L4AO 13

Abstracts and Presentations

Please click on the presentation title to access a PDF version of the presentation.

Jose Luis Alvarez, European Southern Observatory
Multiple Laser Guide Star; performance, operation and maintainability at Paranal Observatory
For more than a decade ago, a Laser Guide Star Facility (LGSF) was installed in the YEPUN (UT4) telescope, at the Paranal Observatory, to provide a reference star to the adaptive optics systems. The LGSF is tuned to the sodium D2 line to use the resonance fluorescence of atomic sodium in the mesospheric layer at an altitude of 90 Km. Since then, major upgrades have been made to the system to increase its availability, simplify its remote operation and improve its performance.
These upgrades include the replacement of the ring dye laser source for a “state of the art” fiber laser. All these changes have made a strong impact in the availability of the system, in its operations and the reduction of maintenance.
Since April 2016, a new “state of the art” Multiple Laser Guide Star Facility (MLGSF), has been fully operational and producing scientific data. The new four lasers are based on fiber Raman laser technology. Each laser delivers 22 watts of power in a beam with a diameter of 30 centimeters.
The four laser sources of the MLGSF were develop by ESO with the cooperation of the industries, including TOPTICA, Germany; TNO, the Netherlands; MPB Communications, Canada, and several European institutes.
In this article we present the technical performance of the MLGSF system, first we identified some critical parameters for the overall performance of each laser unit such as power stability, wavelength stability, and thermal stability of the subsystems. In addition, we show the operation and handling of the system done by telescope operators, and the main problems reported by them during operation.
Finally, we explained the service and maintenance of the whole system as well as some subsystems and the overheads evolution necessary to maintain the LGSF and the MLGSF fully operational.
Martijn van Riel, TNO
Laser Launch Tube for LGS systems
At present, the interest in reliable and accurate laser launch systems intended for creating artificial sodium LGS shows a significant increase. Many of the large telescopes in the world, irrespective of their current state (design, realization, or operational), are investigating what solutions exist for realization of a LGS system. A good example of a fully operational 4 LGS facility is found in ESO’s VLT, for which TNO delivered the laser launch tubes (LLTs). In general, the LLT in an LGS system is considered to be one of the most critical components where it concerns the performance of the total system. The difficulty here is that the LLT needs to combine challenging requirements on the output wavefront (<50 nm RMS and ±0.2 waves thermally induced defocus) and on the pointing accuracy (0.3” (3σ) over ±4.8’) of the output beam with the overall system size (governed by the Ø300 mm (1/e2) output beam) and the environmental conditions (pressure and temperature range, thermal gradients, and varying orientation with respect to gravity amongst others). Moreover, the LLT’s large L1 lens (Ø380 mm) needs to be manufactured such that it only consumes a portion of the WFE budget.
In this presentation, the LLTs that TNO delivered for the VLT-LGSF are discussed. Topics include a short recap of the LLT’s requirements, it’s thermo- and opto-mechanical design, manufacturing of the main lens, and testing of the full LLT. Furthermore a short outlook into the evolving requirements of the next generation LLTs is included.
Celine D'Orgeville, Australian National University
First Laser Guide Star for Space Debris Tracking, Imaging and Manoeuvring in Australia
This year (2019) will see the first Laser Guide Star ever created in Australian skies. The Australian National University (ANU) and industry partners EOS Space Systems and Areté Associates are currently testing and integrating the last components of the Mount Stromlo Laser Guide Star Facility (LGSF) at Mount Stromlo Observatory near Canberra, Australia. The LGSF will provide the EOS laser tracking station 1.8m telescope with a single LGS for use with two ANU-built Adaptive Optics (AO) systems: an AO Imaging system to detect, track and image satellites and space debris at Low Earth Orbit (LEO) and Geostationary Orbits (GEO); and an AO Tracking and Pushing system to assist with maneuvering LEO debris with a high power 20kW infra-red laser used to mitigate the threat of collisions in space.
The Mount Stromlo LGSF is unique in many ways. It includes two laser systems: a fiber-based sum-frequency laser designed and built by EOS Space Systems in Australia, and a Semiconductor Guidestar Laser designed and built by Areté Associates in the USA under contract with the Australian National University. The Beam Transfer Optics (BTO) include Beam Combining Optics (BSO) to enable either simultaneous or separate propagation of the two lasers to create a single LGS on the sky. The innovative 320mm diameter, all reflective, compact projector design of the ANU Laser Launch Telescope (LLT) includes a position sensing device behind the projector secondary mirror to ensure optimum laser beam alignment into the LLT.
This presentation provides an overview of the Mount Stromlo LGSF design, assembly, integration and testing including laser and LLT performance results obtained in the laboratory and on the telescope ahead of the conference. Early commissioning results may be presented as well depending on effective access to clear nights before early June 2019 (winter time in the Southern hemisphere).
Noelia Martinez, Instituto de Astrofisica de Canarias (IAC)
Adaptive Optics for LGS uplink correction: on-sky validation
The Uplink Wavefront Corrector System (UWCS) has been conceived by following the premises from simulation outcomes as a pathfinder instrument to demonstrate Adaptive Optics uplink correction in both applications: Free-Space Optical Communications and generation of Laser Guide Stars. Preliminary designs and expected performance have been analysed via simulations in previous studies.
A Rayleigh LGS is propagated to the sky while the atmospheric wavefront it suffers, is measured by a Shack-Hartmann with 12 x 12 sub-apertures in synchronized operation with the laser pulses. The laser upwards propagation path is then pre-compensated by a 97-actuator deformable mirror. By attaching a score camera to the guider telescope next to the main aperture, the demonstration outcome is assessed in terms of beam power concentration and spot size reduction.
Present paper describes on-sky performance validations of an AO system designed to pre-compensate a Laser Guide Star launched through the Optical Ground Station telescope at Teide Observatory (Tenerife, Spain).
Laure Catala, Gemini Observatory
Toptica laser commissioning at Gemini North
LGS operations at Gemini North have been suspended for 2 years due to multiple issues, preventing stable and reliable operation of the existing laser, a 12W Lockheed Martin Coherent Technologies (LMCT) laser. In August 2018, the installation of a 20W Toptica SodiumStar laser was completed, and we subsequently completed the on-sky commissioning in October. Here, we are presenting the overall layout of the new laser facility system, as well as the results from the on-sky commissioning. We will also review the performance results from the first couple of months of science observations and the transition from block to queue observations.
Michael Hart, University of Arizona
Stable Narrow-line VECSEL Operation for Sodium Guide Star Generation
In the development of laser guide star technology, Vertical External-Cavity Surface-Emitting Lasers (VECSELs) are attractive because of their simplicity and compactness. VECSELs operating at 1178 nm may readily be frequency doubled to the sodium D2 resonance at 589 nm. The output power of VECSELs can be scaled to multiple tens of watts by expanding the spot size on the gain medium, or “lateral scaling”. It can also be scaled by using multiple VECSEL devices in one cavity, or “longitudinal scaling”. In the case of longitudinal scaling, at least one VECSEL device must be at a fold of the cavity. This typically causes problems in longitudinal mode stability. In this paper, we present the results of successful experiments to demonstrate a technique to deliver stable single-frequency operation of a multi-VECSEL cavity at 1178 nm. Single-frequency output of more than 10 W at 1178 nm was demonstrated in a standing-wave VECSEL cavity with two gain mirrors. In comparison, the output power for operation with one gain mirror only was 7.4 W when operating in single frequency. The new technology paves the way for sodium guide star lasers delivering tens of watts that are more compact and substantially cheaper per watt than any existing technology.
Gustavo Rahmer, LBTO
LGS Operations at the Large Binocular Telescope Observatory
While performing LGS operations we are required to deal with airplanes flying overhead and also to prevent the illumination of satellites.
The first requirement is fulfilled by constantly monitoring air traffic in the vicinity of the observatory, and stopping propagation when an airplane gets close to the laser propagation direction, as detected by an automatic aircraft detection system (TBAD). We were approved in 2018 by the FAA to use this automatic system as the only detection measure (i.e. no more human spotters), which has made our operations much more efficient and flexible.
Satellite avoidance is accomplished through coordination with the military-operated Laser Clearinghouse (LCH), which provides a daily list of allowable time windows for every potential target on the sky. Unlike aircraft avoidance, satellite avoidance is predictive and therefore can be integrated in the planning for telescope operations.
I describe and discuss the impact of both avoidance schemes on the operation efficiency of the observatory.
Angel Otarola, TMT International Observatory
LLT optics aberrations and their effect on the Laser Guide Star spot size and excess wavefront error
The ability of an adaptive optics system to correct for the effects of atmospheric turbulence on the propagation of an astronomical source wavefront, is influenced by the ability to accurately compute the wavefront slopes sensed by the wavefront sensors (WFS). In turn, the slopes are derived from an estimation of the laser guide star (LGS) centroids. The TMT uses lasers tuned to the Na D2 line, to create six LGS in the mesosphere, by means of stimulation of Na atoms and their subsequent spontaneous emission of light. Hence, the LGS spot size and their intensity, in the mesosphere, is crucial for an accurate estimation of the various LGS centroid. How sharp the LGS spot becomes, depends on the aberrations in the laser beam source, and those aberrations imprinted on the laser beam through the various optical surfaces along the propagation path, in particular the optics on the laser launch telescope (LLT) that launches the beam into the open atmosphere. Ultimately, the turbulence in the atmosphere also plays a role on the broadening of the laser beam. In this study we present results, for a TMT case study, that shows the aberrations on the laser launch telescope optics are the ones influencing the most the broadening of the LGS spots, and consequently this highlights the need to achieve the best design possible for a Laser Launch Telescope as to assure an optimum operation of the adaptive optics system.
Eduardo Marin, Gemini Observatory
A new Laser Guide Star Facility for Gemini North
The Gemini Observatory is currently in the process of developing a new Multi-Conjugate Adaptive Optics facility for the Gemini North Telescope. A key part of this facility will be a redesign of the current Laser Guide Star Facility (LGSF) to a new facility that will allow for the creation of multiple laser beacons from multiple lasers and multiple Laser Launch Telescopes (LLT). The system must also be able to allow for multiple different on sky configurations of the laser beacons to allow for the MCAO mode at first light and possible GLAO and LTAO modes in the future. We present here the requirements of the system, our first level mechanical and optical designs as we move to our conceptual design review.
Stefan Kuiper, TNO
TNO’s adaptive (secondary) mirror developments for laser-communications and astronomy
Adaptive Optics (AO) has proven itself as a major enabler to meet the ever-increasing imaging performance requirements in various fields: ground-based astronomy, microscopy, and lithography. Also new application areas are emerging such as space telescopes, and laser communication. When applying AO in these highly demanding application areas a deformable mirror (DM) technology is required that not only meets the optical performances but is also highly reliable and serviceable.
TNO is developing deformable mirror technology (DM) targeted for use in these high-end adaptive optics systems. The heart of TNO’s DM technology is a unique actuator concept based on the electromagnetic variable reluctance principle which enables high efficiency in terms of force per volume and force per unit power in combination with a linear and repeatable response. Due to the absence of wear and aging, these actuators are inherently reliable which is essential for many applications.
Recently this DM-technology has been applied in laser-communications experiments under an ESA program, effectively correcting for the atmospheric aberrations in link distances up to 10km and in severe turbulence conditions. Future upgrades of this DM-technology are targeted to increase the actuator density down to 4mm actuator-spacing, and to increase the optical power handling up to several kilowatt.
Furthermore, TNO and industrial partners are aiming to evolve this DM technology towards large adaptive (secondary) mirrors (ASM) for (extremely) large astronomical telescopes. As a first step along this roadmap an ASM is under development, targeted for use in the University of Hawaii 88-inch primary mirror telescope on Mauna Kea. This ASM will have a convex optical surface, a diameter of around 63cm, and around 200 actuators.
In this talk presents the latest experimental results in the field of laser communications, and the ASM design for the UH88-telescope.
Felipe Pedreros Bustos, Helmholtz Institute Mainz
Predicting return flux enhancement with chirped CW laser guide stars
An important issue related to optical excitation of sodium atoms in the mesosphere with a single-frequency laser is the atomic recoil. With increasing irradiance, a larger number of excited atoms migrate from the resonant velocity class as a result of the Doppler frequency shift due to linear momentum transfer from the laser photons to the atomic medium. This effect reduces the absorption efficiency and prevents from obtaining higher return flux with high power lasers.
To counteract recoil, it was suggested that chirped lasers could be employed. In this scheme, the optical frequency of the laser is rapidly swept around the central resonant frequency (D2a, for example) following the shift of recoiled atoms, which would increase the number of excited atoms and therefore, enhance the fluorescence of a sodium laser guide star. Here, we present a study based on Bloch equations to evaluate the enhancement of return flux of laser guide stars with chirped CW lasers. The model includes mesospheric profiles of atomic and molecular densities, temperature, and winds; as well as the transverse intensity profile of the laser beam, and the geomagnetic field. We will discuss our theoretical findings that show a significant enhancement of the return flux, particularly in a regime of high laser irradiance.
Felipe Pedreros Bustos, Helmholtz Institute Mainz
Mesospheric magnetometry experiments in La Palma
An adverse factor of CW sodium laser guide stars (LGS) is the geomagnetic field. At large angles between the laser and the magnetic field in the mesosphere, Larmor precession diminishes optical pumping and reduces the optical absorption efficiency of atomic sodium. As a result, less photon return flux can be obtained as compared to directions where the geomagnetic field is parallel to the laser beam.
One proposed possibility to increase the return flux of LGS in unfavorable directions is through synchronously optical pumping at the Larmor frequency, which produces a net increase in atomic polarization and a sharp fluorescence peak at the pumping rate.
During 2017 and 2018, using the ESO’s Wendelstein Laser Guide Star Unit in La Palma, we performed two successful experiments aimed to detect a magnetic resonance and to evaluate the enhancement in return flux from the sodium layer. In one approach, intensity modulation of the laser beam was employed and an enhancement of ~18% was measured. In the second experiment, the laser intensity was kept constant and polarization modulation was used instead. We present an overview of the experiments and discuss our results and future possibilities.
Camilo Weinberger, Pontificia Universidad Católica de Valparaiso
Generation and characterization of atmospheric tubulence
A fast way to describe the turbulent atmospheric phenomena is through the spectrum analysis of spot light beam motion. This poster focus on how to generate controlled atmospheric conditions in the laboratory, showing how to calibrate, characterize and control the synthetically produced turbulence. We compare the behavior of two home-made devices, namely ""turbulators"". Our results demonstrate that we are able to synthetize a vast variety of atmospheric turbulence scenarios, even replicating anisotropic atmospheric turbulence conditions in the non-Kolmogorov spectrum.
Etsuko Mieda, Subaru Telescope
LGS activity at Subaru Telescope
After 8 years of operation, our current laser, which uses the sum-frequency generation technology, now takes a long time to stabilize (about a day), and can only create an LGS with on-sky brightness of R=14. The laser is still operational, but considering the maintenance effort we need versus the AO performance we get, we have decided to replace the current laser with a new, more powerful, and easier to operate laser, TOPTICA.
In this presentation, we will report the current status of the laser facility, including its performance, TBAD status, and satellite closure situation. We will then talk about the laser upgrade project and describe the requirement and specification of the new design. This new laser will be commissioned first in a single beam configuration, and in a four-beam configuration in the next step in LTAO mode.
At the beginning of this year, we announced the decommission of the current laser in the 2019B semester, and currently we are organizing its decommission plan. Meanwhile, we are finalizing the optical and mechanical design for the new laser system, and we expect to commission the new laser early next year.
Greg Fetzer, Arete Associates
Semiconductor Guide Star Laser
Optically pumped semiconductor lasers (OPSLs), also referred to as Vertical External Cavity Surface Emitting Lasers (VECSELs), represent a technology pathway to achieving Sodium Guide Star Lasers with high performance, compact size, high reliability, and low acquisition and maintenance costs. Efficient excitation of sodium in the upper atmosphere presents unique laser challenges. Stable, high power, narrow linewidth operation at multiple wavelengths tuned to sodium resonances is required to maximize the fluorescence returning to the ground. It is possible to exploit the unique characteristics of VECSEL’s to produce low complexity laser designs. Maintaining low complexity VECSEL advantages while simultaneously meeting performance requirements presents interesting challenges. We describe our efforts to bring this promising technology to an on sky demonstration in 2019. Both laboratory-based and prototype systems destined for observatory deployment will be presented. We will discuss system design and power scaling approaches and results achieved to date. Additionally, we will present plans for laser deployment at the Mt. Stromlo Observatory near Canberra AU.
Domenico Bonaccini Calia, European Southern Observatory
An overview on ESO activities in Laser Systems R&D
In this talk the activities at ESO on Laser Systems R&D will be listed and their rationale discussed in an interactive manner. The activities are related to maturing the LGS technologies, for different aspects, and are conducted in collaboration with several ESO member states institutes. Past results will be highlighted, and an outlook will be given on the ongoing and planned activities. More details on ongoing or planned activities will be given in dedicated topical talks, proposed for the Workshp. rationale and
Domenico Bonaccini Calia, European Southern Observatory
First results on on-sky laser Chirping to enhance LGS brightness
We will present in informal fashion our fresh first results obtained on-sky at the La Palma Observatorio del Roque de los Muchachos. The experimental setup was with the ESO WLGSU, used chirping in emission frequency the Wendelstein laser. The work has been done in collaboration with Toptica, which has developed and implemented the laser chirping HW. The presentation will be splitted between ESO and Toptica, describing the HW/SW setup, the experiment rationale, and the preliminary results obtained.
Domenico Bonaccini Calia, European Southern Observatory
Scaling the 589nm power obtained with fiber amplifiers to 50W+ CW LGS lasers
In the frame of the ESO lead CaNaPy system development,
a 50W+ CW, 589nm laser is needed to be used in both, CW and on-off pulsed mode.
Toward this goal, in collaboration with ESO, MPBC is supporting the development. MPBC has successfully scaled up from 36W to 100W the 1178nm output of the narrow band Raman Fiber Amplifier.
ESO is cloning its 20W CW Wendelstein prototype, scaled up to produce more 589nm power, using also state of the art novel laser components procured from Toptica and High Finesse,
which are interested in monitoring the development and the outcome.
We describe the rationale, the challenges and the new laser prototype, providing experimental results obtained so far in the lab at MPBC and ESO.
The talk will be split between ESO and MPBC.
Philippe Duhoux, ESO
Upgrade of the Laser Traffic Control Software for future collision predictions
With the deployment of the Adaptive Optics Facility at Paranal Observatory and the increasing usage of the lasers (up to 80% of the night time on the Unit Telescope 4), the prediction of the collisions between the laser beams and the line of sight of the other telescopes has become an essential tool for the Observatory now and in the future with the lasers of the ELT.
We will present here the actual observatory tool: the Laser Traffic Control System aka LTCS, the strategy for handling the collisions based on priority rules, and its up-coming upgrade : its capability to predict the laser collisions in the close future for better planning the night observations of all 10 telescopes and even at longer term for allocating telescope time especially in visitor mode.
Pedro Escarate, Universidad Austral de Chile
LGS Research at Universidad Austral
In this work we present a project to recover the laser guide systems (LGS) deprecated by Gemini North and Gemini South, with the purpose of turning them into lasers for research and teaching. The Gemini South laser will be transferred to a laboratory of the Millennium Nucleus of Planetary Formation (NPF) at Universidad Técnica Federico Santa María (USM) and the Gemini North laser will be transferred to a laboratory at Universidad Austral de Chile (UACH) where will be repaired.
The high cost of sodium lasers makes this type of technology very difficult to acquire by universities or research groups in Chile, therefore recovering the sodium lasers discharged by Gemini represents a unique opportunity for UACH and USM to do research in laser systems, optical communications, laser beam shaping, laser tomography and atmospheric studies. Also, these facilities will enable collaboration between research groups in Chile and abroad, to perform experiments that would be impossible to carry out with tLGS systems in use at astronomical observatories.
This proposal will be supported by the Laser Systems Engineer of the Large Binocular Telescope Observatory (LBTO), who will also train students and researchers in the safety and operation of this kind of lasers.
Huizhe Yang, Durham University
Projected Pupil Plane Pattern (PPPP): an alternative LGS wavefront sensing technique
Projected Pupil Plane Pattern (PPPP): an alternative LGS wavefront sensing technique, has been demonstrated a successful method for the purpose of removing the focus anisoplanatism from both the simulation and a laboratory experiment. Compared to the commonly used laser tomography adaptive optics (LTAO), PPPP has several unique features. First, turbulence is sensed during uplink where a laser beam is projected as a collimated beam from the whole telescope primary mirror. This automatically eliminates the cone effect. Secondly, a simple camera is used to measure the wavefront by imaging the back scattered light patterns on sky. No Shack Hartmann or Pyramid WFS is needed. Two types of reconstructors have been developed for PPPP. One is a simple matrix vector multiply operation, and the other is a non-linear method using the Artificial Neural Network (ANN). The latest result shows that ANN reconstructor can improve the SNR significantly and advances this novel technique to a practical level for laser power equalling 200W or even less. With PPPP ready for practical use, we propose to upgrade PPPP to wide field of view (around 1 arcmin) by launching two expanded laser beams of varying degrees of expansion. With larger overlapping area between these two beams, this upgraded wide FOV PPPP is better configured from the perspective of tomography in theory. A similar tomography algorithm as in any other wide field of view AO systems such as MCAO, is required to reconstruct the 3D turbulence profile.
Keith Wyman, Starfire Optical Range
Development of Semiconductor Disk Lasers for Sodium Guidestar Applications
Keith A. Wyman, Alexander R. Albrecht, Garrett D. Cole, and Mansoor Sheik-Bahae
We discuss the progress in our development of high-power semiconductor Disk Lasers (SDLs) for sodium guidestar lasers at 589 nm, by intra-cavity frequency-doubling of a 1178 nm laser, as well as 1140 nm devices for use as polychromatic laser guidestars. Currently, the output power of SDLs in the traditional vertical external-cavity surface-emitting laser (VECSEL) geometry is limited by thermal roll-over. We have developed a novel SDL geometry which does not include the traditional distributed Bragg reflector (DBR), but instead utilizes a semiconductor active region in transmission. This not only reduces cost and complexity of material growth, but also allows much closer contact between active region and heat spreader, promising superior performance. We compare the laser performance using a diamond heat spreader to single and double Silicon Carbide heat spreaders. The latter is available in large areas, with excellent surface quality and low absorption, and opens up the possibility of wafer-scale processing of high power DBR-free SDLs.
John Dewsnap, Serco, Inc.
Strategies for Satellite Safety with Adaptive Optics Lasers
Abstract for 13th Annual Workshop on Laser Technology and Systems for Adaptive Optics (L4AO-13) (June 7-8, 2019 at Université Laval, Québec City, Canada)
Presentation title: Strategies for Satellite Safety with Adaptive Optics Lasers.
Description of presentation/paper: The unique characteristics of lasers can present hazards to certain satellites, in particular those with sensitive, high-gain optical components. Given the proliferation of lasers in society, science/technology and various military mission areas, satellite safety from unintended illumination is an on-going concern for the U.S. Department of Defense (DoD). Since 2000, the U.S. Strategic Command Laser Clearinghouse (LCH) has been the centralized office for fostering safe and responsible laser firings above the horizon or into space. The DoD policy is to provide satellite safety coordination and notification services to the astronomy community when resources allow. U.S. Strategic Command has developed several methods of laser deconfliction that provide analytical means for promoting satellite safety. The suitability of these approaches for use by observatories operating laser guidestars will be explored in this presentation. The LCH uses a six-step systems analysis process to identify laser hazards to satellites, analyze deconfliction approaches for mitigating the risks, and verify the deconfliction processes and capabilities of systems. Based on its work with some 100 laser programs, including observatories and adaptive optics systems, the LCH has developed strategies to streamline and optimize the centralized deconfliction approach. In addition, there are observatories and organizations with relatively mature software controlled solutions for deconfliction.
In this presentation we describe the deconfliction process, explain how various solutions provide satellite safety, discuss the process and documentation involved in working with the LCH, and identify complementary initiatives that could be leveraged by the scientific community. The focus is satellite safety in the context of adaptive optics systems.
Authors’ names/organizations: Ms. Heather Witts (Joint Force Space Component Command - Laser Clearinghouse), John Dewsnap, Dr. Steven Gabriel and Stephen Serniak (Serco, Inc.)
Simone Di Filippo, INAF - OAPD
Ingot wavefront sensor: from the optical design to a preliminary laboratory test
Ingot is a novel pupil-plane wavefront sensor, specifically designed to cope with the elongation typical of the extended nature of the Laser Guide Star (LGS).
In the framework of the ELT, we propose an optical solution suitable for a Laser launch telescope, located outside the telescope pupil.
In this paper, we present the current optical design, based on a reflective roof-shaped prism, which, at the level of the focal plane, splits the light from an LGS producing three beams. The three images of the telescope pupils can be then used for the retrieval of the first derivative of the wavefront.
The 3D nature of such a device requires new alignment techniques to be determined theoretically and verified in the real world. A possible fully automated procedure, relying solely on the illumination observed at the three pupils, to align the prism to the image of the LGS is discussed. Careful attention needs to be put both on the telecentricity of the system and on the reference systems of the Ingot adjustments in the 3D space. This is crucial in order to disentangle all the possible misalignment effects.
In this context, we devised a test-bench able to reproduce, in a scaled manner, the 3D illumination that the ingot will face at the ELT, in order to validate the design and to perform preliminary tests of phase retrieval.
Tom Murphy, Aircraft Avoidance Systems, LLC
TBAD: Protecting Against Accidental Illumination of Aircraft
The Transponder-Based Aircraft Detector (TBAD) detects radio-frequency (RF) transmissions from aircraft transponders, signaling the laser to shutter off when the directional antenna determines a transmission source to be close to telescope/laser boresight. First-generation TBAD systems operating at 1090 MHz are installed on six 8-m-class telescopes using LGAO, providing spotter-free operation. The next-generation air traffic control (ATC) system is poised to be fully operational in 2020, adding a second frequency at 978 MHz for airplanes remaining below 18,000 ft. This talk will introduce TBAD's capabilities and highlight the second-generation instrument that covers both frequencies for full protection against aircraft illumination going forward.
Jussi-Pekka Penttinen, Vexlum Ltd.
Progress in VECSEL technology for laser guide stars
VECSELs (vertical cavity surface emitting lasers) have recently emerged as an attractive laser platform for generating high-brightness visible radiation [1]. In particular, VECSELs can be used to generate high power beams at yellow wavelength range [2], making it attractive for adaptive optics on earth-based telescopes. Their key advantages include ability to reach power levels of more than 10 W in fundamental transversal mode, low-noise single frequency operation, relatively broad wavelength tuning, and simpler and more compact architectures compared to alternative frequency converted solid-state laser systems used to generate visible radiation.
We review progress in developing VECSEL technology with fundamental emission at 1180 nm and steps towards frequency doubled single-frequency systems at 590 nm range. The key enabling technology is the gain mirror, which has reached a maturity level suitable for generating more than 70 W at 1180 nm, with optical efficiencies in the range of 30% [3]. We then introduce a compact package laser system enabling single frequency operation with 100 kHz linewidth at infrared and locking to external reference system. Finally, we presented recent development steps towards demonstrating advanced intracavity converted VECSEL-based guide star lasers.
1. M. Guina et al, “Optically pumped VECSELs: review of technology and progress”
J. Phys. D: Appl. Phys., 2017, 50 , p. 3830001
2. E. Kantola et al, “High-efficiency 20 W yellow VECSEL”, Opt. Express., 2014, 22 (6), p. 6372
3. E. Kantola et al, “72-W VECSEL emitting at 1180 nm for laser guide stars”, Electron. Lett., 2018, 54 (19), p. 1135
Angel Otarola, TMT International Observatory
Enhanced Laser Traffic Control System Operation Mode
The proportion of telescopes using Laser Guide Star (LGS) systems is increasing worldwide. LGS systems generally use either “pulsed lasers” (at 532 nm), creating an LGS in the upper troposphere by means of molecular scattering of light, or “sodium lasers” (at 589 nm), creating an LGS by means of excitation and spontaneous emission of sodium atoms in the mesosphere. Adequate coordination of observations involving non-laser and laser-assisted telescopes is necessary to prevent the laser beams from contaminating the field of view of telescopes operating in the visible. This coordination is done using a Laser Traffic Control System (LTCS), originally designed at the Keck Observatories. A key aspect of the LTCS is the implementation of a set of policies defining the pointing priorities of all telescopes during LGS assisted observations. A simple policy, “lasers always yield”, was to assign the lowest operational priority to the lasing telescope. This basic scheme evolved into the “first-on-target” policy, giving priority to the first telescope pointing in a given direction. In this study we propose an evolution of these policies, the “enhanced LTCS”, which defines pointing privileges according to the scientific priority of the telescopes involved in a collision. This study was made in the context of the Observatorio Roque de Los Muchachos (ORM), the future location of the Cherenkov Telescope Array North (CTA-N). The Thirty-Meter-Telescope (TMT) project has selected ORM as its alternate site, and it is the location of the Gran Telescopio de Canarias (GTC). This study was conducted to assess the operational impact of LGS-equipped telescopes on all existing and future ORM telescopes. Our results show that implementing an enhanced LTCS Mode, based on the scientific priorities of the executed programs, minimizes the disruption imposed on high-priority science programs, maximizing the science impact of all telescopes operating at a given site.
Robert Johnson, Starfire Optical Range
A brief introduction to sodium layer physics
The layer of sodium in the Earth’s mesosphere has been studied for decades through the use of lidar, which has led to a greater understanding of the physics of the upper atmosphere. This talk discusses the characteristics of the sodium layer, with an eye towards those characteristics that a ect the brightness of sodium beacons. These include factors, such as time of year and latitude, which in uence the abundance of atomic sodium in the mesosphere. This talk also introduces basic sodium atomic physics to help explain concepts such as optical pumping and re-pumping of the lower sodium ground state. Finally, this talk discusses brie y laser parameters, such as polarization and bandwidth, which in uence the e ciency in producing return ux from the sodium beacon at the telescope.
M. Olivia Byrd, AFRL/Starfire Optical Range
Demonstration of VECSELs as viable alternatives for 589 nm sodium
We present progress of an ongoing e ort to explore novel techniques to develop 589 nm vertical external cavity surface emitting lasers (VECSELs) to form a sodium beacon. A 589 nm laser is the primary excitation source for all current sodium beacons; however, these lasers are generally very complex, with over 30 optical elements, and very expensive, over $US1M. Because of the wavelength selectability, small footprint, and simplistic design, a VECSEL could provide a cheaper, less complex source for a sodium beacon at 589 nm. Current developments of 589 nm VECSELs have focused on frequency doubling of an 1178 nm VECSEL to provide 589 nm light. Such a system would require narrow linewidth (20 MHz) and > 10 W of output power. We have completed the rst phase of this e ort, where we explored various laser cavity designs and studied growth of VECSEL chips. The second phase, which started in late 2018, constitutes the development of laboratory prototypes with an emphasis on improving system robustness. The third and nal phase would involve the production of a facility-grade laser beacon system, for use with a large-aperture telescope.