Laser Frequency Combs for Astronomical Observations

Abstract

A direct measurement of the universe's expansion history could be made by observing in real time the evolution of the cosmological redshift of distant objects. However, this would require measurements of Doppler velocity drifts of approximately 1 centimeter per second per year, and astronomical spectrographs have not yet been calibrated to this tolerance. We demonstrated the first use of a laser frequency comb for wavelength calibration of an astronomical telescope. Even with a simple analysis, absolute calibration is achieved with an equivalent Doppler precision of approximately 9 meters per second at approximately 1.5 micrometers-beyond state-of-the-art accuracy. We show that tracking complex, time-varying systematic effects in the spectrograph and detector system is a particular advantage of laser frequency comb calibration. This technique promises an effective means for modeling and removal of such systematic effects to the accuracy required by future experiments to see direct evidence of the universe's putative acceleration.

Carbon nanotubes for ultrafast fibre lasers

“…switching, routing and regeneration of optical signals, as well as for generation of ultrashort laser pulses [1]. Femtosecond lasers found applications in high-precision metrology of time and distance [2], spectroscopy [3], nonlinear optics and astrophysics [4] and many other applications [5][6][7]. In this review, we discuss carbon nanotube-based photonic technologies and applications making a particular focus on mode-locking fibre lasers.…”

Testing the Stability of the Fine Structure Constant in the Laboratory

“…In 2008 a first implementation of the laser frequency comb as a calibration tool for the German Vacuum Tower Telescope (VTT) (Schröter et al 1985) has been demonstrated (Steinmetz et al 2008). The very high resolution of the VTT (0.8 GHz) is still too low to resolve individual modes of the erbium fiber laser frequency comb with ω r = 2π × 250 MHz used for its calibration.…”

“…If Doppler shifts on the order of 1 cm s −1 (3 × 10 −11 c) could be measured the presumed acceleration of the cosmic expansion could be verified in real time in a largely model independent way, i.e. without assuming the validity of general relativity (Steinmetz et al 2008). For a typical high-resolution spectrometer used in astrophysics such a resolution corresponds to a physical size of one silicon atom on a CCD substrate.…”

“…It has been demonstrated recently that a frequency comb with a resolvable large mode spacing can be used as an accurate calibration tool for high-resolution spectrographs. This is particular interesting for astrophysical applications (Steinmetz et al 2008). An accurate frequency axis for spectrometers is required for a number of sensitive fundamental measurements like testing the drift of redshifts of different astrophysical objects (Sandage 1962), the search for extrasolar planets by the reflex Doppler motion (Mayor and Queloz 1995;Marcy and Butler 1996;Lovis et al 2006) as well as the search for cosmological variations of fundamental constants (Bahcall and Salpeter 1965;Thompson 1975;Webb et al 1999).…”

“…Li et al 2008). The Fabry-Pérot cavity used by Steinmetz et al (2008) has a free spectral range of mω r where the integer m can be set between 4 and 60. This cavity interferometrically suppresses all modes generated by the laser except every mth.…”