The investigations presented below were originally planned on the background of the DARWIN mission. The demand for improved fibre coupling efficiency yet is a general one for applications that cannot cope with the 20% loss of power upon coupling into a fibre. By shaping the transverse amplitude and phase distributions of the receive beam, the single-mode fibre coupling efficiency can potentially be increased to almost 100% (if the beam shaping is lossless), thus allowing for a gain of more than 20% (or 1.07dB) compared to conventional designs. We show that the use of "tailored freeform surfaces" for purpose of beam shaping reduces the mode mismatch between the Airy pattern of the image and the fibre mode, and therefore allows for achieving a performance close to the physical limitations. As a follow-up to a previously published paper, , we present the design and the results of a proof of concept demonstrator (POCD) for the application of tailored surfaces for building a beam shaping optics that shall enhance fibre coupling performance. The demonstrator consists of two main parts, the input beam support equipment and the POCD core. The first part contains the laser source, the top hat beam generation optics and a deformable mirror. The POCD core is set up as a three mirror system focussing into the fibre with 1064 nm being the design wavelength. The experiments performed with the demonstrator aimed at the principle proof of the beam shaping approach and at the test of deviations from the nominal parameters like field angle and aberrations. The results acquired from the experiments with a proof of concept demonstrator in an "as built" configuration show good agreement with the theoretical performance predictions by wave-optical simulations. Limitations of the available manufacturing technologies and of the operating wavelength regime are discussed.
This paper focuses on optical links from ground to a geostationary satellite, using adaptive optics to pre-compensate the wave-front of the uplink beam. We present the numerical prediction of the irradiance statistics at the satellite. Uplink beam diameters, as well as the turbulence strength in the boundary layer and in the tropopause are varied. Results show, that the choice of uplink beam diameter significantly affects the fading statistics, in particular the shape of the probability density function. In a realistic worst case (30° elevation, day-time turbulence, sea level), the optimum beam diameter, showing minimum irradiance fluctuation at the target, is around 20 cm. In that case, the probability of fades larger than 6 dB (with respect to the diffraction limit) is approx. 10-3.
A breadboard of such a closed-loop adaptive-optics pre-compensation system has been implemented. It comprises a Shack- Hartmann wavefront sensor, two tip/tilt mirrors, a 140-element deformable mirror, a far-field propagation simulator, and two rotating turbulence phase screens (simulating tropopause and atmospheric boundary layer, respectively). The link geometry is representative of the intended application in an optical feeder link for geo-stationary satellites. Preliminary test results confirm the numerical predictions.
For the DARWIN mission the extremely low planet signal levels require an optical instrument design with utmost efficiency to guarantee the required science performance. By shaping the transverse amplitude and phase distributions of the receive beams, the singlemode fibre coupling efficiency can be increased to almost 100%, thus allowing for a gain of more than 20% compared to conventional designs. We show that the use of "tailored freeform surfaces" for purpose of beam shaping dramatically reduces the coupling degradations, which otherwise result from mode mismatch between the Airy pattern of the image and the fibre mode, and therefore allows for achieving a performance close to the physical limitations. We present an application of tailored surfaces for building a beam shaping optics that shall enhance fibre coupling performance as core part of a space based interferometer in the future DARWIN mission and present performance predictions by wave-optical simulations. We assess the feasibility of manufacturing the corresponding tailored surfaces and describe the proof of concept demonstrator we use for experimental performance verification.
A mountain-top-to-valley optical link demonstration was performed in Switzerland between Säntis mountain, 2’502m
altitude, and Dübendorf airfield, 448m altitude. The link distance at very low elevation angle of 2° was 55km. Main goal
was to evaluate an optical communication system for LEO-to-Ground links in realistic atmospheric conditions, though
worst case, comprising the impact on data throughput and on pointing acquisition and tracking performance. Three wavelengths were tested simultaneously, a downlink at both, 1550nm and 808nm together with a 1064nm uplink,
thus allowing for comparison of atmospheric transmission impact over a wide wavelength range. Alongside, all
transmitters were designed to be eye-safe. The mountain top transmitter was installed inside a service building and the 60cm receiver telescope on the airfield was placed in an open stand. The link demonstration forms part of an on-going development activity started at RUAG Space with support from ESA in 2010. This activity is currently in the Engineering Model phase and aims at the Flight Model to be ready in 2016. Goal is to develop an optical downlink terminal that primarily addresses the needs of the emerging market of small satellites, the optical ground terminal and the ground network topology. The overall test approach is presented and explained together with a summary of all activities performed. Test results are presented and the discovered issues are addressed. Furthermore, a general overview is provided on the development activity and its current status.
Science return and high bandwidth communications are key issues to support the foreseen endeavors on spaceflights to
the Moon and beyond. For a given mass, power consumption and volume, laser communications can offer an increase in
telemetry bandwidth over classical RF technology allowing for a variety of new options, like more raw scientific data
being sent back to Earth where data processing can be performed on ground. Recent European activities in the field of
laser communications investigated mission scenarios for deep space and within the Earth's sphere of influence. Various
link topologies have been investigated, involving Lissajous orbits at Libration points of the Earth-Sun and the Moon-
Earth system, and also Martian orbiters. Different types of lasercom terminal concepts have been investigated, either
operating fully autonomously or being attached to dedicated telecom orbiter spacecraft. Enhanced pulse position
modulation formats were tested together with tailored FEC and interleaver technology in inter-island test campaigns
using ESA's optical ground station on Tenerife. The paper summarizes the findings from all activities, highlights the
potential and describes synergy aspects of involved technologies, all in view using lasercom as part of an integrated RF-optical
TT&C subsystem to support enhanced science return.
Oerlikon Space AG has since 1995 been developing the OPTEL family of optical communications terminals. The optical
terminals within the OPTEL family have been designed so as to be able to position Oerlikon Space for future
opportunities open to this technology. These opportunities range from commercial optical satellite crosslinks between
geostationary (GEO) satellites, deep space optical links between planetary probes and the Earth, as well as optical links
between airborne platforms (either between the airborne platforms or between a platform and GEO satellite).
The OPTEL terminal for deep space applications has been designed as an integrated RF-optical terminal for telemetry
links between the science probe and Earth. The integrated architecture provides increased TM link capacities through the
use of an optical link, while spacecraft navigation and telecommand are ensured by the classical RF link. The optical TM
link employs pulsed laser communications operating at 1058nm to transmit data using PPM modulation to achieve a
robust link to atmospheric degradation at the optical ground station. For deep space links from Lagrange (L1 / L2) data
rates of 10 - 20 Mbps can be achieved for the same spacecraft budgets (mass and power) as an RF high gain antenna.
Results of an inter-island test campaign to demonstrate the performance of the pulsed laser communications subsystem
employing 32-PPM for links through the atmosphere over a distance of 142 km are presented. The transmitter of the
communications subsystem is a master oscillator power amplifier (MOPA) employing a 1 W (average power) amplifier
and the receiver a Si APD with a measured sensitivity of -70.9 dBm for 32-PPM modulation format at a user data rate of
10 Mbps and a bit error rate (BER) of 10-6.
Contraves Space AG is currently developing the OPTEL family of optical terminals for free-space optical communications. The optical terminals within the OPTEL family have been designed so as to be able to position Contraves Space for future opportunities open to this technology. These opportunities range from commercial optical satellite crosslinks between geostationary (GEO) satellites, deep space optical links between planetary probes and the Earth, as well as optical links between airborne platforms (either between the airborne platforms or between a platform and GEO satellite). This paper will present an overview of the space based and airborne system architectures that the Contraves Space family of OPTEL terminals have been designed to support, provide a description and performance summary of each OPTEL terminal and the key technologies that have been developed.
The paper presents the general interplay of coarse and fine tracking sub systems for an optical intersatellite link terminal. It briefly describes the hardware items that were designed by the Contraves Space led team to realise the required pointing, acquisition and tracking (PAT) functionality, especially in view of a commercial use of the terminals. Additionally, the control concept is outlined and test results are presented that were obtained during PAT sub system tests, used to verify the acquisition algorithms and the closed loop tracking performance.