Operation of a Broadband Visible-Wavelength Astro-Comb with a High- Resolution Astrophysical Spectrograph
Fig. 3. (a) Dimensions of tapered photonic crystal fiber (PCF, not to scale). A solid-core PCF is tapered to the desired size over a few millimeters, which gives an adiabatic transition between the transverse modes of the large and small fiber cores. (b) Optical micrograph of tapered PCF with a sealed end. End sealing collapses the air holes guiding the light at the beginning and end of the fiber such that there is an adiabatic transition between free propagation in the uniform glass at each fiber end and guided propagation in the solid core of the holey fiber, which approximately halves the numerical aperture.*
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.
In the March, 2015, issue of Optica,* Dr. Ronald Walsworth and colleagues from Harvard, MIT, Idesta Quantum Electronics, and University of Hamburg, germany, 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 astro-comb 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 the researchers to realize HARPS-N wavelength calibration with RV measurement precision and stability < 10 cm/s.
*A.G. Glenday, C.H. Li, N. Langellier, G. Chang, et al., "Operation of a broadband visible-wavelength astro-comb with a high-resolution astrophysical spectrograph," Optica, 2 (2015) http://dx.doi.org/10.1364/OPTICA.2.000250.