First light of a laser frequency comb at SALT

Eric Depagne, Richard A. McCracken, Derryck T. Reid, Rudi B. Kuhn, Nicolas A. Erasmus, Lisa Ann Crause

Research output: Contribution to conferencePaper

Abstract

The High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT) is a dual beam, fiber-fed echelle spectrograph providing high resolution capabilities to the SALT observing community. Its High Stability mode has a resolution of 65000, and its standard calibration sources are a ThAr lamp and an Iodine cell. The ThAr lamp’s emission lines cover two thirds of HRS’ red channel wavelength range, but with sparse and severely uneven spectral line spacing. The line intensity also ranges from only a few counts above the background noise to close to saturation. The Iodine Cell provides narrower wavelength coverage: from 500 to 620 nm (roughly a third of the red channel range), but with improved line spacing and greater homogeneity in line intensity.
Here we report on a Laser Frequency Comb (LFC) which provides complete coverage of the red channel, with comb lines uniformly separated in frequency space, with good intensity homogeneity and o ering unprecedented line occurrence (20 000 lines vs 400 ThAr lamp lines). The LFC is powered by a Nd:YVO4 laser centred at 1064 nm, which is frequency doubled to 532 nm. A mode-locked Ti:Sapphire laser oscillator, pumped by the Nd:YVO4 laser, generates sub-30 femtosecond laser pulses at a 1 GHz repetition rate and centred at 800 nm. The near-infrared pulses are then spectrally broadened into a super-continuum in the 2.7 micron core of a 50 cm long Photonic Crystal Fibre, yielding wavelength coverage from 550 to beyond 1000 nm. A Fabry-Pérot cavity then filters the laser modes such that the individual lines are 15 GHz apart, corresponding to ~10 pixels between successive comb lines on the HRS’s red detector. A narrow linewidth diode laser is stabilised to an atomic transition in a Rb gas cell (at a wavelength of 780.24nm) and co-coupled into the spectrograph with the frequency comb to provide an absolute fiducial point for wavelength calibration. The output beam of the frequency comb is fed into a 10 m transport fibre and directed into the HRS. This optical injection setup makes provision for effortlessly alternating between calibration sources: the ThAr lamp, the frequency comb, or both at the same time. The first on-sky observations were made 5 days after arrival at the telescope. Results obtained duringa week of operation include refinement of the wavelength calibration established using only the ThAr lamp, and a revised determination of the High-Stability mode’s resolution: 67000 rather than 65000. Expected results include: full calibration of the ThAr spectrum due to the possibility of obtaining frames that afford simultaneous detection of lines from the ThAr arc lamp and the LFC, improved understanding of the HRS stability and resultant radial velocity precision, and (from an engineering point of view) practical operational requirements for a LFC on a large telescope.
Original languageEnglish
Pages9908-387
Publication statusPublished - 28 Jun 2016
EventSPIE Astronomical Telescopes + Instrumentation 2016 - Edinburgh International Conference Centre, Edinburgh, United Kingdom
Duration: 26 Jun 20161 Jul 2016

Conference

ConferenceSPIE Astronomical Telescopes + Instrumentation 2016
CountryUnited Kingdom
CityEdinburgh
Period26/06/161/07/16

Fingerprint

telescopes
spectrographs
luminaires
lasers
wavelengths
high resolution
iodine
homogeneity
fibers
cells
spacing
arc lamps
laser modes
background noise
pulses
radial velocity
arrivals
line spectra
sky
repetition

Cite this

Depagne, E., McCracken, R. A., Reid, D. T., Kuhn, R. B., Erasmus, N. A., & Crause, L. A. (2016). First light of a laser frequency comb at SALT. 9908-387. Paper presented at SPIE Astronomical Telescopes + Instrumentation 2016, Edinburgh, United Kingdom.
Depagne, Eric ; McCracken, Richard A. ; Reid, Derryck T. ; Kuhn, Rudi B. ; Erasmus, Nicolas A. ; Crause, Lisa Ann. / First light of a laser frequency comb at SALT. Paper presented at SPIE Astronomical Telescopes + Instrumentation 2016, Edinburgh, United Kingdom.
@conference{bea6007197d44ad3bc748ad48bcb048a,
title = "First light of a laser frequency comb at SALT",
abstract = "The High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT) is a dual beam, fiber-fed echelle spectrograph providing high resolution capabilities to the SALT observing community. Its High Stability mode has a resolution of 65000, and its standard calibration sources are a ThAr lamp and an Iodine cell. The ThAr lamp’s emission lines cover two thirds of HRS’ red channel wavelength range, but with sparse and severely uneven spectral line spacing. The line intensity also ranges from only a few counts above the background noise to close to saturation. The Iodine Cell provides narrower wavelength coverage: from 500 to 620 nm (roughly a third of the red channel range), but with improved line spacing and greater homogeneity in line intensity.Here we report on a Laser Frequency Comb (LFC) which provides complete coverage of the red channel, with comb lines uniformly separated in frequency space, with good intensity homogeneity and o ering unprecedented line occurrence (20 000 lines vs 400 ThAr lamp lines). The LFC is powered by a Nd:YVO4 laser centred at 1064 nm, which is frequency doubled to 532 nm. A mode-locked Ti:Sapphire laser oscillator, pumped by the Nd:YVO4 laser, generates sub-30 femtosecond laser pulses at a 1 GHz repetition rate and centred at 800 nm. The near-infrared pulses are then spectrally broadened into a super-continuum in the 2.7 micron core of a 50 cm long Photonic Crystal Fibre, yielding wavelength coverage from 550 to beyond 1000 nm. A Fabry-P{\'e}rot cavity then filters the laser modes such that the individual lines are 15 GHz apart, corresponding to ~10 pixels between successive comb lines on the HRS’s red detector. A narrow linewidth diode laser is stabilised to an atomic transition in a Rb gas cell (at a wavelength of 780.24nm) and co-coupled into the spectrograph with the frequency comb to provide an absolute fiducial point for wavelength calibration. The output beam of the frequency comb is fed into a 10 m transport fibre and directed into the HRS. This optical injection setup makes provision for effortlessly alternating between calibration sources: the ThAr lamp, the frequency comb, or both at the same time. The first on-sky observations were made 5 days after arrival at the telescope. Results obtained duringa week of operation include refinement of the wavelength calibration established using only the ThAr lamp, and a revised determination of the High-Stability mode’s resolution: 67000 rather than 65000. Expected results include: full calibration of the ThAr spectrum due to the possibility of obtaining frames that afford simultaneous detection of lines from the ThAr arc lamp and the LFC, improved understanding of the HRS stability and resultant radial velocity precision, and (from an engineering point of view) practical operational requirements for a LFC on a large telescope.",
author = "Eric Depagne and McCracken, {Richard A.} and Reid, {Derryck T.} and Kuhn, {Rudi B.} and Erasmus, {Nicolas A.} and Crause, {Lisa Ann}",
year = "2016",
month = "6",
day = "28",
language = "English",
pages = "9908--387",
note = "SPIE Astronomical Telescopes + Instrumentation 2016 ; Conference date: 26-06-2016 Through 01-07-2016",

}

Depagne, E, McCracken, RA, Reid, DT, Kuhn, RB, Erasmus, NA & Crause, LA 2016, 'First light of a laser frequency comb at SALT' Paper presented at SPIE Astronomical Telescopes + Instrumentation 2016, Edinburgh, United Kingdom, 26/06/16 - 1/07/16, pp. 9908-387.

First light of a laser frequency comb at SALT. / Depagne, Eric; McCracken, Richard A.; Reid, Derryck T.; Kuhn, Rudi B.; Erasmus, Nicolas A.; Crause, Lisa Ann.

2016. 9908-387 Paper presented at SPIE Astronomical Telescopes + Instrumentation 2016, Edinburgh, United Kingdom.

Research output: Contribution to conferencePaper

TY - CONF

T1 - First light of a laser frequency comb at SALT

AU - Depagne, Eric

AU - McCracken, Richard A.

AU - Reid, Derryck T.

AU - Kuhn, Rudi B.

AU - Erasmus, Nicolas A.

AU - Crause, Lisa Ann

PY - 2016/6/28

Y1 - 2016/6/28

N2 - The High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT) is a dual beam, fiber-fed echelle spectrograph providing high resolution capabilities to the SALT observing community. Its High Stability mode has a resolution of 65000, and its standard calibration sources are a ThAr lamp and an Iodine cell. The ThAr lamp’s emission lines cover two thirds of HRS’ red channel wavelength range, but with sparse and severely uneven spectral line spacing. The line intensity also ranges from only a few counts above the background noise to close to saturation. The Iodine Cell provides narrower wavelength coverage: from 500 to 620 nm (roughly a third of the red channel range), but with improved line spacing and greater homogeneity in line intensity.Here we report on a Laser Frequency Comb (LFC) which provides complete coverage of the red channel, with comb lines uniformly separated in frequency space, with good intensity homogeneity and o ering unprecedented line occurrence (20 000 lines vs 400 ThAr lamp lines). The LFC is powered by a Nd:YVO4 laser centred at 1064 nm, which is frequency doubled to 532 nm. A mode-locked Ti:Sapphire laser oscillator, pumped by the Nd:YVO4 laser, generates sub-30 femtosecond laser pulses at a 1 GHz repetition rate and centred at 800 nm. The near-infrared pulses are then spectrally broadened into a super-continuum in the 2.7 micron core of a 50 cm long Photonic Crystal Fibre, yielding wavelength coverage from 550 to beyond 1000 nm. A Fabry-Pérot cavity then filters the laser modes such that the individual lines are 15 GHz apart, corresponding to ~10 pixels between successive comb lines on the HRS’s red detector. A narrow linewidth diode laser is stabilised to an atomic transition in a Rb gas cell (at a wavelength of 780.24nm) and co-coupled into the spectrograph with the frequency comb to provide an absolute fiducial point for wavelength calibration. The output beam of the frequency comb is fed into a 10 m transport fibre and directed into the HRS. This optical injection setup makes provision for effortlessly alternating between calibration sources: the ThAr lamp, the frequency comb, or both at the same time. The first on-sky observations were made 5 days after arrival at the telescope. Results obtained duringa week of operation include refinement of the wavelength calibration established using only the ThAr lamp, and a revised determination of the High-Stability mode’s resolution: 67000 rather than 65000. Expected results include: full calibration of the ThAr spectrum due to the possibility of obtaining frames that afford simultaneous detection of lines from the ThAr arc lamp and the LFC, improved understanding of the HRS stability and resultant radial velocity precision, and (from an engineering point of view) practical operational requirements for a LFC on a large telescope.

AB - The High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT) is a dual beam, fiber-fed echelle spectrograph providing high resolution capabilities to the SALT observing community. Its High Stability mode has a resolution of 65000, and its standard calibration sources are a ThAr lamp and an Iodine cell. The ThAr lamp’s emission lines cover two thirds of HRS’ red channel wavelength range, but with sparse and severely uneven spectral line spacing. The line intensity also ranges from only a few counts above the background noise to close to saturation. The Iodine Cell provides narrower wavelength coverage: from 500 to 620 nm (roughly a third of the red channel range), but with improved line spacing and greater homogeneity in line intensity.Here we report on a Laser Frequency Comb (LFC) which provides complete coverage of the red channel, with comb lines uniformly separated in frequency space, with good intensity homogeneity and o ering unprecedented line occurrence (20 000 lines vs 400 ThAr lamp lines). The LFC is powered by a Nd:YVO4 laser centred at 1064 nm, which is frequency doubled to 532 nm. A mode-locked Ti:Sapphire laser oscillator, pumped by the Nd:YVO4 laser, generates sub-30 femtosecond laser pulses at a 1 GHz repetition rate and centred at 800 nm. The near-infrared pulses are then spectrally broadened into a super-continuum in the 2.7 micron core of a 50 cm long Photonic Crystal Fibre, yielding wavelength coverage from 550 to beyond 1000 nm. A Fabry-Pérot cavity then filters the laser modes such that the individual lines are 15 GHz apart, corresponding to ~10 pixels between successive comb lines on the HRS’s red detector. A narrow linewidth diode laser is stabilised to an atomic transition in a Rb gas cell (at a wavelength of 780.24nm) and co-coupled into the spectrograph with the frequency comb to provide an absolute fiducial point for wavelength calibration. The output beam of the frequency comb is fed into a 10 m transport fibre and directed into the HRS. This optical injection setup makes provision for effortlessly alternating between calibration sources: the ThAr lamp, the frequency comb, or both at the same time. The first on-sky observations were made 5 days after arrival at the telescope. Results obtained duringa week of operation include refinement of the wavelength calibration established using only the ThAr lamp, and a revised determination of the High-Stability mode’s resolution: 67000 rather than 65000. Expected results include: full calibration of the ThAr spectrum due to the possibility of obtaining frames that afford simultaneous detection of lines from the ThAr arc lamp and the LFC, improved understanding of the HRS stability and resultant radial velocity precision, and (from an engineering point of view) practical operational requirements for a LFC on a large telescope.

M3 - Paper

SP - 9908

EP - 9387

ER -

Depagne E, McCracken RA, Reid DT, Kuhn RB, Erasmus NA, Crause LA. First light of a laser frequency comb at SALT. 2016. Paper presented at SPIE Astronomical Telescopes + Instrumentation 2016, Edinburgh, United Kingdom.