Using Physical Optics Propagation (POP), Part 1: Inspecting the beams

This article introduces the Physical Optics Propagation (POP) tool in OpticStudio, which propagates electric fields in free space.  It introduces the Beam File Viewer, which is used to inspect the phase and intensity of the beam at every surface.

This article is part one of a three part series. 

Authored By Erin Elliott

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Introduction

The Physical Optics Propagation (POP) tool is one of the only ones in OpticStudio that requires hands-on guidance to get to the correct result. This is because POP uses scalar diffraction theory to propagate an electric field through space, making the tool sensitive to the Fresnel propagation process. This process must balance achieving a high beam resolution with a wide grid width that captures all spatial frequencies. As such, the setup and results of a POP run must be thoroughly checked by the user every time.

This series will work through the correct way to setup and evaluate a simple system using POP. In this article, we will discuss the example system and look at ways to evaluate the POP results.

Physical Optics Propagation Help Files

Before working through this series of articles, you should read the POP information in the OpticStudio Help Files. To do so, navigate to Help...System Help. Under the Contents section, navigate to “The Analyze Tab>Laser and Fibers Group>About Physical Optics Propagation," as shown below. 

OpticStudio_Help_files

Example lens file

The lens file that we’ll evaluate is shown below; it’s a two-lens system. The first lens collimates the beam, and the second lens focuses the beam. Both lenses are aspheric singlets that carry an r4 aspheric term to correct spherical aberration. There is a small central obscuration in the collimated portion of the beam. The wavelength for the system is set to 1 um. 

Two-lens_example_system

Lens_Data_Editor
We want to launch a Gaussian beam into the system, as if it’s fed by a fiber optic. In the System Explorer shown below, the Aperture Type is set to Object Space NA and the Aperture Value to 0.05. This corresponds to a beam divergence angle of about 2.9° and a Gaussian beam waist semi-diameter of about 6.4 µm, using the equations below.

Equation_1

Equation_2

Despite these settings, it is important to note that the layout plot shows geometric rays only!  The layout plot is not a representation of a true Gaussian beam.

For the example system that we’re using, the rays are a good representation of the Gaussian beam at locations just outside the beam waist, so we can use Footprint plots and other ray-based analysis tools to cross-check the POP results as long as we’re not near a focus.

System_Explorer

Initial POP result

Initialize a POP analysis by navigating to Analyze...Physical Optics.

Launch_a_POP_analysis

To access the POP settings, use the down-arrow in the upper left corner of the POP window. Note that the POP analysis does not pick up the object space NA from the System Explorer; the NA must be set manually in POP. The NA in the System Explorer governs the geometric ray trace only.

Under General, ensure that your settings match those below. 

POP_General

Under Beam Definition, select Beam Type: Gaussian Waist and set a waist size that corresponds to the input beam NA of 0.05, which is about 6.36 mm. This corresponds to the setting Waist X/Y: 0.00636. We’ll also set the sampling grid at a density of 1024x1024 grid (X-/Y-Sampling: 1024), with a X-/Y-Width of 0.1 mm.

POP_Beam_Definition

Under the Display tab, check Save Output Beam To: and Save Beam At All Surfaces. These options create a Zemax Beam (.zbf) File at every surface that contains the electric field information.

POP_Display
After running POP, the beam irradiance at the image plane is displayed. There aren’t any obvious problems visible; the beam seems to be sampled well, with plenty of pixels across the beam. There is no aliasing or other strange artifact in the output. However, if we switch to the Prop Report tab, it looks like there are some potential problems. A “Low sampling of pilot beam” warning appears at Surface 2. It should be noted that because of the complexity of the POP calculations, we must always examine the beam information at every surface to ensure the calculation is behaving correctly. 

POP_analysis_output     Prop_Report_tab

Inspecting the Beam Files

To troubleshoot the calculation, we need to look at the beam files that were saved at every surface. Open the Beam File Viewer from Analyze...Beam File Viewer

Beam_File_Viewer

Under the Settings in the Beam File Viewer window, there is a drop-down for selecting the .zbf file. Each file is saved with a “00XX” at the end, to indicate which surface the beam file represents. The file without a surface number is at the image surface and is the same as the result shown in the POP window.

Select_file

The data can be displayed in various ways. We’ll mainly use Data: Irradiance and Phase information, displayed as Show As: False Color. That said, we’ll also use cross-sections, log plots, and the zoom functions.

 

Beam_File_Viewer_2

 

Next Article: Using POP, Part 2: Inspecting the beam intensities where we’ll use the Beam File Viewer to inspect the beam irradiance profiles for the example system.

 

KA-01601

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