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DC readout for Virgo+? E. Tournefier WG1 meeting, Hannover January 23 rd ,2007

DC readout for Virgo+? E. Tournefier WG1 meeting, Hannover January 23 rd ,2007. DC vs AC readout: technical noises Output mode cleaner for DC readout. DC readout for Virgo+ ?. Virgo+ optical parameters: P las =50W, F=150 DC readout:

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DC readout for Virgo+? E. Tournefier WG1 meeting, Hannover January 23 rd ,2007

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  1. DC readout for Virgo+?E. TournefierWG1 meeting, HannoverJanuary 23rd ,2007 • DC vs AC readout: technical noises • Output mode cleaner for DC readout

  2. DC readout for Virgo+ ? • Virgo+ optical parameters: Plas=50W, F=150 • DC readout: • ITF locking point is offset (Loff) from the dark fringe => B1_DC is sensitive to OGL • DC vs AC readout: • Advantages of DC readout: • Shot noise limit smaller by 20% • No oscillator phase noise • No frequency noise at high frequency • Requirements for DC readout: • Need very good power stabilization • Need to eliminate the sidebands from B1_DC (increases shot noise + power noise) => new output mode cleanerneeded => Estimate the AC and DC technical noises (frequency, power noises,…) for Virgo+ Both are non stationary

  3. WSR6 noise budget

  4. Phase noise for Virgo+ 6 MHz gen EOM genx TFIMC gen +LO LO board  • Phase noise on B1_ACp: ACp =  x ACqRMS - Current phase noise:  ~ 0.15rad/Hz (high freq.) (oscillator, modulation/demodulation electronics) - ACqRMS mainly driven by alignment fluctuations • Extrapolation for Virgo+ assuming similar alignment performances Dangerous since non-stationary noise  Should be reduced for Virgo+ Can gain a factor 2 improving electronics?

  5. Frequency noise • Frequency noise (i) coupling to dark fringe: h = CMRF(f) x i/ • CMRF depends on Fabry-Perot cavities asymmetries: • Finesse asymmetry F/F • Induces a phase difference like OG - Losses asymmetry P and beam matching M Arm reflectivity difference - Equivalent to phase difference for AC readout - Not present for DC readout (checked with SIESTA) Advantages of DC readout: - CMRF drops at high frequencies Drawback of AC readout: - Losses vary with the cavities alignment => non-stationary noise (BoBs…)

  6. Frequency noise for Virgo+ • h = CMRF(f) x i/ • i ? • Assume only limited by B5 shot noise (optimistic) • CMRF: Losses asymmetry Now equivalent to P =50 ppm Can be improved? • assume for Virgo+ : P = 25 ppm • CMRF: Finesse asymmetry: F/F=2% • Will be very difficult to reach the shot noise above 100 Hz for AC readout • Need a small finesse asymmetry for both

  7. Power noise • Coupling: • AC readout: Couples through locking accuracy: • Assumes equivalent to LRMS = 2x10-13 m (i.e. 10 times better than C6 measurement) • DC readout: Directly proportional to carrier and sidebands power (reduce SB power with smaller modulation depth and small TOMC,SB) • Which power noise now? -Smaller for carrier than for SB - Power noise due to ITF angular/long controls • For Virgo+ DC: Power should be stabilized inside ITF: • reduce control noises • use B5_DC => photodiode under vacuum Sensor noise

  8. Power noise dP/P carrier • Which power noise for Virgo+? Carrier: Will need to be stabilized inside ITF Use B5_DC (low freq) + IMC_Tra (high freq) Assume for Virgo+: - reach B5 shot noise at 100 Hz optimistic? LIGO reached 3x10-9 at 20Hz - use IMC_Tra for high frequencies (P noise filtered by double cavity) Sidebands: Not directly controllable • Rely on control noise reduction remember: control noises should be reduced by more than 100 to reach Virgo design! Assume for Virgo+: - low freq: 10 times better - > 1kHz: ~ identical (Virgo error signal) (B5) assumed dP/P sidebands (Virgo)

  9. LIGO laser power noise

  10. Power noise for Virgo+ • With previous assumptions and assuming OMC transmission = 3% for sidebands Note: carrier power noise filtered by double cavity (pole at 3 Hz for Virgo+) - High frequency: SB power noise Should be ok for AC and DC - Low frequency: Both AC&DC need low control noises DC: carrier power noise - need good power stab - need to address power noise due to the jitter of the beam on the OMC

  11. Output mode cleaner for DC readout OMC transmission vs frequency F=50 F=1500 MHz MHz TBL~3% • Need to remove sidebands power from B1_DC: • SB would increase shot noise • Sidebands power noise could limit sensitivity => with m=0.15 and TSB,OMC=3%: PSB= 2% Pcar • New OMC? • Current OMC: SB and carrier transmitted in same Airy peak • Need to increase finesse and/or length and/or modulation frequency For Virgo+: minimize changes => Keep same fmod => Keep same OMC geometry/control • Increase F? could reach F=1000-2000 (now F=50) TSB,OMC~3% for F=1500

  12. New Output mode cleaner specifications • Increase finesse by ~30-40  F = 1500 – 2000. => need low losses material with good uniformity: Suprasil 311 • Potential problems/difficulties: • Losses: need < few 10 ppm per round trip for losses < few % on transmission • Absorption: < 1 ppm/cm => OK • Roughness < 2-3 A => difficult • Birefringence: difficult to estimate, request best uniformity  to be measured • Thermal effects • Control: temperature increase with P0=100 mW: T=10-3oC  Should not disturb the temperature control • Thermal lensing: f~20m => no problem for P0=100mW and absorption = 1ppm/cm • Control with temperature: less constraint than for AC readout but more difficult with higher finesse  Should be ok, to be tested

  13. Backup slides

  14. ITF control with offset on dark fringe? ~ 20 pm ~5 mW carrier • We do it at every lock: When ITF is controlled with B1p there is an offset of the order of 10 pm - Example of switch from B1p to B1 Loff ~ 20 pm: - expect B1 carrier increase by ~3.9 mW, observe 5mW! - OG roughly as expected • For real DC readout: Just need to switch the control from B1_AC to B1_DC signal B1_DC B1_2f

  15. OMC control

  16. Control noises • Example: BS longitudinal control noise Assumptions for Virgo+ : • Subtraction at 2% (now efficient at 8%) • B5 shot noise reached (now: >100 times above) => looks very optimistic => Would need subtraction at 0.5% to reach the level of the fundamental noise !

  17. AC and DC technical noises Tentative projection of technical noises for both readout schemes: Virgo+ case • High frequencies: • DC readout (assuming OMC SB transmission= 3%) a priori easier: onlySB power noise - AC readout: dangerous non-stationary noises: frequency noise+ phase noise • Low frequencies: • DC readout: might be more difficult due to carrier power noise => need to understand the possible reduction of carrier power noise • Control noises similar for both schemes AC DC

  18. Virgo+ optical parameters • Virgo+ optical parameters used to estimate technical noises: DC Powers P0 F losses Gcar GSB TSB TOMC,SB m Loff B1 B5 Virgo+ AC 25W 150 300 ppm 32 16 0.1 0.85 0.30 -97mW 160mW Virgo+ DC 25W 150 300 ppm 32 16 0.1 0.03 0.15 12x10-12 m35mW 160mW  to reduce sidebands power on B1 And for both schemes: - 1-C= 10-5 (current upper limit) - sidebands recycling gain 2/3 from optimal (C6-C7 case, assumes thermal compensation) - sidebands transmission half from expected (as observed for Virgo) • Technical noises projections: - use same analytical formulae as for Virgo noise budget - rescale shot noise and optical gains according to P0, F, m and recycling gains.

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