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

TL;DR

This study detects laser-induced Raman scattering from uplink laser beams at Cerro Paranal, revealing spectral signatures that impact astronomical observations and emphasizing the need for mitigation strategies at observatories using laser guide stars.

Contribution

First detection of laser-induced Raman scattering by atmospheric molecules at an astronomical observatory, highlighting its implications for observational contamination and instrument calibration.

Findings

Detected Raman scattering from N2, O2, CO2, H2O, and CH4 molecules.

Spectral signatures are 480-2210 Å redder than original laser wavelength.

Raman scattering is common at LGS-equipped observatories, affecting spectrograph observations.

Abstract

(Abr.) 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 UT4 at the VLT, we characterize the spectral signature of the uplink beams from the 22W lasers to assess the impact of laser scattering from the 4LGSF on science observations. We use the MUSE optical integral field spectrograph to acquire spectra at a resolution of R~3000 of the uplink laser beams over the wavelength range of 4750\AA\ to 9350\AA. 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 480\AA\ to 2210\AA\ redder than the original laser wavelength of 5889.959\AA\ -- 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 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. 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.

Tables

Table 1. Table 1: Integrated fluxes for the different laser-induced Raman lines from the uplink beam at an altitude of 27.6 km above sea level, normalized to the power of a (continuous wave) 1 W laser shining at 5889.959 Å. Uncertainties on the quoted fluxes are ≲ less-than-or-similar-to \lesssim 15%. For the fundamental Raman lines of N 2 and O 2 , the fluxes refer to the central peak only (i.e. the Q − b ​ r ​ a ​ n ​ c ​ h 𝑄 𝑏 𝑟 𝑎 𝑛 𝑐 ℎ Q-branch ). The flux associated with the main Rayleigh line of the laser in the same observational setup is also indicated.

Raman line		${\lambda }_{\text{4LGSF}}$	CO2	CO2	O2(${\nu }_{1\leftarrow 0}$)	N2(${\nu }_{1\leftarrow 0}$)	CH4	O2(${\nu }_{2\leftarrow 0}$)	H2O	N2(${\nu }_{2\leftarrow 0}$)
Raman shift $\left[{\text{cm}}^{-1}\right]$		–	1285.8	1388.1	1556.4	2330.7	2914.2	3089.2	3651.7	4631.2
${\lambda }_{obs}$ $\left[\text{Å}\right]$		5889.959	6372.57	6414.39	6484.39	6827.17	7110.43	7200.02	7503.93	8099.23
Flux $\left[\text{10}\text{<sup><span>-20</span></sup>}\text{ erg\hspace{0.17em}s}\text{<sup><span>-1</span></sup>}\text{ cm}\text{<sup><span>-2</span></sup>}\text{ W}\text{<sup><span>-1</span></sup>}\right]$		$1.9\cdot {10}^7$	11.3	18.9	$6.8\cdot {10}^3$	$2.0\cdot {10}^4$	$\lesssim 1.1$	3.2	2.7	16.1

Table 2. Table 2: Wavelength of the main atmospheric, laser-induced Raman scattering lines as a function of the LGS wavelength λ LGS subscript 𝜆 LGS \lambda_{\text{LGS}} at different astronomical telescopes.

Telescope	LGS system	${\lambda }_{\text{LGS}}$	CO2	CO2	O2(${\nu }_{1\leftarrow 0}$)	N2(${\nu }_{1\leftarrow 0}$)	CH4	O2(${\nu }_{2\leftarrow 0}$)	H2O	N2(${\nu }_{2\leftarrow 0}$)
		[nm]	[nm]	[nm]	[nm]	[nm]	[nm]	[nm]	[nm]	[nm]
Gemini North	ALTAIR (Boccas et al., 2006; Christou et al., 2010)
Gemini South	GeMS (d’Orgeville et al., 2012; Rigaut et al., 2014; Neichel et al., 2014)
W. M. Keck	LGSAO (Wizinowich et al., 2006; van Dam et al., 2006)
Lijang 1.8-m Telescope	LGS-AOS (Wei et al., 2016)
Shane 3-m Telescope	ShaneAO (Gavel et al., 2014, 2016)
Subaru	AO188 (Hayano et al., 2010; Minowa et al., 2012)
VLT (UT4)	4LGSF (Arsenault et al., 2013; Bonaccini Calia et al., 2014a)
VLT (UT4)	PARLA (Lewis et al., 2014)
Thirty Meter Telescope	LGSF (Boyer et al., 2014; Boyer and Ellerbroek, 2016)
Giant Magellan Telescope	GMT AO (Bouchez et al., 2014)
Extremely Large Telescope	ATLAS & MAORY (Diolaiti, 2010; Thatte et al., 2014; Davies et al., 2016; Neichel et al., 2016)
Large Binocular Telescope	ARGOS (Rabien et al., 2010; Orban de Xivry et al., 2016)	532	571	574	580	607	630	637	660	706
William Hershel Telescope	GLAS (Benn et al., 2008)	515	552	555	560	585	606	612	634	676
Hale 5-m Telescope	PULSE (Baranec et al., 2014)
Southern Astrophysical	SAM (Tokovinin et al., 2010)
Research Telescope

Equations

$${\lambda }_{\text{4LGSF}}=\frac{{\lambda }_{\text{4LGSF,vacuum}}}{n}$$

$$n=1+\frac{8.34254}{10^5}+\frac{2.406147}{10^2\times (130-s^2)}+\frac{1.5998}{10^4\times (38.9-s^2)}$$

$$s=\frac{10^4}{{\lambda }_{\text{4LGSF,vacuum}}}$$

$${\Delta {\nu }_{1\leftarrow 0}|}_S=\Delta {\nu }_0+6B_1+(5B_1-B_0)J+(B_1-B_0)J^2,$$

$${\Delta {\nu }_{1\leftarrow 0}|}_Q=\Delta {\nu }_0+(B_1-B_0)J+(B_1-B_0)J^2,$$

$${\Delta {\nu }_{1\leftarrow 0}|}_O=\Delta {\nu }_0+2B_1-(3B_1+B_0)J+(B_1-B_0)J^2,$$

$$B_i=B_e-{\alpha }_e(i+0.5),$$

$$B_{e,{\mathrm{O}}_2}=1.44562{\mathrm{cm}}^{-1},{\alpha }_{e,{\mathrm{O}}_2}=0.0159305{\mathrm{cm}}^{-1},$$

$$B_{e,{\mathrm{N}}_2}=1.998241{\mathrm{cm}}^{-1},{\alpha }_{e,{\mathrm{N}}_2}=0.017318{\mathrm{cm}}^{-1}.$$

$$H$$

$$d_{L,0}$$

$$R_L$$

$$k=H\tan \kappa$$

$$L=\sqrt{(R_L-k{)}^2+H^2},$$

$$x_L$$

$$y_L$$

$$x_B$$

$$y_B$$

$$\tan \xi =\frac{R_L-k}{H}$$

$$h_L$$

$$\eta =\frac{\pi }{2}-\arctan \left(\frac{h_L}{\sqrt{d{x}^2+d{y}^2}}\right)$$

$$dx$$

$$dy$$

$$x_{MUSE}$$

$$y_{MUSE}$$

$$\tan \delta =\frac{dy}{dx}$$

Full text

**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 is a 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 given area of the sky – on the Unit 4 Telescope of the VLT to record the spectral signature of the lasers over the entire optical range. These 22-watt lasers, each tuned to a wavelength of 5889.959 Å, excite sodium atoms at 90 km above the ground. We identified contaminating Raman spectral lines from molecular nitrogen, molecular oxygen, carbon dioxide, water, and tentatively methane. This detailed characterization of the spectral signature of laser-induced Raman scattering, and the identification of the molecules involved, is crucial to anticipating, reducing and correcting the possible contamination of scientific observations obtained at any observatory using a laser guide star.

I Introduction

The past few years have seen the emergence of numerous Laser Guide Star (LGS) systems at astronomical observatories (Liu, 2006, 2008). These provide artificial wavefront reference sources to help alleviate the impact of atmospheric turbulence on astronomical observations (Foy and Labeyrie, 1985; Davies and Kasper, 2012). A cleaner wavefront does not only lead to an enhancement in image quality: it also improves the detection limit of the telescope by further concentrating the light of point sources, a crucial advantage for the detection (from the ground) of galaxies at the highest redshifts, for example. Most of the existing LGS systems are available to the general community of observers, and thus used regularly in operations (d’Orgeville and Fetzer, 2016). Such is the case at the W.M. Keck telescopes (Wizinowich et al., 2006; van Dam et al., 2006), the Gemini North and South telescopes (Boccas et al., 2006; Christou et al., 2010; d’Orgeville et al., 2012; Rigaut et al., 2014; Neichel et al., 2014), the Subaru telescope (Hayano et al., 2010; Minowa et al., 2012) and the Very Large Telescope (VLT) (Lewis et al., 2014).

The 4 Laser Guide Star Facility (4LGSF; Bonaccini Calia et al., 2014a) consists of four laser guide star units (LGSUs), each comprised of a standalone 22 W laser (Feng et al., 2009; Taylor et al., 2009; Bonaccini Calia et al., 2010), a launch telescope, steering mechanisms and dedicated control electronics. These systems are an integral part of the Adaptive Optics Facility (AOF; Arsenault et al., 2013) currently being installed on the Unit Telescope 4 (UT4) of the VLT on Cerro Paranal in Chile. In addition to the 4LGSF, the AOF also comprises a new deformable secondary mirror (Arsenault et al., 2006a; Briguglio et al., 2014) and the adaptive optics modules GALACSI (Stuik et al., 2006; La Penna et al., 2016) & GRAAL (Paufique et al., 2010, 2016; Hibon et al., 2016).

The AOF is intended to feed corrected wavefronts to the MUSE optical integral field spectrograph (spectral range: 4750 Å $\rightarrow$ 9350 Å; Bacon et al., 2010) and the HAWK-I near-IR wide-field imager (Kissler-Patig et al., 2008; Siebenmorgen et al., 2011). Altogether, these systems allow to correct on-the-fly the effect of atmospheric turbulence over several arcminutes using a square asterism of 4 LGSs, the spatial extent of which is adjustable to the respective field-of-views of MUSE (7.5 arcseconds in Narrow-Field-Mode$\equiv$NFM; 60 arcseconds in Wide-Field-Mode$\equiv$WFM) and HAWK-I (7.5 arcminutes). Specifically, the adaptive optics corrections to be provided by the AOF are a) laser tomography for the MUSE NFM, and b) ground-layer correction for the MUSE WFM and HAWK-I (Arsenault et al., 2006b).

As the power of LGS systems increase, so is their potential for collateral damage on the associated astrophysical observations. In the case of Cerro Paranal, the 22 W of the 4LGSF lasers represent a factor of four increase in the on-sky power compared to the PARLA laser (Lewis et al., 2014) used with the SINFONI infrared integral field spectrograph (Eisenhauer et al., 2003; Bonnet et al., 2004) on UT4 since March 2013. This power increase, the fact that the 4LGSF lasers will be used (at times) with an optical spectrograph, and the presence of numerous telescopes in the immediate vicinity of UT4 on Cerro Paranal motivated the spectral analysis of the 4LGSF uplink beams presented in this Article. In particular, the present analysis distinguishes itself from previous observations targeting LGS systems (Hayano et al., 2003; Coulson and Roth, 2010; Amico et al., 2015) in that 1) it does not restrict itself to the immediate spectral vicinity of the laser wavelength, and 2) it focuses on the existence and consequences of inelastic Raman scattering physics (in addition to elastic Rayleigh and Mie scattering) associated with the use of LGS systems, a phenomena hitherto often overlooked within the astrophysics community.

This Article is structured as follows. Our initial detection of laser-induced Raman scattered photons by N2 and O2 molecules above Cerro Paranal is described in Sect. II. We discuss our search for the signatures of Raman scattering from additional molecules in Sec. III, and quantify the fluxes of the different Raman lines in Sec. IV. We conclude by discussing the impact of laser-induced Raman scattering physics on astrophysical observations in Sec. V. Unless mentioned otherwise, all wavelengths are quoted in air at a pressure of 1 atm and a temperature of 15*∘*C.

II The first detection of laser-induced atmospheric Raman scattering at Cerro Paranal

We undertook dedicated observations combining the newly installed 4LGSF systems with the MUSE integral field spectrograph (located on the Nasmyth B focus of UT4) in 2016, June, November & December, as part of the 4LGSF/AOF ongoing commissioning activities. These observations were designed to derive an initial characterization of the possible impact of laser scattering from the 4LGSF on science observations. We note that these observations were performed prior to the installation of the GALACSI AO module (that has since then been inserted directly ahead of MUSE in the light path). All observations were reduced using biases, flat fields (including both lamp and sky flats) and arc lamp exposures acquired as part of the regular calibration plan of MUSE. The data were flux-calibrated using dedicated observations of flux standard stars (one per night) taken from the MUSE list of standard stars.

At the laser wavelength, the laser uplink beams are visible through elastic, non-isotropic, polarization-preserving Rayleigh scattering of the laser photons by the air molecules, and through non-isotropic, higher intensity Mie scattering by dust particles. In the specific case of the 4LGSF, 18 W are emitted by each LGSU at the laser main carrier wavelength $\lambda_{\text{4LGSF,vacuum}}=5891.5912$ Å to excite the D2a transition of sodium atoms. An additional 4 W are emitted in two lines located 1.713 GHz on either side of the main line at 5891.5714 Å and 5891.6110 Å (with 2 W in each line). Emitting 10% of the laser power at 5891.5714 Å is specifically designed to counteract the depopulation of the sodium atoms from the ${}^{2}S_{1/2}$ F=2 to F=1 ground state by re-pumping them via the D2b transition (where F is the total atomic angular momentum quantum number, see Holzlöhner et al., 2010). These three lines are essentially unresolved at the spectral resolution of MUSE, so that in the remainder of this article we simply refer to the 4LGSF laser carrier wavelength in air $\lambda_{\text{4LGSF}}=5889.959$ Å, where:

[TABLE]

with $n$ the refractive index of dry air at a pressure of 1 atm and a temperature of 15*∘*C defined by Birch and Downs (1994):

[TABLE]

with:

[TABLE]

In normal operations, MUSE will employ dedicated $\sim$225 Å-wide notch filters – centered on the LGS frequency – to block $99.7\pm 0.2\%$ of the light emitted by the uplink laser beams. Comparing two back-to-back 60 sec MUSE exposures (acquired with and without the 4LGSF lasers propagating) readily reveals the presence of a residual narrow & unresolved emission line at $\lambda_{\mathrm{4LGSF}}$ (see Fig. 1). The presence of the notch filter in the light path (leading to the $\sim$225 Å gap in the spectrum) reduces the intensity of this laser line, without fully blocking it. As such, this line effectively offers an excellent wavelength and spectral resolution reference, given that the laser wavelength is controlled with a root-mean-square error of 0.03 mÅ.

We also detect two additional spectral features associated with the use of the 4LGSF at $\lambda\cong 6484$ Å and $\lambda\cong 6827$ Å. These features consist of 1) a sharp and unresolved emission line with a peak flux density similar to that of the brightest sky emission lines in the 7000 Å$\rightarrow$ 8000 Å spectral range, and 2) broad wings at a level of $<\above 0.0pt\sim$ $5\times 10^{-20}$ erg s*-1* cm*-2* Å*-1* spanning $\sim$130 Å and $\sim$190 Å, respectively. These spectral features evidently result from the inelastic Raman scattering by the O2 and N2 molecules of the laser photons while on their way up to the sodium layer: a process seen easily at high peak powers with e.g. pulsed Light Detection and Ranging (LIDAR) lasers, and exploited for many years in atmospheric physics experiments (see e.g. Leonard, 1967; Cooney, 1968; Melfi et al., 1969; Collis, 1970; Melfi, 1972; Farah et al., 2002; Turner and Whiteman, 2006). Specifically, laser photons lose through the inelastic collision with O2 or N2 molecules a quantized amount of energy through the excitation of the fundamental $\nu_{1\leftarrow 0}$ rotational-vibrational (rovibrational) modes of these diatomic molecules (Rasetti, 1929; Herzberg, 1950; Whiteman et al., 1992). The nature of Raman scattering is such that the molecule responsible for a specific spectral line is not tied to the absolute wavelength of the line, but instead to its shift from the exciting (laser) wavelength. Indeed, the spectral lines in our MUSE observations are in perfect agreement with the tabulated Raman shifts of $\Delta\nu_{0,\mathrm{O}_{2}}=1556.4$ cm*-1* and $\Delta\nu_{0,\mathrm{N}_{2}}=2330.7$  cm*-1* for O2 and N2 (Herzberg, 1950), implying a resulting (post-collision) photon wavelength of $6484.39$ Å and $6827.17$ Å.

In normal operations, the four LGSs are located outside of the MUSE field-of-view ($\sim$62 arcseconds from the center in WFM; see Fig. 2). As such, the Rayleigh and Raman signals in the MUSE WFM observations presented in Fig. 1 (i.e., with the LGSs outside of the instrument field-of-view) are best described as a residual glow throughout the field-of-view. As illustrated in Fig. 3, we note that 1) the intensity of the Rayleigh (and Raman) glow is strongly dependent on the size of the 4LGSF asterism (probed out to its physical limit of $\sim$440 arcseconds in radius set by the opto-mechanical range of the 4LGSU field steering devices), and 2) there is no evidence for the Rayleigh (and Raman) glow to be dependent on the altitude of the observation. Preliminary, follow-up observations performed on 2017, April 15 during the second commissioning run of GALACSI reveal that the installation of this AO module does not have any noticeable influence on the intensity of the Raman lines seen by MUSE. The commissioning of GALACSI and the AO mode of MUSE is ongoing; a detailed and comprehensive characterization of the intensity and impact of Raman lines on MUSE AO observations (both in the WFM and NFM) is thus clearly not feasible just yet. This aspect will be thoroughly investigated during commissioning observations, and reported on separately.

The absence of GALACSI in the light path during the 4LGSF commissioning runs of 2016 June, November & December did not, however, preclude a detailed characterisation of the Raman emission lines themselves. For test purposes, we acquired a 60 sec MUSE exposure with one LGS guide star voluntarily placed within the $1\times 1$ square arcminute field-of-view of MUSE in WFM-N (see Fig. 4). At the laser wavelength $\lambda_{\mathrm{4LGSF}}$, this observation reveals the out-of-focus LGS & thus the UT4 pupil (in this case prior to the installation of the DSM), as well as the uplink laser beam. At the mean wavelength of the rovibrational Raman lines, the same dataset reveals a clear view of the laser beam, but not of the LGS donut. The LGS is created through the excitation of Na atoms at $\sim$90 km of altitude, so that the lack of Raman emission associated with the LGS itself is to be expected. We note that the laser beam is not continuously visible (either through Rayleigh or Raman scattering) all the way to the sodium layer. Using a model of the UT4 telescope and 4LGSF launch telescope (see the Appendix A for details), we find that the Rayleigh and Raman emission from the laser beam is undetected in our observations for altitudes beyond $30\pm 5$ km above ground, consistent with atmospheric LIDAR observations (Keckhut et al., 1990; Liu and Yi, 2014), and typical atmospheric density profiles 111See for example the international Standard Atmosphere model; ISO 2533:1975..

Although not representative of normal operations, the on-axis observation of one of the LGS and its associated uplink beam with MUSE allows the construction of a high signal-to-noise view of the Raman rovibrational lines associated with O2 and N2. The mean spectrum of the regions $\mathrm{R}_{1}-\mathrm{R}_{0}$ (each 6$\times$6 square arcseconds in size) presented in Fig. 4 is built to that end. Following Herzberg (1950), the Raman spectral shifts of the ro-vibrational lines $\Delta\nu_{1\leftarrow 0}|_{S}$ (for the $S$-branch), $\Delta\nu_{1\leftarrow 0}|_{O}$ (for the $O$-branch) and $\Delta\nu_{1\leftarrow 0}|_{Q}$ (for the Q-branch) for a vibrating & rotating homonuclear diatomic molecule are:

[TABLE]

[TABLE]

with $J\in[0,1,2,\ldots]$, and:

[TABLE]

with $J\in[2,3,4,\ldots]$. $B_{i}$ is the rotational constant of the diatomic molecule at the vibrational level $i$, which can be approximated as:

[TABLE]

with $B_{e}$ the rotational constant in equilibrium position and $\alpha_{e}$ the first order term. From Huber and Herzberg (1979); Irikura (2007) and references therein:

[TABLE]

The spectral location of the observed rotational side-bands (visible up to rotational quantum numbers $J=17$ in Fig. 4) are in perfect agreement with the theoretical predictions of the Raman shifts of the $S$- and $O$-branch, while the $Q$-branch remains unresolved by MUSE. We also note that for O2, only rotational side-lines for odd values of $J$ are visible, while for N2, rotational side-lines for even values of $J$ are stronger than for odd values of $J$ (Herzberg, 1950; Barrett and Adams, 1968).

III Extending the search for Raman scattering beyond the $\nu_{1\leftarrow 0}$ fundamental mode of O2 and N2 molecules

Virtually all of the Rayleigh, Mie and Raman scattering of laser photons occurs within the footprint of the uplink beams: narrow cylinders of 30 cm in diameter extending from the laser launch telescopes up the sodium layer at an altitude of $\sim$90 km, and beyond. As illustrated in Fig. 4, looking directly at the uplink beam with MUSE offers an ideal way of boosting our detection ability for the Raman scattering signatures of different molecules. Beyond N2 and O2 that make up (by volume) 78.09% and 20.95% of air, CO2 (0.04%), H2O (variable, typically $<$1%) and CH4 ($\sim$0.0002%) are the next best candidates for triggering laser-induced Raman scattering events in the sky of Cerro Paranal, given the clean air of the surrounding Atacama desert.

To further increase the signal of Raman-scattered photons compared to the observations described in Fig. 4 (with one LGS within the MUSE field-of-view), we acquired an additional series of observations with MUSE in which we crossed the laser beams over the field-of-view. One must stress once more that this is a setup clearly not representative of normal science operations. We performed these observations in four individual steps:

setup the 4LGSF asterism to the MUSE WFM size (62 arcseconds in radius), 2. 2.

stop the propagation of all lasers but LGSU2, and place its LGS at the location of LGSU4, 3. 3.

without modifying the position of LGSU2, start the propagation of LGSU4 and place its LGS at the original location of LGSU2, and 4. 4.

repeat the same procedure for the LGSUs 1 & 3.

This process is illustrated in Fig. 5 by the means of images from the Laser Positioning Camera (LPC; Bonaccini Calia et al., 2014b; Centrone et al., 2016) located on the top-ring of UT4. The unusual geometry of the beams in the crossed configurations is a direct consequence of the off-axis location of this camera. The lack of a single intersection point for the four laser beams is due to the rectangular distribution of the launch telescopes, compared to the square shape of the LGS asterism. From the MUSE perspective, this implies that the different beams will together excite several altitude layers of the atmosphere below 10 km within the instrument’s WFM field-of-view of 1 square arcminutes.

The mean spectra extracted from all the spatial pixels (spaxels) located within 20 arcseconds from the center of the MUSE WFM field-of-view is presented in Fig. 6, both for a 60 s exposure with the four laser beams crossed and a subsequent reference 60 s exposure with no laser propagating. The Raman lines associated with the fundamental $\nu_{1\leftarrow 0}$ rovibrational excitation of the O2 and N2 molecules are clearly detected, with rotational side-bands visible up to $J=23$ (see Eq. 1 and 2).

Beyond the fundamental Raman lines, we also detect the first overtone lines (corresponding to the $\nu_{2\leftarrow 0}$ transition; Creek and Nicholls, 1975; Knippers et al., 1985; Heaps and Burris, 1996) from O2 and N2 molecules at 7200.02 Å ($\Delta\nu^{\prime}_{0,\mathrm{O}_{2}}=3089.2\,\mathrm{cm}^{-1}$) and 8099.23 Å ($\Delta\nu^{\prime}_{0,\mathrm{N}_{2}}=4631.2\,\mathrm{cm}^{-1}$). These second order lines are 3-4 orders of magnitude fainter than the peak intensity of the fundamental vibrational line of N2.

At similar intensity levels, we unambiguously detect the presence of Raman scattering from CO2 molecules at 6372.57 Å and 6414.39Å ($\Delta\nu_{0,\mathrm{CO}_{2}}=\left\{1285.8\,\mathrm{cm}^{-1};1388.1\,\mathrm{cm}^{-1}\right\}$; Herzberg, 1945) and H2O molecules at 7503.93 Å ($\Delta\nu_{0,\mathrm{H}_{2}\mathrm{O}}=3651.7\,\mathrm{cm}^{-1}$; Herzberg, 1945; Penney and Lapp, 1976). We note that these observations were acquired with low levels of precipitable water vapor ($\sim$1 mm) and humidity ($\sim$10%). The signature of Raman scattering from CH4 molecules at 7110.43 Å ($\Delta\nu_{0,\text{CH}_{4}}=2914.2$ cm*-1*; Herzberg, 1945) is only tentatively detected with a signal-to-noise S/N$<$3.

IV Quantifying the intensity of laser-induced Raman lines

Predicting the exact impact of Raman scattering from a LGS system on a given astronomical observation is not straightforward. The number of laser photons that are subject to Raman scattering is proportional both to the power (on-sky) of the laser system employed and the density of molecules within the footprint of the uplink beam, which varies as a function of the altitude. In addition, the intensity of the contamination for any given telescope at any given time also depends on the collision parameters (i.e. the scattering angle and the distance to the impacting portion of the uplink laser beam from the telescope). Finally, the intensity of the Raman contamination also depends heavily on the characteristics of the instrument involved: for example, the central wavelength and width of the filter for an imager, and the spectral range and resolution for a spectrograph. The specificities of the telescope (incl. the overall throughput and aperture) also play a role. Altogether, this implies that it is very difficult to summarize with precision the impact of laser-induced Raman scattering for all possible instruments, telescopes, LGS systems and observatories.

These factors notwithstanding, we provide in Table 1 indicative fluxes of the different Raman lines identified in Sec. III. These fluxes are scaled to the brightness of the uplink beam of a (continuous wave) 1 W laser guide star with $\lambda_{\text{LGS}}=\lambda_{\text{4LGSF}}$ observed at an altitude of 25 km above ground on Cerro Paranal, equivalent to 27.6 km above sea level. The appropriate scaling for the different line fluxes (measured from the spectrum presented in Fig. 6) was derived using dedicated observations of one of the 4LGSF uplink beam using the FORS2 (Appenzeller et al., 1998) spectrograph mounted on the Cassegrain focus of UT1 at the VLT, performed during the evening twilight of 2016, November 28 under clear conditions. During the test, UT4 was pointed directly away from UT1 with LGSU1 (alone) propagating in center-field at an altitude of 89.000*∘. UT1, located at a (Vincenty) distance of 130.22 m from UT4, was pointed directly towards UT4 at an altitude of 88.694∘*. In this configuration (with both telescopes idle during the entire duration of the test), UT1 was effectively pointing towards the uplink laser beam at an altitude of 25 km above ground.

The resulting FORS2 long-slit spectrum for a 60 s exposure acquired with a slit-width of 0.28 arcseconds and the 600RI+19 grism is presented in Fig. 7. The data were processed using the ESO Reflex (Freudling et al., 2013) workflow (v 5.0.20) for FORS2. We use Gaussian components to fit the main laser line and the fundamental Raman lines associated with O2 and N2 molecules, combined with a constant level to account for the existing continuum residual post sky subtraction. We note that fitting the main laser line at $\lambda_{\text{4LGSF}}$ requires 3 Gaussian components. The narrowest (unresolved) component is the brightest, and corresponds to the laser photons that experienced elastic Rayleigh and Mie scattering. The intermediate component, with a full-width-at-half-maximum FWHM = 44.2$\pm$0.6 Å, corresponds to the laser photons that experienced Raman scattering and excited the rotational mode of molecules (only, as opposed to the rovibrational modes described until now). This component is as such a collection of narrow emission lines –blended at the spectral resolution of FORS2 of R$\cong$2900– associated with the different air molecules present in the footprint of the laser beam. The existence of (some of) these narrow emission lines is best seen in the fit residual $\Delta$. Finally, the broadest Gaussian component associated with the main laser line is an instrumental artefact of the FORS2 spectrograph. A similar glow is present, for example, around the brightest lines nearing saturation in arc lamp exposures. The exact origin of this glow is under investigation by the FORS2 instrument team.

We derive the appropriate scaling factor for all the Raman line fluxes presented in Table 1 by comparing the FORS2 observations of the N2 and O2 fundamental Raman lines with the MUSE observations described in Sec. III. The resulting fluxes thus correspond to the integrated flux of a single laser uplink beam observed at an altitude of 25 km, under the (reasonable) assumption that the composition of the atmosphere above Cerro Paranal does not vary significantly up to heights of $\sim$25 km above ground (i.e. the intensity ratio between the different laser-induced Raman lines is constant).

The line fluxes provided in Table 1 are intended for observatories and their users to assess the possible impact of laser-induced Raman scattering on regular operations. We stress however that these fluxes must be treated with caution. They provide a realistic measure of the different Raman line fluxes, but their validity is limited to the meteorological specificities of a given site and seasonal variations in the temperature profile and water vapor content of the atmosphere. For reference and comparison purposes, the IR temperature of the sky was ($-88\pm 5$)*∘C during the FORS2 observations, with a temperature at ground level of ($16\pm 1$)∘*C. For the H2O line in particular, the precipitable water vapor during the (scaled) MUSE observations was $\sim$1 mm and the humidity $\sim$10% at ground level.

V Implications of laser-induced Raman scattering for astrophysical observations

Elegant from the perspective of the physics involved, the possible contamination of observations via laser-induced rovibrational Raman scattering may become a clear concern from the operational perspective of an astronomical observatory. As illustrated in Fig. 4, rovibrational Raman scattering leads to spectrally complex and broad structures in optical regions containing important astrophysical spectral lines (see Fig. 8). Although the structure of the Raman lines associated with diatomic molecules like N2 and O2 is well known, removing these lines from the data a posteriori will necessarily induce some residual photon noise affecting the quality of observations, in particular for deep fields and/or extremely faint targets. The intensity of the Raman rovibrational lines is also temperature-dependent (Cooney, 1972; Keckhut et al., 1990), and thus potentially time-dependent. The use of notch filters, such as in the case of MUSE, is only viable (from a scientific efficiency perspective) to alleviate the impact of the main Rayleigh line and rotational Raman lines in its immediate surroundings. It is thus crucial to handle the impact of rovibrational Raman contamination by first ensuring the proper baffling of the instrument, in addition to designing appropriate calibrations, observation procedures, and data reduction pipelines.

Certainly, the existence of laser-induced Raman scattering is not unique to Cerro Paranal, but common to any astronomical observatory employing similar devices. The majority (today) of astronomical telescopes rely on 589 nm lasers, but the emergence of so-called Rayleigh LGS systems with shorter lasing wavelengths implies that at specific sites, laser-induced Raman scattering will affect different spectral regions. For completeness, we list in Table 2 the wavelengths of the atmospheric rovibrational Raman lines described in Sec. III as a function of the laser wavelength $\lambda_{\text{LGS}}$ for the main astronomical telescopes equipped with an LGS system. The efficiency of Raman scattering increases at shorter wavelengths, so that Rayleigh LGS systems are likely to be more strongly affected by it, modulo the on-sky power the lasers involved.

In most cases, LGS systems are used to feed (near-)IR instruments, oblivious to the existence of Raman-scattered photons by the O2 and N2 molecules (which are the most abundant in the atmosphere and thus give rise to the strongest signal). Yet, the presence of laser-induced Raman-scattered photons can also affect other near-by telescopes and instruments at sites hosting multiple telescopes, such as Mauna Kea or Cerro Paranal. The distance between the different telescopes is a key factor influencing the frequency of so-called laser collisions: for the case of Cerro Paranal, the probability of at least one collision between the uplink laser beams from UT4 and any of the UTs and the VLT Survey Telescope (VST; Arnaboldi et al., 1998; Kuijken, 2011) is estimated to be $>$90% on any given night (Amico et al., 2010). To avoid the accidental contamination of laser-sensitive observations at near-by telescopes, a laser collision prediction tool was initially developed for the W.M. Keck LGS system (Summers et al., 2003), and eventually ported to the observatory of La Palma (Summers et al., 2006) and Cerro Paranal (Amico et al., 2015). One essential component of this tools is that the different telescopes on any given site must declare themselves as being laser sensitive, or not. On Cerro Paranal, the sensitivity of the different instruments mounted on all the UTs was assessed individually for each system, filter set, grating and grism, depending whether a given setup can see (or not) the main laser Rayleigh line at 589 nm. In other words, the system has so far been oblivious to the possibility of rovibrational Raman contamination. In particular, laser collisions for instrumental setups sensitive to the rovibrational Raman lines only (but not the main laser line) do not trigger any red flag. As discussed in Sect. IV, each instrument will be affected by rovibrational Raman contamination at a different level. Broad-band imagers can be expected to be less impacted than high-resolution spectrographs, and each case must undoubtedly be assessed individually. At the time of publication of this article, the individual Raman sensitivity assessment for all affected instruments on Cerro Paranal is underway.

With respect to the next generation of extremely large telescopes, our observations of laser-induced Raman scattering on Cerro Paranal will be particularly applicable to the HARMONI (Thatte et al., 2014) visible and near-IR integral field spectrograph to be installed as a first-light instrument on the ELT at Cerro Armazones. HARMONI will be fed corrected wavefronts by the ELT ATLAS LGS system (Fusco et al., 2010) that relies on six 589 nm lasers similar to those of the 4LGSF (according to the current design). HSIM (Zieleniewski et al., 2015) is a dedicated software designed to simulate observations with HARMONI. This tool thus likely offers the ideal means for 1) evaluating the impact of laser-induced Raman scattering on HARMONI observations, and 2) implementing suitable observing strategies.

Finally, the 4LGSF lasers and associated laser-induced Raman emission are currently under (ongoing) close scrutiny, including via dedicated observations using the VST, FLAMES (Pasquini et al., 2002) on UT2, as well as with the Gran Telescopio Canarias using the Wendelstein Laser Guide Star (Bonaccini Calia et al., 2012). Combined with existing and upcoming MUSE & GALACSI tests, these datasets are intended to assess directly the impact of laser-induced Raman scattering on specific instruments on Cerro Paranal and at the Observatorio del Roque de los Muchachos.

Acknowledgements.

We thank the AOF Builders (Arsenault et al., 2013) for the construction and installation of the AOF on UT4 at the VLT. We are grateful to Alain Smette for stimulating discussions over the course of the commissioning tests, to George Hau, Julien Girard, Thomas Pfrommer, Steffen Mieske, Andreas Kaufer, Tim de Zeeuw and Roland Bacon for their feedback on this article, to Claudia Reyes Saez, Claudia Cid, Susana Cerda and Diego Parraguez for their operational support, and to Joe Anderson for his input regarding the behavior of the FORS2 spectrograph. We also thank the two anonymous referee for their constructive reviews. This research has made use of the following python packages: matplotlib (Hunter, 2007), astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al., 2013), aplpy, an open-source plotting package for python (Robitaille and Bressert, 2012), and mpfit, a Python script that uses the Levenberg-Marquardt technique (Moré, 1978) to solve least-squares problems, based on an original fortran code that is part of the minpack-1 package. This research has also made use of montage, funded by the National Science Foundation under Grant Number ACI-1440620 and previously funded by the National Aeronautics and Space Administration’s Earth Science Technology Office, Computation Technologies Project, under Cooperative Agreement Number NCC5-626 between NASA and the California Institute of Technology, of the aladin interactive sky atlas (Bonnarel et al., 2000), of saoimage ds9 (Joye and Mandel, 2003) developed by Smithsonian Astrophysical Observatory, and of NASA’s Astrophysics Data System. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory. All the observations described in this article are freely available online from the ESO Data Archive.

Appendix A Constraining the altitude of the Raman emission with a toy model of the UT4 and 4LGSF systems

The geometry and spatial extent of the Rayleigh emission from the laser beam visible in Fig. 4 is directly related to the altitude at which the scattering of the laser photons occurs (either via Rayleigh or Raman processes). The geometry of the problem is such that a simple model of the UT4 telescope and 4LGSF launch system can be used to constrain the altitude at which the laser beam is visible within the MUSE field-of-view. A schematic of the 4LGSF systems and UT4 is presented in Fig. 9, along with all the relevant quantities of our model.

We define $R_{m}=4.1$ m, the radius of the primary mirror of UT4. Let us further assume that:

[TABLE]

where $H$ is the altitude of the 4LGSF guide star (i.e. the altitude of the sodium layer above the primary mirror), $d_{L,0}$ is the diameter of the laser beam, and $R_{L}$ is the distance between the optical axis of UT4 and the location where the 4LGSF laser beam would cross the mirror plane (if extended backward from the launch telescope). We also assume that the laser beam (which is effectively a gaussian beam) has a constant diameter as a function of the altitude, and that the observation is performed at zenith.

We define $\kappa$ as the angle between the optical axis of UT4 and the location of the 4LGSF guide star, as measured from the center of the primary mirror. This angle corresponds to a physical offset projected onto the mirror surface $k$:

[TABLE]

When the laser guide star is placed at the center of the MUSE field-of-view, $\kappa=0\Rightarrow k=0$. The total path length of the laser beam to the sodium layer $L$ is:

[TABLE]

To reconstruct the footprint of the laser beam in the MUSE field-of-view, we perform a crude ray-tracing exercise and construct the observed position (in the MUSE field-of-view) of all sky locations illuminated by the 4LGSF beam. Each location within the laser beam can be described with $z_{L}$, its distance to the sodium layer (along the beam propagation axis), $\rho_{L}$ its distance from the beam center, and $\phi_{L}$ its azimuth. In the subsequent analysis, we sample the laser beam at intervals of 1 km along its propagation axis, and each slice with 3 radial and 10 azimuthal steps. For a chunk of atmosphere illuminated by the laser with coordinates ($z_{L}$;$\rho_{L}$;$\phi_{L}$), its projected coordinates in the mirror plane ($x_{L}$;$y_{L}$) are:

[TABLE]

with:

[TABLE]

Here, we assume a geometry in which the laser beam is launched from the $\vec{y}$-axis at the location (0;$R_{L}$), with the laser $\vec{e}_{L,x}$-axis in the $\vec{x}-\vec{z}$ plane, the $\vec{e}_{L,y}$-axis in the $\vec{y}-\vec{z}$ plane, and the angle $\phi_{L}$ increasing counter-clockwise from the $\vec{e}_{L,x}$-axis. The angle $\xi$ is the angle between the vertical and the laser propagation axis as seen from the altitude $H$, i.e.:

[TABLE]

The altitude $h_{L}$ of the sky element located at ($z_{L}$;$\rho_{L}$;$\phi_{L}$) can be written as:

[TABLE]

so that the opening angle $\eta$ between the center of the MUSE field-of-view and the location on-the-sky of the said element is, as seen from the location ($x_{m}$;$y_{m}$) on the primary mirror:

[TABLE]

with:

[TABLE]

Finally, this can be translated to coordinates in the MUSE frame (in radians):

[TABLE]

with $\delta$ the azimuth (measured in the mirror plane) between the location ($x_{m}$;$y_{m}$) and the projected chunk of atmosphere at ($x_{L}$;$y_{L}$), which is:

[TABLE]

The prediction of this model – sampling the beam in steps of 10 km – is presented in Fig. 10 (bottom). The associated MUSE observation was performed at an altitude of 82*∘*, so that the beam altitudes $h_{L}$ computed via our model are off by only $\cos(90^{\circ}-82^{\circ})\equiv 1\%$. The size of the out-of-focus LGS donut of 18 arcseconds in diameter is accurately reproduced. For clarity, we sampled the primary mirror with only its center point and 60 locations on the outer edge of the mirror (see Fig. 11), giving rise to the specific ring-like patterns visible in Fig. 10. We note that to match the orientation of the beam in the observation, we rotated our final model accounting for the value of the parallactic angle $\eta$ at the time of the observation. The Rayleigh (and Raman) beam is best matched by the model out to $30\pm 5$ km above ground (bottom right panel). For higher altitudes, we do not detect any evident sign of emission. For completeness, a movie illustrating the prediction of our model – sampling the full laser beam at 1 km intervals – is made available online as supplementary material to the published article; in open access at http://link.aps.org/supplemental/10.1103/PhysRevX.7.021044.