Researchers have developed a new technology that can shape the spectrum of light emitted from a laser frequency comb across the visible and near-infrared wavelengths with more precision than previously possible. This advance could provide an important new tool in the hunt for Earth-like planets outside our solar system.
When searching for exoplanets, astronomers use high-precision spectroscopy to detect tiny shifts in starlight that reveal a star’s subtle “wobble” due to an orbiting planet. But for Earth-sized planets, these wavelength changes are smaller than the spectrograph’s natural instabilities, so laser frequency combs - lasers that emit thousands of evenly spaced spectral lines - are needed to provide a reference, acting like precise wavelength rulers.
“For astronomers, the big prize would be to find a planet with a mass similar to the Earth and orbiting a star similar to our Sun,” said research team leader Derryck T. Reid, from Heriot-Watt University in the UK. “Our spectral shaper can make the lines on a laser frequency comb more uniform, which allows the spectrograph to detect smaller stellar motions, such as those from Earth-like planets, that would otherwise be hidden in the noise.”
In Optica, Optica Publishing Group’s journal for high-impact research, the researchers show that using their new spectral shaping method with a lab-based astronomical spectrograph can precisely control 10,000 individual lines of light, a roughly 10-fold improvement in performance over previous approaches.
“Although there is an immediate application in astronomy instrumentation, spectral shapers are versatile tools,” said Reid. “This technology could also benefit fields such as telecommunications, quantum optics and advanced radar, where precise control over the shape of light across broad bandwidths can improve signal fidelity, enable faster data transfer and enhance the manipulation of quantum states.”
Shaping the Spectrum
Spectral shapers are used to fine-tune 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 these wavelengths to produce a spectrum with a more balanced power distribution.
This type of spectral shaping might, for example, be accomplished using a prism, which splits white light into various wavelengths along a line, forming a single spectrum. However, this one-dimensional line spectrum is not well matched to the two-dimensional grid of pixels in a spatial light modulator. Spatial light modulators enable programmable, pixel-by-pixel control of the light’s intensity and phase across the 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 the astronomical spectrographs on large telescopes, which split up the spectrum of light into many rows, a format that makes more efficient use of high-resolution two-dimensional camera sensors,” said Reid. “By substituting a spatial light modulator for the camera typically used in spectrographs, we could control the spectrum of light across 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 to any shape they wanted.
Next-Level Frequency Control
Since it wasn’t possible to develop the technology on a true telescope-based astronomical spectrograph, the researchers built a version of one in their lab. They wrote an algorithm that compared the measured spectrum to a chosen target shape and 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 programed various photos as target shapes on the two-dimensional spectrograph, mapping the pixels of each photo to individual laser comb lines.
These experiments showed that they could accomplish precise amplitude control of 10,000 comb modes - the “teeth” of the frequency comb - spanning 580 to 950 nm, with a bandwidth:resolution ratio exceeding 20,000. For comparison, previous demonstrations of line-by-line modulation reported the control of hundreds of comb modes, with bandwidth:resolution ratios of a few thousand.
The team is now working to test the spectral shaper at the Southern African Large Telescope, where they will assess its performance during actual observations.