University's research advances ultra-precision laser tech

University of Auckland research may help the world to miniaturise the laser tools used for ultraprecise measurements such as atomic clocks and quantum computing.

There are big scientific and commercial implications.

So far, the laser tools called optical frequency combs have been trapped in the lab because of their size and expense. Optical frequency combs are the standard tools for measuring the frequency – or colours – of light. They are sometimes called `light rulers’ or ‘rainbow rulers’.

But research by University of Auckland and US scientists, reported in the prestigious journal Nature, suggests a new path for shrinking the technology to chip size for future navigation systems, communications networks, precision sensors and portable atomic clocks.

The breakthrough is a culmination of work that started in 2021, when Professor Miro Erkintalo, who is the head of the Department of Physics at the University and a researcher at the Dodd-Walls Centre for Photonic and Quantum Technologies, began exploring a potential new way to generate chip-scale frequency combs.

With the help of his students, he predicted that two laser beams launched into a chip-scale ring could generate a new type of frequency comb.

The prediction became reality in 2024, when the Auckland researchers teamed up with Grégory Moille and Kartik Srinivasan from the University of Maryland and the US National Institute of Science and Technology to successfully demonstrate the idea in the lab.

While the 2024 demonstration was a groundbreaking physics proof-of-concept, breakthrough applications were still unrealized.

Now, two years later, the team has taken the next steps. They found that the new comb can be generated by two lasers an octave apart. (The term octave, borrowed from music, refers to a doubling of frequency.)

The resulting miniaturised frequency comb immediately unlocks a host of applications.

To showcase the platform, the researchers used the chip-based system to perform the three core tasks associated with frequency combs: generating precise optical frequencies, producing low-noise millimetre-wave signals and integrated optical clock readout.

“This work is the culmination of several years of work, involving a fantastic international collaboration that brings together the world-leading knowledge and innovation of New Zealand researchers with equally world-leading folks in the US,” says Erkintalo. “It also shows how new technological applications with commercial implications emerge from research that is originally driven by curiosity.”

The original laboratory-scale optical frequency comb was pioneered in the late 1990s by John Hall and Theodor Hänsch, who won the 2005 Nobel Prize in Physics. 

Just as a school ruler has evenly spaced black lines to measure distance, an optical frequency comb creates millions of perfectly spaced, precise colours (frequencies) of light to measure time, distance, and chemical signals with extreme accuracy.

Optical frequency combs have enabled the creation of the most precise clocks in the world – optical atomic clocks – and the many other applications have included experiments in basic physics.

Miniature versions can provide the basis for bringing laboratory-grade precision into everyday technologies.

Portable optical atomic clocks could help map underground variations in mineral deposits and enable navigation systems that don’t rely on GPS satellite signals. 

“Though we have been working on chip-integrated optical frequency combs for many years, their control and stabilization – essential for many applications – has often been complicated and difficult,” says the University of Maryland’s Moille. “With this new approach, we finally see a viable path for their use in deployable atomic timekeeping, which is one of their most demanding and important applications.”

The advance dreamed up by Erkintalo and his collaborators flips the design of the optical frequency comb.

In a conventional lab system and existing miniaturised architectures, light from a single laser cascades and stretches outward to cover a wide spectrum.

By way of contrast, the new design starts with two lasers placed far apart at opposite ends of the spectrum. The system then automatically fills in all the light frequencies between those two boundaries, overcoming challenges that hampered the performance of existing architectures.

Future uses could include synchronisation in telecommunications, and sensors capable of detecting extremely small changes in their environments.

Reducing the size, weight, power, and cost of optical frequency combs can lead to mass production and move the technology from lab to consumer, Erkintalo and his collaborators write in the Nature paper.

Erkintalo, Moille and Srinivasan have submitted a provisional patent application based on aspects of the work.