Spectral Shaper sculpts 10,000 laser comb lines for detection of exoplanets and beyond

Researchers have developed new technology that can shape the spectrum of light emitted by a laser frequency comb across visible and near-infrared wavelengths with more precision than before. This breakthrough could provide an important new tool in the search for Earth-like planets outside our solar system.

When searching for exoplanets, astronomers use high-precision spectroscopy to detect tiny changes in starlight that reveal the subtle “wobble” of a star due to an orbiting planet. But for Earth-sized planets, these wavelength changes are smaller than the natural instabilities of the spectrograph. Laser frequency combs (lasers that emit thousands of evenly spaced spectral lines) are therefore necessary to provide a reference, acting as precise wavelength rulers.

“For astronomers, the big challenge would be to find a planet with a mass similar to Earth and orbiting a star similar to our sun,” said research team leader Derryck T. Reid of Heriot-Watt University in the United Kingdom. “Our spectral shaper can make the lines of a laser frequency comb more uniform, allowing the spectrograph to detect smaller stellar movements, such as those of Earth-like planets, that would otherwise be hidden in noise.”

In their article published in OpticalThe researchers show that using their new spectral shaping method with an astronomical spectrograph in the laboratory, they can precisely control 10,000 individual beamlines, about 10 times better performance than previous approaches.

“While there is an immediate application in astronomy instrumentation, spectral shapers are versatile tools,” Reid said. “This technology could also benefit areas such as telecommunications, quantum optics and advanced radar, where precise control of the shape of light over wide bandwidths can improve signal fidelity, enable faster data transfer and improve the manipulation of quantum states.”

Shaping the spectrum

Spectral shapers are used to refine light to produce precisely defined spectral characteristics. For example, if a light source had more intensity in the longer wavelength red part of the spectrum, a spectral shaper could be used to attenuate those wavelengths to produce a spectrum with a more balanced power distribution.

This type of spectral shaping could, for example, be achieved using a prism that splits white light into different wavelengths along a line, thus forming a single spectrum. However, this one-dimensional line spectrum does not correspond well to the two-dimensional grid of pixels in a spatial light modulator. Spatial light modulators enable programmable pixel-by-pixel control of light intensity and phase across the entire spectrum, enabling high-resolution shaping of complex sources such as laser frequency combs, where each mode can be adjusted independently.

“For our spectral shaper, we took inspiration from astronomical spectrographs in large telescopes, which divide the spectrum of light into multiple rows, a format that allows more efficient use of high-resolution two-dimensional camera sensors,” said Reid. “By replacing the camera typically used in spectrographs with a spatial light modulator, we could control the spectrum of light over a wide bandwidth much more precisely than ever before.”

By mapping each frequency comb line to a unique group of pixels, the researchers were able to control each line independently, giving them the ability to sculpt the spectrum into any shape they wanted.

Next level frequency control

Since it was not possible to develop the technology on a real telescope-based astronomical spectrograph, the researchers built a version in their laboratory. They wrote an algorithm that compared the measured spectrum to a chosen target shape, then adjusted the spatial light modulator until it matched.

They tested the spectral shaper’s ability to shape the spectrum into different patterns, including flattening or isolating different comb lines. For demonstration purposes, they also programmed various photos as target shapes on the two-dimensional spectrograph, mapping the pixels in each photo onto individual laser comb lines.

These experiments showed that they could achieve precise amplitude control of 10,000 comb modes (the “teeth” of the frequency comb) spanning from 580 to 950 nm, with a bandwidth-to-resolution ratio exceeding 20,000. For comparison, previous line-by-line modulation demonstrations reported control of hundreds of comb modes, with bandwidth-to-resolution ratios in the low thousands.

The team is currently working to test the spectral shaper at the Southern African Large Telescope, where they will evaluate its performance in real-world observations.