Single-Pump Ring-Core Amplifier for Balanced Modal Gain in Mode-Division Multiplexing Systems

Today’s digital world is moving at lightning speed, and the demand for data is skyrocketing. The systems that carry our data—these tiny, single-mode fiber cables running under oceans and across countries—are being pushed to their very limits. The techniques that have kept up with data traffic so far, like wavelength-division and polarization-division multiplexing, are simply running out of room. Imagine trying to pour more and more water into a bucket that’s already full; that’s essentially what’s happening to these traditional methods. To find a way forward, researchers are exploring something new: mode-division multiplexing, or MDM. MDM isn’t just about squeezing more data through a fiber; it’s about creating multiple “lanes” inside the fiber where different data streams can travel at the same time. Think of it as adding more lanes to a highway to reduce traffic. But to make MDM work, the signal strength in each of these “lanes” needs to stay balanced; otherwise, the whole system risks breaking down. This is where amplifiers come in—they boost the signals to make sure they stay strong over long distances. The challenge, however, is that conventional amplifiers struggle to keep things even across multiple lanes, or modes. This unevenness, called differential modal gain (DMG), causes some signals to overpower others, leading to distorted data and, ultimately, crosstalk that affects the overall quality. Traditional solutions have tried to balance these gains by adding multiple pump sources, each tailored to boost a specific mode. But this multi-pump setup is complicated, costly, and doesn’t lend itself well to the compact, on-chip designs that modern optical systems are aiming for.

In response to this problem, recent research paper in Optics Express and conducted by doctoral candidate Cheng Yu, Fei Wang, Lizhan Gao, Jiahui Shi, Changlong Li, Dan Zhao, Meiling Zhang, and led by Professor Guijun Hu from Jilin University developed novel amplifier design that gets rid of the need for multiple pumps altogether. They came up with a ring-core structure where the erbium doping—the substance that makes amplification possible—is carefully layered. By adjusting the doping in specific parts of the waveguide, they’ve managed to balance the amplification for each mode, or data lane, with just one pump source. This ring-core design doesn’t just make the amplifier simpler; it also makes it practical for the compact, chip-based designs that are increasingly important in today’s tech landscape. This breakthrough isn’t just technical—it has far-reaching implications for the future of data networks. It means more efficient and affordable optical systems that can handle the demands of a data-driven world. And for researchers and engineers, it opens the door to even more innovations in how we design the systems that keep our digital world connected and running smoothly.

To tackle the challenge of DMG in few-mode erbium-doped waveguide amplifiers (FM-EDWAs), a research group from Jilin University embarked on a series of experiments centered around their innovative ring-core design. They started by carefully building an FM-EDWA with a specialized ring-core structure, precisely doped with both erbium and ytterbium ions. Using advanced lithography techniques, they achieved a layered doping approach within the polymer waveguide. The outer layer was enriched with a higher concentration of erbium ions, while the inner layer held a lower concentration, creating a well-thought-out spatial distribution aimed at equalizing gain across modes. This layered setup was particularly advantageous for higher-order modes, which generally receive less amplification, allowing them to perform more comparably to the fundamental mode (LP01).

Once the FM-EDWA was fabricated, the team set up a detailed experimental system to test the amplifier’s performance across different signal modes and power levels. They used a 976 nm laser as the pump source, with a tunable laser (spanning 1510 to 1590 nm) providing the signal input. This setup let them analyze the amplifier’s behavior over a wide wavelength range, which is essential for understanding its practical applications in communications. With the help of mode-selective photonics lanterns, they could efficiently split and recombine signal modes, ensuring precise coupling with the waveguide. They measured the gain for three main modes—LP01, LP11a, and LP11b—at each stage. Significantly, the amplifier achieved gains of over 14 dB for all three modes at the target wavelength of 1529 nm, with an impressively low DMG of just 0.73 dB when pumped with LP01 at 200 mW. This result demonstrated that their ring-core design could consistently produce high, balanced gains across multiple modes using a single pump source. The researchers extended their analysis across the broader 1525-1565 nm range to assess whether the amplifier could maintain steady performance over an even wider bandwidth. They found that it delivered stable gains above 10 dB across this entire range, while the DMG stayed low at less than 1.3 dB. This consistency shows the amplifier’s potential for reliable performance in real-world optical networks. By examining the near-field profiles of the modes both before and after amplification, the team verified that the ring-core structure maintained the purity of each mode, with minimal distortion or loss. This preservation of mode integrity is essential in high-capacity optical systems, as it reduces interference between channels.

The authors also measured modal crosstalk to make sure that the amplified modes stayed distinct from each other. They observed low crosstalk levels, below -12.47 dB between modes post-amplification. These findings underscore the ring-core structure’s effectiveness in keeping each mode isolated within the waveguide, allowing them to be amplified separately without affecting one another. Low crosstalk is crucial for maintaining high-quality signal fidelity in scalable MDM systems. A key finding from their experiments came from examining how gain responded to different input power levels. They measured gain for each mode at various input power levels (0 dBm, -5 dBm, and -10 dBm) while gradually increasing the pump power. As expected, the gain for each mode initially rose with the pump power and then leveled off, showing typical saturation behavior. Notably, the amplifier reached a maximum gain of 14.91 dB with an input signal power of -10 dBm, and the DMG remained very low, affirming that the layered structure effectively balanced the gain across modes. When the team compared the ring-core structure’s performance with older designs using different pump modes (like LP21b), they found that the LP01 pump mode delivered much more balanced gain for the three signal modes. In previous designs with uniform doping, DMG could rise to as high as 5 dB when higher-order modes were pumped. However, with the ring-core structure—optimized using a genetic algorithm—the DMG stayed within 1 dB, demonstrating a clear advantage in achieving uniform gain across modes.

In conclusion, Professor Guijun Hu and his team at Jilin University have made a game-changing discovery in optical amplifier technology, particularly in how we manage the huge amounts of data flowing through our networks every second. Traditional systems use complex techniques to keep signals strong across different channels, or “modes,” but these methods have always come with trade-offs. To balance signal strength, or “gain,” across these channels, most setups need multiple pumps, each working to amplify a specific mode. Not only does this make the system bulky and expensive, but it also limits how easily these amplifiers can be adapted to smaller, on-chip devices. This is where Professor Hu’s team took a fresh approach. They designed a ring-core structure with carefully arranged layers of erbium ions—a key ingredient in boosting signal strength. By adjusting how these ions are distributed, the team discovered a way to amplify multiple modes almost evenly with just a single pump source. Imagine replacing a complicated multi-pump setup with a single, streamlined system that does the same job. This new approach isn’t just simpler; it’s a major step forward in making these amplifiers smaller, cheaper, and much easier to integrate into advanced, compact systems. The real impact of this design goes far beyond just simplifying things. As demand for data surges, networks are struggling to keep up, and finding ways to handle large volumes of data without overloading the system is essential. The ring-core FM-EDWA design developed by this team is incredibly efficient, paving the way for smaller, high-capacity devices that won’t eat up tons of energy. This makes it an ideal candidate for the next generation of communication networks, especially in dense environments like data centers, where saving space and energy is crucial. What’s more, this study opens up a whole new way of thinking about how we design these devices. The authors’ innovative approach—fine-tuning the placement of materials within the amplifier to control how it performs—could be applied to a wide range of other optical devices. Rather than simply adding more parts to improve performance, this method allows engineers to work smarter with what’s already there, making small adjustments that have big impacts. It’s a reminder that sometimes, the best innovations come from looking at things in a new way, finding creative ways to do more with less. This discovery is likely to inspire further research and advancements, not only in amplifiers but in the entire field of photonics.