The well-defined spacing between frequency spikes makes the frequency comb a valuable light measurement tool. Just as the evenly spaced lines on a ruler provide a way to measure distance, the evenly spaced spikes of a frequency comb allow the measurement of unknown frequencies of light. On-chip optical frequency combs enable numerous applications, but the predominant use of single-ring microresonators in on-chip combs can limit the comb’s frequency range and optical power. A new path to generating optical frequency combs, developed by a team from the University of Maryland (UMD), resulted in a nested comb-within-a-comb that could lead to smaller, more efficient frequency combs for atomic clocks, rangefinders, quantum sensors, and other applications that require precise measurement of light. Researchers at UMD developed a silicon nitride-based chip with hundreds of microscopic rings. According to the researchers, the device is the world’s first topological frequency comb. Courtesy of E. Edwards. The nested frequency comb is based on a topological platform with a 2D array of coupled microresonator rings (i.e., microrings). The light from the comb is confined to the edges of the array. The array accommodates fabrication-robust topological edge states with linear dispersion. To create the nested comb, the researchers designed a chip with hundreds of microrings arranged in a 2D grid. The team engineered a complex pattern of interference that takes input laser light and circulates it around the edge of the chip while the material of the chip itself splits the light into many frequencies. Because continuous wave lasers delivered too much heat to the chip, the researchers used a custom-built pulsed laser to deliver light to the comb. They found that pulses from off the shelf pulsed lasers were too short and contained too many frequencies to provide the edge-constrained light that was needed to support the design of the topological frequency comb. The team also went through multiple chip iterations before it arrived at a chip design that could support the topological frequency comb. The individual microrings form cells that allow the light to jump from ring to ring — that is, from one pathway to another. The microrings are designed to create specific forms of interference between the paths. Collectively, the rings disperse the input light into the many teeth of the comb and guide the light along the edge of the grid. The individual microrings together form a super-ring. The presence of both individual microrings and a super-ring causes the comb to have two different time and length scales, because it takes longer for light to travel around the super-ring than the microrings. This phenomenon is what leads to the generation of two nested frequency combs. The microrings produce a coarse comb, with frequency spikes spaced widely apart. Within each coarsely spaced spike is nested a finer comb, produced by the super-ring. This nested, comb-within-a-comb structure could be useful in applications that require precise measurements of two different frequencies that are separated by a wide gap. The team placed an infrared camera above the chip to capture images of the light circulating around the edge of the chip. By pumping the edge states, the researchers generated a nested frequency comb that showed oscillation of multiple edge state resonances across approximately 40 longitudinal modes, while being spatially confined at the 2D lattice edge. A schematic of the new UMD experiment on nested topological frequency combs is shown. In an experiment, pulsed laser light (the pump laser) was sent into the chip while an infrared camera placed above captured images of light circulating around the chip's edge. The team used a spectrum analyzer to detect a nested frequency comb in the circulating light. Courtesy of C. Flower et al./University of Maryland. The researchers performed a high-resolution analysis of the light frequencies from the comb. Using a spectrum analyzer, they detected one comb with relatively broad teeth and, nestled within each tooth, a hidden, smaller comb. Although the nested comb developed by the UMD team is only a proof of concept — its teeth are not evenly spaced and are a bit too noisy to be called pristine — the new device could ultimately lead to more efficient frequency combs for quantum sensing, ranging, and metrology. The work also provided an opportunity for the team to explore the interplay between topological physics and nonlinear frequency comb generation in a nanophotonic platform. While the experiment with nested optical frequency combs was performed on a chip made from silicon nitride, according to the researchers, the design could easily be translated to other photonic materials to create combs in different frequency bands. The team believes that the on-chip topological frequency comb could serve as a platform for the study of topological photonics, especially in applications where a threshold exists between relatively predictable behavior and more complex effects — like the generation of a frequency comb, for example. The research was published in Science (www.doi.org/10.1126/science.ado0053).
