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">Aurélien Benoît</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 Sánchez</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 für Astronomie der Universität 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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