A research team has put two frequency comb generators on a single millimeter-size chip, miniaturizing the dual comb and thus making it portable enough to perform sensing and spectroscopy in field environments in real time. Researchers from Columbia University used a single laser to simultaneously generate two microresonator combs on the same chip, reducing complexity and removing the need for complicated electronics. A compact, integrated, silicon-based chip used to generate dual combs for extremely fast molecular spectroscopy. Courtesy of A. Dutt, A. Mohanty, E. Shim, G. Patwardhan/Columbia Engineering. To guide and enhance light while maintaining ultralow loss, they used silicon nitride to build rings just tens of micrometers in diameter. By combining the silicon nitride with platinum heaters, they were able to finely tune the rings to work in tandem with a single input laser. Professor Michal Lipson said that silicon is a widely used material in the semiconductor industry. “By leveraging the capabilities of this mature industry, we can foresee reliable fabrication of these dual comb chips on a massive scale at a low cost,” Lipson said. The team demonstrated broadband optical spectra spanning 51 Thz and low-noise operation of both combs, resulting in narrow (less than 10 kHz) microwave beat notes. The team further used one comb as a reference to probe the formation dynamics of the other comb, introducing a technique to investigate comb evolution without auxiliary lasers or microwave oscillators. Experiments showed high signal-to-noise ratio absorption spectroscopy spanning 170 nm, using the dual-comb source over a 20-ms acquisition time. The compact, chip-scale dual-comb spectrometer was used to measure a broad spectrum of dichloromethane in just 20 ms — a task that would have taken several seconds using a conventional spectrometer. “By mixing two frequency combs instead of a single comb, we can increase the speed at which measurements are made by thousandfolds or more,” said researcher Avik Dutt. Researchers said that the miniaturized spectrometer is well suited to measuring liquids and solids, which have broader absorption features than gases. “Our very broad dual combs have a moderate spacing between the successive lines of the frequency comb, as compared to gas spectrometers, which can get away with a less broad dual comb, but need a fine spacing between the lines of the comb,” said professor Alexander Gaeta. Conventional dual-comb spectrometers are bulky tabletop instruments and are not portable because of their size, cost and complexity. In contrast, the chip-scale dual comb can easily be carried around and used for spectroscopy in the field. “There is now a path for trying to integrate the entire device into a phone or a wearable device,” said Gaeta. The team is working on further broadening the frequency span of the dual combs and on increasing the resolution of the spectrometer by tuning the lines of the comb. Its dual-comb spectrometer could provide the foundation for compact, yet robust, spectrometers at nanosecond time scales enabled by large beat-note spacings. “One could also envision integrating the input laser into the chip for further miniaturizing the system, paving the way for commercializing this technology in the future,” said Dutt. The research was published in Science Advances (doi:10.1126/sciadv.1701858).