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  • April 2012

Optics InfoBase > Spotlight on Optics > Demonstration of on-sky calibration of astronomical spectra using a 25 GHz near-IR laser frequency comb

Demonstration of on-sky calibration of astronomical spectra using a 25 GHz near-IR laser frequency comb

Published in Optics Express, Vol. 20 Issue 6, pp.6631-6643 (2012)
by Gabriel G. Ycas, Franklyn Quinlan, Scott A. Diddams, Steve Osterman, Suvrath Mahadevan, Stephen Redman, Ryan Terrien, Lawrence Ramsey, Chad F. Bender, Brandon Botzer, and Steinn Sigurdsson

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Spotlight summary: The mode-locked laser frequency comb (LFC) provides a broad spectrum of regularly-spaced emission lines, which can be stabilized by an atomic clock and serve as a coherent link between the optical and radio-frequency regimes. With intense technological development over the last decade, LFCs have revolutionized optical precision measurement and control, enabling broadband ultra-fast molecular spectroscopy, ultra-stable optical atomic clocks, and optical waveform synthesis. One of the most recent and exciting LFC applications is in astrophysics, and in particular the search for Earth-like planets around other stars (exoplanets) using the radial velocity (RV) technique. The presence of an exoplanet can be indirectly revealed by measuring the small periodic Doppler shift of a star’s broad spectrum, typically including ~105 atomic absorption lines, caused by the exoplanet’s gravitational pull and the associated periodic variation in the RV of the star relative to observatories on the Earth. The sensitivity of the RV technique is often limited by the wavelength calibrator used to correct for systematic errors and slow drifts in the astrophysical spectrograph integrated with the observing telescope. Existing astrophysical wavelength calibrators, such as thorium-argon emission lamps and molecular iodine absorption cells, have been used successfully in the discovery of more than 600 large exoplanets (of the size of Jupiter or Neptune) orbiting a wide variety of stars, but these calibrators are inadequate for measurements of the tiny stellar RV shifts induced by small, rocky exoplanets like the Earth or Mars.

Astrophysical wavelength calibrators based on the broad, stable LFC spectrum (“astro-combs”) may solve this problem, improving the accuracy and stability of astrophysical spectroscopy by two orders of magnitude and thereby enabling the discovery and characterization of Earth-like exoplanets. Over the last few years, several groups have developed prototype astro-combs operating in the visible spectrum, including a collaboration of which we are members between the Harvard-Smithsonian Center for Astrophysics (CfA) and MIT, as well as a collaboration between the Max Planck Institute for Quantum-Optics (MPQ) and the European Southern Observatory (ESO). The work of the CfA/MIT and MPQ/ESO teams is directed primarily at the search for Earth-like exoplanets orbiting in the liquid-water habitable zone around Sun-like stars, which are brightest in the visible spectrum. Promising initial tests of these visible-wavelength astro-combs have been performed with high-resolution astrophysical spectrographs located at the Whipple Observatory in Arizona and the La Silla Observatory in Chile, for the CfA/MIT and MPQ/ESO collaborations, respectively. With further development and optimization, visible-wavelength astro-combs should enable astrophysical spectroscopy with accuracy and long-term stability sufficient to measure slow changes in stellar RV smaller than 5 cm/s, which is equivalent to <100 kHz shifts in the stellar visible emission spectrum. As a comparison, the Earth induces an RV change in the Sun, relative to a distant observer, of ±9 cm/s with a period of one year.

In the present paper, Ycas et al. report a complementary RV technique for the search for Earth-like exoplanets. They developed an astro-comb operating in the near-infrared (NIR) and used it for wavelength calibration of the high-resolution PathFinder spectrograph operated with the 9.2 m Hobby-Eberly telescope at the McDonald Observatory in southwest Texas. NIR astrophysical spectroscopy is optimal for the search for small exoplanets orbiting M dwarf (also known as red dwarf) stars, which have peak emission in the NIR and are the most numerous type of stars (~60% of stars near our solar system). The liquid-water habitable zone is much closer to an M dwarf than to a Sun-like star, because M dwarfs are relatively cool and low mass. Consequently, the RV change caused by an Earth-mass exoplanet in the habitable zone around an M dwarf star is more than an order of magnitude larger (>1 m/s) than for a Sun-like star, greatly easing the technical challenge of detection.

Ycas et al. used an erbium-fiber LFC, two Fabry-Perot mode-filtering cavities, and nonlinear fibers to produce a broadband (1450-1700 nm) NIR frequency comb with 25 GHz line spacing, optimized for the resolution of the PathFinder spectrograph. Long-term stability was guaranteed by referencing all control loops to an atomic clock governed by the Global Positioning System (GPS). Laboratory tests of the NIR astro-comb demonstrated optical frequency measurements equivalent to an RV accuracy of 6 cm/s, which would be more than sufficient for the detection of an Earth-like exoplanet around an M dwarf star. Ycas et al. then employed the NIR astro-comb in proof-of-principle calibrations of the PathFinder spectrograph during a two week program of observations of several nearby stars, yielding a stellar RV precision of 10 m/s limited by technical issues such as guiding of the star and astro-comb calibration light to the spectrograph. The authors believe that the lessons learned from their work will inform the development of a broader-bandwidth NIR astro-comb (covering ~950 to 1700 nm) to be used as the wavelength calibrator for the planned Habitable Zone Planet Finder spectrograph, which is expected to achieve RV accuracy <1 m/s, suitable for the detection of Earth-like exoplanets in the habitable zone around M dwarf stars.

--Chih-Hao Li and Ronald Walsworth

Technical Division: Light–Matter Interactions
ToC Category: Nonlinear Optics
OCIS Codes: (190.7110) Nonlinear optics : Ultrafast nonlinear optics
(300.6340) Spectroscopy : Spectroscopy, infrared
(350.1270) Other areas of optics : Astronomy and astrophysics

Posted on April 13, 2012

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