Two million microscopic mirrors help astronomy laser technology shape light

Heriot-Watt University scientists have significantly advanced a laser technology to precisely manipulate the spectrum of light.

Their approach could be an important new tool for astronomy, where lasers are used to measure the wavelength of light from distant stars.

The team, from the School of Engineering and Physical Sciences, develops laser frequency combs for astronomers. They have shown how to control thousands of individual 'comb teeth', each of which serves as a precision wavelength marker for astronomers.

Their technique could improve the performance of spectrographs in telescopes used in the search for Earth-like planets orbiting distant stars.

The breakthrough is reported in the journal Optica and was funded by the Science and Technology Facilities Council.

Rulers that measure shifts in starlight

Laser frequency combs act as ultra-accurate rulers for light, with each comb tooth resembling one division on the scale of a ruler that astronomers can use to measure tiny shifts in the wavelength of starlight.

These shifts can reveal the presence of an orbiting planet, but are so small for an Earth-sized planet that they can easily be lost amid tiny instabilities in telescope instruments.

The new system gives researchers far greater control over the light produced by the comb. Using an array of two million microscopic mirrors, the Heriot-Watt team can now fine-tune each of 10,000 individual laser frequencies — the “teeth” of the comb — across a broad range of colours.

Professor Derryck Reid, from Heriot-Watt’s School of Engineering and Physical Sciences, said: "This is about making the wavelength rulers that astronomers rely on much clearer and more uniform.

“The more stable and precise the calibration, the better our chances of detecting small planets — perhaps even one like our own.

“We use a grid of two million mirrors, with each mirror the same size as a red blood cell (0.008 mm).

“By devising a way of mapping each comb tooth to a unique group of mirrors, we can make all of the comb teeth in our laser spectrum the same intensity, removing variations that would otherwise reduce the calibration accuracy of the comb.

“Imagine finding a rusty old tape measure, with millimetre divisions and a length of ten metres.

“Some of the divisions are very faint and others are too fat and bold. Our technique would be like restoring each division to its original clarity.”

The upgraded system will soon be tested at the Southern African Large Telescope, one of the largest optical telescopes in the southern hemisphere.

Transforming optical technologies on Earth

While astronomy remains the immediate application, the technology could also support advances in telecommunications, quantum computing and precision measurement.

Professor Reid added: "Laser frequency combs have transformed how we measure light. This new control takes us another step forward — that could ultimately expand our understanding of the universe and shape future optical technologies."