1. 1 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

2. 2 DC readout for Virgo+ ? Virgo+ optical parameters: P las =50W, F=150 DC readout: –ITF locking point is offset (  L off ) 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 cleaner needed => Estimate the AC and DC technical noises (frequency, power noises,…) for Virgo+ Both are non stationary

3. 3 WSR6 noise budget

4. 4 Phase noise for Virgo+ Phase noise on B1_ACp:  ACp =  x ACq RMS - Current phase noise:  ~ 0.15  rad/  Hz (high freq.) (oscillator, modulation/demodulation electronics) - ACq RMS 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? 6 MHz  gen EOM  gen +  LO  gen x TF IMC  LO board

5. 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. 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. 7 Power noise Coupling:  AC readout: Couples through locking accuracy: –Assumes equivalent to  L RMS = 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 T OMC,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. 8 Power noise 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 dP/P carrier dP/P sidebands (B5) (Virgo error signal) (Virgo) assumed

9. 9 LIGO laser power noise

10. 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. 11 Output mode cleaner for DC readout Need to remove sidebands power from B1_DC: –SB would increase shot noise –Sidebands power noise could limit sensitivity => with m=0.15 and T SB,OMC =3%: P SB = 2% P car 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 f mod => Keep same OMC geometry/control  Increase F? could reach F=1000-2000 (now F=50) T SB,OMC ~3% for F=1500 MHz OMC transmission vs frequency T BL ~3% F=50 F=1500 MHz

12. 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: OK Roughness difficult Birefringence: difficult to estimate, request best uniformity  to be measured –Thermal effects Control: temperature increase with P 0 =100 mW:  T=10 -3 o C  Should not disturb the temperature control Thermal lensing: f~20m => no problem for P 0 =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. 13 Backup slides

14. 14 ITF control with offset on dark fringe? ~ 20 pm ~5 mW carrier B1_DC B1_2f 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 L off ~ 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

15. 15 OMC control

16. 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. 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: only SB 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 DCAC

18. 18 Virgo+ optical parameters Virgo+ optical parameters used to estimate technical noises: DC Powers P 0 F losses G car G SB T SB T OMC,SB m L off 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 m 35mW 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 P 0, F, m and recycling gains.