Modal noise mitigation for high-precision spectroscopy using a photonic reformatter. (arXiv:2001.09211v1 [astro-ph.IM])

Modal noise mitigation for high-precision spectroscopy using a photonic reformatter. (arXiv:2001.09211v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pike_F/0/1/0/all/0/1">Fraser A. Pike</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Benoit_A/0/1/0/all/0/1">Aur&#xe9;lien Beno&#xee;t</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+MacLachlan_D/0/1/0/all/0/1">David G. MacLachlan</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Harris_R/0/1/0/all/0/1">Robert J. Harris</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Sanchez_I/0/1/0/all/0/1">Itandehui Gris S&#xe1;nchez</a> (3 and 4), <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_D/0/1/0/all/0/1">David Lee</a> (5), <a href="http://arxiv.org/find/astro-ph/1/au:+Birks_T/0/1/0/all/0/1">Tim A. Birks</a> (3), <a href="http://arxiv.org/find/astro-ph/1/au:+Thomson_R/0/1/0/all/0/1">Robert R. Thomson</a> (1) ((1) Heriot-Watt University, Edinburgh, UK, (2) Zentrum f&#xfc;r Astronomie der Universit&#xe4;t Heidelberg, Germany, (3) University of Bath, UK, (4) Currently with ITEAM Research Institute, Valencia, Spain, (5) STFC UK Astronomy Technology Centre, UK)

Recently, we demonstrated how an astrophotonic light reformatting device,
based on a multicore fibre photonic lantern and a three-dimensional waveguide
component, can be used to efficiently reformat the point spread function of a
telescope to a diffraction-limited psuedo-slit [arXiv:1512.07309]. Here, we
demonstrate how such a device can also efficiently mitigate modal noise — a
potential source of instability in high resolution multi-mode fibre-fed
spectrographs). To investigate the modal noise performance of the photonic
reformatter, we have used it to feed light into a bench-top near-infrared
spectrograph (R {approx} 9,500, {lambda} {approx} 1550 nm). One approach to
quantifying the modal noise involved the use of broadband excitation light and
a statistical analysis of how the overall measured spectrum was affected by
variations in the input coupling conditions. This approach indicated that the
photonic reformatter could reduce modal noise by a factor of six when compared
to a multi-mode fibre with a similar number of guided modes. Another approach
to quantifying the modal noise involved the use of multiple spectrally narrow
lines, and an analysis of how the measured barycentres of these lines were
affected by variations in the input coupling. Using this approach, the photonic
reformatter was observed to suppress modal noise to the level necessary to
obtain spectra with stability close to that observed when using a single mode
fibre feed. These results demonstrate the potential of using photonic
reformatters to enable efficient multi-mode spectrographs that operate at the
diffraction limit and are free of modal noise, with potential applications
including radial velocity measurements of M-dwarfs.

Recently, we demonstrated how an astrophotonic light reformatting device,
based on a multicore fibre photonic lantern and a three-dimensional waveguide
component, can be used to efficiently reformat the point spread function of a
telescope to a diffraction-limited psuedo-slit [arXiv:1512.07309]. Here, we
demonstrate how such a device can also efficiently mitigate modal noise — a
potential source of instability in high resolution multi-mode fibre-fed
spectrographs). To investigate the modal noise performance of the photonic
reformatter, we have used it to feed light into a bench-top near-infrared
spectrograph (R {approx} 9,500, {lambda} {approx} 1550 nm). One approach to
quantifying the modal noise involved the use of broadband excitation light and
a statistical analysis of how the overall measured spectrum was affected by
variations in the input coupling conditions. This approach indicated that the
photonic reformatter could reduce modal noise by a factor of six when compared
to a multi-mode fibre with a similar number of guided modes. Another approach
to quantifying the modal noise involved the use of multiple spectrally narrow
lines, and an analysis of how the measured barycentres of these lines were
affected by variations in the input coupling. Using this approach, the photonic
reformatter was observed to suppress modal noise to the level necessary to
obtain spectra with stability close to that observed when using a single mode
fibre feed. These results demonstrate the potential of using photonic
reformatters to enable efficient multi-mode spectrographs that operate at the
diffraction limit and are free of modal noise, with potential applications
including radial velocity measurements of M-dwarfs.

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