Scientists at the Starfire Optical Range (SOR) have been researching Rayleigh beacons and mesospheric sodium beacons for adaptive optics (AO) for nearly three decades. We developed four different sodium-wavelength lasers, all of which were based on diode-pumped, sum-frequency Nd:YAG oscillators. In 2016, we combined light from two commercial 22-watt, sodium-wavelength lasers to form a single beacon. These commercial lasers, which use resonant-frequency doubling of light from a Raman fiber-amplifier, were built by MPB Communications and Toptica Projects. In 2019, we started to develop and procure a 75-watt sodium-wavelength laser to enable better correction of turbulence in poor seeing. In conjunction with Toptica Projects and European astronomers, we have increased the return flux from sodium beacons by shifting or chirping the laser wavelength to compensate for recoil of optically pumped sodium atoms. In addition, we have demonstrated a single-sideband D2b re-pumper. In this talk, we review the development our new sodium beacon laser and discuss results from an on-sky test in 2023 that demonstrates the improvement in beacon brightness by using these techniques.
We report on the novel ALASCA (Advanced LGS AO for Satellite Communication Assessment) facility being built for ESA by a consortium of industry and national research institutes under the ScyLight program. The aim of ALASCA is to create a facility for Optical Feeder Links (OFL) field tests, as well as to demonstrate at the ESA Optical Ground Station in Tenerife, starting in 2023, 24/7 reliable operation of optimal Optical Feeder Links based on Laser Guide Star Adaptive Optics (LGS-AO) to solve the point-ahead problem on ground-space laser communications. Space optical communication represents a technological challenge due to its specific requirements and merit parameters; the consortium’s extensive experience in LGS-AO in the astronomical field allows an expert technology transfer to earth-space communication. This will enhance the review of the ALASCA’s main requirements, their implementation by a proper tailoring of the modular solutions that will be adopted by the design, facing the new challenges at system level posed by the OFL applications compared to astronomical solutions. The ALASCA project will, last but not least, provide a technology assessment and a development roadmap towards the industrial exploitation of a 24/7 operational Optical Ground Station (OGS). We will provide an overview of the ALASCA project, its goals, phases and planned timeline up to the field experiments; the presentation will then focus on the project status, including also the simulations results of LGS-AO assisted OFL.
In this invited talk we report on the CaNaPy facility laser being developed at ESO, in collaboration with MPBC and Toptica projects.
The MPBC breakthrough of high power narrow band Raman Amplifiers based on the ESO patented technology has allowed to demonstrate and commission, at the time of this writing, a 63W CW laser with the same optical beam and functional properties as the commercial 22W CW 589nm Toptica-MPBC lasers.
To avoid saturation effects due to the high irradiance at the mesospheric sodium layer, Toptica projects has developed and integrated remarkable frequency chirping capabilities, on a laser architecture which remains similar to the commercial 22W version. In the talk, besides the presentation of the design and of the test results, there will be an outlook on commercial availability, further laser power scalability and other emerging developments, for applications related to astronomical adaptive optics as well as space awareness and satellite communications.
TOPTICA Projects GmbH is the leading supplier of sodium guide star laser technology to all major ground-based VIS/IR observatories. SodiumStar lasers combine a single-frequency diode-based seeder, Raman fiber amplifier technology and resonant frequency doubling. They emit > 20 W of narrow-linewidth (~ 5 MHz) continuous-wave radiation at sodium D2a resonance and include an integrated D2b re-pumping scheme for maximum return flux. Apart from the unique optical characteristics, emphasis has been placed on supplying astronomers with modular turn-key systems that are absolutely reliable even under harsh environmental conditions and in remote locations. Applications beyond astronomy include atmospheric magnetometry, space situational awareness, and free-space optical communication, in particular feeder links to LEO and GEO satellites as well as deep space probes.
The performance of large ground-based optical telescopes is limited due to wavefront distortions induced by atmospheric turbulence. Adaptive optics systems using natural guide stars with sufficient brightness provide a practical way for correcting the wavefront errors by means of deformable mirrors. Unfortunately, the sky coverage of bright stars is poor and therefore the concept of laser guide stars was invented, creating an artificial star by exciting resonance fluorescence from the mesospheric sodium layer about 90 km above the earth’s surface. Until now, mainly dye lasers or sumfrequency mixing of solid state lasers were used to generate laser guide stars. However, these kinds of lasers require a stationary laser clean room for operation and are extremely demanding in maintenance. Under a development contract with the European Southern Observatory (ESO) and W. M. Keck Observatory (WMKO), TOPTICA Photonics AG and its partner MPB Communications have finalized the development of a next-generation sodium guide star laser system which is available now as a commercial off-the-shelf product. The laser is based on a narrow-band diode laser, Raman fiber amplifier (RFA) technology and resonant second-harmonic generation (SHG), thus highly reliable and simple to operate and maintain. It emits > 22 W of narrow-linewidth (≈ 5 MHz) continuous-wave radiation at sodium resonance and includes a re-pumping scheme for boosting sodium return flux. Due to the SHG resonator acting as spatial mode filter and polarizer, the output is diffraction-limited with RMS wavefront error < λ/25. Apart from this unique optical design, a major effort has been dedicated to integrating all optical components into a ruggedized system, providing a maximum of convenience and reliability for telescope operators. The new remote-pumping architecture allows for a large spatial separation between the main part of the laser and the compact laser head. Together with a cooling-water flow of less than 5 l/min and an overall power consumption of < 700 W, the system offers a maximum of flexibility with minimal infrastructure demands on site. Each system is built in a modular way, based on the concept of line-replaceable units (LRU). A comprehensive system software, as well as an intuitive service GUI, allow for remote control and error tracking down to at least the LRU level. In case of a failure, any LRU can be easily replaced. With these fiber-based guide star lasers, TOPTICA for the first time offers a fully engineered, off-the-shelf guide star laser system for groundbased optical telescopes providing convenient, turn-key operation in remote and harsh locations. Reliability and flexibility will be beneficial in particular for advanced satellite and space debris tracking as well as LIDAR applications.
Large telescopes equipped with adaptive optics require high power 589-nm continuous-wave sources with emission linewidths of ~5 MHz. These guide-star lasers should be highly reliable and simple to operate and maintain for many years at the top of a mountain facility. After delivery of the first 20-W systems to our lead customer ESO, TOPTICA and MPBC have begun series production of next-generation sodium guide-star lasers. The chosen approach is based on ESO’s patented narrow-band Raman fiber amplifier (RFA) technology [1]. A master oscillator signal from a TOPTICA 50-mW, 1178-nm diode laser, with stabilized emission frequency and linewidth of ~ 1 MHz, is amplified in an MPBC polarization-maintaining (PM) RFA pumped by a high-power 1120-nm PM fiber laser. With efficient stimulated Brillouin scattering suppression, an unprecedented 40 W of narrow-band RFA output has been obtained. This is spatially mode-matched into a patented resonant-cavity frequency doubler providing also the repumper light [2]. With a diffraction-limited output beam and doubling efficiencies < 80%, all ESO design goals have been easily fulfilled. Together with a wall-plug efficiency of < 3%, including all system controls, and a cooling liquid flow of only 5 l/min, the modular, turn-key, maintenance-free and compact system design allows a direct integration with a launch telescope. With these fiber-based guide star lasers, TOPTICA for the first time offers a fully engineered, off-the-shelf guide star laser system for ground-based optical telescopes. Here we present a comparison of test results of the first batch of laser systems, demonstrating the reproducibility of excellent optical characteristics.
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