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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 δL

• 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(

2δ

λπ

π

Both are non stationary

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WSR6 noise budget

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Phase noise for Virgo+

• Phase noise on B1_ACp: δACp = δϕ x ACqRMS

- Current phase noise: δϕ ~ 0.15µrad/√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 MHzδϕgen

EOM

δϕgen +δϕLO

δϕgenx TFIMC

δϕLO

boa

rd

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Frequency noise

• Frequency noise (δνi) coupling to dark fringe: h = CMRF(f) x δνi/ν

• 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…)

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Frequency noise for Virgo+

• h = CMRF(f) x δνi/ν

∀ δνi ?- 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 vacuum

Sensor 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.