Combing light with sharper teeth

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. And the discovery was made almost by coincidence.

A ruler made of light. That is the simplified comparison that is commonly used to describe what a microcomb is. In short, the principle is based on a laser sending light that circulates inside a small cavity, a so-called micro resonator. There, the light is divided into a variety of colours, or frequencies. The frequencies are precisely located, similar to the markings on a ruler.

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.

New insights on the frequency comb’s lines
"Laser frequency combs have revolutionised research that relies on frequency metrology," says Victor Torres Company, professor at the Department of Microtechnology and Nanoscience, MC2, 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 micro resonator were thought of as the reason for this.

When Fuchuan Lei, researcher at MC2, tested these theories and ran the experiments with devices fabricated at the MC2 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.

A new theory in place
"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.

Possible to measure more precisely
“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.

More about the scientific article and the research

• The article “Optical linewidth of soliton microcombs” was published in Nature Communications and written by Fuchuan Lei, Zhichao Ye, Óskar B. Helgason, Attila Fülöp, Marcello Girardi and Victor Torres Company at the Department of Microtechnology and Nanoscience, Chalmers University of Technology, Sweden.
• The devices demonstrated in this work were fabricated at Myfab Chalmers. The research has been funded by the European Research Council, Knut Alice Wallenbergs Foundation, and the Swedish Research Council.

For more information, please contact

Victor Torres Company
Professor, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Sweden
+46317721904, torresv@chalmers.se

Fuchuan Lei
Researcher, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Sweden
fuchuan@chalmers.se

More about: Frequency combs and microcombs

• A frequency comb is a special laser where the emission frequencies are evenly spaced. It functions as a ruler made of light, where the markers set the frequency scale across a portion of the electromagnetic spectrum, from the ultraviolet to the mid infrared. The location of the markers can be linked to a known reference. This was achieved in the late 90s, and it signified a revolution in precision metrology – an achievement recognised by the Nobel Prize in Physics in 2005.
• A microcomb is a modern technology, alternative to mode-locked lasers, that can generate repetitive pulses of light at astonishing rates. They are generated by sending laser light to a tiny optical cavity called a microresonator. Thus, microcombs have two important attributes that make them extremely attractive for practical purposes: the frequency spacing between markers is very large (typically between 10 – 1,000 GHz), that is much higher than the spacing in mode-locked laser frequency combs, and they can be implemented with photonic integration technology. The compatibility with photonic integration brings benefits in terms of reduction of size, power consumption and the possibility to reach mass-market applications.