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The Virgo Experiment

The Virgo Experiment. Michele Punturo INFN Perugia 3 rd VESF School on Gravitational Waves. Build up the interferometer. “Kilometric” detector. NS/NS collapse @ Virgo cluster. Quadrupolar nature of the gravitational wave: A Michelson interferometer seems a very appropriate detector.

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The Virgo Experiment

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  1. The Virgo Experiment Michele Punturo INFN Perugia 3rd VESF School on Gravitational Waves

  2. Build up the interferometer “Kilometric” detector NS/NS collapse @ Virgo cluster • Quadrupolar nature of the gravitational wave: • A Michelson interferometer seems a very appropriate detector E1 Ein E2 Interference term Eout 3rd VESF school - Michele Punturo - Virgo

  3. … Build up the interferometer • Let suppose that TGW>>2L/c: • The largest high vacuum system in Europe: • About 7000 m3 • 1.2 m diameter pipe @ 10-7mbar (H2 partial pressure) • Reduction of light fluctuation given by air flux • 7 long towers (9m long) with differential vacuum: • Usual 10-7 mbar vacuum in the upper part • 10-9 mbar in the lower part 3rd VESF school - Michele Punturo - Virgo

  4. Why a Fabry-Perot? • In a Michelson the sensitivity to an arm length difference DL=h·L is given by the slope, in the gray fringe, of Resonant cavity: l/2 -l/2 0 laser 3rd VESF school - Michele Punturo - Virgo

  5. Why power recycled? • The gray fringe working point is not the right choice: • The ITF is not a “Null Instrument”, that is the output is not null when the input is null: large DC • We want to operate in the dark fringe: no DC if zero input • What to do with the light wasted in the input port? Recycle it! Shot noise reduced by the recycling factor, but how to extract the GW signal if we work at the dark fringe, where laser 3rd VESF school - Michele Punturo - Virgo

  6. l/4 PBS EOM PHD LO Modulation-Demodulation • To operate in the dark fringe, but converting the FP in a linear instrument we need to adopt a modulation-demodulation scheme: • Pound-Drever technique Out carrier sidebands 3rd VESF school - Michele Punturo - Virgo

  7. …modulation-demodulation • Let suppose that there is a GW signal that modulates the phase of the incoming field. Its effect is present only in the carrier, because it is resonant in the cavity • The carrier is resonant in the cavity, but not the sidebands (p shift). Hence, the reflected beam is • At the output of the interferometer, the photodiode reads the power, averaged over Wc, hence we must evaluate the square of • The mixed product term gives: • Demodulating the wmod disappears and the output is proportional to the gravitational signal: • We build a linear null instrument 3rd VESF school - Michele Punturo - Virgo

  8. GW interferometer as aDouble-Superheterodyne Receiver GW Laser h(wgw) E(Wlaser) V(wgw) E(Wlaserwgw) Antenna ITF mechanical part Sig. RF Mixer Parametric transducer ITF optical part i(wmodwgw) V(wmodwgw) Preamp Sig. DL(wgw) Out Photodiode L.O. L.O. E(Wlaserwmod) V(wmod) DL(wmod) R.F. Oscillator V(wmod) Pockel cell wmod Sig. = Signal L.O. = Local Oscillator 3rd VESF school - Michele Punturo - Virgo

  9. Virgo simplified Optical Scheme 140m 3km 3rd VESF school - Michele Punturo - Virgo

  10. The injection system: The Laser • 20 W, Nd:YVO4 laser, two pumping diodes • Injection locked to a 0.7 W Nd:YAG laser • Required power stability: dP/P~10-8 Hz-1/2 • Required frequency stability: 10-6 Hz1/2 Slave Nd:YVO4 Laser ModeCleanerL = 143 m Diode pump 1W master laser Injection bench Telescope ITF 22 W slave laser ULE monolithic Reference cavity Nd:YAG l=1.064 mm

  11. Gaussian beams • Until now we considered, for the light, the plane wave approximation • But the beam, coming from a laser, shows a finite size and a approximately gaussian shape • In effect the propagation law of an electromagnetic field in an homogeneous medium gives: • Where n are the Hermite-gaussian functions: Minimum beam waist • w(x) represents the beam size: • R(x) represents the curvature radius of the beam: • and  is defined by: 3rd VESF school - Michele Punturo - Virgo

  12. Transverse Modes 00 and 01 3rd VESF school - Michele Punturo - Virgo

  13. Transverse Modes 11 and 22 3rd VESF school - Michele Punturo - Virgo

  14. Fabry Perot as mode cleaner • The Fabry-Perot cavity is a resonator that can be tuned to select the desired resonant frequency • In fact, the resonance condition is defined by the request that the complete round trip phase delay of the light inside the cavity is a integer multiple of 2p. • Let suppose for simplicity: • The resonance conditions becomes: 3rd VESF school - Michele Punturo - Virgo

  15. … FP as mode cleaner • If we want to select the gaussian mode 00, we choose the length of the cavity in such a way exists a p1 index satisfying the previous resonance condition: l00p1 resonant • The mode 00 is then transmitted by the cavity 3rd VESF school - Michele Punturo - Virgo

  16. Input beam Transm. beam Refl. beam Input Mode Cleaner • Mode cleaner cavity: filters laser noise, selects TEM00 mode Input mode-cleaner: curved mirror • Mode cleaner cavity: filters laser noise, select TEM00 mode Input mode-cleaner: dihedron

  17. Output Optics • Light filtering: output mode cleaner, 3.6 cm long monolithic cavity • Light detection: InGaAs photodiodes, 3 mm diameter, 90% quantum efficiency • Suppression of TEM01 by a factor of 10 • Length control via temperature (Peltier cell) Detection bench 3rd VESF school - Michele Punturo - Virgo Output Mode-Cleaner

  18. OMC filtering effect before OMC after OMC 3rd VESF school - Michele Punturo - Virgo

  19. Seismic Noise • The correct and usual way to realize an interferometer in an University Lab is to rigidly clamp the optics to the table • We cannot adopt this solution, mainly, because of the seismic noise: • The simplest seismic filter is an harmonic oscillator, for frequencies larger than the resonant one: • A pendulum is an harmonic oscillator of natural frequency: • A cascade of N pendulums is a multistage filter whose transfer function is:

  20. Virgo “Superattenuators” XYZ pendulum chains to reduce seismic motion by a factor 1014 above 10 Hz Magnetic anti-springs Blades

  21. Passive Isolation performance • Expected seismic displacement of the mirror (TF measured stage by stage): • Thermal noise is dominant above 4 Hz • Isolation sufficient also for “advanced” interferometers Residual motion too high at the resonances (tens of microns): could be a problem for the ITF operation  need of damping! 3rd VESF school - Michele Punturo - Virgo

  22. Working conditions • A FP is mainly a non-linear device. It can be used only at resonance where it is sensitive and linear: Pr_B8_DC • Keep the main cavity locked to enhance the phase response 3rd VESF school - Michele Punturo - Virgo 4 seconds

  23. … working conditions 2 • The power recycling cavity must be kept locked to reduce the shot noise • Keep the ITF in the dark fringe to reduce the dependence on the power fluctuation 3rd VESF school - Michele Punturo - Virgo

  24. Interferometer Control • To push the ITF in the working conditions we need to know the status of the cavities • Mainly, we need to know 4 length and the angles of the mirrors respect to the beams • Photodiodes Bx provide the error signals to control the 4 independent length of the interferometer • Quadrant photodiodes provide the error signals to control the angular positions of the mirrors 3rd VESF school - Michele Punturo - Virgo

  25. NE correction B7 (Cavitypower) NI NE BS B7 B1p Locking a cavity correction error Locking trials Locked 3rd VESF school - Michele Punturo - Virgo

  26. B8_phase/B8_DC West arm North arm Michelson B5 B5_phase/B7_DC B1_quad B2 Locking the ITF 3rd VESF school - Michele Punturo - Virgo

  27. DC-0.01 Hz 0.01-8 Hz 8-50 Hz How the correction is applied? • Three application points • Top of the inverted pendulum (Filter 0) • Marionette • Mirror • Locking requirement: • dL  10-12 m • Tidal strain over 3 km: • dL 10-4 m • Resonant motions of the mirrors • dL  10-4 m • Need to control the mirror position in a large range of frequency and displacement: • Need of hierarchical control 3rd VESF school - Michele Punturo - Virgo

  28. Mirror Actuators 3rd VESF school - Michele Punturo - Virgo

  29. Tides • Tides stretch Virgo arms up to 200 mm in 6 hours • The coils at the mirror level can support up to 10V corresponding to 100 mm displacement • The tide displacement causes a saturation of the coil voltage and a consequent delock of the ITF Tide prediction cavity power Coil voltage Tide effect saturation Loss of lock

  30. Tide compensation • Example of hierarchical strategy: • The mirror level coil drivers haven’t enough dynamical range to compensate the tidal effect • This effect is very low frequency • Moving the inverted pendulum (IP) is easy and soft • Use the low frequency part of the interferometer signal as error signal 3rd VESF school - Michele Punturo - Virgo

  31. Designing the detector sensitivity • Best seismic damping with low Q suspension • But not all the stages can be highly dissipative because of the suspension thermal noise • The ITF mirrors are suspended by an oscillator (the pendulum) that vibrates (Brownian motion) because of its finite temperature • The mirror mass itself is a system of oscillators (internal modes) that oscillate because T>0 • How to evaluate the thermal noise contribution? • You surely know the Nyquist theorem that defines the voltage noise at the end of a resistor of impedance R: • Translating the electrical impedance into mechanical one we have the fluctuation-dissipation theorem:

  32. Fluctuation-Dissipation theorem where • The previous formulation of the FD theorem is equivalent to: • Introducing the transfer function of a mechanical system H(w): • For an harmonic oscillator: • Thermal noise issues require mechanical Q as high as possible 3rd VESF school - Michele Punturo - Virgo

  33. L3 y L2 L1 Qy Qz Qx z x Special feature of a Pendulum • A pendulum is an harmonic oscillator where the restoring force is mainly given by the gravitation (lossless force). The dissipation is, instead, due to the elastic force of the suspension wire. • Very thin (and strong) suspension wires • Low loss steel wires

  34. North tube 3 km West tube 3 km Contribution to the pendulum Q • The loss angle that enters in the thermal noise formula is not only due to the “intrinsic” material loss, but other excess losses must be taken in account: • Clamping losses • Frictional losses / eddy currents • Residual gas losses 3rd VESF school - Michele Punturo - Virgo

  35. Mirror thermal Noise • In the modal expansion approximation we can consider the Virgo mirrors as composed by an infinite number of oscillators. • In the limit w<<w1 the thermal noise displacement power spectrum is given by: • A better estimation of that power spectrum can be obtained by a direct application of the FD theorem (Levin et al.): • Where U is the strain energy in the mirror due to a static pressure having the distribution of the beam profile • Other dissipation should be taken in account: • Coatings • Thermo-elastic effects • Excess losses due to the clamping and control system • … 3rd VESF school - Michele Punturo - Virgo

  36. Virgo Mirrors • The Virgo mirrors are the largest (and more expensive) mirrors in the current GW detectors • Since the substrate specifications are very stringent a special fused silica (Suprasil 311 SV) have been realized on purpose for Virgo: • Low absorption: 0.7 ppm/cm • Low OH content (<< 50ppm) • Low birefringence (<5·10-4 rad/cm) • The coating specifications are still more stringent • @1064nm the nominal absorption should be about 1 ppm • The scattering should be lower than 5ppm • As usual the effective realization is more difficult than expected: • Excess optical losses in the mounted mirrors (pollution?) • Thermal effects 3rd VESF school - Michele Punturo - Virgo

  37. 350 mm 100 mm 3rd VESF school - Michele Punturo - Virgo

  38. Optical read-out noise • Optical read-out noise is the (incoherent) sum of the shot noise and radiation pressure • In the current ITFs only the shot noise plays a relevant role, but it is instructive to see a formalism (KLMTV) that reports the two noises in a single expression: Radiation pressure Shot noise PBS is the power impinging the beam splitter: 3rd VESF school - Michele Punturo - Virgo

  39. Virgo Nominal Sensitivity 3rd VESF school - Michele Punturo - Virgo

  40. The real life! • The design of a GW interferometric detector is an hard job, but the attainment of the design sensitivity is even harder • In fact, a GW detector is a complex machine that needs a deep tuning of many parameters • Methods and technologies are completely new • 5 years of commissioning needed in LIGO • Similar time spent in Virgo 3rd VESF school - Michele Punturo - Virgo

  41. Commissioning evolution • Phase A: Commissioning of interferometer arms • Test all aspects of control systems with a simple optical configuration • - locking, automatic alignment, second stage of frequency stabilization and suspension hierarchical control (tidal and marionette) • First shake of the sub-systems 3rd VESF school - Michele Punturo - Virgo

  42. Commissioning evolution • Phase A: Commissioning of interferometer arms • Test all aspects of control systems with a simple optical configuration • - locking, automatic alignment, second stage of frequency stabilization and suspension hierarchical control (tidal and marionette) • First shake of the sub-systems 3rd VESF school - Michele Punturo - Virgo

  43. Commissioning evolution • Phase A: Commissioning of interferometer arms • Test all aspects of control systems with a simple optical configuration • - locking, automatic alignment, second stage of frequency stabilization and suspension hierarchical control (tidal and marionette) • First shake of the sub-systems • Phase B: Commissioning of interferometer in ‘recombined mode’ • Useful intermediate step towards full interferometer lock • Verify functioning of BS longitudinal control • Re-run all aspects of control system in a more complex configuration • Start noise investigations 3rd VESF school - Michele Punturo - Virgo

  44. Commissioning evolution • Phase A: Commissioning of interferometer arms • Test all aspects of control systems with a simple optical configuration • - locking, automatic alignment, second stage of frequency stabilization and suspension hierarchical control (tidal and marionette) • First shake of the sub-systems • Phase B: Commissioning of interferometer in ‘recombined mode’ • Useful intermediate step towards full interferometer lock • Verify functioning of BS longitudinal control • Re-run all aspects of control system in a more complex configuration • Start noise investigations • Phase C: Commissioning of Recycled Fabry-Perot interferometer • Run full locking acquisition process • Verify functioning of PR mirror longitudinal control • Re-run SSFS, tidal control and marionette control • Implement complete wave-front sensing control • Continue noise investigations 3rd VESF school - Michele Punturo - Virgo

  45. Commissioning evolution • Phase A: Commissioning of interferometer arms • Test all aspects of control systems with a simple optical configuration • - locking, automatic alignment, second stage of frequency stabilization and suspension hierarchical control (tidal and marionette) • First shake of the sub-systems • Phase B: Commissioning of interferometer in ‘recombined mode’ • Useful intermediate step towards full interferometer lock • Verify functioning of BS longitudinal control • Re-run all aspects of control system in a more complex configuration • Start noise investigations • Phase C: Commissioning of Recycled Fabry-Perot interferometer • Run full locking acquisition process • Verify functioning of PR mirror longitudinal control • Re-run SSFS, tidal control and marionette control • Implement complete wave-front sensing control • Continue noise investigations • Phase D: Noise hunting 3rd VESF school - Michele Punturo - Virgo

  46. Sensitivity Improvement 3rd VESF school - Michele Punturo - Virgo

  47. Sensitivity of the global network NSNS detection distance ~6Mpc 3rd VESF school - Michele Punturo - Virgo

  48. Noise Budget Unclear excess noise sources Residual light scattering Shot noise dominated

  49. Scientific case:Thermal effects in the Virgo mirrors LM • The 3km Virgo arms are a long Fabry Perot cavity: LA AR HR ~10-3 – 10-4 • Hence, actually, each arm is a double FP cavity: • Etalon effect 3rd VESF school - Michele Punturo - Virgo

  50. Etalon Effect • Hence, the Finesse of the cavity and all the fundamental parameters of the ITF are affected by the input mirror optical thickness variation • But, why the mirror optical thickness fluctuates? • Temperature!! • Hence, knowing the mirror temperature it is possible to predict some of the ITF performances • OK, but how to measure the mirror temperature? 3rd VESF school - Michele Punturo - Virgo

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