Jun 21, 2021
Using Physical Optics Propagation (POP), Part 1: Inspecting the beams
Physical Optics Propagation (POP) is a powerful tool for modelling many types of laser systems and is one of the only ones in OpticStudio that requires hands-on guidance to get to the correct result. This blog introduces how to correctly use 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 beam's phase and intensity at every surface. This blog post is part of a series that can be found on the Zemax Knowledgebase.
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.
Physical Optics Propagation Help Files
Before working with Physical Optics Propagation, you should read the OpticStudio Help Files' POP information. 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.
Example lens file
The lens file that we will evaluate is shown below; it is 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.
We want to launch a Gaussian beam into the system as if a fiber optic feeds it. 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.
Despite these settings, it is essential to note that the layout plot shows geometric rays only! The layout plot is not a representation of an accurate Gaussian beam.
For the example system that we are using, the rays represent the Gaussian beam at locations just outside the beam waist. We can use Footprint plots and other ray-based analysis tools to cross-check the POP results if we are not near a focus.
Initial POP result
Initialize a POP analysis by navigating to Analyze...Physical Optics.
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.
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 will also set the sampling grid at a density of 1024x1024 grid (X-/Y-Sampling: 1024), with an X-/Y-Width of 0.1 mm.
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.
After running POP, the beam irradiance at the image plane is displayed. There are not any apparent 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.
Inspecting the Beam Files
To troubleshoot the calculation, we need to look at the beam files saved at every surface. Open the Beam File Viewer from Analyze...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 a result shown in the POP window.
The data can be displayed in various ways. We will mainly use Data: Irradiance and Phase information displayed as Show as False Color. That said, we will also use cross-sections, log plots, and zoom functions.
You can find out how to use the Beam File Viewer to inspect the beam irradiance profiles for the example system in POP, Part 2: Inspecting the beam intensities.
It is essential to thoroughly check the setup and results of every POP run at every surface. To help you do this, make sure to save the beam files at every surface during a run. Then, the Prop Report tab can be reviewed to find any warnings produced during computation, and the Beam File Viewer can be used to inspect the beams at every surface in the system.
There are Knowledgebase articles that explain these next steps in the process. Part 2 will cover beam intensities their potential problems, and Part 3 will look at the beam phases and their potential problems.
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