Operation of a broadband visible-wavelength astro-comb with a high-resolution astrophysical spectrograph

Abstract

Searches for Earth-like exoplanets using the periodic Doppler shift of stellar absorption lines require 10 cm∕s precision in the measurement of stellar radial velocity (RV) over timescales of years. Current techniques have led to the discovery of short-period exoplanets that induce RV wobbles as small as ≈1 m∕s on their parent stars. It has been suggested that order-of-magnitude improved RV precision may be achievable using an astro-comb, a laser frequency comb optimized for astrophysical spectrograph wavelength calibration. Here we report the development of a broadband visible-wavelength astro-comb and its operation with the HARPS-N spectrograph at the Telescopio Nazionale Galileo in the Canary Islands. This green astrocomb has >7000 narrow (<1 MHz) spectral lines spaced by 16 GHz with relatively uniform line power from 500 to 620 nm. The line frequencies are locked to GPS, enabling us to realize HARPS-N wavelength calibration with RV measurement precision and stability <10 cm∕s.

Free-running performance and full control of a passively phase-stable Er:fiber frequency comb

“…Tuning the output of the DFG to the Er:fiber gain window allows for reamplification via standard telecom components, providing an intense and fully coherent output of phase-stable pulses [15,16]. Inherent stability of the CEP is especially appealing for space-based applications [17,18] and high-power fiber systems aiming at sensitive experiments in extreme nonlinear optics [19]. Very recently, a DFG comb from an Yb:fiber oscillator mode locked via nonlinear polarization rotation was stabilized to a linewidth of 1 kHz by coupling to an external reference cavity [20].…”

A crucial test for astronomical spectrograph calibration with frequency combs

“…The HARPS spectrograph 22 is one of several cutting-edge spectrographs that have recently been equipped with an LFC 1,5,10,[23][24][25] . After a number of test campaigns to demonstrate the technique 2,3, 26 , HARPS has been given a permanently installed LFC (LFC1) as its future routine calibrator in May 2015.…”

An astro-comb calibrated solar telescope to search for the radial velocity signature of Venus

“…14,15 This green astro-comb produces 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 (Fig. 1).…”

“…15 We have also measured the line profile of HARPS-N. We fit a convolution of the fiber profile (a skewed half circle), optical aberrations (16 orders of Hermite-Gaussian modes), and the CCD pixel response (top hat function) to these observed astro-comb spectra. The results indicate an instrument line profile consistent with the design of HARPS-N: a line profile varying slowly in width and shape across each order of the spectrograph.…”

“…On the blue end of each order the PSF is narrower and skewed to the blue relative to pixel center and on the red end the PSF is broader and skewed to the red relative to the pixel center. 15 The astro-comb has been partially automated with future work planned to turn the astro-comb into a fully automated, turn-key instrument.…”