- The project will create a TRL6 operational OFL test facility at OGS Dome Coudé focus in Tenerife, initially to be used with Geostationary (GEO) Alphasat; thanks to its modular approach, the ALASCA facility forms the basis for future experiments related to LGS AO 24/7 with Low Earth orbit (LEO) satellites;

- the test facility targets 24/7 operation, night- and day-time configurations; the latter is particularly challenging due to the stronger atmospheric turbulence and the background light compensation in severe turbulence conditions;

- the possibility of obtaining the tip-tilt correction via LGS-AO is proactively pursued via R&D in the project institutes and will benefit from the ALASCA facility. According to our End2End numerical simulations, see section 4.2 for details, the detection of the uplink tip-tilt from the LGS will significantly improve the fading rate; in addition, this possibility would represent a key milestone for optical communications [1]. Hence the R&D for the detection of atmospheric tip-tilt and focus from the LGS itself constitutes an important synergy between space and astrophysics developments of LGS-AO.

- Microgate (IT)

- A.D.S. International (IT)

- LumiSpace (UK) – Durham University (UK)

- Toptica Projects (DE)

- INAF (IT) – in collaboration with ESO for all aspects related to the CaNaPy project

- Two lasers, a single frequency 75W CW 589nm for LGS and a 100W NIR OFL 1055nm communication fiber laser from IPG Photonics, both propagated in monostatic configuration for uplink pre-compensation; the LGS unit will be used in long-pulsed mode to avoid blinding the wavefront sensor cameras.

- Reliable 75W, 589nm laser, built at ESO in collaboration with MPBC – which has provided the unique 1178nm, 100W narrow-band RFA, based on the ESO patented Raman Fiber Amplifier technologies [6]. The ALASCA facility, thanks to Toptica Projects, will further exploit and improve the ESO CaNaPy 75W CW 589nm laser, with emission spectral and temporal formats finely tuned to maximize the LGS return flux. ALASCA introduces laser frequency chirping and fundamental improvements in the efficiency of the amplitude modulation needed for the uplink correction. The LGS return flux is enhanced by optical pumping, the emission of repumper lines and by frequency chirping based on Toptica proprietary technology, thus suppressing LGS saturation effects at high mesospheric irradiances due to spectral hole burning effects – which is a must at high laser irradiances at the mesosphere.

- LGS and IR pyramid WFS, built on INAF decadal experience. The usage of the Py-WFS is a result of a multiparametric trade-off vs Shack-Hartmann WFS. A SH-WFS is foreseen for Natural Guide Star operation, used for the initial optimization studies of the uplink pre-compensation, for the LGS-AO loop commissioning with the Py-WFS to detect NGS tilt and focus, and as reference benchmark for the loop performance.

- Use of LGS-AO for adaptive optics correction at Point-ahead angles, with tip-tilt and defocus optical modes sensed using as reference the satellite itself – via an Infrared Pyramid WFS. R&D towards sensing the same modes via the LGS itself is part of synergic ongoing activities with ESO, to be continued with ALASCA.

- Optimal LGS-AO operation by uplink pre-compensation of the 589nm laser beam, producing small LGS spot sizes – optimally coupled with a Pyramid wavefront sensor. This requires monostatic propagation geometry for both, the LGS-AO system and the OFL system.

- Possibility to run the ALASCA system without LGS-AO, i.e. in ‘classical mode’ with Adaptive Optics based solely on the Satellite IR reference source, to be used also as bench-mark comparison.

- 24/7 operation. Two operation modes for nighttime and daytime: monostatic propagation with telescope aperture diameters of 100 cm with central obstruction (ESA OGS telescope in Tenerife) and 30cm without central obstruction, respectively. LGS-AO closed loop in daytime using ultra-narrow band magneto-optical filters to suppress the sky background [8]

- Flexibility of using double Axicons module to create annular Bessel laser beams and avoid vignetting the Gaussian laser beams from the telescope central obstruction

- Automatic, Laser pointing smart cameras from Astrel Instruments, to control uplink laser beams pointing, monitor LGS brightness, presence of cirrus clouds, sodium abundance in real time.

- Fast and highly efficient AO loops with minimum latency, to cope with hard seeing conditions. Adaptive modal control with provision for predictive control algorithms, including machine learning. The AO loop rate at any time will depend on the atmospheric conditions and the LGS return flux. In order for the system to be operated at frame rates of up to 3 kHz, especially for the day-time operation of the system, a highly deterministic and extremely low latency image processing, as well as the minimization of the pixel processing latency incurred by the RTC and the very low jitter of the control loop, are fundamental. It is also mandatory to guarantee a very accurate synchronization of the different components (cameras, mirrors, including also the Py WFS beam steering ones, laser and optical beam shutters), in the order of μs, to avoid detrimental effects on the loop performance. In the ALASCA design, this will be assured by the Microgate interface module that centralizes all sensors acquisition, actuators command and synching on a FPGA-based platform.

- Instrument Control Software (ICS) regulates and commands at high level with graphical user interfaces the facility, with the AO systems, the lasers, the OGS telescope, the laser pointing camera and the external turbulence monitoring unit. ICS monitors also environmental parameters in the ALASCA instrument, and stores non-real-time observing data.

- Soft and hard Real Time Computer software optimizes performance via refined methods of adaptive calibrations and control, coming from two decades of LGS-AO experience in astronomical applications. It displays and stores user-selected data during operation.

- True modular design by subsystems: ALASCA is a TRL6 class facility allowing multiple operation modes, multiple configurations and changes in experimental setups.

- Gravity invariant, stable optomechanical system optimized for maximum throughput at the ESA OGS Dome Coudé focus.

- Dust-free, overpressured when not in operation, thermally controlled environment regulated at 13 °C ± 3°C on the ALASCA optical table, with provisions for clean-room maintenance (see Figure 6).

- Atmospheric monitoring module (24h turbulence profiler) produced by Lumi Space, to continuously evaluate the atmospheric turbulence, giving feedback to the LGS-AO control. In-depth knowledge of the vertical structure of the atmospheric turbulence during operation is of critical importance in FSOC system. As pointed out by J. Osborn in [2], if the turbulence level can not only be measured, but also forecasted in advance, this will enhance the throughput of the FSOC service allowing a better planning and schedule of the communication scheme, especially for 24/7 operation.

- Chirping: 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 that remains similar to the commercial 22 W version. The goal of the chirping system is to apply an optimized sawtooth modulation with fundamental frequencies in the 1 – 10 kHz range and shift amplitudes up to a few hundred MHz to the guide star laser frequency, to overcome the spectral hole burning effect and increase the number of atoms the laser is interacting with – hence the LGS brightness. This is done by performing repeated excitation/emission cycles with slower atoms while changing (chirping) the laser frequency synchronously to their recoil Doppler effect. This leads to a pile-up of atoms in a single velocity class during the chirp, analogous to the effect of a snow plough. Experiments to anchor the numerical models with reality are being done jointly by ESO and Toptica at the Wendelstein Laser Guide Star facility in La Palma.

- Amplitude modulation: ALASCA system uses a monostatic configuration for the lasers. The usage of the same telescope to propagate the guide star laser and collect the return signals allows for Adaptive Optics pre-compensation of the guide star laser beam itself. This is a pre-requisite for the stronger turbulence conditions expected during the daytime. The drawback of this setup is the scattering of laser light within the telescope, and as this straylight has the same frequency as the fluorescence light, it cannot be filtered using any spectral filtering technique. The facility implements a pulsing unit into the laser system based on electro-optical modulators and detects the return flux only when the laser is not emitting, alternating between LGS laser launch and detection with a laser pulsing frequency and a specific duty cycle tailored according to the projects’ needs. Due to the extremely high sensitivity of the LGS wavefront sensor, laser photons entering the optics mainstream Tx-Rx common path need to be completely blocked during WFS detection periods. A combination of three methodologies for attenuating the laser amplitude is developed and implemented by Toptica Projects: a free-space electro-optic amplitude modulator, a rotating-disk optical chopper, and a technique based on fast frequency tuning of the laser resulting in the laser frequency being out of resonance with the frequency-doubling resonator (a.k.a. second-harmonic generation cavity).

- The ICS controls and manages the system, interacting with the subsystems, the safety and cooling systems and the external workstations for the control. See Figure 10 for details. The ICS GUI will allow the control and monitoring of all subsystems, and will coordinate the acquisition and observation sequences. It is foreseen to implement the Observing Block mode, which allow to user to specify automated acquisition and observing sequences, with data collection and pre-analysis display.

- The Real Time control (RTC) software and hardware controls the high-speed components of the adaptive optics, being responsible for calculating an estimate of the incident wavefront, and computing corrections for DM and TTMs. It also encompasses the routines responsible for keeping the system stable and optimised for the atmospheric conditions. The overall RTC system will provide functionality to retrieve real-time telemetry from the HRTC and displays for control and diagnostics of the system. This is highly enhanced by the presence of the proprietary Microgate control Electronics that will be responsible for the WFS data acquisition, transfer and pre-processing, as detailed in the following section. See Figure 11 for details.

- Microgate control electronics will interface the AO hardware (such as the mirrors and the cameras) to the RTC unit through a single deterministic link based on a standard UDP network interface and direct memory transfer to the processing core memory; this will relieve the RTC from the real/time interfaces implementation complexity, thus enabling high/speed operation, up to 3 kHz or more control loop frequency with very low jitter of the execution time of the whole real/time pipeline. Besides that, the Microgate electronics will also provide very flexible and accurate synchronization to fast beam modulation mirrors in the Pyramid WFS. See Figure 12 for details.