The ring design has potential applications in telecommunications, medicine, and other fields.

Scientists from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with the Vienna University of Technology (TU Wien), have developed a groundbreaking semiconductor laser that offers broad and precise wavelength tuning in a compact, chip-sized format. This new laser design brings together the most effective features of today’s leading laser technologies.

Tunable lasers, which allow users to adjust and control the color (wavelength) of the light they produce, are essential in a wide range of fields. These include fast data transmission in telecommunications, advanced medical testing, and the detection of leaks in gas pipelines. However, current laser systems often involve compromises. Lasers with wide color ranges typically lose precision, while those that can finely tune to specific wavelengths tend to be complex and costly because they rely on mechanical components.

The new Harvard device could one day replace many types of tunable lasers in a smaller, more cost-effective package.

A Collaborative Breakthrough in Laser Technology

The research is published in Optica and was co-led by Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, and Professor Benedikt Schwarz at TU Wien with whom Capasso’s group has maintained a longstanding research partnership.

The researchers have initially demonstrated a laser that emits light in the mid-infrared wavelength range because that’s where quantum cascade lasers, upon which their architecture is based, typically emit. “The versatility of this new platform means that similar lasers can be fabricated at more commercially relevant wavelengths, such as for telecommunications applications, for medical diagnostics, or for any laser that emits in the visible spectrum of light,” said Capasso, who co-invented the quantum cascade laser in 1994.

How the Laser Works: Tiny Rings, Wide Range

The new laser consists of multiple tiny ring-shaped lasers, each a slightly different size, and all connected to the same waveguide. Each ring emits light of a different wavelength, and by adjusting electric current input, the laser can smoothly tune between different wavelengths. The clever and compact design ensures the laser emits only one wavelength at a time, remains stable even in harsh environments, and can be easily scaled. The rings function either one at a time or all together to make a stronger beam.

“By adjusting the size of the ring, we can effectively target any line we want, and any lasing frequency we want,” said co-lead author Theodore Letsou, an <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

” data-gt-translate-attributes=”[{"attribute":"data-cmtooltip", "format":"html"}]” tabindex=”0″ role=”link”>MIT graduate student and research fellow in Capasso’s lab at Harvard. “All the light from every single laser gets coupled through the same waveguide and is formed into the same beam. This is quite powerful, because we can extend the tuning range of typical semiconductor lasers, and we can target individual wavelengths using a different ring radius.”

Fabrication Simplicity and Operational Stability

“What’s really nice about our laser is the simplicity of fabrication,” added co-lead author Johannes Fuchsberger, a graduate student at TU Wien, where the team fabricated the devices using the cleanroom facilities permanently provided by the school’s Center for Micro and Nanostructures. “We have no mechanically movable parts and an easy fabrication scheme that results in a small footprint.”

The laser’s unique architecture also guards against common problems like optical feedback, or when some laser light gets reflected backward into the source and can cause destabilization. Since the new laser’s rings emit unidirectionally, either clockwise or counterclockwise, there’s no chance of back-reflection.

The new ring laser could possibly replace current technologies for different types of tunable semiconductor lasers that each have strengths and drawbacks depending on the application. For example, distributed feedback lasers make smooth and accurate beams and are therefore used in telecommunications fiber to send optical signals long distances, but their tuning range is narrow.

External cavity lasers, on the other hand, have broader tuning ranges but more complex designs and moving parts, which makes their laser lines tend to skip around. These are commonly used in gas sensors that test for leaks in pipelines, because they can detect gases like methane and carbon dioxide which absorb light at distinct wavelengths.

Reference: “Continuously and widely tunable semiconductor ring lasers” by Johannes Fuchsberger, Rolf Szedlak, Dmitry Kazakov, Theodore P. Letsou, Federico Capasso and Benedikt Schwarz, 19 July 2025, Optica.
DOI: 10.1364/OPTICA.559884

The paper was co-authored by Dmitry Kazakov and Rolf Szedlak. The team has worked in collaboration with the Harvard Office of Technology Development and the TU Wien Patent and License Management Office to protect the underlying intellectual property, with the goal of commercializing this idea in the future.

U.S. federal funding in support of this research came from the Department of Defense and the National Science Foundation (Grant No. ECCS-2221715).

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