Researchers in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Technical University of Vienna (TU Wien) have invented a new type of tunable semiconductor laser that combines the best attributes of today's most advanced laser products, demonstrating smooth, reliable, wide-range wavelength tuning in a simple, chip-sized design.
Tunable lasers, or lasers whose light output wavelengths can be changed and controlled, are integral to many technologies, from high-speed telecommunications to medical diagnostics to safety inspections of gas pipelines. Yet laser technology faces many tradeoffs – for example, lasers that emit across a wide range of wavelengths, or colors, sacrifice the accuracy of each color. But lasers that can precisely tune to many colors get complicated and expensive because they commonly require moving parts.
The new Harvard device could one day replace many types of tunable lasers in a smaller, more cost-effective package.
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.
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 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."
"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.
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).
