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ADAPTIVE OPTICS METHODS IN GW INTERFEROMETRIC DETECTORS A PERSPECTIVE

ADAPTIVE OPTICS METHODS IN GW INTERFEROMETRIC DETECTORS A PERSPECTIVE. M. Lorenzini , L. Aiello, E. Cesarini, V. Fafone , D. Lumaca, Y. Minenkov , I. Nardecchia , A. Rocchi, V. Sequino. GRASS - GRAvitational-wave Science&technology Symposium Padova, March 1 st and 2 nd 2018.

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ADAPTIVE OPTICS METHODS IN GW INTERFEROMETRIC DETECTORS A PERSPECTIVE

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  1. ADAPTIVE OPTICS METHODS IN GW INTERFEROMETRIC DETECTORSA PERSPECTIVE M. Lorenzini, L. Aiello, E. Cesarini, V. Fafone, D. Lumaca, Y. Minenkov, I. Nardecchia, A. Rocchi, V. Sequino GRASS - GRAvitational-waveScience&technologySymposium Padova, March 1st and 2nd 2018

  2. Optical aberration budget Sources • imperfections in the production process of the mirrors (cold defects); • absorption of optical power in the coatings and substrates of the optics (dynamical effects). Effects • Manifacturingerrors (mismatch of RoC…) • Inhomogeneity of refractiveindex, impurities … • Thermal lensing • Thermo-elastic effect OPL distorsionsdue to dependence of refractiveindex from temperature variation Change of RoC of mirrors due to the absorption of laser power GRASS18 - Padova

  3. The case of Advanced Virgo • Advanced Virgo hasmarginallystablerecyclingcavities • The wholeaberration budget isrelevant (cold/ dynamicaleffects) • Transmissionmaps of core opticsallowsimulation of impact • Requirement on residual OPL distortion RMS<2 nm • Spatialfrequencyspectrumlimitssize of aberrations Sideband power as a function of aberration spatial frequency. From VIR-0389A-11 Sideband power as a function of residual aberration RMS. From VIR-0128A-12 GRASS18 - Padova

  4. AdV Thermal Compensation System • TCS actuators : • CO2 laser projector (50 W) • Ring Heater • TCS sensing: • HWS-RC • HWS-HR + PhaseCameras (notshown) GRASS18 - Padova 4

  5. Wavefrontsensing: HWS • Itmapswavefrontgradients in deformations of a spot array on CCD • Differentialsensor: very sensitive to changes in OPL • Noise can be reduced by optimizing spot size and by takingaverages • Sametechnologyshared by LIGO and Virgo • Developed at the University of Adelaide [Opt. Express 15 (16), (2007)] ON-AXIS DOUBLE PASS matched to OPL matched to HWS OFF-AXIS DOUBLE PASS x 10-10 m Measured wave-front averaged over 100 frames:RMS 0.1 nm or l/6500 @ 650 nm GRASS18 - Padova

  6. Wavefrontsensing: HWS • Accuracyand RMS noisefloorare impressive • However, thereisstill room for improvements of the sensing: • HWS plate sensitive to room temperature changes (T must be controlled, alreadytested in UToV) • Thermal defocusin telescopesadds a fake curvature: a secondarybeam with differentl can be used to measure the defocus (two color system) • Currentspatialresolutionisabout 7 mm (20 cm wide area sensed on TM/ number of holes in a row) • Nottrivial to improve, keeping the presentlevel of performance HWS A. Brooks DT GRASS18 - Padova

  7. Actuators: ring heaters • RHsable to change the surface curvature (reducingRoC) and induce a thermallensinside the TM • RH used in aLIGO, GEO and AdV • Ultra high vacuumcompatible • Can be integrated in the payloadstructure • Magneticcouplingnoise due to currentfluctuations can be reduced by usingtwocounter-propagatingheating coils • Thermal lens can provide HWS with a TM centre reference HWS-DET GRASS18 - Padova

  8. Actuators: ring heaters TM tuning of RoC in perspective • Using RH: still a choiceatleast for room temperature detectors • Thermal lens and induceddeformation are sphericalto high degree • Thermal lens/surfaceRoCchange can be decoupled by exploiting CO2actuators (annular and centeredprofiles) on CP • Issue: long thermaltimes • Virgo-likeCentral Heating Radius of Curvature Correction: • It allows increase of mirror RoC • Deformation of HR surface is less spherical • (CHRoCC - CQG 30, 055017, 2013) GRASS18 - Padova

  9. Actuators: CO2 • CO2wavelengthoptimallyabsorbed by silica • Actuation on dedicatedopticsbeforecavity input: compensationplates • Multi-actuationapproach: • Central Heatingfor axi-symmetriccentered pattern (also, keepingthermal state durignafterunlocks) • DAS (double axiconsystem) for axi-symmetricannularcorrection • Notaxi-symmetricresidualpatterns (scanning system) • The powerdelivered by eachactuator can be separatelycontrolled and monitored • DAS pattern can be tuned online spuriouslens CH correction on CP beampowerabsorbed DAS pattern on CP residualdisortion SS pattern on CP GRASS18 - Padova

  10. Actuators: CO2 Wavefrontcorrection in perspective • Wavelengthdepends on CP substratematerial • CH spotsize online tuningcouldprovideadditionalflexibility • Quality of compensationdepends on the possibility to get a correction pattern ascloseaspossible to optimal, therefore: • Precise beamshapingrequiresgoodquality of laser output • Spatialfilteringalreadyused in AdV CO2beams, where output isnotclean • Single-mode optical fibre? • Design of a mode cleanerongoing in UToV GRASS18 - Padova

  11. Cleaning the CO2beam Work in collaboration with E. Genin, G. Pillant, M. Bawaj Design of a mode cleaner for CO2beams (50 W) • No Faraday isolatorsavailablefor CO2: • triangularconfigurationavoids back-reflection • HOMsreduction with F ~100 • Compactness: round trip < 1 m • Thermal effects on core optics • Feasibilitystudy with a 15 W CO2 source • PDH technique, but EOM crystals from usual companies toomuchexpensive: • AOM frequencyshifting? • Othercrystals/companies? Not for now… • Invar housing, finesse 103, FWHM = 5.8 MHz, g1g2= 0.75, Round-trip LP = 0.5 m (credits A. Bazzichi) GRASS18 - Padova

  12. TeTis: testing TCS strategies • Scaled down version of Advanced Virgo TCS @UToV • Wavefrontsensing with HWS in on-axis/off-axisconfigurations • RHactuation on a scaled TM • CO2DASwavefrontcorrection on a scaled CP • Pickoff of the CO2beam for CHactuation on TM • Test bed for AdV TCS integration • Room for studyingdifferentconfigurations/strategies (two color system) • Sensingparametersnotaccessible on AdV (temperatures, thermal de-focus…) GRASS18 - Padova

  13. TeTis: testing TCS strategies GRASS18 - Padova

  14. Example I: spatialfiltering in TeTis CO2beamqualitynot so good in TeTis 9 W source: • DAS qualitylargelyaffected • 300 mm pinholeinsertedafterbeam source • Pinhole position optimized to getgoodqualityat the axicon position • Complete study of the beamprofileafterpinhole Spot sizewithoutspatialfiltering Spot size with spatialfiltering GRASS18 - Padova

  15. Example II: HWS temperature control • Temperature control of the HWS platetested on TeTis: • PID control with temperature sensedusing PT100 • Peltiercellsprovideactuation • RMS of T variationwithin0.01°C • As a consequence, AdV HWS T control possible(yetnotneeded) a=1,2 10-61/K invar plateexpansioncoefficient L=1 cm distancebetweenplate and CCD GRASS18 - Padova

  16. Example III: DAS optimization • observednon uniformintesitydistributioninside the rings (particularly in the outerone), due to divergentbeam on the recombiningthin film polarizer (TFP) • solution: implemented a telescopestraddlingthe TFP GRASS18 - Padova

  17. TeTisand FEA, improvedknowledge • RH thermallensingmeasurementscompared with FEA transientsimulation: • One copper ring with manganine wire (R=28.8 Ω) and teflonwrapping to increase the emissivity; • Copper shield • Measured telescope magnification 4.039±0.029 • Characterization of the curvature induced on TM by CH. Possible error sources accounted for: • Measured telescope magnification (4.039±0.029) • CO2 power fluctuations • Uncertainty on the CO2 beam size on TM (most relevant) GRASS18 - Padova

  18. Future actuators DC actuators: MEMS (Micro Electro-Mechanical Systems) System Titan144 • DC correctionavoidsfrequencynoise in scanning systems • Deformablemirrorsto shape the CO2 laser beam; • Mirrordeformation by varying the voltage on the piezo-electricalactuators; • In UToV: 12x12 actuatorsneeded to applycorrectionsover lengthshigherthan 1 cm (asAdVrequired), applied on a mirror of 25 mm diameter. Control electronicspeed: >1kHz • Preliminar test of actuatorsongoing(measurement of influencefunctions) Credits Stefano Bonora TARGET DM 12x12 GEO heater array (LIGO-G1500461) Simulation of 12x12 DM, heatingprofilereproduced via Gerchberg-Saxton recursive algorithm DC actuatorswithout CO2: heater arrays (GEO) GRASS18 - Padova

  19. What to expect for ET? • ET conceived as a 2-tone configuration (CQG 28, 094013, 2011) • ET-HF uses • Helical LG33 modes • Fused silica test masses • Considering 3MW in the FP cavity and coating absorptions of 0.5ppm, absorbed power is 1.5W (3 times higher than in AdVirgo) • AdV-like TCS: • CPs+ properheatingpatterns • RHs to correctRoC of core optics • Moreover, LG33 modes extremely sensitive to low-order optical aberrations: • Current polishing techniques are not enough to guarantee the required cavity mode quality (PRD 87, 082003, 2013); • Need for an additional actuator to correct these defects. GRASS18 - Padova

  20. A key word: versatility • Experience gained from AdV: TCS equipmentused to face commissioningneeds • CH for correction of CP coldtransmissionmaps, improvement of contrastdefect • Thermal lens due to RH serve asreference for the centre of mirrors in HWS maps • HWS SLED beamusedasreference for detectionparabolicmirrorreplacement… • TCS actuators can be useful in taklingforeseenissues • RH on end mirrorscould be employed to reduce the impact of parametricinstabilities… • TCS methods can help in approachingotherproblems • Adaptive mode matching for injection of squeezedbeam • Reduction of round trip losses in filter cavities Flexibility of concepts, within the requirementsdictated by applications, will be a keyfeatureof adaptiveoptics in future detectors. GRASS18 - Padova

  21. Summary • The integration and pre-commissioning of Thermal Compensation System in Advanced Virgo providedhints for a vision of future improvements: • Precise beamshaping and mode cleaning • Advanced HWS with higherspatialresolution and reducedsensitivity to temperature fluctuations • DC actuators for wavefrontcorrection • Flexible design (PI, adaptive mode matching) • TeTisprovides a test bed for validatingcurrent and improvedmethods, and for studying new possibilities • TCS provides a valid approach to adaptive optic correction for 3rd generation GW detectors with LG33 modes GRASS18 - Padova

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