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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