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Designing an Arbitrary Array Beam Splitter in VirtualLab Fusion

🎬 Overview

This tutorial walks you through the complete design workflow for a beam splitter that generates spots at the apexes of a regular hexagon plus one at the center. We employ the Arbitrary Array Beam Splitter Session Editor in VirtualLab Fusion to achieve this. The Session Editor guides you through defining a target intensity pattern, configuring the optical system, setting merit functions and then provides configured documents for IFTA (Iterative Fourier Transform Algorithm) design and analysis as well as an Optical Setup for simulation.

The workflow consists of four main steps:

  1. Prepare the target pattern – prepare the beam splitter orders and the required corresponding weights.
  2. Configure the Session Editor – set up the source, optical system configuration, and Diffractive Optical Element parameters.
  3. Run the IFTA design and multiple runs – execute a single IFTA design, then generate and filter multiple design candidates.
  4. Simulate the optical system – adjust engine settings and evaluate the final irradiance pattern.

🚀 Step‑by‑Step Tutorial

Step 1: Prepare the Target Pattern

We require an intensity distribution in the shape of a regular hexagon (including the center spot). In the Session Editor, the amplitude weights for each diffraction order are specified using a bitmap. The Session Editor accepts order weights from a Harmonic Field, ASCII file, or bitmap.

The IFTA algorithm works with discrete diffraction orders, each located at the center of its corresponding pixel. For this example, we use an externally generated bitmap created with a graphics program (e.g., Paint, GIMP). The pattern consists of a 5×3 rectangular grid of diffraction orders, where the bright pixels are selected so that their arrangement forms the desired hexagon.

  • The image size is 5×3 pixels, matching the grid of orders.
  • Pixels that belong to the hexagon are set to white (maximum intensity); all others are black (zero intensity).
Target bitmap - hexagon on 5x3 grid
Target bitmap – a regular hexagon defined on a 5×3 order grid

Download the order weights as a png image

Step 2: Configure the Session Editor

2.1 Start the Session Editor:

Step 2.1: Start the session editor
Step 2.1: Start the session editor

2.2 Configure the optical system:

  • Set the input beam parameters.
Step 2.2a: Input beam parameters
Step 2.2a: Input beam parameters
Step 2.2b: Input beam parameters
Step 2.2b: Input beam parameters
  • Set the optical system parameters.
Step 2.2c: Optical system parameters
Step 2.2c: Set optical system parameters
Step 2.2d: Optical system parameters
Step 2.2d: Optical system parameters

2.3 Import order weights:

  • In the session editor, choose "Weights from Bitmap file" and import the required orders from the file manager.
Step 2.3: Import order weights
Step 2.3: Import order weights

2.4 Configure the output field parameters:

  • Set order separation: As the hexagon lies on a rectangular grid, we use different order separations for the X and Y directions, with the scaling \(\Delta x : \Delta y = 1 : \sqrt{3}\) to maintain a regular hexagonal geometry.
Step 2.4a: Order separation
Step 2.4a: Set order separation
Step 2.4b: Output field parameters
Step 2.4b: Output field parameters

2.5 Define Merit Functions

  • Define merit functions to be evaluated
Step 2.5: Merit functions to be evaluated
Step 2.5: Merit functions to be evaluated

2.6 Set Diffractive Optical Element Parameters

  • In this step, the diffractive optical element (DOE) can be configured: the aperture shape (rectangular or elliptical) and diameter can be set, and the transmission type (amplitude-only, phase-only, or complex) can be selected. The period, pixel size, and number of pixels can also be defined, with the Session Editor automatically calculating these from the desired angular resolution and maximum deflection angle.

  • In this example, we use an 8-Level quantization, while keeping the other parameters as provided by the Session Editor.

Step 2.6a: DOE Aperture Parameters
Step 2.6a: DOE Aperture Parameters
Step 2.6b: DOE Transmission Parameters
Step 2.6b: DOE Transmission Parameters
Step 2.6c: DOE Period
Step 2.6c: DOE Period

2.7 Design Summary

  • After evaluating the design summary, click the "Finish" button to finish the Session Editor setup.
Step 2.7a: Design Summary
Step 2.7a: Design Summary
  • This creates the IFTA optimization and the Optical Setup Documents.
Step 2.7b: Optical Setup Document
Step 2.7b: Optical Setup Document
Step 2.7c: IFTA Optimization Document
Step 2.7c: IFTA Optimization Document

Step 3: Run the IFTA Design

  • In the Design tab of the IFTA Design document, the number of IFTA iterations may be adjusted. For this example we use the default settings. If the results are not satisfactory, try increasing the number of iterations.

  • Click “Start Design”.

  • After the iterations finish, use the Analysis tab to evaluate the merit functions. Clicking "Recalculate" on the Analysis tab outputs:

    • The checked merit function values
    • The output field in its mathematical representation: In this data, derived via a Fourier transform, each pixel corresponds to one diffraction order. For paraxial cases, the squared amplitude of each pixel is proportional to its efficiency.
Step 3.1: Analysis Tab
Step 3.1: Analysis Tab

  • VirtualLab Fusion offers you the Multiple Run document for automatic generation and evaluation of many designs with preset result filters. Multiple runs are useful because the IFTA algorithm may converge to different local optima depending on the initial random phase; running many designs increases the chance of finding a better solution.

    For this example we use the following filters:

    Merit Function Requirement
    Conversion Efficiency > 80 %
    Uniformity Error < 5 %
    Max. Stray Light < 5 %
Step 3.2: Multiple Runs
Step 3.2: Multiple Runs
  • The multiple run document saves the CA2 files of the designed transmission functions together with an overview CSV file. Selected transmission functions can be set in the IFTA document for some tolerance evaluations.
Step 3.2: Multiple Runs Result
Step 3.2: Multiple Runs Result

Step 4: System Simulation

4.1 Engine

Apart from the IFTA document, the session editor has prepared a preconfigured system, the optical setup (OS). To improve the speed‑accuracy balance, we adjust the default settings of the preconfigured optical setup:

  1. Simulation engine – Switch from the older CFT (Classic Field Tracing) engine to the newer Field Tracing engine. This provides a more modern and flexible framework.
  2. Diffraction handling – Set all propagation options to “Automatic”. This enables diffraction wherever it is needed, while the engine intelligently selects the most efficient method.

  3. Sampling limit – Reduce the sampling limit from 10,000 to 1000 points squared. This significantly speeds up the simulation while keeping reasonable accuracy.

    Important: If the result appears undersampled (e.g., aliasing or missing details), increase the limit step‑by‑step until the solution converges. The optimal value depends on your specific system.

The image below shows the corresponding dialog settings:

Step 4.1: Optical Setup Configuration
Step 4.1: Optical Setup Configuration

Note: These recommended settings will become the default in a future release of VirtualLab Fusion.

4.2 Run the system

Run the system to view the result at the camera detector. Shown below, is the result in "Real Color" and "False Color" modes:

4.2a: Result in Real Color
4.2a: Result in Real Color
4.2b: Result in False Color
4.2b: Result in False Color

Downloads

💡 What’s next?
The IFTA design gives you the optimized phase distribution. You can now export this phase map or convert it into a real diffractive optical element structure (e.g., a binary or multilevel surface) using VirtualLab Fusion’s Structure Design tools.

Last updated: April 16, 2026
Tags: beam splitter IFTA diffractive optics array generation hexagon pattern VirtualLab Fusion