Validation of a novel ultraviolet lidar system with relative Raman-scattering cross sections determined from atmospheric measurements

Farah, A.; Venable, D. D.; Thorpe, A. N.; Marsh, F.S; Heaps, William S.

doi:10.1364/ao.41.000407

Cited by 2 publications

(1 citation statement)

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“…pulsed Light Detection and Ranging (LIDAR) lasers, and exploited for many years in atmospheric physics experiments [see e.g. [39][40][41][42][43][44][45]. Specifically, laser photons lose through the inelastic collision with O 2 or N 2 molecules a quantized amount of energy through the excitation of the fundamental ν 1←0 rotational-vibrational (rovibrational) modes of these diatomic molecules [46][47][48].…”

Section: The First Detection Of Laser-induced Atmospheric Raman Scatt...mentioning

confidence: 99%

Detection and Implications of Laser-Induced Raman Scattering at Astronomical Observatories

et al. 2017

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Laser guide stars employed at astronomical observatories provide artificial wavefront reference sources to help correct (in part) the impact of atmospheric turbulence on astrophysical observations. Following the recent commissioning of the 4 Laser Guide Star Facility (4LGSF) on the Unit Telescope 4 (UT4) of the Very Large Telescope (VLT), we characterize the spectral signature of the uplink beams from the 22 W lasers to assess the impact of laser scattering from the 4LGSF on science observations. We use the Multi-Unit Spectroscopic Explorer (MUSE) optical integral field spectrograph mounted on the Nasmyth B focus of UT4 to acquire spectra at a resolution of R ∼ =3000 of the uplink laser beams over the wavelength range of 4750Å → 9350Å. We report the first detection of laser-induced Raman scattering by N2, O2, CO2, H2O and (tentatively) CH4 molecules in the atmosphere above the astronomical observatory of Cerro Paranal. In particular, our observations reveal the characteristic spectral signature of laser photons -but 480Å to 2210Å redder than the original laser wavelength of 5889.959Å -landing on the 8.2m primary mirror of UT4 after being Raman-scattered on their way up to the sodium layer. Laser-induced Raman scattering, a phenomenon not usually discussed in the astronomical context, is not unique to the observatory of Cerro Paranal, but common to any astronomical telescope employing a laser-guide-star (LGS) system. It is thus essential for any optical spectrograph coupled to a LGS system to handle thoroughly the possibility of a Raman spectral contamination via a proper baffling of the instrument and suitable calibrations procedures. These considerations are particularly applicable for the HARMONI optical spectrograph on the upcoming Extremely Large Telescope (ELT). At sites hosting multiple telescopes, laser collision prediction tools also ought to account for the presence of Raman emission from the uplink laser beam(s) to avoid the unintentional contamination of observations acquired with telescopes in the vicinity of a LGS system.Popular summary: Atmospheric turbulence strongly affects the sharpness of astronomical observations from the ground. Specially-equipped telescopes (and their associated instruments) can reduce this effect by directing lasers up into the sky, causing sodium atoms in the upper atmosphere to glow. Deformable mirrors can then use these "artificial guide stars" to help correct for the impact of turbulence on observations.Four such lasers were recently installed at the Very Large Telescope (VLT) at Cerro Paranal in Chile. For the first time, we characterized the astronomical consequences of laser-induced inelastic Raman scattering, a process through which the laser photons lose energy by exciting air molecules. This isa possible source of contamination for astrophysical observations, appearing in the data as complex groups of emission lines.We used the Multi-Unit Spectroscopic Explorer (MUSE) integral field spectrograph -an instrument that obtains a spectrum for each pixel of an image in a...

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