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Mitigating Outages in a 4.6-km FSO Link via Mode-Diverse Reception: An Experimentally Validated Digital Twin Approach

This paper experimentally validates a digital twin channel model against a 4.6-km free-space optical link to demonstrate that a 6-mode receiver significantly reduces turbulence-induced outage probabilities to 2.02e-5.

Original authors: Jonas Krimmer, Vincent van Vliet, Menno van den Hout, Eduward Tangdiongga, Chigo Okonkwo, Sebastian Randel

Published 2026-06-23

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Jonas Krimmer, Vincent van Vliet, Menno van den Hout, Eduward Tangdiongga, Chigo Okonkwo, Sebastian Randel

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Sending Light Through a Stormy Window

Imagine you are trying to shine a laser pointer through a window to hit a tiny target on the other side of the street. If the air outside is perfectly still, the beam goes straight, and you hit the target easily.

But in the real world, the air is never perfectly still. Heat rising from the ground, wind, and traffic create invisible "ripples" in the air (called atmospheric turbulence). These ripples act like a wobbly, funhouse mirror. They bend and twist the laser beam, sometimes so much that the light misses the tiny target entirely. This is called a signal outage.

This is a huge problem for Free-Space Optical (FSO) communication, which is a way to send internet data using light through the air instead of fiber optic cables. While fiber is great, it's hard to lay down everywhere. FSO is the "wireless fiber," but the wobbly air causes the connection to drop frequently.

The Problem: The "Single-Mode" Bottleneck

The researchers in this paper were working with a system that tries to catch this laser light and funnel it into a single-mode fiber (a glass strand so thin it's like a human hair).

Think of this single-mode fiber as a very narrow straw.

When the air is calm, the laser beam is a straight, thin stream of water that fits perfectly into the straw.
When the air is turbulent, the beam gets distorted. It might get wider, or it might wiggle.
Because the straw is so narrow, even a tiny wobble means the water (the light) spills over the sides and is lost. This causes the internet connection to crash.

The Solution: The "Mode-Diverse" Umbrella

The researchers proposed a clever fix: instead of using one tiny straw, use a bundle of straws (a few-mode fiber) and a special splitter that catches the light in different shapes.

Imagine the laser beam as a chaotic splash of water hitting a wall.

Old way (Single straw): You try to catch the splash with one tiny cup. If the splash moves, you miss.
New way (Mode-diverse): You use a large umbrella made of 6 different sections. Even if the water splashes wildly, at least one (or several) of the umbrella sections will catch it. You then combine the water from all sections to get a full drink.

This "umbrella" approach is called Mode-Diverse Reception. The more sections (modes) you have, the better you can catch the light, even when the air is very turbulent.

The Experiment: The "Digital Twin"

You might ask: "How do we know this works before we build expensive equipment?"

The researchers built a Digital Twin. Think of this as a hyper-realistic video game simulation of the real world.

The Real World: They have a real 4.6-km (about 3 miles) laser link running between two buildings in Eindhoven, Netherlands. They measured how much the light faded there over time.
The Simulation: They created a computer model that mimics the physics of light traveling through that specific stretch of air. They fed it real data about how "bumpy" the air was (using a device called a scintillometer).
The Match: They compared the computer's predictions with the real measurements. The simulation was accurate, proving their "Digital Twin" was a trustworthy tool.

The Results: From "Often Down" to "Almost Never Down"

Once they trusted the simulation, they used it to test their "umbrella" idea. They asked: How many straws (modes) do we need to make the connection reliable?

1 Straw (Standard): The connection fails (goes down) about 13% of the time during bad weather. That's like having your internet cut out for 3 hours every day.
3 Straws: The failures drop significantly.
6 Straws: This is the magic number they found. With 6 modes, the connection failure rate drops to 0.002%.

To put that in perspective:

1 Mode: The link is down frequently.
6 Modes: The link is down so rarely that it's practically perfect. It reduces the chance of a crash by 10,000 times (four orders of magnitude).

The Conclusion

The paper proves that by using a "Digital Twin" to simulate the messy air, we can design better receivers. By catching light with multiple "modes" (like using a 6-section umbrella instead of a single straw), we can make wireless light internet as reliable as fiber optic cables, even when the air is turbulent.

Key Takeaway: You don't need to wait for the air to be calm. You just need a bigger, smarter net to catch the light when it gets wobbly.

Technical Summary: Mitigating Outages in a 4.6-km FSO Link via Mode-Diverse Reception

Problem Statement
Terrestrial Free-Space Optical (FSO) communication is a critical candidate for bridging the gap between fixed fiber backhaul and dynamic endpoints in next-generation 6G and AI-driven networks. However, its deployment is severely limited by atmospheric turbulence. While macroscopic weather phenomena (fog, rain) cause slow attenuation manageable by static margins, turbulence induces rapid, millisecond-scale spatio-temporal refractive index fluctuations. These distortions cause deep, catastrophic signal fades that exceed Forward Error Correction (FEC) interleaving depths and force coherent Digital Signal Processing (DSP) tracking loops to re-lock. Crucially, these phase distortions severely impair coupling efficiency into Single-Mode Fibers (SMF), which are required for integration into coherent optical networks. While mode-diverse reception using Few-Mode Fibers (FMF) has been proposed to mitigate this, system designers lack experimentally validated guidelines on the specific number of modes required to meet long-term reliability targets under real-world conditions.

Methodology
To address this validation gap, the authors propose and validate a "Digital Twin" channel model against a deployed 4.6-km terrestrial FSO link (the Reid Photonloop) in Eindhoven, The Netherlands.

Experimental Setup: The testbed connects the Eindhoven University of Technology campus to the High Tech Campus at an altitude of ~40 m, traversing an urban environment that promotes strong turbulence. The system operates at 1560 nm with automated pointing and tracking.
Data Acquisition: A scintillometer (Scintec BLS900 Neo) runs in parallel to measure the path-averaged refractive index structure constant ( $C_n^2$ ). Long-term data is used to isolate representative atmospheric states (median, 90th, and 99th percentiles).
Dual-Branch Measurement: The receiver splits the signal into two paths:
1. Fiber Branch: Coupled into an SMF monitored by a high-speed power meter to measure coupling efficiency.
2. Free-Space Branch: Focused onto a large-area detector (9.7 mm active area) to capture aperture-averaged amplitude fluctuations, serving as a calibration target independent of fiber coupling dynamics.
Digital Twin Simulation: The authors simulate wave propagation using the split-step Fourier method with phase screens adhering to the Frehlich spectrum. The simulation incorporates:
- 30 propagation steps with a 1 mm transverse grid resolution.
- Monte Carlo iterations ( $N_{iter} = 10,000$ ) to generate statistical distributions.
- Numerical evaluation of the overlap integral between the received field and backpropagated fiber modes to calculate coupling efficiency.
- Idealized tip-tilt correction to isolate fundamental atmospheric phase distortions from hardware-specific mechanical jitter.

Key Contributions

Experimental Validation of a Digital Twin: The paper rigorously validates a numerical channel model against experimental fading statistics from a real-world 4.6-km link. The model successfully reproduces both large-aperture irradiance fluctuations and SMF coupling dynamics across distinct turbulence regimes.
Quantification of Mode-Diversity Trade-offs: By integrating the validated simulation with long-term scintillometer data, the authors quantify the engineering trade-off between the complexity of the receiver (number of modes) and the resulting long-term outage probability.
Design Framework: The work provides a validated framework for determining the necessary mode count to guarantee specific reliability targets, moving beyond theoretical assumptions to data-driven design.

Results

Model Accuracy: The numerical simulation aligns closely with experimental data. For the median turbulence regime ( $C_n^2 \approx 1.5 \cdot 10^{-15} \text{ m}^{-2/3}$ ), the scintillation index ( $\sigma^2_I$ ) for the free-space branch was 0.12 (simulated) vs. 0.157 (experimental), and for the fiber branch, 0.14 vs. 0.194. The slight discrepancies in fiber coupling are attributed to residual jitter in the pointing-and-tracking system and short-term turbulence fluctuations within the 10-minute measurement window.
Outage Probability Reduction: Using a loss threshold of $L_{th} = 10$ $L_{t h} = 10$ dB and integrating over the observed $C_n^2$ $C_{n}^{2}$ probability density function:
- Single-Mode Fiber (SMF): Total long-term outage probability is 0.131.
- 3-Mode Receiver: Reduces outage probability to $1.76 \cdot 10^{-2}$ .
- 6-Mode Receiver: Reduces outage probability to $2.02 \cdot 10^{-5}$ .

Significance
The paper demonstrates that mode-diverse reception can reduce turbulence-induced outage probabilities by four orders of magnitude compared to single-mode systems. By establishing a "Digital Twin" that is rigorously validated against a real-world link, the authors provide a reliable baseline for designing robust FSO receivers. This approach allows system designers to move from theoretical speculation to empirically grounded decisions regarding the complexity (mode count) required to achieve fiber-like reliability in turbulent atmospheric channels.