in the download section of this KBA.

 

 

The physical structure of step index fiber modelled are created using the STRUCTURES tab in the Layout Editor. Press the arrow on the STRUCTURES button and select CIRCLE from the pull-down menu.

 

 

Now edit the object properties.

 

 

Set the properties of the circle clad and core according to the following table.

Core

From the Structures section of the Design tab, select a CIRCLE to be added to the Objects Tree. Select the circle in the Objects Tree and click on the "Edit Properties" button    to edit the properties of the circle according to the following table. 

 

tab	property	value
	name	Core
	x (μm)	0
	y (μm)	0
	z (μm) / z span (μm)	1
	radius (μm)	9
Material		Object defined dielectric
Index		1.44
Mesh		2

 

Cladding

From the Structures section of the Design tab, select another CIRCLE to be added to the Objects Tree. Select the circle in the Objects Tree and click on the "Edit Properties" button   to edit the properties of the circle according to the following table. Note the mesh order is set to 5 (a higher number than the default 2) so that the "cladding" structure will not fill the volume where it overlaps with the "core" structure. The z span of the "cladding" is set to be slightly smaller than that of the "core" so that the latter is not hidden by the former in the viewport; alternatively, you can use an alpha value less than 1 in the Graphical rendering tab of the "cladding" properties to make the object semi-transparent.

 

tab	property	value
	name	cladding
	x (μm)	0
	y (μm)	0
	z (um) / z span (um)	0 / 1
	radius (μm)	26.389
Material		Object defined dielectric
Index		1.4
Override Mesh		5

 

Simulation Region

Click on in the Objects Tree and click on the    button (on the left of the Objects Tree) to edit its properties according to the following table. Note that the simulation region size is such that the cladding cylinder is completely inside, and the simulation region boundaries are all set to open. The purpose is to assign the simulation boundary conditions to 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

 

 

Index profile of Step Index fiber

We can now view the index profile as shown below. To view the index profile, click on mesh structure.

 

 

Import Zemax Beam File

This section describes the Finite Difference Eigenmode Solver (FDE) analysis component of MODE, once the physical structure and the simulation region have been defined. Now that we have created the structure, we can proceed with setting up the launch field. We have already exported Zemax Beam File as single mode coupler.ZBF. Now we will import the single mode coupler.ZBF file in the Deck of Eigen solver Analysis Window.

Load the ZBF file from Zemax LLC\Documents\Zemax\Samples\POP\BEAMFILES.

 

 

Calculate Modes

For viewing the field in Lumerical, you need to calculate the modes from Zemax Beam File. We need to set the wavelength same as OpticStudio 1.55 micron.  Click Calculate Modes then we will have a Modal area. As we can see in the image below.

 

 

Power Coupling and Overlap analysis with a Gaussian beam

The overlap, defined as the fraction of electro-magnetic fields that overlap between 2 field profiles,
is calculated using this formula, where E1, H1 are the fields from Mode 1, and E2, H2,
the fields from Mode 2. The power coupling represents the fraction of power that couples from one mode to another. If we consider the input mode (of power Pin) and the ith mode (of power Pi), the power
coupling is given by this formula. Power coupling gives the total input coupling, considering both the mode overlap and the mismatch in effective indices between modes.

input      : E_input and H_input

ith Mode: E_i and H_i   

We will now calculate the overlap with an ideal Gaussian beam to determine the best spot-size we should use to couple with the fundamental modes in Zemax Beam File produced from OpticStudio. Choosing the "Overlap" tab results in the following screenshot. After clicking Calculate button calculates the overlap and the power coupling of the currently selected mode with the currently selected D-CARD. 

 

 

As we can see previously that Mode 2 has maximum power coupling and overlapping with Zemax Beam File. In the Beam tab, we want to check the Gaussian parameters so that we can match the effective areas. The effective area of a Gaussian beam is πw₀²  exported from Zemax Beam File is w₀ = sqrt(122.557/π) um = 6.3 um and Mode 2 have w₀ = sqrt (122.557/π) um = 6.7 um. To match the modal area of 144.713 um², we have a waist radius, w₀ = sqrt (1.2/π) um = 6.7 um. We will export Mode 2 as Zemax Beam File to OpticStudio and check the Fiber coupling efficiency.

 

 

Export Zemax Beam File to OpticStudio

Export the Mode as d-card in Deck. The d-card with better efficiency from Lumerical can be saved as Zemax Beam File. Now we import this beam to OpticStudio to check the coupling efficiency. 

 

 

Import new Zemax Beam File in OpticStudio

 Please import the new Zemax Beam File created in Lumerical deck back to OpticStudio Beam File Viewer. As we can see the waist Radius of 6.544 um suggested from Lumerical Mode 2 when imported to Zemax Beam File Viewer. After changing the waist size in POP Beam Definition and Fiber Data increased the fiber coupling effiecincy to 96.02 percent. As previously the Beam waist was 6 um with fiber coupling efficiency 95.47 percent.