Search

The Ingot WFS represents an innovative and indispensable class of sensors conceived to overcome some limitations due to the LGSs geometry, which is significantly different from the point-object originated by a NGS. Here we overview the project, aiming at investigating the performance of an ELT-like telescope equipped with the Ingot WFS, facing different aspects of the program: the needs for numerical simulations and laboratory experiments, the prototype and, finally the future plan for the verification on sky., Comment: 7 pages, 5 figures; SPIE 2022 conference proceeding

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.

This publisher's note corrects an error in the author listing of Opt. Lett.43, 5825 (2018)OPLEDP0146-959210.1364/OL.43.005825.

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.

CANARY is a wide-field AO on-sky test facility which has been operated annually on the 4.2m William Herschel Telescope since 2010. CANARY has the stated goal of testing and demonstrating AO technologies that are critical for ELT AO performance. It has seen four distinct phases where new AO technologies have been developed and demonstrated, including NGS MOAO in 2010 (phase A), Rayleigh LGS and NGS MOAO in 2012 and 2013 (phase B, with LGS commissioning in 2011), LTAO operation in 2014 and 2015, and finally operation with a single Sodium laser guide star launched far off axis in 2016 and 2017 (phase D). By launching this laser guide star 40m off axis, extremely elongated laser guide star spots are created in the CANARY LGS Shack-Hartmann wavefront sensor. Therefore, the 7×7 sub-apertures of CANARY can be used to test wavefront sensing performance of a sub-pupil of the ELT located furthest from the laser launch axis. We present an overview of CANARY in its phase D configuration. Depending on where in the sky the LGS is pointing, the projected baseline between the on-axis LGS wavefront sensor and the laser launch location, as seen by the wavefront sensor, will vary from about 20-40m, allowing us to artificially generate different degrees of elongation. Additionally, the well sampled CANARY sub-apertures have 30×30 pixels each and a 20 arcsecond field of view, using an OCAM2S EMCCD camera. This means that by shrinking sub-apertures, and optionally by binning pixels, we are able to investigate different pixel scales and fields of view for the ELT systems, thus determining the optimal design parameters. Here we discuss the closed loop tests that were performed to investigate the effect of spot truncation and extreme elongation. We include different correlation techniques, including standard FFT-based correlation, brute force correlation and correlation by difference squared. We also mention dynamic and automatic updates of the correlation reference images while the AO loop is engaged that have previously been reported. The matched filter algorithm is also mentioned, with a pointer to our prior on-sky investigations. We give our recommendation for the ELT wavefront sensing algorithm of choice, and our evidence based reasons for this recommendation, which may come as a surprise to some. Finally we also present the future experiments to be performed with CANARY, give details of the OPTICON funded programme which enables the hosting of AO experiments on CANARY, allowing the AO community to get involved.

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

Ground-based optical telescopes, in particular large ones, require adaptive optics to overcome the atmospheric seeing limit due to turbulence. Correcting the distorted wavefront necessitates bright stars in the field of view. The sky coverage can be greatly increased by using artificial sodium laser guide stars in addition to natural guide stars. We describe the underlying physics and technical considerations relevant to such systems before discussing the design of the four-laser guide star facility (4LGSF) which is currently under development for the ESO Very Large Telescope (VLT) on Cerro Paranal, Chile. The focus is upon the justification of the requirements and their technical solution.

The German Aerospace Center (DLR) and the European Southern Observatory (ESO) performed a measurement campaign together in April and July 2016 at Teide-Observatory (Tenerife), with the support of the European Space Agency (ESA), to investigate the use of laser guide stars (LGS) in ground to space optical communications. Atmospheric turbulence causes strong signal fluctuations in the uplink, due to scintillation and beam wander. In space communications, the use of the downlink channel as reference for pointing and for pre-distortion adaptive optics is limited by the size of the isokinetic and isoplanatic angle in relation to the required point-ahead angle. Pointing and phase errors due to the decorrelation between downward and upward beam due to the point-ahead angle may have a severe impact on the required transmit power and the stability of the communications link. LGSs provide a self-tailored reference to any optical ground-to-space link, independently of turbulence conditions and required point-ahead angle. In photon-starved links, typically in deep-space scenarios, LGSs allow dedicating all downlink received signal to communications purposes, increasing the available link margin. The scope of the joint DLR-ESO measurement campaign was, first, to measure the absolute value of the beam wander (uplink-tilt) using a LGS, taking a natural star as a reference, and, second, to characterize the decrease of correlation between uplink-tilt and downlink-tilt with respect to the angular separation between both sources. This paper describes the experiments performed during the measurement campaigns, providing an overview of the measured data and the first outcomes of the data post-processing.

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.

The use of sodium laser guide star for Extremely Large Telescopes (ELT) adaptive optics systems is a key concern due to the perspective effect that produces elongated images in the Shack-Hartmann pattern. In order to assess the feasibility of using an elongated sodium beacon on an ELT, an on-sky experiment reproducing the extreme off-axis launch conditions of the European ELT is scheduled for summer and autumn 2016. The experiment will use the demonstrator CANARY installed on the William Herschel Telescope and the ESO transportable 20W CW fiber laser, embedded in the Wendelstein LGS unit. We will discuss here the challenges this experiment addresses as well as the details of its implementation and the derivation of the error budget.

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

We present a fast and robust ad-hoc method to analyze a set of short exposure images of a close bi- nary star taken by the adaptive optics system of the ESO 3.6 m telescope. At rst each image is tted, assuming as a radially symmetrical approximation of its \instan- taneous" PSF, the superposition of a central Gaussian and of a Moat torus. Owing to the coral variability and asymmetry of the PSF the t gives a poor estimate of the component's luminosity ratio; nevertheless it reproduces satisfactorily the global structure of the PSF and above all gives the accurate sub-pixel positions of the two stars. This allows to obtain, by the shift-and-add procedure, the mean set image from which the nal parameters of the two stars and the mean PSF during the observing run are derived. The results from a set of AO images of the close binary system Canis Maioris are consistent with those previously obtained by one of us using the iterative blind deconvolution. Extensive simulations give evidence that the magnitude dierence and the separation of the components can be estimated within about 0.05 mag and 0: 005 respectively when the secondary star image does not overlap a bright artifact of the AO system.

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 report on a 39 W SBS-suppressed narrow-linewidth (