Microcombs have widely differing appli­cation areas – they can help us discover planets outside our solar system as well as track diseases in our bodies. New research results at Chalmers University of Techno­logy, Sweden, now give a deeper under­standing of how the line width in the combs works, something that will, among other things, enable even more precise measure­ments in the future. 

Today, virtually all optical measure­ments can be linked to light frequencies, and this gives the microcombs a plethora of different appli­cation areas – everything from cali­brating instruments that measure signals at light-years distances, to identifying and keeping track of our health via the air that we exhale. “Laser frequency combs have revo­lutionized research that relies on frequency metrology,” says Victor Torres Company at the Department of Micro­technology and Nanoscience at Chalmers University of Technology.

A key question when working with microcombs is how narrow the frequency comb lines are. The pre­vailing view until a few years ago was that the lines cannot be narrower than the input light from the laser. When researchers began to examine this more in depth, it was discovered that the lines located farther out from the laser are a little wider than the centrally located lines. Noise sources in the micro­resonator were thought of as the reason for this. When Fuchuan Lei tested these theories and ran the experiments with devices fabricated at the Chalmers Nano­fabrication Labora­tory facilities, he discovered that some of the lines were in fact narrower than the light of the laser source itself. He traced all noise sources that can influence the linewidth or the purity of the lines, repeated the experiments and continued to receive the same result.

“We didn't understand why but based on these results we developed a theo­retical model that explained what happened, did simulations, and confirmed via experiments that our model was correct”, says Victor Torres Company. “Earlier on it was not clear how the different noise mechanisms would affect the linewidth of the comb lines in the micro comb.” “At first we thought something must be wrong, but once we had our theory in place every­thing was clear”, says Fuchuan Lei. How narrow the markings are in a microcomb has great signi­ficance in how it can be used. A microcomb with narrowly placed markings allows for even more precise measurements, and that is why under­standing why the lines are narrower is a key issue in the development of microcombs.

Victor Torres Company compares it to rulers made of different kinds of materials. “Imagine you would draw markers with some chalk versus if you would do it with a pencil. You can define a grid, you can define the spacing, but with a pencil you can measure more precisely because then you have your ruler with very well-defined marks”, he says. What was originally an interes­ting curiosity discovered by the researchers, came to reveal the physical mechanisms of what causes the lines in the microcomb to vary in linewidth. “Thanks to our research and publi­cation, those who work with the design of this type of devices will understand how the different noise sources affect the different parameters and the per­formance of the micro­comb”, says Victor Torres Company. (Source: Chalmers U.)

Reference: F. Lei et al.: Optical linewidth of soliton microcombs, Nat. Commun. 13, 3161 (2022); DOI: 10.1038/s41467-022-30726-5

Link: Photonics Laboratory, Dept. of Microtechnology and Nanoscience, Chalmers University of Technology, Gothenburg, Sweden