1. LISA symp 19-24 July 02, PSU 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 ……

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

3. LISA symp 19-24 July 02, PSU 3 Issues in MF range Mirror thermal noise limit : - Q of test-mass (substrate, coatings) - T of test-mass, M of test-mass Shot noise limit : - directly by laser power - indirectly by optical imperfections  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 ?

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

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

6. LISA symp 19-24 July 02, PSU 6 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) 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 Mitsubishi: > 200 W achieved in TEM00 output with transverse diode-pumped rod laser

7. 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

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

9. LISA symp 19-24 July 02, PSU 9 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) 1.46 kW obtained in multimode operation with YAG ceramic 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) 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.

10. LISA symp 19-24 July 02, PSU 10 High power lasers: Fiber lasers Erbium doped Silica Ytterbium doped all glass (eff > 80%) Ytterbium doped Silica (eff 85%) 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. Fiber lasers based on rare-earth doped silica: very high output powers up to 2 kW cw operation in June 02 (IPG Photonics).

11. 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 = 7.10 5 to 4.10 7 : 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 10 7 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 2x10 8 confirmed for a variety of samples, thermal noise improves at low T

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

13. LISA symp 19-24 July 02, PSU 13 Coatings: optical performances (1) Optical performances achieved today in Virgo-SMA: 199219942000 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 Scattering at 1064 nm - 2 ppm 0,6 ppm 4 ppm over  150 mm Wavefront - -- 3.8 nm rms over  mm Components diameter 25 mm 50 mm25 mm350 mm 4 ppm

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

15. LISA symp 19-24 July 02, PSU 15 Coatings : mechanical loss Levin (98) showed coatings could be a limiting source of loss Preliminary measurements at Glasgow, Stanford & Syracuse: f coating = 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): f coating < 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 Ta 2 O 5 layer has higher loss than SiO 2 What is the way forward?Other high index materials than Ta 2 O 5 ? Will it be a trade-off between absorption and mechanical loss ?

16. LISA symp 19-24 July 02, PSU 16 Thermal effects 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

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

18. LISA symp 19-24 July 02, PSU 18 Laser cooling of solids Cooling a 3-level atom E2 E1 E3 Radiative transitions Laser pumping Phonon absorption 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 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

19. LISA symp 19-24 July 02, PSU 19 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 Experimental demonstration in 98 by Sun & Byer in a Sagnac configuration

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

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

22. 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 ?