Alexander G. Glenday

A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1

Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s-1 (ref. 1), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earth-like orbit, a precision of ∼5 cm s-1 is necessary. The combination of a laser frequency comb with a Fabry–Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration, with recent encouraging results. Here we report the fabrication of such a filtered laser comb with up to 40-GHz (∼1-Å) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or ‘astro-comb’, is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s-1 in astronomical radial velocity measurements.

Visible wavelength astro-comb

We demonstrate a tunable laser frequency comb operating near 420 nm with mode spacing of 20-50 GHz, usable bandwidth of 15 nm and output power per line of ~20 nW. Using the TRES spectrograph at the Fred Lawrence Whipple Observatory, we characterize this system to an accuracy below 1m/s, suitable for calibrating high-resolution astrophysical spectrographs used, e.g., in exoplanet studies.

In-situ determination of astro-comb calibrator lines to better than 10 cm s −1

Improved wavelength calibrators for high-resolution astrophysical spectrographs will be essential for precision radial velocity (RV) detection of Earth-like exoplanets and direct observation of cosmological deceleration. The astro-comb is a combination of an octave-spanning femtosecond laser frequency comb and a Fabry-Pérot cavity used to achieve calibrator line spacings that can be resolved by an astrophysical spectrograph. Systematic spectral shifts associated with the cavity can be 0.1-1 MHz, corresponding to RV errors of 10-100 cm/s, due to the dispersive properties of the cavity mirrors over broad spectral widths. Although these systematic shifts are very stable, their correction is crucial to high accuracy astrophysical spectroscopy. Here, we demonstrate an in-situ technique to determine the systematic shifts of astro-comb lines due to finite Fabry-Pérot cavity dispersion. The technique is practical for implementation at a telescope-based spectrograph to enable wavelength calibration accuracy better than 10 cm/s.

Green astro-comb for HARPS-N

We report the design, installation and testing of a broadband green astro-comb on the HARPS-N spectrograph at the TNG telescope. The astro-comb consists of over 7000 narrow lines (<10-6 nm width) spaced by 16 GHz (0.02 nm at 550 nm) with wavelengths stabilized to the Global Positioning System (GPS) and with flat power from 500 to 620 nm. The narrow lines are used to calibrate the spectrograph and measure its line profile. The short term sensitivity of HARPS-N is measured to be less than 2 cm/s and the long-term drift of the spectrograph is approximately 10 cm/s/day. The astrocomb has been partially automated with future work planned to turn the astro-comb into a fully automated, push button instrument.

HARPS-N @ TNG, two year harvesting data: performances and results

The planet hunter HARPS-N[1], in operation at the Telescopio Nazionale Galileo (TNG)[13] from April 2012 is a highresolution spectrograph designed to achieve a very high radial velocity precision measurement thanks to an ultra stable environment and in a temperature-controlled vacuum. The main part of the observing time was devoted to Kepler field and achieved a very important result with the discovery of a terrestrial exoplanet. After two year of operation, we are able to show the performances and the results of the instrument.

Conjugate Fabry-Perot cavity pair for astro-combs

We propose a new mode-filtering scheme for astro-combs using two Fabry-Perot cavities: a “conjugate Fabry-Perot cavity pair”. Simulations suggest that this scheme improves astro-comb accuracy in the presence of errors from nonlinear fibers.

A green astro-comb for Earth-like exoplanet searches

Our astro-comb, providing >7000 lines spaced by 16 GHz from 500-620 nm, has been deployed at TNG telescope as a wavelength calibrator for HARPS-N spectrograph. It provides sub-10 cm/s calibration accuracy required for exo-Earth searches.

Spectrally flat, broadband visible-wavelength astro-comb

We demonstrate a broadband visible-wavelength astro-comb enabled by two key technologies: dispersion-managed, fiber-optic Cherenkov radiation for green-to-red source-comb generation and complementary chirped-mirror pairs for constructing broadband Fabry-Perot filtering cavities.

Deploying comb and tunable lasers to enable precision radial velocity surveys

We describe recent progress toward developing optical frequency laser combs and tunable laser to the problem of more precise calibration of high dispersion astronomical spectra, thus permitting radial velocity determinations in the few cm/sec regime. We describe two programs in progress to calibrate both a cross dispersed echelle spectrograph with a laser comb and to calibrate a multiobject echelle spectrograph with a tunable laser.

Tunable blue astro-comb

We developed a tunable, visible frequency comb near 420nm with 22GHz mode spacing and 20nm spectral width, which is able to calibrate astronomical spectrographs to search Earth-like exoplanets orbiting around stars similar to the Sun.