This paper describes the technical challenges and the experimental set-up for an optical feeder-link demonstration between ESA’s optical ground station in Tenerife, Spain, and the Alphasat satellite in geostationary orbit (25º East). In the absence of clouds, reliable optical feeder-uplinks require pre-distortion of the transmit beam to minimize the influence of the continuously changing atmospheric turbulence effect. Without beam pre-distortion, the quality of service (minimum impact of scintillations and outages) required for commercially viable feeder-links is hardly achievable. Power efficient feeder-links require the wave-front distortions of an optical downlink beam from a satellite to be inversely applied onto the optical feeder-uplink beam to the satellite with certain adaptations due to the difference in the uplink and downlink paths. The underlying assumption is that pre-distortion and atmospheric turbulence cancel each other out and the main goal of the planned experiment is to determine to which extent this is true. The closed-loop bandwidth of pre-distortions has to be higher than the atmospheric turbulence fluctuations and optical beam reception and transmission should use the same telescope aperture. The former requires a fast controller and the later poses stringent requirement on the stray-light performance of the set-up such that the powerful transmit beam does not blind the receiver. Avoidance of receiver blinding is a particularly difficult task when doing feeder-uplink tests with the coherent laser communication terminal on board the Alphasat satellite as will be explained. The paper will describe the design of the feeder-uplink system that will be installed in the OGS as well as initial measurements that have been performed to evaluate feasibility.
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, [1], 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.
The European Data Relay System (EDRS) is operational, optically transferring data from currently four LEO Earth observation satellites to the geo-stationary EDRS-A spacecraft at 1.8 Gbps. The demand has increased to extend these point-to-point optical links towards a full optical network in space and enable high data rate links between space assets and between ground and space. This article presents the ESA developments towards high data rate optical free space feeder links. The performance of an optical link from a ground station to a geostationary relay spacecraft experiences major limitations by atmospheric turbulence. To overcome this limitation, a free-space optical link experiment over 13 km is being set up. It shall assess the gain in irradiance and corresponding reduction of the scintillation index by pre-distortion of the optical “uplink” beam based on the measured wave-front disturbances of the “downlink” beam using an adaptive optics system. A second experiment will answer the question if the isoplanatic angle covers the point ahead angle in a ground to GEO link. This was/will be done by correlation measurements on double stars separated between 3.6 and 4.1 arcsec in varying elevation angles and atmospheric turbulence conditions. A third experiment shall address the potential gain and limitations of the implementation of Wavelength Division Multiplexing (WDM) into optical inter-satellite links. WDM being a standard technique to increase the data handling capacity of fibre networks by injecting multiple data streams into one single fibre using only one set of transmit and receive optics.
The Aeolus mission will take an innovative wind lidar called ALADIN (Atmospheric LAser Doppler INstrument) into space to measure wind profiles in the lowermost 30 km of the Earth's atmosphere. ALADIN is a direct detection wind lidar capable of using the backscatter signal from both molecular (Rayleigh-) and aerosol (Mie-) scattering to retrieve independent wind information. To achieve the mission goal, two separate spectrometers have been manufactured. The Rayleigh spectrometer is using a Fabry-Perot etalon with 2 paths and works like 2 narrow band filters. The detector measures the power reflected by the atmosphere for each thin band. The Mie spectrometer core is a Fizeau etalon. A CCD matrix measures directly the spectral response with a very fine resolution.
For both etalons, the critical parameters are the FWHM (Full Width Half Maximum) and the Finesse. High optical quality and extremely narrow FWHM are needed to achieve mission performance but also request high quality system for the verification of those performances.
The optical performance predictions, the verification philosophy and the test results are presented. The description of the different measurement setups including a system able to do spectral measurement with a resolution of some femtometers, the characteristic of the equipment and mathematical method used for calibration and to optimize the measurement accuracy are described.
For the 2 spectrometers, a numerical model has been developed to analyse and predict the spectral response. The model and the results of the analysis are presented in the documents. The comparison between analysis and measurement results is discussed.
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.
LISA Pathfinder is an ESA experiment to demonstrate the key technologies needed for the LISA mission to detect gravitational waves in space. The LISA Pathfinder spacecraft represents one arm of the LISA interferometer, containing an optical metrology system and two proof masses as inertial references for the drag-free control system.
The LISA Pathfinder payload consists of two drag-free floating test masses located in the inertial sensors with their control electronics and an optical metrology subsystem. The optical metrology subsystem monitors the movement of both test masses relative to each other and to the spacecraft with very high sensitivity and resolution. This is achieved with a heterodyne Mach- Zehnder interferometer. This interferometer requires as input two coherent laser beams with a heterodyne frequency difference of a few kHz.
To generate the two laser beams with a heterodyne frequency difference a Nd:YAG laser is used together with the Laser Modulator. The Nd:YAG laser generates a single coherent laser signal at a wavelength of 1064nm which is fibre coupled to the Laser Modulator. The Laser Modulator then generates the two optical beams with the required heterodyne frequency offset. In addition, the Laser Modulator is required to perform laser amplitude stabilization and optical path difference control for the two optical signals.
The Laser Modulator consists of an optical unit – the LMU – and RF synthesiser, power amplification and control electronics. These electronics are all housed in the Laser Modulator Electronics (LME).
The LMU has four primary functions:
• Splitting of the input laser beam into two paths for later superposition in the interferometer.
• Applying different frequency shifts to each of the beams.
• Providing amplitude modulation control to each of the beams.
• Providing active control of the optical path length difference between the two optical paths.
The present paper describes the design and performance of the LMU together with a summary of the results of the Laser Modulator engineering model test campaign.
Remote sensing is a priority activity for the European Space Agency and detector performance is a crucial factor in determining how well this role is performed. Consequently, the Agency has a strong interest in continuous improvement of both detector capabilities and availability within Europe. To this end, ESA maintains a number of strategic detector development plans combining both technology-push and technology-pull. The visible and infrared wavebands are of particular interest for remote sensing activities and this paper sets out the requirements for current and future missions and presents details of the Agency’s current and planned detector developments.
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