Why are multimode fibers exciting for classical and quantum optical communication?
A multimode fiber is like a multi-lane highway, where light travels carrying information, much like the cars moving along different lanes. The different lanes correspond to the spatial modes of the fiber, and the number of available modes increases with the size of the fiber core. Even with a fiber core just 100 micrometers in diameter—about the thickness of a human hair—we can have over a thousand modes, allowing us to multiplex information, meaning we can send multiple signals in parallel.
Now, this is where things get both fascinating and challenging. If I inject a light wave at a specific spot (about 1 micrometer in diameter) on the fiber’s input, it doesn’t simply travel straight through. Instead, it spreads across many different paths, leading to complex interference effects, much like overlapping ripples in water. Instead of receiving a clear signal at the end of the fiber, we will see a random light pattern, scrambling the transmitted information. However, this seemingly chaotic interference pattern is not entirely unpredictable. In fact, each fiber produces a unique but deterministic pattern, which we can precisely measure and correct. This is where wavefront shaping comes in.
Imagine using an intelligent traffic control system that doesn’t just manage cars but actively directs them along optimized routes. By carefully shaping the light before it enters the fiber, we can control how different modes interfere. This spatial encoding effectively steers light waves towards controlled positions at the fiber output upon transmission, ensuring the information arrives in an organized way rather than getting scrambled. Unlike a simple fiber, which has only one mode for transmission, a multimode fiber—when combined with wavefront shaping—becomes a high-capacity superhighway for data, dramatically increasing the amount of information that can be transmitted!
What happens when we send short pulses of light instead of continuous-wave laser light over multimode fibers?
A short pulse contains many different frequencies, not just a single color like the continuous-wave laser. Because of the intricate way light scatters and interferes inside the fiber, these frequencies don’t behave independently—they are correlated. In simple terms, this means that the light pattern that we shape at one frequency is not entirely independent of nearby frequencies. These frequency correlations set a fundamental limit on how precisely we can control wavefront-shaped light through the fiber. The wider the range of correlated frequencies, the more colors can be effectively focused using the same wavefront shaping pattern.
Why does this matter?
Because these frequency correlations also set the limit on how short a pulse we can transmit. If we try to compress a pulse beyond this limit, its different frequency components won’t stay aligned in a controlled way, causing the pulse to spread out and lose its sharpness at the output. Essentially, the wider the range of frequency correlations, the shorter the pulse we can successfully transmit while keeping it focused at the receiver. The geometry of the fiber plays a crucial role in determining how these frequency correlations behave. We investigated two types of fiber cores: circular and rectilinear (rectangular or square-shaped). In circular-core fibers, we observed a weakening of spatial correlations near the edges, which force us to use longer pulses to maintain a reliable transmission, thereby slowing down data rates. In contrast, rectilinear-core fibers provide more consistent frequency correlations, enabling shorter pulses and potentially faster data transmission.
We are particularly excited about its potential in photonic quantum hardware. Our group actively develops single- and entangled-photon sources for quantum technologies, and one major challenge in scaling up photonic quantum computing is the efficient transport of quantum light between semiconductor chips. Rectilinear-core fibers are ideal for this purpose because their rectangular geometry naturally matches photonic chip architectures while providing the high channel capacity benefits.