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The performance of adaptive optics systems employing sodium laser guide stars can be improved by continuously monitoring the vertical density structure of mesospheric sodium along the line of sight. We demonstrate that sodium density profiles can be retrieved by amplitude modulation of continuous wave (CW) lasers. In an experiment conducted at the Large Zenith Telescope (LZT), ESO's Wendelstein Raman-fibre laser was amplitude-modulated with a pseudo-random binary sequence and profiles were obtained by cross-correlation of the modulation pattern with the observed return signal from the laser guide star. For comparison, high-resolution profiles were obtained simultaneously using the lidar system of the LZT. The profiles obtained by the two techniques show noise contamination, but were found to agree to within the measurement error. As a further check, a comparison was also made between several lidar profiles and those obtained by simultaneous observations using a remote telescope to image the laser plume from the side. The modulated CW lidar technique could be implemented by diverting a small fraction of the returned laser light to a photon counting detector. Theoretical analysis and numerical simulations indicate that, for 50 per cent modulation strength, the sodium centroid altitude could be retrieved every 5 s from a single laser guide star, with an accuracy which would induce a corresponding wavefront error of 50 nm for the ELT and less than 30 nm for the TMT and GMT. If multiple laser guide stars are employed, the required modulation amplitude will be smaller., 11 pages, 14 figures

International audience; Context. Laser guide stars (LGS) allow adaptive optics (AO) systems to reach greater sky coverage, especially for AO systems correcting the atmospheric turbulence on large fields of view. However LGS suffer from limitations, among which is their apparent elongation which can reach 20 arcsec when observed with large aperture telescopes such as the European Southern Observatory 39 m telescope. The consequences of this extreme elongation have been studied in simulations and laboratory experiments, although never on-sky, yet understanding and mitigating those effects is key to taking full advantage of the Extremely Large Telescope (ELT) six LGS. Aims: In this paper we study the impact of wavefront sensing with an ELT-scale elongated LGS using on-sky data obtained with the AO demonstrator CANARY on the William Herschel telescope (WHT) and the ESO Wendelstein LGS unit. CANARY simultaneously observed a natural guide star and a superimposed LGS launched from a telescope placed 40 m away from the WHT pupil. Methods: Comparison of the wavefronts measured with each guide star allows the determination of an error breakdown of the elongated LGS wavefront sensing. With this error breakdown, we isolate the contribution of the LGS elongation and study its impact. We also investigate the effects of truncation or undersampling of the LGS spots. Results: We successfully used the elongated LGS wavefront sensor (WFS) to drive the AO loop during on-sky operations, but it necessitated regular calibrations of the non-common path aberrations on the LGS WFS arm. In the off-line processing of the data collected on-sky we separate the error term encapsulating the impact of LGS elongation in a dynamic and quasi-static component. We measure errors varying from 0 nm to 160 nm rms for the dynamic error and we are able to link it to turbulence strength and spot elongation. The quasi-static errors are significant and vary between 20 nm and 200 nm rms depending on the conditions. They also increase by as much as 70 nm over the course of 10 m. We do not observe any impact when undersampling the spots with pixel scales as large as 1.95″, while the LGS spot full width half maximum varies from 1.7″ to 2.2″; however, significant errors appear when truncating the spots. These errors appear for fields of view smaller than 10.4″ to 15.6″, depending on the spots' elongations. Translated to the ELT observing at zenith, elongations as long as 23.5″ must be accommodated, corresponding to a field of view of 16.3″ if the most elongated spots are put across the diagonal of the subaperture. Data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/649/A158

SAMM (Solar Activity MOF Monitor) is a ground based robotic instrument that has been developed to study and constantly monitor the magnetic activity of the Sun, focusing on Active Regions (ARs). These regions are characterized by complex magnetic structures that may result in explosive events usually associated with large amount of particle and matter ejections in the space environment. When interacting with the Earth magnetosphere they can represent a threat for our infrastructures both in space (satellites, navigation systems) and on the ground (power plants and electrical grids). Based on Sodium (Na) and Potassium (K) magneto optical filters (MOFs), SAMM provides a “tomographic” view of the magnetic structures delivering high cadence magnetograms and dopplergrams at different heights of the solar atmosphere thus providing a unique dataset with the aim to push forward the current space weather forecasting capabilities. Being able to forecast these events enough in advance (even few hours) is a fundamental task to put in place mitigation strategies to reduce the potential catastrophic impact on vital infrastructures on earth. In this scenario the SAMM observatory has been realized to be a “node” that can be replicated in a world-wide network with the aim to give a continuous coverage of the Sun situation. This project has been initially funded by the Italian Ministry of Economic Development (MiSE) in 2015 through a soft loan grant and its development and operation is carried on within a scientific collaboration between the INAF – Rome and Naples Astronomical Observatories and the Italian small enterprise (SME) Avalon Instruments. After three years of development, SAMM is in the commissioning phase. In this paper we are presenting a final instrument description along with the first light images.

We report experimental results on the first on-sky observation of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. The magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 km and 100 km altitude. The experiment was performed at La Palma, and the uncertainty in the measured Larmor frequency ($\approx$260 kHz) corresponded to an error in the geomagnetic field of 0.4 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere.

Next-generation extremely large telescopes will be equipped with adaptive optics systems that use laser guide stars produced by resonant excitation of mesospheric sodium. The wavefront sensing of these systems could be enhanced by measurements of the vertical density profile of the mesospheric sodium along the line of sight, i.e. the sodium profile producing the LGS itself. Normally in AO system a few percent ( 3%) of the light from the LGS can be retrieved as leftover from the dichroic or the optics feeding the LGS WFS. This light, filtered and sent to a photon counter could be used for retrieving sodium density profiles. One approach is to partially modulate the intensity of the continuous wave lasers with a pseudo-random binary sequence of pulses. The sodium density profile can then be retrieved by cross-correlation of the measured LGS flux with the modulation sequence. With sufficient return flux, this technique can provide profiles every few seconds, useful for matching filters applied on elongated LGS, and can provide the centroid of the sodium layer profile, needed for the adaptive optics proper focus offset corrections. We have tested the continuous wave lidar concept by making simultaneous measurements using the modulation technique and a conventional lidar system, which provides the 'truth' in the comparison of the profiles. The experiments were conducted using the ESO Wendelstein laser guide star unit and the 6 m Large Zenith Telescope's sodium lidar system, located near Vancouver, Canada. This paper describes the experiment and presents some preliminary results. The possible implementation and potential performance of this technique for extremely large telescopes will be discussed.

The return flux from a sodium laser guide star suffers, at large angles between the geomagnetic field and the laser beam, from the reduction in optical pumping due to spin-precession of sodium atoms. This detrimental effect can be mitigated by modulating the circular polarization of a continuous-wave laser beam in resonance with the Larmor frequency of sodium atoms in the mesosphere. We present an investigation based on numerical modeling to evaluate the brightness enhancement of a laser guide star with polarization modulation of a continuous-wave laser beam at different observatories.

Six Laser Guide Stars (LGS) are included in the design of the European Extremely Large Telescope (ELT), with all of its current instruments taking advantage of them using Shack-Hartmann (SH) wavefront sensors (WFS). However, this implementation raises new issues related to the unprecedented elongation that results from the perspective effect combined to the thickness of the sodium layer. In order to investigate wavefront sensing with an elongated LGS on a SH WFS, we are taking advantage of the presence of the multi-object adaptive optics demonstrator CANARY on the William Herschel Telescope (WHT), in La Palma island, that was upgraded with a sodium LGS WFS for our experiment. The LGS is generated by ESO’s transportable Wendelstein LGS unit and the elongation is obtained by positioning the laser launch telescope 40 meters away from the WHT. With this experiment we are able to measure wavefronts using an elongated LGS WFS. In this paper, we present results obtained during the latest run of observations in September 2017. In these results is comprised an error breakdown of wavefront measurement on elongated LGS. The performances of several centroiding methods are compared thanks to this error breakdown. Additionally, we take advantage of varying observation conditions with respect to seeing and sodium profile to establish the robustness of the different centroiding methods. Finally, these performances are evaluated for different SH designs, to explore which compromises can be reached with respect to pixel scale and sub-aperture field of view.

(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., Comment: 17 pages, 11 figures, published in Physical Review X. v2: Fixed typo in Eq. 9 and y-labels in Fig. 6

Encouraged by imaging of faint galaxies around bright stars using ALFA on the 3.5-m telescope at Calar Alto, we have begun a survey to identify a large number of candidate sources near bright stars. In this contribution we report the status of this survey and show our preliminary results from deep imaging around one of these stars during the early phases of CONICA and NAOS on the 8.2-m VLT. We outline the exciting prospects for this type of work in terms of number counts, morphology, and rotation curve analyses.

We develop an algorithm to retrieve mesospheric Na profiles from the ESO-IAC LGS experiment at Teide Observatory (OT). We are using a bistatic configuration for Na LGS profiling between the ESO Wendelstein LGS Unit (WLGSU) and the IAC80 telescope with a baseline of 126 m. We describe the geometry of the problem and discuss the errors. The inputs are the observer pointing coordinates and the azimuth of the launcher, avoiding the refraction effect on the beam. Accuracy in the coordinates is a must and the images should be astrometrized. With an accuracy of 1" in the launcher azimuth, the absolute Na heights can be obtained with a resolution better than 200 m (ZD=40°). We also propose a double observer telescope, 90° shifted, to avoid the effect of a divergent solution when launching in the azimuth subtended between observer and launcher.

Measurement of magnetic fields on the few 100-km length scale is significant for many geophysical applications including mapping of crustal magnetism and ocean circulation measurements, yet available techniques for such measurements are very expensive or of limited accuracy. We propose a method for remote detection of magnetic fields using the naturally occurring atomic sodium-rich layer in the mesosphere and existing high-power lasers developed for laser guide star applications. The proposed method offers a dramatic reduction in cost and opens the way to large-scale, parallel magnetic mapping and monitoring for atmospheric science, navigation, and geophysics.

The first observations of laser guide star photons Raman-scattered by air molecules above the Very Large Telescope (VLT) were reported in June 2017. The initial detection came from the Multi-Unit Spectroscopic Explorer (MUSE) optical integral field spectrograph, following the installation of the 4 Laser Guide Star Facility (4LGSF) on the Unit Telescope 4 (UT4) of the VLT. In this Letter, we delve further into the symbiotic relationship between the 4LGSF laser guide star system, the UT4 telescope, and MUSE by monitoring the spectral contamination of MUSE observations by Raman photons over a 27 month period. This dataset reveals that dust particles deposited on the primary and tertiary mirrors of UT4 -- responsible for a reflectivity loss of ~8% at 6000{\AA} -- contribute (60$\pm5)% to the laser line fluxes detected by MUSE. The flux of Raman lines, contaminating scientific observations acquired with optical spectrographs, thus provides a new, non-invasive means to monitor the evolving scatter properties of the mirrors of astronomical telescopes equipped with laser guide star systems., Comment: 5 pages, 3 figures, 1 table, accepted for publication in A&A Letters

The two first light adaptive optics (AO) instruments for the European Extremely Large Telescope (E-ELT) will both rely on several Sodium Laser Guide Stars (LGS). In using this technology, the E-ELT diameter comes with new challenges mainly raised by spot elongation. Before the final design studies of the E-ELT instruments, a Sodium LGS wavefront sensing (WFS) on-sky experiment at this scale is mandatory to provide meaningful spatial and temporal measurement error and performance evaluation. For this purpose, we propose to use CANARY, the Multi-Object AO demonstrator installed at the WHT (4.2m). CANARY is now undergoing a Rayleigh LGS upgrade and also provides natural guide star WFS. It could easily be adapted to the Sodium LGS case. Additionally, a transportable laser system, such as the one developed at ESO, positioned at a varying distance from the WHT could be used to provide off-axis launching (up to 40m), simulating the whole range of spot elongations that will be obtained on the E-ELT. Full scale simulations of a Sodium LGS WFS on the E-ELT have guided us in the definition of the experiment we are proposing. Previous simulation results in the literature have stressed that an estimate of the Sodium profile is necessary to reduce measurement error to an affordable level. Hence parallel Sodium profiling is mandatory in the proposed experiment. In this paper, we present the simulation results as well as the specifications for both the Sodium LGS WFS and profiler to be used on Canary.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The ESO Adaptive Optics Facility (AOF) consists in an evolution of one of the ESO VLT unit telescopes to a laser driven adaptive telescope with a deformable mirror in its optical train. The project has completed the procurement phase and several large structures have been delivered to Garching (Germany) and are being integrated (the AO modules GRAAL and GALACSI and the ASSIST test bench). The 4LGSF Laser (TOPTICA) has undergone final design review and a pre-production unit has been built and successfully tested. The Deformable Secondary Mirror is fully integrated and system tests have started with the first science grade thin shell mirror delivered by SAGEM. The integrated modules will be tested in stand-alone mode in 2012 and upon delivery of the DSM in late 2012, the system test phase will start. A commissioning strategy has been developed and will be updated before delivery to Paranal. A substantial effort has been spent in 2011-2012 to prepare the unit telescope to receive the AOF by preparing the mechanical interfaces and upgrading the cooling and electrical network. This preparation will also simplify the final installation of the facility on the telescope. A lot of attention is given to the system calibration, how to record and correct any misalignment and control the whole facility. A plan is being developed to efficiently operate the AOF after commissioning. This includes monitoring a relevant set of atmospheric parameters for scheduling and a Laser Traffic control system to assist the operator during the night and help/support the observing block preparation. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Almost all sodium laser guide star (LGS) systems in the world are based on pulsed lasers. We review the relevant sodium physics and compare different laser pulse formats. Selected formats are discussed on the basis of numerical simulation results. One of the key findings is that the brightness of most existing LGS facilities could be boosted at, as we argue, reasonable expense. Recommendations are presented to enhance the LGS return flux and to design future LGS lasers, including those suitable for spot tracking in th e mesosphere to mitigate the spot elongation problem. Keywords: Laser guide stars, Atmospheric propagation, Laser beam transmission, Wavefront sensing 1. INTRODUCTION Sodium laser guide stars (LGS) are used as artificial beacons for Adaptive Optics (AO) in many large telescopes. The upcoming generation of 30+m telescopes will rely for their operation on LGS-assisted AO. An LGS is a luminous column of 10–20 km height in the mesosphere, centered at about 93 km altitude, where naturally occurring sodium is excited with resonant light at 589 nm (orange), which is followed by fluorescen ce. The LGS facilities require lasers with excellent beam quality to minimize the spot size and a power of 5–25 watt per star. In order to optimize the brightness of the guide stars, detailed understanding of light-atom interaction physics is required. We find that the excitation efficiency depends nontrivially on the laser spectrum, polariza tion, the geomagnetic field, and other parameters. While a format optimization is already a challenging task fo r continuous-wave (cw) lasers, any modulation of the laser power introduces an additional level of complexity to the simulation. When modeling installed systems, a good knowledge of the pulse format both in time and frequency domain is required. However, only limited data are usually available from observatories; in particular, highly resolved optical spectra and their stability in time have seldom been measured. The return flux from sodium LGS depends on, among various other parameters, the geomagnetic field, atomic collision rates and mesospheric temperatures. Moreover, the photon return flux per atom is in general a nonlinear function of irradiance (power density) in the sodium layer.

In this paper we present the rationale and design of a compact, transportable, modular Laser Guide Star Unit, comprising a 589nm laser mounted on a 300mm launch telescope, to be used in future experiments probing the mesospheric sodium properties and to validate existing LGS return flux simulations. The 20W CW 589nm Laser is based on the ESO developed concept of 589nm lasers based on Raman Fiber Amplifiers, refined and assembled together with industry. It has the same laser architecture as the laser which will be used for the VLT Adaptive Optics Facility. We have added to the 20W CW laser system the capabilities of changing output polarization, D2b emission levels, power level, linewidth and to operate as pulsed laser with amplitude modulation. We focus in this paper on the laser description and test results.

We extend previous sodium LGS models by integrating the return flux across the mesosphere, taking into account variable mesospheric gas density, temperature, and local sodium density. This method allows us to produce accurate predictions of the actual return flux on the ground, relevant for determining the performance of adaptive-opticsassisted instruments. We find that the flux distribution across the sky depends strongly on geographic location and laser parameters. Almost independent of location, future sodium LGS will be about three times brighter at zenith than at the observing horizon.

The quest of the astronomical instrumentation community for high-power, narrow-band CW laser guide stars (LGS) has been a challenge to the laser community for more than two decades now. Only recently, a new generation of rugged laser system developments has started to provide the laser infrastructure for the next generation earth-bound telescopes. We report on the system design of four 20W CW diode-seeded fiber-amplified laser guide star for deployment at the VLT in 2013.

We report on a 25 W continuous wave narrow linewidth (2.3 MHz) 589 nm laser by efficient (95%) coherent beam combination of two narrow linewidth (1.5 MHz) Raman fiber amplifiers with a Mach-Zehnder interferometer scheme and frequency doubling in an external resonant cavity with an efficiency of 86%. The results demonstrate the narrow linewidth Raman fiber amplifier technology as a promising solution for developing laser for sodium laser guide star adaptive optics.

We report on a recent breakthrough in the generation of high power laser light at 589 nm that is needed for sodium laser guide stars and medical purposes. Large telescopes require 20–25W CW at 589 nm with good beam quality and linewidths of 50 MHz and below. Due to the virtual absence of useful solid state resonances at 589nm or 1178nm, and since laser guide star systems require high reliability and, optimally, small size, fiber lasers are an interesting option. We have focused on Raman fiber lasers. However, the early onset of stimulated Brillouin scattering (SBS) has previously thwarted their successful operation at narrow bandwidths and high power.

We present the results of extensive Bloch equation numerical simulations, both for cw and for various pulsed laser formats and applications.

We use physical optics to simulate LGS propagation and imaging in a Shack-Hartmann wavefront sensor (WFS). We model different launch telescope (LT) sizes and realistic LT aberrations, the turbulent atmosphere, a sodium layer of finite thickness, the downlink propagation of the return light, an 8m-telescope, and finally the planned Very Large Telescope (VLT) Adaptive Optics Facility 40×40 GRAAL WFS. We study both long-exposure and instantaneous images on the WFS and compute spot size statistics. The results agree with observations obtained in the VLT telescope guider camera and enable us to optimize the LT diameter and devise design rules.

We report our works on fiber Raman amplifiers with linewidth of ~10MHz. Up to 1 W, < 12.5 MHz 1178 nm laser is obtained by Raman amplification of a distributed feedback diode laser seed in standard fibers, limited by stimulated Brillouin scattering. By introducing temperature non-uniformity along fiber, we are able to push back the threshold of stimulated Brillouin scattering and increase the output to 1.5 W. ©2008 Optical Society of America

We investigate the potential of fiber Raman lasers in the visible wavelength region, which have hardly been explored yet, motivated by developing a fiber based 589nm source for laser guide star adaptive optics.

The Adaptive Optics Facility is a project to convert one VLT-UT into a specialized Adaptive Telescope. The present secondary mirror (M2) will be replaced by a new M2-Unit hosting a 1170 actuators deformable mirror. The 3 focal stations will be equipped with instruments adapted to the new capability of this UT. Two instruments are in development for the 2 Nasmyth foci: Hawk-I with its AO module GRAAL allowing a Ground Layer Adaptive Optics correction and MUSE with GALACSI for GLAO correction and Laser Tomography Adaptive Optics correction. A future instrument still needs to be defined for the Cassegrain focus. Several guide stars are required for the type of adaptive corrections needed and a four Laser Guide Star facility (4LGSF) is being developed in the scope of the AO Facility. Convex mirrors like the VLT M2 represent a major challenge for testing and a substantial effort is dedicated to this. ASSIST, is a test bench that will allow testing of the Deformable Secondary Mirror and both instruments with simulated turbulence. This article describes the Adaptive Optics facility systems composing associated with it.

We present the design of a 0.5 GHz linewidth 589 nm master laser by direct Raman shift from a commercially available solid state green laser, done in a single mode fiber. The laser design would be a breakthrough because it would allow a large number of currently unavailable lasing wavelengths to be reached in solid state devices. Our purpose is to produce 0.2-0.5 W CW at 589 nm, to seed a Raman amplifier scheme and achieve 10-15 W CW output at 589 nm. Our simulations show that > 0.5 W may be obtained with a 10 W 532 nm pump laser and sets of appropriately steep Fiber Bragg Gratings which resonate only the Raman Stokes.

As part of a collaboration between Durham University and ESO, an experimental platform is presented whose purpose is for testing generic laser-based wavefront sensors (WFS) for adaptive optics. The Rayleigh Technical Demonstrator (RTD) has been designed to allow a laser launch and Rayleigh back-scatter collection by installing components solely on a Nasmyth platform of the William Herschel Telescope, La Palma. The aim is to provide a WFS testing port within the RTD, permitting new WFS concepts to be tested rapidly in conjunction. This means that the RTD only requires small modifications for each concept to be tested. The second goal is to permit near-contemporaneous comparison of WFS data with that from a tomographic WFS. Currently, the RTD is planned to trial with three "cone effect-free" WFS concepts as part of the CALDO project. Presented here is an overview of the RTD design with detailed information on novel components and design choices.

The Laser Guide Star Facility for the VLT will be commissioned during 2003. This, of course, can only be the first step towards enhancing the scientific output of the VLT through adaptive optics (AO). So in this contribution we propose the development of a laser guide star (LGS) multi-conjugate adaptive optics (MCAO) system. We consider geometries for the adaptive optics system, and discuss the technology needed to project up to 5 laser beacons. The rationale for such a project is provided by an outline of two primary science drivers.

In this paper, we developed an all-fiber laser system for generating a 589 nm source for laser-guided adaptive optics. The design is based on sum-frequency mixing two fiber lasers in a nonlinear material to produce a CW output at the desired wavelength. Combining an Er/Yb:doped fiber laser operating at 1583 nm with a 938 nm Nd:silica fiber laser, one can generate 589 nm via sum-frequency mixing in a periodically poled crystal. The integrated system design and performance is presented

We did calculations structure function of phase fluctuations in signal from real star and set of spherical waves, formed in plane of LGS. We developed the approach on formation laser guide star free from focal anisoplanarity.

We are developing an all-fiber laser system for generating 589 nm light for laser-guided adaptive optics. If only natural stars can be used to measure the turbulence in the Earth's atmosphere, at most a few percent of the sky is accessible to adaptive optics correction. Laser guide stars are therefore crucial to the broad use of adaptive optics, because they facilitate access to a large fraction of possible locations on the sky. In particular, lasers tuned to the 589 nm resonance line of atomic sodium are able to create an artificial beacon at altitudes of 95-105 km, thus coming as close as possible to reproducing the light path of starlight. The deployment of multiconjugate adaptive optics on large aperture telescopes world-wide will require the use of three to five sodium laser guide stars in order to achieve uniform correction over the aperture with a high Strehl value. Current estimates place the minimum required laser power at 10 W per laser for a continuous wave source. In addition, the lasers need to be compact, efficient, robust and turnkey.

We report in this paper on the current activities of the Laser Guide Star group at ESO. We are building the Laser Guide Star Facility on the VLT UT4, we are pushing new technologies in view of multiple LGS and future ELT LGS-AO systems. We are studying new LGS propagation/sensing schemes, to get rid of the LGS cone effect. We are also fostering in member states countries the development of fast sensors to deal with pulsed lasers atmospheric layer sensing.

The goal of the CALDO experiments is to demonstrate Laser Guide Star technologies which can scale directly to a 100m diameter primary aperture, and which are not compromised by the cone-effect at very large telescope diameters. The laser guide star group at ESO and the adaptive optics group at Durham have proposed two different laser wavefront sensing methods designed to meet this goal. Though based on quite different physical principles, the two methods achieve their scalability through the use of a parallel sensing beam projected from the whole of the telescope primary mirror. They can therefore both be demonstrated by performing a scaled-down projection and sensing experiment on a smaller telescope. The CALDO experiments evaluate the ESO and Durham methods concurrently and provide a comparison with Natural Guide Star wavefront sensing, and with each other, without the uncertainty introduced into a separate evaluation by changing atmospheric conditions. The location for CALDO is the 4.2m William Herschel Telescope, which has the advantage of the GHRIL Nasmyth facility for adaptive optics experiments and which has already been used by the Durham group for shared-optics launch experiments with a laser guide star. We describe the ESO and Durham methods, the current progress on the experimental subsystems, and the projected timescales for the experiments.

11W of 938nm light was produced in an Nd3+ fiber laser. Optimization of the ratio of the fiber core and cladding areas permitted operation of the laser at room temperature by minimizing 1088nm gain.

We have developed a Nd:doped cladding pumped fiber amplifier, which operates at 938nm with greater than 2W of output power. The core co-dopants were specifically chosen to enhance emission at 938nm. The fiber was liquid nitrogen cooled in order to achieve four-level laser operation on a laser transition that is normally three level at room temperature, thus permitting efficient cladding pumping of the amplifier. Wavelength selective attenuation was induced by bending the fiber around a mandrel, which permitted near complete suppression of amplified spontaneous emission at 1088nm. We are presently seeking to scale the output of this laser to 10W. We will discuss the fiber and laser design issues involved in scaling the laser to the 10W power level and present our most recent results.

We report on our research to develop a fiber Raman laser for multiple sodium laser guide star adaptive optics. Our goal is a fiber laser source at 589 nm with a diffraction-limited continuous-wave output power of at least 10 W and a linewidth of less than 3 GHz. In this paper we present and model our concept of a frequency-doubled fiber Raman amplifier. Recent advances in commercially available pump and seed laser technologies are making the amplifier approach a highly attractive solution for a turn-key sodium-guide star laser system.

We report on the ongoing VLT Laser Guide Star Facility project, which will allow the ESO UT4 telescope to produce anartificial reference star for the Adaptive Optics systems NAOS-CONICA and SINFONI. A custom developed dye laserproducing >10W CW at 589nm is installed on-board of the UT4 telescope, then relayed by means of a single mode opticalfiber behind the secondarymirror, wherea 500mm diameter lightweight, f/1 launch telescope is projecting the laser beam at90 km altitude.We described the design tradeoffs and provide some details of the chosen subsystems. This paper is an update includingsubsystems results, to be read together with our previous paper on LGSF design description 3 .Keywords: adaptive optics, sodium laser guide star, largetelescopes 1. INTRODUCTION It has been demonstrated that AO systems can indeed work effectively with Laser Guide Stars (LGS), in the visible 1 and inthe NIR 2 . Recentlythe Lick Observatoryhasachieved K-band Strehl values of ~0.6 with LGS-AO. Thistechnologyisbecoming progressivelymature, and solid statelasers at 589nm with adequatepower and spectral format to produce LGS,are within reach in the next few years [other papers at this conference].It is common experience that both the AOand the LGS generation facilityhave to be well tuned andreliable to obtain goodresults, in other words, good engineering of all subsystems isneeded from the verystart. Oneshould avoid going over thesecomplex systems by refurbishing them on the telescope. We have developed the LGSF based on previous systemsgenerations' lessons learned, introducing more stability of the laser system, integration in the telescope, ease of operationand abundant diagnostics at several steps of thelaser light mainstream path, with thegoal of improving reliabilityandoperations.ThestrategybehindtheESOLGSFistobuildasystemtoproduceasingleLGS,usefultorunAOsystemscorrectingforthe Near Infra-Red bands, with the built-in provision to upgrade tomultiple LGS for MultiConjugate AO. Wehave pushedthe design, development and integration phases to complete them within a three year period, and introduced several featureson thedye laser to be able torun it smoothlyand without a laser specialist for extended periods of time.

One of the critical design drivers for the PARSEC laser was that by the time it is integrated into the VLT Laser Guide Star Facility in 2003 it should be able to run remotely without any hands-on tuning for a period of at least one week. In this contribution we describe some of the methods we have employed to achieve this and take a brief look at the proposed operating procedure.

The use of multi-conjugate adaptive optics (MCAO) for large telescopes will be most efficient with multiple laser guide stars. To implement a single laser guide star (LGS) at 589 nm requires the use of bulk and complex dye lasers at the moment. Solid-state lasers will be necessary to reduce to decent levels the infrastructure impact and operational complexity of multiple guide star adaptive optics systems. Development programs for discrete solid-state lasers are in place at GEMINI, the USAF Starfire Optical Range and University of Chicago. ESO has chosen the path of the development of fiber lasers, which is complementary to the other programs. The advantages of fiber lasers are manifold: they are very compact, inherently rugged, most reliable and have a potential for low cost. In this paper we present the research and development activity started at ESO on fiber lasers for MCAO. The aim is to produce > 10-W continuous-wave (CW) output power at 589 nm with a linewidth < 3 GHz. We report here on our fiber Raman laser conceptual design.

We report on the ongoing VLT Laser Guide Star Facility project, which will allow the ESO UT4 telescope to produce an artificial reference star for the Adaptive Optics systems NAOS-CONICA and SINFONI. A custom developed dye laser producing >10W CW at 589nm is installed on-board of the UT4 telescope, then relayed by means of a single mode optical fiber behind the secondary mirror, where a 500mm diameter lightweight, f/1 launch telescope is projecting the laser beam at 90 km altitude. We described the design tradeoffs and provide some details of the chosen subsystems. This paper is an update including subsystems results, to be read together with our previous paper on LGSF design description 3

The European Southern Observatory is developing a medium order curvature adaptive optics system designed to be operable with minimal modification at any focus of the Very Large telescope (VLT). The first application of this AO system (MACAO) is to equip all four VLT Unit Telescope (UT) Coude foci with 60 element AO systems capable of delivering to the VLT Interferometer (VLTI) > 50% K band Strehl. The AO system being used by an interferometer is constrained to introduce minimal piston and operating as a sub-system of a large and complex instrument to have a robust architecture and simple operation. Installation of the first AO system is scheduled to begin first Quarter 2002 with completion of all four UTs by early 2004. Other applications of the MACAO system will be for use by the CRIRES and SPIFFI spectrographs.

We report on efficient transportation of visible high power (> 5 W) continuous-wave laser radiation in polarization maintaining, medium-long (> 10 m) optical fibers with single-mode output. Solutions are presented to achieve both maximum launching efficiency of a free-space laser beam as well as efficient reduction of stimulated Brillouin back- scattering together with optimum excitation of mesospheric sodium atoms. The fiber throughput experiments carried out so far with shorter fibers agree well with the theoretical predictions. Single-mode transmission above 70% have been achieved in 5 m and 10 m long test fiber units at input powers of 4.2 W at 532 nm. We do not experience fiber glass damage nor changes in throughput over 200 hours operation, at power densities of 25 MW/cm2. An outlook is given on the final development steps required for successful implementation of a single-mode fiber relay system within ESO's laser guide star facility.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The generation of sodium guide stars through laser excitation imposes very specific requirements on the laser. It has been argued that the most effective sodium guide star laser would operate CW with a mode spacing of less than the 10 MHz Doppler broadening profile of the mesospheric sodium. However, achieving high power in free-space optics with these requirements becomes very difficult because of the limited active gain volume and exceedingly long cavity requirement. The clear solution to these impediments is the development of a fiber raman laser (FRL) to shift the output of a solid state laser to a harmonic of the 589 nm sodium transition. The specific wavelength and bandwidth are selected from the large Raman gain spectrum by application- specific Distributed Bragg Reflectors. The guided-wave nature of the FRL allows for very large interaction volumes, while the long fiber length allows for narrowly-spaced modes which easily fill the sodium D 2 continuum. This paper reviews the theory and design of the Fiber Raman Guidestar Laser being developed for Astronomical Adaptive Optical systems.

We describe the current planning and activities for the laser Guide Star Facility of the VLT observatory. The current activity aims at a feasibility study and to the experimental verification of innovative ideas in the choice of the laser and in the relay system.

We present the preliminary study of a low cost, high performance spectrograph for the VLT, for observations in the V, R and I bands. This spectrograph is meant for intermediate (R equals 16,000) resolution spectroscopy of faint (sky and/or detector limited) sources, with particular emphasis on the study of solar-type (F-G) stars belonging to the nearest galaxies and to distant (or highly reddened) galactic clusters. The spectrograph is designed to use the adaptive optics (AO) systems at the VLT Telescope. Even if these AO systems will not provide diffraction limited images in the V, R and I bands, the photon concentration will still be above approximately 60% of the flux in an 0.3 arcsecond aperture for typical Paranal conditions. This makes the construction of a compact, cheap and efficient echelle spectrograph possible. AVES will outperform comparable non adaptive optic instruments by more than one magnitude for sky- and/or detector-limited observations, and it will be very suitable for observations in crowded fields.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

SINFONI, the SINgle Faint Object Near-infrared Investigation, is an instrument for the Very Large Telescope (VLT), designed to provide spectroscopy at the telescope diffraction limit in the near-infrared. This unique capability is achieved by combining two state-of-the-art developments, an integral field spectrometer (SPIFFI) and a curvature sensor based adaptive optics system (MACAO). SINFONI is a collaborative effort by the Max-Planck-Institut fur extraterrestrische Physik (MPE) and the European Southern Observatory (ESO).SINFONI will operate at the Cassegrain focus of Unit Telescope 1 (UT1) of the VLT, in conjunction with a Laser Guide Star (LGS) for almost complete sky coverage. It will provide integral field data cubes, with a hexagonal field of view ranging from similar to 1 " to 8 ", with corresponding pixel sizes of 0." 03 to 0." 25. The field of view contains 1024 spatial pixels, with similar to 100% filling factor in the focal plane. Spectra are obtained for each of the 1024 pixels. Spectral resolutions of R=2000 to R=4500 will be available, covering the J, H and K spectral windows. The high spectral resolution made will allow software OH suppression in the J and H bands. The detector is a 1024(2) HgCdTe HAWAII array from Rockwell. Spectroscopy of faint objects (m(K) < 21 and m(H) < 22) will be easily feasible.

We report on the calibration and field test results obtained with the ESO/Meudon Adaptive Optics System, Adonis. This 7x7, 50cm subaperture system is now routinely in use at the ESO La Silla 3.6m telescope, offered to the astronomers together with two infrared cameras covering the 1-5 µm wavelength range. The calibration activity summarized here has investigated the field performances achieved so far with Natural Guide Stars (NGS) using the either the Reticon or the EBCCD wavefront sensors.

New details of the star formation process have been revealed by a coordinated use of the ADONIS system on the 3.6 meter telescope at La Silla with data from the WFPC2 Camera of the Hubble Space Telescope. In this very preliminary report, we illustrate some of the unique capabilities of the ADONIS system for high-resolution observations of the stellar formation processes in the near infrared region.

Designs for the large binocular telescope (LBT) beam combiner optics, collimators, and reimaging cameras for the visible and infrared over a range from 0.5 to 13 microns are presented and discussed. To achieve good images at the combined focus of the LBT, an adaptive correction will be made independently for each of the two telescopes prior to beam combination. For interferometry the image planes of each telescope are first collimated. The two beams then pass to two flats. The two flats will be shifted to remove the remaining piston differences between the two telescope beams, using an error signal derived from a fringe tracker further down the optical path. The flats output a pair of parallel collimated beams with the collimator exit pupils and separation scaled to the original LBT entrance pupil geometry. Because the pixel matching, diffraction limits and isoplanatic fields are different in the different wavelength bands, a series of cameras have been designed covering the different bands from the visible through 12 microns. Both refractive and reflective optical designs are considered.