DC readout for Virgo ? E. Tournefier

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DC readout for Virgo ? E. Tournefier

  1. 1. DC readout for Virgo+? E. Tournefier WG1 meeting, Hannover January 23 rd ,2007 <ul><li>DC vs AC readout: technical noises </li></ul><ul><li>Output mode cleaner for DC readout </li></ul>
  2. 2. DC readout for Virgo+ ? <ul><li>Virgo+ optical parameters: P las =50W, F=150 </li></ul><ul><li>DC readout: </li></ul><ul><ul><li>ITF locking point is offset (  L off ) from the dark fringe </li></ul></ul><ul><ul><li>=> B1_DC is sensitive to OG  L </li></ul></ul><ul><li>DC vs AC readout: </li></ul><ul><ul><li>Advantages of DC readout: </li></ul></ul><ul><ul><ul><li>Shot noise limit smaller by 20% </li></ul></ul></ul><ul><ul><ul><li>No oscillator phase noise </li></ul></ul></ul><ul><ul><ul><li>No frequency noise at high frequency </li></ul></ul></ul><ul><ul><li>Requirements for DC readout: </li></ul></ul><ul><ul><ul><li>Need very good power stabilization </li></ul></ul></ul><ul><ul><ul><li>Need to eliminate the sidebands from B1_DC (increases shot noise + power noise) </li></ul></ul></ul><ul><ul><ul><li>=> new output mode cleaner needed </li></ul></ul></ul><ul><li>=> Estimate the AC and DC technical noises (frequency, power noises,…) for Virgo+ </li></ul>Both are non stationary
  3. 3. WSR6 noise budget
  4. 4. Phase noise for Virgo+ <ul><li>Phase noise on B1_ACp:  ACp =  x ACq RMS </li></ul><ul><li>- Current phase noise:  ~ 0.15  rad/  Hz (high freq.) </li></ul><ul><li>(oscillator, modulation/demodulation electronics) </li></ul><ul><li>- ACq RMS mainly driven by alignment fluctuations </li></ul><ul><li>Extrapolation for Virgo+ assuming similar alignment performances </li></ul><ul><ul><ul><li> Dangerous since non-stationary noise </li></ul></ul></ul><ul><ul><ul><ul><ul><li> Should be reduced for Virgo+ </li></ul></ul></ul></ul></ul><ul><ul><li>Can gain a factor 2 improving electronics? </li></ul></ul>6 MHz  gen EOM  gen +  LO  gen x TF IMC  LO board
  5. 5. Frequency noise <ul><li>Frequency noise (  i ) coupling to dark fringe: h = CMRF(f) x  i /  </li></ul><ul><li>CMRF depends on Fabry-Perot cavities asymmetries: </li></ul><ul><ul><li>Finesse asymmetry  F/F </li></ul></ul><ul><ul><ul><li>Induces a phase difference like OG </li></ul></ul></ul><ul><li>- Losses asymmetry  P and beam matching  M </li></ul><ul><li> Arm reflectivity difference </li></ul><ul><li> - Equivalent to phase difference for AC readout </li></ul><ul><li>- Not present for DC readout (checked with SIESTA) </li></ul><ul><li>Advantages of DC readout: </li></ul><ul><li>- CMRF drops at high frequencies </li></ul><ul><li>Drawback of AC readout: </li></ul><ul><li>- Losses vary with the cavities alignment </li></ul><ul><li>=> non-stationary noise (BoBs…) </li></ul>
  6. 6. Frequency noise for Virgo+ <ul><li>h = CMRF(f) x  i /  </li></ul><ul><li> i ? </li></ul><ul><ul><li>Assume only limited by B5 shot noise (optimistic) </li></ul></ul><ul><li>CMRF: Losses asymmetry </li></ul><ul><ul><li>Now equivalent to  P =50 ppm </li></ul></ul><ul><ul><li>Can be improved? </li></ul></ul><ul><ul><li>assume for Virgo+ : </li></ul></ul><ul><ul><li> P = 25 ppm </li></ul></ul><ul><li>CMRF: Finesse asymmetry: </li></ul><ul><li> F/F=2% </li></ul><ul><li>Will be very difficult to reach the shot noise above 100 Hz for AC readout </li></ul><ul><li>Need a small finesse asymmetry for both </li></ul>
  7. 7. Power noise <ul><li>Coupling: </li></ul><ul><ul><li>AC readout: </li></ul></ul><ul><ul><li>Couples through locking accuracy: </li></ul></ul><ul><ul><li>Assumes equivalent to  L RMS = 2x10 -13 m (i.e. 10 times better than C6 measurement) </li></ul></ul><ul><ul><li>DC readout: </li></ul></ul><ul><ul><li>Directly proportional to carrier and sidebands power </li></ul></ul><ul><ul><li>(reduce SB power with smaller modulation depth and small T OMC,SB ) </li></ul></ul><ul><li>Which power noise now? </li></ul><ul><li>- Smaller for carrier than for SB </li></ul><ul><li>- Power noise due to ITF angular/long controls </li></ul><ul><li>For Virgo+ DC: </li></ul><ul><li>Power should be stabilized inside ITF: </li></ul><ul><ul><li>reduce control noises </li></ul></ul><ul><ul><li>use B5_DC </li></ul></ul><ul><ul><li>=> photodiode under vacuum </li></ul></ul>Sensor noise
  8. 8. Power noise <ul><li>Which power noise for Virgo+? </li></ul><ul><li>Carrier: </li></ul><ul><li>Will need to be stabilized inside ITF </li></ul><ul><li>Use B5_DC (low freq) + IMC_Tra (high freq) </li></ul><ul><li>Assume for Virgo+: </li></ul><ul><li>- reach B5 shot noise at 100 Hz </li></ul><ul><li>optimistic? LIGO reached 3x10 -9 at 20Hz </li></ul><ul><li>- use IMC_Tra for high frequencies </li></ul><ul><li>(P noise filtered by double cavity) </li></ul><ul><li>Sidebands: </li></ul><ul><li>Not directly controllable </li></ul><ul><ul><li>Rely on control noise reduction </li></ul></ul><ul><ul><li>remember: control noises should be reduced </li></ul></ul><ul><ul><li>by more than 100 to reach Virgo design! </li></ul></ul><ul><ul><li>Assume for Virgo+: </li></ul></ul><ul><ul><li>- low freq: 10 times better </li></ul></ul><ul><ul><li>- > 1kHz: ~ identical </li></ul></ul>dP/P carrier dP/P sidebands (B5) (Virgo error signal) (Virgo) assumed
  9. 9. LIGO laser power noise
  10. 10. Power noise for Virgo+ <ul><li>With previous assumptions and assuming OMC transmission = 3% for sidebands </li></ul><ul><li>Note: carrier power noise filtered by double cavity (pole at 3 Hz for Virgo+) </li></ul><ul><li>- High frequency: SB power noise </li></ul><ul><li>Should be ok for AC and DC </li></ul><ul><li>- Low frequency: </li></ul><ul><li>Both AC&DC need low control noises </li></ul><ul><li>DC: carrier power noise </li></ul><ul><li> - need good power stab </li></ul><ul><li> - need to address power noise due </li></ul><ul><li> to the jitter of the beam on the OMC </li></ul>
  11. 11. Output mode cleaner for DC readout <ul><li>Need to remove sidebands power from B1_DC: </li></ul><ul><ul><li>SB would increase shot noise </li></ul></ul><ul><ul><li>Sidebands power noise could limit sensitivity </li></ul></ul><ul><ul><li>=> with m=0.15 and T SB,OMC =3%: P SB = 2% P car </li></ul></ul><ul><li>New OMC? </li></ul><ul><ul><li>Current OMC: SB and carrier </li></ul></ul><ul><ul><li>transmitted in same Airy peak </li></ul></ul><ul><ul><li>Need to increase finesse and/or length </li></ul></ul><ul><ul><li>and/or modulation frequency </li></ul></ul><ul><ul><li>For Virgo+: minimize changes </li></ul></ul><ul><ul><li>=> Keep same f mod </li></ul></ul><ul><ul><li>=> Keep same OMC geometry/control </li></ul></ul><ul><ul><li>Increase F? </li></ul></ul><ul><ul><li>could reach F=1000-2000 (now F=50) </li></ul></ul><ul><li> T SB,OMC ~3% for F=1500 </li></ul>MHz OMC transmission vs frequency T BL ~3% F=50 F=1500 MHz
  12. 12. New Output mode cleaner specifications <ul><li>Increase finesse by ~30-40  F = 1500 – 2000. </li></ul><ul><li>=> need low losses material with good uniformity: Suprasil 311 </li></ul><ul><li>Potential problems/difficulties: </li></ul><ul><ul><li>Losses: need < few 10 ppm per round trip for losses < few % on transmission </li></ul></ul><ul><ul><ul><li>Absorption: < 1 ppm/cm => OK </li></ul></ul></ul><ul><ul><ul><li>Roughness < 2-3 A => difficult </li></ul></ul></ul><ul><ul><ul><li>Birefringence: difficult to estimate, request best uniformity </li></ul></ul></ul><ul><ul><ul><li> to be measured </li></ul></ul></ul><ul><ul><li>Thermal effects </li></ul></ul><ul><ul><ul><li>Control: temperature increase with P 0 =100 mW:  T=10 -3 o C </li></ul></ul></ul><ul><ul><ul><li> Should not disturb the temperature control </li></ul></ul></ul><ul><ul><ul><li>Thermal lensing: </li></ul></ul></ul><ul><ul><ul><li>f~20m => no problem for P 0 =100mW and absorption = 1ppm/cm </li></ul></ul></ul><ul><ul><li>Control with temperature: less constraint than for AC readout </li></ul></ul><ul><ul><li>but more difficult with higher finesse </li></ul></ul><ul><ul><ul><li> Should be ok, to be tested </li></ul></ul></ul>
  13. 13. Backup slides
  14. 14. ITF control with offset on dark fringe? <ul><li>We do it at every lock: </li></ul><ul><li>When ITF is controlled with B1p there is an offset of the order of 10 pm </li></ul><ul><li>- Example of switch from B1p to B1 </li></ul><ul><li>L off ~ 20 pm : </li></ul><ul><li>- expect B1 carrier increase by ~3.9 mW, observe 5mW! </li></ul><ul><li>- OG roughly as expected </li></ul><ul><li>For real DC readout: </li></ul><ul><li>Just need to switch the control </li></ul><ul><li>from B1_AC to B1_DC signal </li></ul>B1_DC B1_2f ~ 20 pm ~5 mW carrier
  15. 15. OMC control
  16. 16. Control noises <ul><li>Example: BS longitudinal control noise </li></ul><ul><ul><li>Assumptions for Virgo+ : </li></ul></ul><ul><ul><li>Subtraction at 2% (now efficient at 8%) </li></ul></ul><ul><ul><li>B5 shot noise reached (now: >100 times above) </li></ul></ul><ul><ul><li>=> looks very optimistic </li></ul></ul>=> Would need subtraction at 0.5% to reach the level of the fundamental noise !
  17. 17. AC and DC technical noises <ul><li>Tentative projection of technical noises for both readout schemes: Virgo+ case </li></ul><ul><li>High frequencies: </li></ul><ul><ul><li>DC readout (assuming OMC SB transmission= 3%) a priori easier: only SB power noise </li></ul></ul><ul><ul><li>- AC readout: dangerous non-stationary noises: frequency noise + phase noise </li></ul></ul><ul><li>Low frequencies: </li></ul><ul><ul><li>DC readout: might be more difficult due to carrier power noise </li></ul></ul><ul><ul><li>=> need to understand the possible reduction of carrier power noise </li></ul></ul><ul><ul><li>Control noises similar for both schemes </li></ul></ul>DC AC
  18. 18. Virgo+ optical parameters <ul><li>Virgo+ optical parameters used to estimate technical noises: </li></ul><ul><ul><ul><ul><ul><li> DC Powers </li></ul></ul></ul></ul></ul><ul><li> P 0 F losses G car G SB T SB T OMC,SB m L off B1 B5 </li></ul><ul><li>Virgo+ AC 25W 150 300 ppm 32 16 0.1 0.85 0.30 - 97mW 160mW </li></ul><ul><li>Virgo+ DC 25W 150 300 ppm 32 16 0.1 0.03 0.15 12x10 -12 m 35mW 160mW </li></ul><ul><li>  to reduce sidebands power on B1 </li></ul><ul><li>And for both schemes: </li></ul><ul><li>- 1-C= 10 -5 (current upper limit) </li></ul><ul><li>- sidebands recycling gain 2/3 from optimal (C6-C7 case, assumes thermal compensation) </li></ul><ul><li>- sidebands transmission half from expected (as observed for Virgo) </li></ul><ul><li>Technical noises projections: </li></ul><ul><li>- use same analytical formulae as for Virgo noise budget </li></ul><ul><li>- rescale shot noise and optical gains according to P 0 , F, m and recycling gains. </li></ul>

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