Mini-QCL Frequency Combs Provide Solution for Chemical Sensing

Researchers at the Vienna University of Technology (TU Wien) are working with laser frequency combs to enable chemical analysis on a chip. This new patent-pending technology will enable frequency combs to be created on a single chip in a simple, robust manner.

“It is relatively easy to build a spectrometer with two frequency combs,” researcher Benedikt Schwarz said. “It is possible to make use of beats between different frequencies, similar to those that occur in acoustics, if you listen to two different tones with similar frequency. We use this new method, because it does not require any moving parts and allows us to develop a miniature chemistry lab on a millimeter scale.”

The laser system developed at TU Wien creates many frequencies with equal spacing between them. Courtesy of TU Wien.

The team produces its frequency combs using quantum cascade lasers (QCLs). These lasers are semiconductor structures that consist of many different layers. When electrical current is sent through the structure, the laser emits light in the IR range. The properties of the light can be controlled by tuning the geometry of the layer structure.

“With the help of an electrical signal of a specific frequency, we can control our quantum cascade lasers and make them emit a series of light frequencies, which are all coupled together,” researcher Johannes Hillbrand said.

The researchers compared this phenomenon to swings rocking on a frame. If the frame is made to wobble at the right frequency, all the swings will oscillate in certain coupled patterns, instead of individually. Without the team’s technique, the lasers would be extremely sensitive to disturbances outside of a lab environment, such as temperature fluctuations or reflections that could send some of the light back into the laser.

“The big advantage of our technology is the robustness of the frequency comb,” Schwarz said. “Because of its robustness, our system has a decisive advantage over all other frequency comb technologies: it can be easily miniaturized. We do not need lens systems, no moving parts and no optical isolators; the necessary structures are tiny. The entire measuring system can be accommodated on a chip in millimeter format.”

The researchers created a prototype — a dual-comb setup consisting only of an injection-locked dual-comb chip, a lens, and a mirror — to demonstrate the potential for on-chip dual-comb spectroscopy using their approach. They demonstrated coherent electrical injection locking of the repetition frequency to a stabilized radio-frequency oscillator and showed that the injection-locked QCL spectrum could be phase-locked, resulting in the generation of a frequency comb. They also showed that injection locking could mitigate the effect of optical feedback.

The team at TU Wien: Benedikt Schwarz, Aaron Maxwell Andrews, Gottfried Strasser, Johannes Hillbrand, and Hermann Detz (left to right). Courtesy of TU Wien.

“Our technology can be realized with very little effort and is therefore perfect for practical applications even in difficult environments,” Schwarz said. “Basically, the components we need can be found in every mobile phone.”

Another advantage is that the QCL generates a frequency comb in the IR range, where many of the most important molecules can be most easily detected. “Various air pollutants, but also biomolecules, which play an important role in medical diagnostics, absorb very specific infrared light frequencies,” Hillbrand said. “So, when we measure which infrared frequencies are absorbed by a gas sample, we can tell exactly which substances it contains.”

The results could open the way to miniaturized and all-solid-state MIR spectrometers. For example, the chip could be placed on a drone to measure air pollutants. Chips glued to a wall could search for traces of explosive substances in buildings. The chips could also be used in medical equipment to detect diseases by analyzing chemicals in the respiratory air.

The research was published in Nature Photonics (https://doi.org/10.1038/s41566-018-0320-3).