Introduction to Physical Optics Propagation (POP) in Zemax: Unlocking Precision in Optical System Design
In the world of optical system design, where precision is paramount, understanding the wave nature of light becomes crucial for accurate modeling. Physical optics propagation (POP) in Zemax bridges this gap by simulating optical fields in scenarios where diffraction, interference, and wavefront effects play a significant role. Unlike geometric ray tracing, which simplifies light as rays, POP models light as waves, making it indispensable for applications like laser beam propagation, fiber coupling, and diffractive optics.
What is Physical Optics Propagation (POP)?
POP offers a detailed wave-based approach to optical design, tracking the propagation of an optical field through free space, lenses, and other optical components. This method accounts for:
- Diffraction Effects: Essential for understanding light behavior in apertures or obstacles.
- Interference Patterns: Crucial in systems with overlapping beams.
- Beam Wavefronts: Key for applications requiring precise alignment, such as fiber coupling.
This advanced modeling capability ensures that engineers can predict and optimize optical system performance in ways that traditional ray tracing cannot.
Why Use POP for Fiber Coupling?
Fiber coupling is a challenging yet critical task in optical engineering, where a laser beam must be directed into an optical fiber with high efficiency. POP allows designers to simulate how the beam propagates, interacts with optical elements, and aligns with the fiber's parameters, including:
- Core Diameter: Ensuring the focused beam spot matches the fiber core size.
- Numerical Aperture (NA): Matching the beam's cone angle with the fiber's acceptance angle.
- Beam Positioning: Aligning the beam precisely at the fiber core.
By using POP, engineers can visualize and refine these aspects to achieve optimal coupling efficiency. This is especially vital for single-mode fibers, where the small core diameter demands precise sub-micron alignment and mode matching between the laser and the fiber.
Benefits of POP in Optical System Design
POP in Zemax provides engineers with robust tools to:
- Optimize beam parameters like waist size and wavefront phase.
- Simulate coupling efficiency under varying configurations.
- Evaluate and refine system, receiver, and coupling efficiencies through detailed merit function analysis.
What to Expect in This Blog
In the following sections, we’ll walk you through a practical example of using POP for fiber coupling. You’ll learn how to:
- Set up and simulate a laser beam's propagation.
- Analyze critical parameters such as coupling efficiency and phase shift.
- Optimize system performance for maximum efficiency.
Whether you're working on laser systems, fiber-optic communications, or advanced optical designs, mastering POP is a game-changer. Dive in to explore Zemax’s powerful capabilities and take your optical system design to the next level!
Physical optics propagation (POP) is used in Zemax to model the propagation of optical fields in systems where diffraction, interference, and wavefront effects are significant. Unlike geometric ray tracing, which simplifies light as rays, POP considers the full wave nature of light. This is crucial for modeling applications such as laser beam propagation, fiber coupling, and diffractive optics. Figure below shows a typical POP scheme that reveals Gaussian beam wave layout at different locations of laser propagation.
Using POP for fiber coupling in Zemax involves modeling how a laser beam propagates through free space and optical elements, such as lenses or collimators, and then couples into an optical fiber. The figure below shows how physical optics conduct laser mode in fiber coupling procedure. Figure below shows a schematic of coupling of light into a multimode or single-mode optical fiber (left). Launching conditions in a multimode optical fiber resulting in an overfilled (upper right) and underfilled (lower right) fiber.
For optimal coupling efficiency, the characteristics of the focused beam (typically a laser) must align with the fiber's parameters. The general rules are:
- The focused spot size should be comparable to the fiber's core diameter.
- The focused beam should be precisely centered on the fiber core.
- The incident beam's cone angle should not exceed the fiber's numerical aperture (NA).
Conditions (1) and (2) are shown on the left side of Figure 1, while condition (3) is depicted on the right side. Multimode fibers, with their large core diameters, make it easier to satisfy the first two conditions, allowing for good coupling efficiency by matching the coupling lens to the fiber's NA. However, coupling into single-mode fibers is significantly more challenging due to their small core diameters. Achieving efficient coupling in single-mode fibers requires additional opto-mechanical components for sub-micron positioning of the focused beam. Additionally, the incident laser beam’s mode must match the fiber's mode, meaning the coupling efficiency is determined by the overlap integral between the Gaussian mode of the input laser and the fiber’s nearly Gaussian fundamental mode.
The example below demonstrates the analysis steps of using POP to optimize fiber coupling. The layout of output and input fiber tips are shown below.
From the POP setup, the waist sizes in both X and Y before the entrance face are set as 0.0046 mm below. X and Y sampling are set as 256. When clicking Automatic, the X- and Y width are calculated, as 0.13 mm below.
The Display tab requested the Cross-X section view of phase distribution at the image plane. The column displays the center of the overall image.
The Fiber Data set the propagation from the waist X and Y at surface 1. The POP window displays fiber coupling results, as shown in the screenshot. In Merit Function, the POPD operand provides all the Physical Optics data within the Merit Function Editor, making it a more useful reference in many cases.
In the Merit Function Editor (MFE), the first three POPD operands yield coupling efficiency (data=0), system efficiency (data=1) and receiver efficiency (data=2) respectively. The next four POPD operands output the beam waist size (data=10), beam radius size (data=23) and M2 value (data=26). Other than the first, the other 5 POPD command weight are set as 0.
Here we optimize the system to get the best coupling efficiency in command 2. From the POP window, the best coupling can also be read as 0.993780. From the phase shift plot below and it numbers in text, the maximum phase shift is 1.2352E-1 rad.
A universal plot can be launched to plot the relation of distance of the fiber tips to the coupling efficiency (Data=0 @ POPD). From the curve, the peak is at thickness of surface 3 equals to 2 mm.
