Chalmers researchers make optical microcombs 10 times more efficient

21 Sep 2023

Trick is using two micro-resonators, instead of one, to boost laser power of soliton microcomb.

Optical microcombs can help with the discovery of planets outside the solar system or track new diseases in the human body. But current microcombs are inefficient and unable to reach their full potential.

Now, researchers at Chalmers University of Technology in Sweden have scored a world first with their solution to make microcombs ten times more efficient. They say that their breakthrough “opens the way to new discoveries in space and healthcare and paves the way for high-performance lasers in a range of other technologies.”

Laser frequency combs can measure frequencies with high precision and are considered the most disruptive technological advance in the field since the birth of the laser. Simply put, a microcomb is like a ruler made of light.

The principle is based on a laser sending photons that circulate inside a small cavity, a so-called microresonator, where the light is divided into a wide range of frequencies. These frequencies are precisely positioned in relation to each other, like the markings on a ruler. Thus, a new kind of light source can be created consisting of hundreds – or even thousands – of frequencies, like lasers beaming in unison.

Since virtually all optical measurements are connected to light frequencies, the microcomb has a multitude of applications – from calibrating instruments that measure signals at light-year distances in space in the search for exoplanets, to identifying and keeping track of our health via the air we exhale.

Unprecedented efficiency

Up to now, a fundamental problem with microcombs has been that their efficiency was too weak to produce a wider technological impact on society. The conversion efficiency between the laser and the microcomb was too weak, meaning that only a fraction of the power contained in the laser beam was usable.

But now, researchers at Chalmers have developed a method to increase the effect of the microcomb’s laser beams tenfold. “We’ve developed a new method that breaks what was previously thought to be a fundamental limit for optical conversion efficiency,” said Victor Torres Company, Professor of Photonics at Chalmers.

“Our method increases the laser power of the soliton microcomb by ten times and raises its efficiency from around 1 percent to over 50 percent.”

The method uses two micro-resonators, as opposed to just one. They form a unique ensemble with properties that are greater than the sum of its parts. One of the resonators enables the light coming from the laser to couple with the other resonator; rather like impedance matching in electronics.

Promising high-performance lasers

Described in Nature Photonics, the method opens up completely new areas of application for high-performance lasers. The technology was recently patented by the project’s researchers. They have founded Iloomina AB, a startup company which will launch the technology onto a wider market.

“The new microcombs have transformative potential because they make high-performance laser technology available to many more markets. For example, frequency combs could be used in lidar modules for autonomous driving, or in GPS satellites and environmental sensing drones, or in data centres to enable bandwidth-intensive AI apps,” said Torres Company.