Technologies for the Future of interferometric detectors

1. Technologies for the Future of interferometric detectors C. Nary MAN UMR 6162, Observatoire Cote d’Azur, BP 4229, 06304 Nice Cedex 4, France • Introduction : fundamental limits of ground-based detectors • Possible solutions in the MF range: • High power lasers • new materials for optics • controls of optics behaviour • A lot of ideas to extract better signals with sophisticated configurations of signal recycling…. • Other optical configurations …… LISA symp 19-24 July 02, PSU

2. Pendulum thermal noise Mirror thermal noise Shot noise 20W Seismic wall Quantum limit Gravity gradients Sensitivity curve and fundamental limitations LISA symp 19-24 July 02, PSU

3. Shot noise limit : - directly by laser power - indirectly by optical imperfections Mirror thermal noise limit : - Q of test-mass (substrate, coatings) - T of test-mass, M of test-mass • Increase laser power but increase also thermal effects (radiation pressure problem : larger masses ) • New materials for mirrors, high Q even at low T, large size, optical quality • Coatings of high Q ? Issues in MF range LISA symp 19-24 July 02, PSU

4. > 500 W ? 50 W Front End Power stages • Rod systems (LZH) • Stable-unstable slab oscillator (Adelaide) • MOPA type (Stanford) • Ceramic laser • Fiber laser 1-3 W 50 W Medium power slave Low power master High power single-frequency laser Stringent demands on frequency stability 10-6 Hz/√Hz (of ground-based detectors): LISA symp 19-24 July 02, PSU

5. LZH: Laser medium is rod, end-pumped by fibre-coupled diode lasers, good wall-plug efficiency, delivers @ 20W Rod Laser systems LISA symp 19-24 July 02, PSU

6. Mitsubishi: > 200 W achieved in TEM00 output with transverse diode-pumped rod laser LZH: Power scaling of End Pumped rod to 100W • Modeling : • 100 W of output power will be achieveable • aberrations , to be compensated for • aberrations comparable in end pumped and transversally pumped rod • Realized (02): • 4 diode boxes have been set up (1200 W of pump power) • temperature stabilization • pump light homogenization has been demonstrated • 45 W single mode and 75 W multi mode laser has been demonstrated (single rod, no compensation) LISA symp 19-24 July 02, PSU

7. Adelaide 100 W slab laser configuration Nd:YAG slab pumped by 520 W fibre-coupled diode lasers Resonator stable in the zig-zag H direction, unstable in V direction LISA symp 19-24 July 02, PSU

8. amplification goal > 100W with 2 zig-zag slab amplifiers and 20W master oscillator 27 W stable operation achieved at 1st stage Stanford MOPA design LISA symp 19-24 July 02, PSU

9. 98 in Japan, development of highly transparent Nd:YAG ceramic: efficiency comparable to single xtal lasers , 1.5 kW cw output ( Ueda et al,2001) 1.46 kW obtained in multimode operation with YAG ceramic High power lasers: ceramic lasers Ceramic laser : any size (23 cm long max for YAG xtals, twice this length for ceramic), any shape, high Nd doping, mass production… first Nd:YAG ceramic laser gives 300 mW output (Ikesue et al. in 1995) Quality of the beam has to be worked out Wavefront quality, distributions of Nd ions to be compared vs xtals… Possibility of having Nd:Y2O3 ceramic where thermal conductivity twice of YAG with similar thermal expansion coef. LISA symp 19-24 July 02, PSU

10. Used as power amplifier with NPRO, emits 20 W on single-frequency output (Jena, 2001) Possibility of scaling up to 100 W with 9m fiber. High power lasers: Fiber lasers Erbium doped Silica Ytterbium doped all glass (eff > 80%) Ytterbium doped Silica (eff 85%) Fiber lasers based on rare-earth doped silica: very high output powers up to 2 kW cw operation in June 02 (IPG Photonics). LISA symp 19-24 July 02, PSU

11. Substrate for future mirrors • low absorption material with good conductivity, high Q, good optical quality …. • Fused Silica (today substrate): • Absorption: best quality has 0.7 ppm/cm • Numata et al (Amaldi 01): measured Q of 13 kinds of FS, Q = 7.105 to 4.107 : no simple correlation with known specs, seems to increase with annealing process… • Homogeneity and roughness of polishing: meet specs • Sapphire: • Absorption : around 20 ppm/cm, vary following samples • Q = 6.5x 107 at room temperature and low temperatures behavior studied extensively, • but direct measurement of thermal noise necessary • Homogeneity: need to be improved by factor 5 to 10 (Caltech, CSIRO) • Silicon: • Used in reflection only (suitable for all-reflective interferometers) • Q around 2x108 confirmed for a variety of samples, thermal noise improves at low T LISA symp 19-24 July 02, PSU

12. Low absorption, high resistance to thermal & mech shocks, high Q, good candidate for cryogenic solution (Silicate bonding not working ) New Candidate Materials for mirrors: CaF2 (VIRGO, Elba 2002 ) LISA symp 19-24 July 02, PSU

13. 1992 1994 2000 to Virgo Absorption at 633 nm 20 ppm 10 ppm < 5 ppm 4 ppm Absorption at 1064 nm  - 2 - 3 ppm 0,5 ppm 0.6 ppm Scattering at 633 nm 50 ppm 5 ppm 1,2 ppm 4 ppm Scattering at 1064 nm -  2 ppm 0,6 ppm 4 ppm over F150 mm Wavefront -   - -  3.8 nm rms over   F 150 mm Components diameter 25 mm 50 mm 25 mm 350 mm Coatings: optical performances (1) Optical performances achieved today in Virgo-SMA: LISA symp 19-24 July 02, PSU

14. 80 mm high reflectivity mirror wavefront before and after corrective coating Ion Source Robot SiO2 target Y Mask X Sputtered Atoms Mirror Interferometer Wavefront control Coatings : optical performances (2) LISA symp 19-24 July 02, PSU

15. Levin (98) showed coatings could be a limiting source of loss • Preliminary measurements at Glasgow, Stanford & Syracuse: fcoating = 2.5 x10-4 • To be used in avanced/future detectors, loss factor < 10 –5 • Coating program initiated to measure thin and thick substrates with different number of coating layers , …. • Loss factor at low T (Yamamoto, Elba 02): fcoating < 10- 4 without change of reflectivity • First conclusions: • First interface between layers is not dominant source of loss • Interfaces between multi-layer are not dominant source of loss • Interface substrate-coating is not a signicant source of loss • Ta2O5 layer has higher loss than SiO2 • What is the way forward? Other high index materials than Ta2O5? • Will it be a trade-off between absorption and mechanical loss ? Coatings : mechanical loss LISA symp 19-24 July 02, PSU

16. Thermal lensing of test-mass: • large efforts to reduce thermal lensing by reducing absorption in sapphire, but not very reliable ? (Fejer 2001 LSC, Blair 97, Benabid 00) • Tomaru et al (Amaldi 01) reported efficient reduction of thermal lensing in the cryogenic sapphire mirrors • Wavefront distorsion of optical components: • Active wavefront corrections via direct thermal actuation are being developed at MIT • R&D to measure aberrations (Shack-Hartmann type sensors, and correct with deformable mirrors (Stanford) the wavefront distorsion of high power lasers. • Reshaping of laser beams with intracavity deformable mirrors • Reshaping of laser wavefronts with deformable mirrors outside the lasers Thermal effects LISA symp 19-24 July 02, PSU

17. Ottaway PAC 12 Compensation of wavefront deformations Mirror heating with outer ring and scanned beam heating (MIT) M.Zucker LSC meeting 02 LISA symp 19-24 July 02, PSU

18. Three-level atom example: • Laser pumps atom from E2 to E3 • Radiative deexcitation from E3 to E2 • Fluorescence from E3 to E1 • => absorption of a phonon E2-E1 • => decreasing the thermal energy E3 Radiative transitions Laser pumping E2 Phonon absorption E1 Cooling a 3-level atom Laser cooling of solids 1929: anti-Stokes fluorescence is basis of optical refrigeration cycle. 60 ’s: GaAs, Nd:YAG,… 90 ’s; Yb doped ZBLAN: up to 48°C (Los Alamos) • Applications to GW detectors: • Identify materials also with high Q, high homegeneity • Recycle the anti-Stokes fluorescence to remove its th.effects out of the solid LISA symp 19-24 July 02, PSU

19. Experimental demonstration in 98 by Sun & Byer in a Sagnac configuration All-reflective interferometers • Advantages: • Higher light power because no bulk absorption • Use of test mass materials giving lower thermal noise such as xtal silicon • Drawbacks come from use of gratings: • Conversion of laser frequency noise to pointing noise: retroreflecting compensator • Laser center frequency drift < max deviation • Distort spatial profile of diffracted beam • Scattered light • Improvement needed LISA symp 19-24 July 02, PSU

20. High Power stages (with deformable mirror) Single -frequency front end 50W 500W Wavefront sensor Pre-mode-cleaner Faraday isolators Phase modulators Wavefront correction Long Input mode cleaner Power stabilisation Correction by Deformable mirrors + signal recycling configurations Future detector: with thermal correction/compensation LISA symp 19-24 July 02, PSU

21. Future detector: all-reflective Sagnac High Power stages (with deformable mirror) Single -frequency front end 50W 500W Wavefront sensor Pre-mode-cleaner Faraday isolators Phase modulators Wavefront correction Long Input mode cleaner Power stabilisation Correction by Deformable mirrors Transmission port M2 SR + thermal compensation of mirrors M3 grating M1 LISA symp 19-24 July 02, PSU

22. Intelligent digital controls • Digital electronics to monitor and control the complex seismic isolation • (gain and phase re-adjusted automatically with the drift /ageing of mechanics due to environment…..) • Low noise digital electronics for all position controls (test-mass, laser beam, beam shape, beam pointing, etc…) • Fast digital electronics to lock the laser parameters (frequency, amplitude) • Neural networks  to manage all the controls , from the locks sequence, the automatic relocks of each servo, the electronic gain/phase adjustments due to the ageing of mechanical actuators, etc….., also the kind of signal extraction ? LISA symp 19-24 July 02, PSU