ESA-NASA Working Meeting on LIDAR ALADIN Instrument: Key Issues & Technical Challenges

Transcription

1 ESA-NASA Working Meeting on LIDAR ALADIN Instrument: Key Issues & Technical Challenges D. Morançais ALADIN Project Manager Page 1

2 ALADIN Overview Baffle Telescope Primary Mirror (1.5 m diameter) Receiver Laser Heads Laser Radiator (on Platform) Mass: 500 Kg Power: 840 W Page 2

3 Functional architecture Transmitted beam Received beam The laser beam is transmitted and received through the telescope (1.5 m) Two laser heads are embarked (redundancy) Two spectrometers and associated CCD detectors are within the instrument (Mie & Rayleigh receiver) Thermal control and synchronisation is performed by the instrument (hardwired logic) ALADIN I/F with S/L Survival Survival Laser Thermal Control Structure & Thermal Control Heaters Thermistors Flip-flop mechanism TxA Laser Heads (RLH & PLH) Transmitter Laser Electronic (TLE) Electrical Panel Telescope T/R Diplexer & Relay Optics Mie & Rayleigh spectrometers DFU1 Aladin Control and Data Management unit (ACDM) Instrument Core Chopper mechanism DFU2 Detection Detection Electr. Unit (DEU) Mech/therm. IF to PF Mech/therm. IF to PF Page 3 Unreg. Power Unreg. Power Reg. Power PCDU Reg. Power Reg. Power TM/TC bus CDMU Science Data

4 Key issues at instrument level Contamination risk on laser optics Page 4 - Bake-out of all glued components within the Power Laser Head and Transmit/Receive Optics - Bake-out of all materials (structures, MLI,..) located close to the PLH and TRO - Purging of PLH from box closing until launch «Laser straylight» within the instrument - No damage/saturation on detector during firing -> Chopper mechanism and anti-backreflection surface on M2 mirror - No laser «hot spots» within the instrument on optics or structure -> Specific protections (field stops, baffles) to avoid high energy illumination outside laser optics and low energy source for laser alignment Thermal control - High laser power dissipation -> Numerous Heat pipes on instrument and platform: orientation versus gravity vector to be managed thoughout all integration and test programme

5 Transmitter Laser Assembly (TxA) The TxA is composed of: Power Laser Head (PLH) Outer Space Thermal - Diode-pumped Nd-YAG laser - Emits 150 mj nm Laser Cooling System (LCS) - Pulse repetition frequency 100 Hz - 12 s bursts every 28 s Reference Laser Head (RLH) - Highly stable seeder laser (a few MHz) - Tunable over 7 GHz PLH and RLH conductively cooled Transmitter Laser Electronics (TLE) Thermal Reference Laser Head (RLH) Electrical I/F Transmitter Assembly Electrical I/F Fiber Thermal Power Laser Head (PLH) Electrical Transmitter Laser Electronics (TLE) Electrical I/F ALADIN Control & Data Management Unit (ACDM) Laser beam Reg. Power Bus (S/C) Unreg. Power Bus (S/C) Reg. Power Bus (S/C) - High current and voltage driver - Transmitter control and synchronisation Page 5

6 Transmitter Laser PLH The Power Laser Head (PLH) includes: Injection seeded Master Oscillator Section (MO) Amplifier Section with twoslabamplifiers Harmonic Generation Section with doubling and tripling crystals Transmitter issues: see specific presentation from A. Cosentino Page 6

7 Receiver The Receiver is composed of two channels (Mie and Rayleigh) each composed by an etalon spectrometer and a CCD Front-End Unit. It also includes a polarisation diplexer to separate Transmit/Receive paths and a Chopper Mechanism to shut the receiver during laser firing The optical architecture allows to feed Mie and Rayleigh channels with maximum optical efficiency The spectral registration between the Mie and Rayleigh channels is performed with thermal tuning of the Rayleigh spectrometer : - Thermal hood around the RSP - Tuning on a range of +/- 3 K - 1 mk accuracy Detection modules are based on Accumulation CCD (Astrium patent) allowing quasi photon-counting performance with a Si-CDD - Read-out noise < 4e- (equivalent to 0.5 e- noise per shot) Page 7

8 Mie Spectrometer Fringe imaging technique Page 8 - An interferometer provides a fringe whose position is proportional with the spectral shift - The energetic distribution of the fringe is sampled (16 channels) - A specific processing allows sub-sample resolution to be achieved (e.g. centroiding) Physical implementation - Fizeau spectrometer : multiple beam interferometer with a wedge which generates the fringe as output - Coupling optics - Detector: Accumulation CCD: quasi-photon counting with 80% quantum efficiency using on-chip shots accumulation Receiver spectral transmission Rerturn signal (Fringe) Fizeau spectrometer Collimated beam from receiving telescope Interférence order n Receiver usefull spectral range Detector array. Fringe imaging optics Interference fringe Interférence order n+1 Mie Fringe Wavelength Rayleigh + Earth Radiance level Detector array Wavelength shift (wind velocity) x = K λ ~10 m/s per pixel

9 Mie Spectrometer Fizeau etalon Mie Spectrometer during integration Page 9

10 Rayleigh Spectrometer Double edge technique - Two filters are implemented aside the Rayleigh spectrum. - The flux through each filter varies with the spectral shift - The detected flux is processed with an ecartometric-like function : (A-B)/(A+B) Physical implementation - Sequential Fabry-Perot cavity (Astrium patent) - Single detector in order to eliminate the errors due to the gain of the detection chains. - The detector is the same as for the Mie channel. Filter A Polarization diplexer Quarter wave plate Filter B Mirror er Full pupil sequential filter Page 10

11 Rayleigh spectrometer Page 11

12 Detection Front-end Unit Page 12

13 Transmit/Receive Optics Page 13 Left : Titanium bench Right : Diplexer Centre : TRO during integration

14 Receiver Engineering Model Page 14

15 Key Issues on Receiver High energy laser optics on Transmit/Receive Optics - Same issues as for the Transmitter (Laser Induced Damage & Contamination) Very high stability of Spectrometers - Etalons assembled by optical contact and sealed under vacuum -> nm stability - Instrument calibration of spectral response -> allows to remove long term effects Very low noise detection - High optical isolation between transmit and receive path - Specific CCD architecture developed for lidar applications - Proton radiation effect on CCD noise verified to be acceptable Complex alignment and integration - High alignment accuracy (several 10 µrad per component) - Requires UV source Page 15

16 Telescope Design Ultra-lightweight Telescope all in silicon carbide (SiC) Sic secondary mirror Sic parts linking legs and M2 Upper Strut Thermal Protection Low mass / high stiffness SiC legs with tubular section Diameter: 1.5 m Afocal optics Mass: 75 Kg First frequency > 60 Hz Titanium brackets SiC Primary mirror Thermal re-focusing capability Titanium Isostatic mounts Page 16

17 Telescope Page 17 M1 flight mirror Tripod during vibration test

18 Key Issues on Telescope High stiffness / high mechanical load - Mechanical tests allowed to demonstrate compatibility to above 50 g level High reflectivity - Custom enhanced metallic coating at 355 nm developed for the mirrors Good required optical quality - Long polishing time (~1 year) - Requires mechanical decoupling from instrument / platform structure - Control of wavefront error at various steps of integration Thermal control - Use of a single material (SiC) with high conductivity to limit gradient - Thermal refocusing (avoids use of mechanism): 1 µm accuracy - Sun illumination: specific protections close to M1 focus Page 18

19 Conclusions ALADIN is the first space Lidar ever built in Europe Specific design solutions and integration methods have been developed with regards to laser aspects Qualification issues (e.g. optical materials, laser components) are discovered during the development phase and difficult decisions have to be taken Future R&D programs at Agency level should include qualification activities in order to secure future lidar programs Page 19