The two rings in the image are microresonators. The bigger ring is the one where the microcomb is generated. The microcomb is formed by a pulse of light—here illustrated with a red spike and also known as a soliton—that recirculates in the cavity forever. The smaller ring helps couple the light from the straight waveguide, the straight orange line at the bottom, into the bigger ring. [Image: Illustration: Óskar Helgason]
In the past decade, optical microcombs have surged in popularity, enabling advances in diverse fields such as metrology, telecommunications, astronomy and quantum optics. However, a fundamental limitation of the technology is that the conversion efficiency scales inversely with the number of comb lines generated.
Now, researchers at Chalmers University of Technology, Sweden, have developed a new approach that, according to the team, makes optical microcombs 10 times more efficient (Nat. Photon., doi: 10.1038/s41566-023-01280-3). The resulting devices have a high conversion efficiency and uniform spectrum, making them especially relevant for applications in optical communications and dual-comb spectroscopy.
A critical next step
Conventional frequency combs, a Nobel Prize–winning technology, employ a special mode-locked laser to create a large number of very evenly spaced frequencies. They can act like an optical ruler, measuring frequencies with unprecedented precision.
An alternative way to generate combs of frequencies, called optical microcombs, leverages the Kerr nonlinearity in high-Q optical resonators. These devices—driven instead by a continuous-wave (CW) laser—feature compact footprints, low power consumption and high repetition rates in broad optical bandwidths.
One major drawback of optical microcombs up to this point was a low conversion efficiency between the CW laser and the microcomb, meaning that only a fraction of the power contained in the laser beam was usable. Improving the conversion efficiency is a critical step toward realizing fully integrated, on-chip microcomb-based systems.
“The motivation for this study was to improve the efficiency of microcombs and make them useful for practical applications,” said study author Victor Torres Company. “Possible real-world applications include datacenter interconnects for bandwidth-demanding applications such as AI, optical clocks for improved navigation systems, precision spectroscopy for accurate diagnosis, and lidar for autonomous driving, among others.”
Overcoming a fundamental limit
Victor Torres Company, Professor, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Sweden [Image: Chalmers / Mikael Nystås]
The key was to shift the CW pumped resonance of the microcavity by using two microresonators instead of just one. An auxiliary resonator coupled to the main cavity induces a shift in one cavity mode, so that the CW pump laser is resonant with the cavity when the solition is generated. Essentially, one of the microresonators enables the light coming from the laser to couple with the other microresonator, allowing the pump to be coupled more efficiently into the cavity.
“We’ve developed a new method that breaks what was previously thought to be a fundamental limit for optical conversion efficiency,” said Torres Company. “Our method increases the laser power of the soliton microcomb by 10 times and raises its efficiency from around 1% to over 50%.”
Next, the researchers aim to develop reliable, wafer-scalable laser pump sources with high power and high efficiency. Torres Company and his colleagues also recently patented the optical microcomb technology and founded Iloomina AB, a company that will launch a related product to a wider market.