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DC readout for Virgo+?
E. Tournefier
WG1 meeting, HannoverJanuary 23rd ,2007
• DC vs AC readout: technical noises
• Output mode cleaner for DC readout
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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 cleaner needed
=> Estimate the AC and DC technical noises (frequency, power noises,…) for Virgo+
SBoffrec
B PLLF
CGP
P
2
01 )(
42
2
1)1(
2
Both are non stationary
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WSR6 noise budget
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Phase noise for Virgo+
• Phase noise on B1_ACp: ACp = x ACqRMS
- Current phase noise: ~ 0.15rad/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?
6 MHzgen
EOM
gen +LO
genx TFIMC
LO
boa
rd
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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 MArm 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…)
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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
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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 vacuumSensor noise
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Power noise
• Which power noise for Virgo+? Carrier:
Will need to be stabilized inside ITFUse B5_DC (low freq) + IMC_Tra (high freq)
Assume for Virgo+: - reach B5 shot noise at 100 Hzoptimistic? 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 reductionremember: control noises should be reducedby 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
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LIGO laser power noise
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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 noiseShould 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
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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 TSB,OMC=3%: PSB= 2% Pcar
• New OMC?– Current OMC: SB and carriertransmitted in same Airy peak Need to increase finesse and/or lengthand/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 MHz
OMC transmission vs frequency
TBL~3%
F=50F=1500
MHz
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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=10-3 oC
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
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Backup slides
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ITF control with offset on dark fringe?
~ 20 pm
~5 mW carrier
B1_DCB1_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
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
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OMC control
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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 !
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AC and DC technical noisesTentative 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
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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 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 P0, F, m and recycling gains.