Laser combover: Calibrating exoplanets

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  • Published: Feb 1, 2016
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Laser combover: Calibrating exoplanets

Optical rules

A segment of the near infrared (IR) spectrum of a cool star as observed by the Keck II telescope's near infrared spectrometer (right) compared with (left) the near IR spectrum of the laser frequency comb. Credit: Emily Martin

Optical rulers, more correctly known as laser frequency combs, could be used to calibrate the infra-red spectra recorded by astronomers hunting for Earth-like exoplanets. The combs facilitate precision measurements that can reveal the gravitational pull of planets orbiting distant stars.

The tick marks on an optical ruler serve as stable reference points when making precision measurements such as those of the small shifts in starlight caused by planets pulling gravitationally on their parent stars. Unfortunately, the commercially available instruments have a major drawback. Because their tick marks are so finely spaced, the light output of these combs must be filtered to produce useful reference lines. This extra step adds complexity to the system and requires costly additional equipment. In order to resolve this and other problems, a team at Caltech, California Institute of Technology, Pasadena, has investigated a different type of comb, one not used before by astronomer. This type of comb produces lines that can be easily resolved without the need for additional filters. Moreover, it can be constructed from readily available components commonly used across the telecommunications industry.


"We have demonstrated an alternative approach that is simple, reliable, and relatively inexpensive," explains Kerry Vahala. The team points out that they had used this kind of frequency comb in earlier work involving the generation of high-stability microwaves. "We believe members of the astronomical community could greatly benefit in their exoplanet hunting and characterization studies with this new laser frequency comb instrument," adds graduate student Xu Yi.

Laser frequency combs have been in common use as precision rulers since the late 1990s in metrology and spectroscopy. The technology's developers, John Hall of JILA and the National Institute of Standards and Technology (NIST) and Theodor Hänsch of the Max Planck Institute of Quantum Optics and Ludwig Maximilians University Munich, Germany, received half of the Nobel Prize in Physics in 2005 for their efforts. More recently, astronomers have begun to use these combs in the radial velocity, or "wobble" method, of exoplanet detection. The wobble refers to the periodic changes in a star's motion, accompanied by starlight shifts due to the Doppler Effect induced by the gravitational pull of an exoplanet orbiting that star. The size of the stellar wavelength shift, usually a few quadrillionths of a metre, together with the period of the wobble can be used to determine an exoplanet's mass and its distance from the star.

Cool red dwarf stars

Mode-locked laser combs have been most useful in this work, but 49 out of every 50 tick marks must be filtered out using temperature- and vibration-insensitive filtering equipment. The Caltech team's new electro-optical comb exploits the microwave modulation of a continuous laser source, rather than a pulsed laser and produces comb lines with a tens of gigahertz separation, which precludes the need for such filtering.

Field tests at Mauna Kea in Hawaii on NASA's Infrared Telescope Facility (IRTF) and with the Near Infrared Spectrometer on the W. M. Keck Observatory's Keck II telescope provided steady calibration at room temperature for more than five days at IRTF and again six months later at Keck. "From a technological maturity point of view, the frequency comb we have developed is already basically ready to go and could be installed at many telescopes," explains team member Scott Diddams of NIST.

The Caltech comb produces spectral lines in the infrared, making it ideal for studying red dwarf stars, the most common stars in the Milky Way. "Our goal is to make these laser frequency combs simple and sturdy enough that you can slap them onto every telescope, and you don't have to think about them anymore," adds Charles Beichman, executive director of the NASA ExoPlanet Science Institute at Caltech. "Having these combs routinely available as a modest add-on to current and future instrumentation really will expand our ability to find potentially habitable planets, particularly around very cool red dwarf stars," he says.

Related Links

Nature Commun 2016, 7, 10436: "Demonstration of a near-IR line-referenced electro-optical laser frequency comb for precision radial velocity measurements in astronomy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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