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Noise sources at high frequency in Virgo

E. Tournefier(LAPP-CNRS)

ILAS WG1 meeting, HannoverDecember 12th ,2005

• Recycled ITF sensitivities

• Noise sources– phase noise– frequency noise– environmental noise– laser noises

• Summary

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Recycled locking scheme

• B1_ACp -> Arms differential mode• B5_ACq -> Small Michelson differential mode• B5_ACp -> Arms common mode (frequency stabilisation)• B2_3f_ACp -> Recycling cavity length

Laser0

B2_3f phase B1

phase

+

-

Differential Mode control loop

B5

Recycling mirror

Beam Splitter

SSFS

B5 phase

B5 quad

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Recycled ITF sensitivities

C5 sensitivity C6 sensitivity ~25 W on BS C7 sensitivity Virgo design (500 W on BS)

• Input power on ITF: ~ 1 Watt

• C5 run: 5-7 Dec. 2004– no automatic alignment

• C6 run: 29 Jul – 12 Aug 2005

– partial automatic alignment

• C7 run: 14-18 Sep 2005– automatic alignment on 5 mirrors (NE, WE, NI, BS, PR)– ‘hierarchical’ control– modulation index: 0.16 ->

0.3

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C5 sensititivy

• At high frequencies: C5 sensitivity ~ 30 times higher than B1 shot + electronic noise=> it was explained with phase noise

C5 recycled sensitivity

B1 Electronic noise

B1 Shot noise

Phase noise ( model with = 0.45 rad/(Hz) )

~ x 30

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Phase noise?

• Observation: At high frequency, the noise is proportionnal to the signal amplitude on

the other quadrature of B1: B1_ACp = x B1_ACqrms

B1_ACp mean noise at high frequency

B1_ACq integrated RMS

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Phase noise

Signal arriving on the photodiode:

S= Sp + Sq = sp cos (t) + sq sin(t) where =2fmod

Demodulation process: ‘multiplies’ S by the oscillator (LO) signal: LO=cos (t+0)

ACp = S x LO0 = (sp cos (t) + sq sin(t)) x cos (t) = sp/2 +… (0 = 0)

ACq = S x LO90 = (sp cos (t) + sq sin(t)) x sin (t) = sq/2 +… (0 = 90)

If there is phase noise : LO = cos (t + + 0) then:

ACp = (sp cos (t) + sq sin(t+ )) x cos (t) = (sp+ sq ) /2 + …

ACp contains phase noise proportionally to the ACq level.

Estimation of with the C5 data: = ACp noise / ACq total rms <=> ~ 0.4 rad/Hz

6 MHz

EOM

ACp = x ACq

LO

boa

rd

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LO board phase noise

• Measurement of the phase noise introduced by the LO board LO ~ 0.3 rad/Hz

• there are 2 LO boards used in cascade B1 ~ 2 x LO = 0.45 rad/Hz

this corresponds to the phase noise observed during C5 C5 sensitivity is limited by LO board phase noise

• Improvement of LO board:– board contains:

• phase shifter• splitter 1 -> 8 outputs• amplitude loop to keep output level constant

– noise comes from the ‘amplitude loop’ removed (was not absolutely needed)

noise well decreased: LO < 0.1 rad/Hz

6 MHz

EOM

ACp = x ACq

LO

boa

rd

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Oscillator phase noise

6 MHz

gen

EOM

gen

genx TFIMC

= gen x (1-TFIMC)

• Oscillator (6MHz) phase noise: gen

– Filtered through IMC => does not cancel in the demodulation process: = gen x (1-TFIMC)

Marconi (gen)

()

After demod.

• Oscillator used up to now: Marconi generator ~ qq 0.1 rad/Hz

=> might be too high for Virgo• Replaced by LNFS-100 during this autumn shutdown

=> expected phase noise: < 0.03 rad/Hz

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Phase noise: from C5 to C6

• Two possibilities to reduce the impact of phase noise: B1_ACp = x B1_ACqrms

1/ reduce the phase noise at the source (generator / LO board)2/reduce the amplitude of the signal on the other quadrature: B1_ACq

1/ New LO board => reduced by at least 3

2/ Partial linear alignment during C6

=> ACq signal reduced by ~ 20 to 50 !

phase noise expected to be reduced by at least 150 for C6 !

C5C6

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From C5 to C6

Phase noise reduction thanks to:- Automatic alignment => reduced ACq signal- Improved LO board=> C6 sensitivity is not limited by phase noise

C5 sensitivity C5 phase noise (from LO board) C6 sensitivity C6 phase noise (from Marconi)

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C6 noise budget

• C6 (high frequency) sensitivity is limited by: - frequency noise (dominant)- shot and electronic noise to a smaller extent

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2f50 %

d1

d2

25 %

25 %

B5to SSFS

C6 frequency noise

B5 shot noise (and electronic noise): are seen by the frequency stabilisation loop (SSFS) and introduced in the ITF as frequency noise

=> frequency noise on B1_ACp: / x L x CMRR

CMRR = common mode rejection ratio

To reduce this effect:

- optimise the shot noise on the photodiode used for the SSFS:

* use most of B5 beam on this photodiode

* increase the modulation depth => larger signal but same shot noise

- improve the CMRR (alignment, symmetry of the arm defects)

Laser

B5_ACp

Laser frequency control loop (SSFS)

B1_ACp

/ x L x CMRR

done between C6 and C7

2f10 %

d1

d2

80 %

10 %

B5

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C7 noise budget

• B5 shot noise reduced ~ /2• modulation index x 2• but CMRR was slightly worse

Frequency noise /~2 ~ lower than B1 shot noise

Estimated freq. noise for Virgo design (500 Watts on BS):

=> frequency noise should still be reduced=> will need to improve CMRR

(B5 shot noise)

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Environmental noise

• Seismic / acoustic noise couples to the optical benches (laser and detection labs)

Environmental noise in detection lab: Vacuum pump (600Hz) on detection tower => harmonics + structures at ~ 2

kHz

will better isolate the detection bench from the pump vibrations

Vacuum pump ONVacuum pump OFF

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Environmental noise

Environmental noise in laser lab (acoustic, seismic)=> beam jitter (r , )=> converted into power noise (P ) and frequency noise ( ) by IMC

Power noise (P/P ) couples to dark fringe proportionally to the locking accuracy (Lrms ):

L = Lrms x P/P

Frequency noise ()couples through the common mode rejection ratio: L/L = CMRR x /

laserP

r

L P/P , /

P/P + / +

with: = r/w0 , = /0

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Power noise during C6 (1/2)

Power noise (P/P ) couples to dark fringe proportionally to the locking accuracy (Lrms ):

L = Lrms x P/P

Power noise projection using Lrms = 2.10-12 m (realistic value)

=> Power noise explains well the structures between 200 Hz and 1 kHz during C6

B1_ACpL x P/P

coherence between P noise and B1_ACp

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Power noise during C6 (2/2)

• Improvement of the power stabilisation at the end of C6 run

laserP r

P stab

Power after IMC

Sensitivity

Old P stab

New P stab

Good improvement of the sensitivity in the 200 Hz – 1kHz region

Power noise should not limit the final Virgo sensitivity

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C7 environmental noise

• Many structures above 400 Hz

• They disappear when the pumps of the Input Bench tower are switched OFF

• What are they?

– power noise?was well reduced during C6 It should not be

– frequency noise?

IB pump OFF

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Environmental noise during C7

Freq noise IB_tx error signal

Coherences with B1_ACp- IB Pumps ON- IB Pumps OFF

Structures above 400 Hz look like frequency noise

• Foreseen improvements: - better isolation from environmental noise - better alignment control - improved frequency stabilisation?

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Summary of C5, C6 and C7 noises

• C5 : – dominated by phase noise

reduced (/~100) with automatic alignment + LO board improvement next: new generator (Marconi -> LNFS-100) <- done

• C6 : – > 1kHz dominated by frequency noise (B5 shot noise)

reduced (/2) with more beam on B5 photodiode + increased modulation index

– 200 – 1kHz : power noise (from environmental noise) reduced (/10-100) with improved power stabilisation

• C7 : – > 1kHz: mixture of B1 shot noise + frequency noise (B5 shot noise)

next: - more power (new input bench) - improved CMRR

– 200 – 1kHz: frequency noise (from environmental noise) next: better isolation of the input bench + improved CMRR

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Summary of C5, C6 and C7 noises

C5 sensitivity C6 sensitivity 25 W on BS C7 sensitivity Virgo design (500 W on BS)

Extrapolation of B1 shot noise + frequency noise (B5 s.n.) for 500 W on BS

What else between C7 and Virgo design?

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Power incident on BS: can we reach 500 Watts?

• Power incident on BS before shutdown:

– Incident power on PR: P0 = 0.8 -1 W

– Recycling gain (all modes) R = 31– Beam matching ~ 94%

• Expected at the restart:

– Incident power on PR: P0 = 8 -10 W

– Recycling gain (new PR: 92 -> 95%) R00 = 43

– Negligible mismatching

R00 = 33

PBS = 25 W

PBS ~ 350 W

new optics / cleaning ? can be improved with better

alignment new IMC mirrors

If losses reduced by 2: Px1.25 => PBS ~ 440 W

•Are there possibilities to increase the input power?–Laser power = 22 Watts, but more than 50% is lost between laser and PR–Losses:

•25% on laser benches•17% due to mismatching•30% due to IMC losses

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What about laser technical noise at 6 MHz ?

• Laser technical power noise at 6 MHz couples to B1_ACp:B1_ACp = 2 x P/P x PB1_DC (Note: a good contrast is important)

=> P/P ~ 1.5 10-9 Hz @ 6.26 MHz

• PBS = 500 Watts: (=> ~ 100 mW on B1) laser noise : 19 10-11 W/Hz

B1 shot noise: 27 10-11 W/ Hz

A pre-mode cleaner will be installed to reduce the laser technical noise

• C7: laser noise : B1_ACp = 2 x 1.5 10-9 x 4.5 mW = 1 10-11 W/HzB1 shot noise : ~ 6 10-11 W/ Hz

=> not seen in C7

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Conclusion

• Identified noises at high frequency and foreseen improvements:

– Phase noise better generator (+ improved electronics?)

– Frequency noise (injected by the frequency stabilisation) better rejection of the common mode

– Environmental noise => power noise and frequency noise better acoustic/seismic isolation of the benches

– Laser technical noise (not yet observed) Pre-mode cleaner

– Shot noise: Increase input power / recycling gain

– …?

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C7 noise budget: high frequency floor

• Noise sources above ~ 300 Hz: estimated contribution @ 1 kHz- B1 electronic noise 2.6 x 10-22

- B1 shot noise 4.1 x 10-22

- B5 shot noise (frequency noise) 4.3 x 10-22

- Phase noise (6MHz oscillator: Marconi) 2.6 x 10-22

- Laser power noise at the 6MHz 0.7 x 10-22

7. 10-22 /Hz

(design: 7.2 10-23)

Scaling of noises with thepower incident on BS:For PBS x n

- elec noise / n - shot noise / n - phase & power noise:

idem

After shutdown, expect n > 10

=> Phase and laser power noises become important

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C7 noise budget below 200 Hz (1/2)• Coherences with the angular correction signals:

WI NI

NE

WE

PR

BS Above 50 Hz: - NI & PR ty (error signal originates

from the same quadrant photodiode)- WI ty up to ~ 200 Hz !

Below 50 Hz: mixture of most of the correction signals

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B1 shot noise

• The shot noise limited sensitivity depends on:

– the contrast defect: 1-C ~ 5 10-5 (B1) and 1-C ~ 5 10-4 (B1p) – the modulation depth: m=0.16 until C6 , m=0.3 for C7– the transmission of the sidebands: T ~ 0.15 (design: T=0.4) – the recycling gain: R=30

The contrast defect on B1 is good: It should allow to reach the optimum sensitivity for m=0.2-0.3

Shot noise limited sensitivity

m=0.16

~B1p

B1

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CMRR and frequency noise

• The CMRR is given by the asymmetry of the two arms:– finesse asymmetry– losses asymmetry – and the quality of the alignment

• Simulation result (R. Gouaty) for F/F=4% and round trip loss asymmetry=200ppmCMRR dominated by loss asymmetry

=> In good alignment conditions the simulated CMRR explains well the measured sensitivity

Sensitivity from Aug 27

B1 electronic noise

B1 shot noise

Simulated frequency noise (B5 sn)

0.15 %

SIESTA Simulation

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CMRR

Evolution of the CMRR (arbitrary units) with LA configuration:

The CMRR is less stable (37mHz,…) but can reach smaller values with 10 LA loops The net effect is a higher frequency noise

Possible to tune the LA + damp 37mHz in order to keep a small CMRR?

C6 (12 Aug) (drift control)Aug 27th (4 LA loops + drift control)Aug 31st (10 LA loops)

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mirror

piezoCorundum ½ sphere

SpiderZeroduractuator

Brewster window

Front mirrors

Vacuum tank

0.35m

- Triangular zerodur cavity * avoid thermal control & provide low mechanical Q

- Controlled by 1 piezo* Corundum half sphere glued on piezo * Pushing a “telescope spider” shaped like support* 6mm thick/12mm diam mirror, glued on spider-> Avoid piezo bending transfer to mirror displacement)( simulation by F. Richard)

- Vacuum tank, Brewster window

PMC Mechanical

126m

m

74mm

Glued mirrors

Mirror glued on piezo