An Astro-Comb Based on a Harmonically
Dr John McFerran
Frequency Standards and Metrology Group,
University of Western Australia
3:00 pm, Friday 12 December 2008, EN101, Ground Floor, Engineering Building, Hawthorn.
The development of an astronomical frequency comb, suitable as a calibration source for échelle spectrographs, has
recently been initiated by several research groups worldwide. The outcomes of such a device, integrated with the
next generation of Extremely Large Telescopes, may well include the detection of earth-size planets in solar-mass
star systems, a measurement of the universe's rate of change of expansion that is model independent, and reduced
uncertainty associated with drift-rate measurements of fundamental constants. While present developments of the
astro-comb are centred on the use of femtosecond (fs)-pulsed solid-sate lasers, here we propose creating an
equivalent device based on a fs-pulsed fibre laser. The likely advantages of the fibre-laser based astro-comb
include: compactness, maneuverability, lower energy requirements, reliability and turn-key capability.
Operating in a harmonically mode-locked condition, a fs-pulse Yb fibre laser will produce a frequency comb with 5
to 10 GHz (1.5 to 3Å) marker separation in the near infrared. Doubling the frequency of the light in a suitable
nonlinear medium will generate ultrashort pulses centred at ~520nm. A photonic crystal fibre can then be used to
extend the light spectrum across the majority of the visible and near infrared wavelengths, making it suitable for
spectrograph calibration. While the resulting spectrum may not be octave spanning, the two key parameters of the
comb: the offset frequency and repetition rate, will still be controllable and tied to suitable atomic frequency
standards. This presentation will include an overview of astro-combs as well as elucidate some of the finer points of
this astro-comb proposal.
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