Transcript Document

Noise sources at high frequency in Virgo
E. Tournefier
(LAPP-CNRS)
ILAS WG1 meeting, Hannover
December 12th ,2005
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Recycled ITF sensitivities
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Noise sources
– phase noise
– frequency noise
– environmental noise
– laser noises
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Summary
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Recycled locking scheme
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B1_ACp
-> Arms differential mode
B5_ACq
-> Small Michelson differential mode
B5_ACp
-> Arms common mode (frequency stabilisation)
B2_3f_ACp -> Recycling cavity length
+
Recycling
mirror
Beam Splitter
Laser
0
-
B5
B2_3f
phase
B1
phase
B5 quad
SSFS
Differential Mode
control loop
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B5 phase
Recycled ITF sensitivities
C5 sensitivity
C6 sensitivity
~25 W on BS
C7 sensitivity
Virgo design (500 W on BS)
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Input power on ITF: ~ 1 Watt
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C5 run: 5-7 Dec. 2004
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C6 run: 29 Jul – 12 Aug 2005
–
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no automatic alignment
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
C5 recycled sensitivity
B1 Electronic noise
B1 Shot noise
Phase noise ( model with  = 0.45 rad/(Hz) )
~ x 30
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At high frequencies:
C5 sensitivity ~ 30 times higher than B1 shot + electronic noise
=> it was explained with phase noise
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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
6 MHz
Signal arriving on the photodiode:
S= Sp + Sq = sp cos (t) + sq sin(t)
where  =2fmod
EOM
ACp = S x LO0 = (sp cos (t) + sq sin(t)) x cos (t) = sp/2 +…
ACq = S x LO90 = (sp cos (t) + sq sin(t)) x sin (t) = sq/2 +…
LO board
Demodulation process:
‘multiplies’ S by the oscillator (LO) signal: LO=cos (t+0)

(0 = 0)
(0 = 90)
ACp =  x ACq
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
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LO board phase noise
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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
6 MHz
Improvement of LO board:
– board contains:
• phase shifter
EOM
• 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
LO board
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
ACp =  x ACq
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Oscillator phase noise
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Oscillator (6MHz) phase noise: gen
– Filtered through IMC => does not cancel in the demodulation process:
 = gen x (1-TFIMC)
6 MHz
gen
EOM
Marconi (gen)
genx TFIMC
After demod.
()
gen
 = gen x (1-TFIMC)
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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
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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 !
C5
C6
 phase noise expected to be reduced by
at least 150 for C6 !
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From C5 to C6
C5 sensitivity
C5 phase noise (from LO board)
C6 sensitivity
C6 phase noise (from Marconi)
Phase noise reduction thanks to:
- Automatic alignment => reduced ACq signal
- Improved LO board
=> C6 sensitivity is not limited by phase noise
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C6 noise budget
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C6 (high frequency) sensitivity is limited by:
- frequency noise (dominant)
- shot and electronic noise to a smaller extent
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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 SSFS
d2
B5
25 %
80
d1
Laser

/ x L x CMRR
25 %
10
50 %
10
2f
To reduce this effect:
B5_ACp
B1_ACp
Laser frequency control loop (SSFS)
- 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)
done between
C6 and C7
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C7 noise budget
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B5 shot noise reduced ~ /2
modulation index x 2
but CMRR was slightly worse
Frequency noise /~2
~ lower than B1 shot noise
(B5 shot noise)
Estimated freq. noise for
Virgo design (500 Watts on BS):
=> frequency noise should still
be reduced
=> will need to improve CMRR
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Environmental noise
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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
Vacuum pump ON
Vacuum pump OFF
 will better isolate the detection bench from the pump vibrations
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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
P/P    +  
/    +  
with:  = r/w0 ,
laser
r
P


 = /0
L  P/P , /
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 /
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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)
B1_ACp
L x P/P
coherence between P noise and B1_ACp
=> Power noise explains well the structures between 200 Hz and 1 kHz during C6
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Power noise during C6 (2/2)
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Improvement of the power stabilisation at the end of C6 run
Power after IMC
r
laser
Old P stab
P
New P stab

P stab
 Good improvement of the sensitivity in the
200 Hz – 1kHz region
Sensitivity
 Power noise should not limit the final Virgo
sensitivity
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C7 environmental noise
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Many structures above 400 Hz
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They disappear when the pumps of
the Input Bench tower are switched
OFF
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What are they?
IB pump OFF
– power noise?
was well reduced during C6
 It should not be
– frequency noise?
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Environmental noise during C7
Freq noise
- IB Pumps ON
IB_tx error signal
Coherences with B1_ACp
- IB Pumps OFF
 Structures above 400 Hz look like frequency
noise
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Foreseen improvements:
- better isolation from environmental noise
- better alignment control
- improved frequency stabilisation?
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Summary of C5, C6 and C7 noises
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C5 :
– dominated by phase noise
 reduced (/~100) with automatic alignment + LO board improvement
 next: new generator (Marconi -> LNFS-100) <- done
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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
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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?
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Power incident on BS before shutdown:
– Incident power on PR: P0 = 0.8 -1 W
– Recycling gain (all modes) R = 31
R00 = 33
– 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
PBS = 25 W
PBS ~ 350 W
•Are there possibilities to increase the input power?
–Laser power = 22 Watts, but more than 50% is lost between laser and PR
–Losses:
 new optics / cleaning ?
•25% on laser benches
If losses reduced by 2: Px1.25 => PBS ~ 440 W
 can be improved with better alignment
•17% due to mismatching
 new IMC mirrors
•30% due to IMC losses
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What about laser technical noise at 6 MHz ?
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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)
C7: laser noise : B1_ACp = 2 x 1.5 10-9 x 4.5 mW = 1 10-11 W/Hz
=> not seen in C7
B1 shot noise :
~ 6 10-11 W/ Hz
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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
=> P/P ~ 1.5 10-9  Hz @ 6.26 MHz
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Conclusion
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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
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Noise sources above ~ 300 Hz:
- B1 electronic noise
- B1 shot noise
- B5 shot noise (frequency noise)
- Phase noise (6MHz oscillator: Marconi)
- Laser power noise at the 6MHz
estimated contribution @ 1 kHz
2.6 x 10-22
4.1 x 10-22
7. 10-22 /Hz
4.3 x 10-22
2.6 x 10-22
0.7 x 10-22
(design: 7.2 10-23)
Scaling of noises with the
power 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)
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Coherences with the angular correction signals:
 Below 50 Hz: mixture of most of the correction signals
WI
NI
NE
PR
 Above 50 Hz:
- NI & PR ty (error signalWE
originates from
the same quadrant photodiode)
- WI ty up to ~ 200 Hz !
BS
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B1 shot noise
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The shot noise limited sensitivity depends on:
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–
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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
Shot noise limited sensitivity
m=0.16
 The contrast defect on B1 is good:
It should allow to reach the optimum
sensitivity for m=0.2-0.3
~B1p
B1
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CMRR and frequency noise
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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=200ppm
CMRR dominated by loss asymmetry
Sensitivity from Aug 27
B1 electronic noise
SIESTA Simulation
B1 shot noise
Simulated frequency noise (B5 sn)
0.15 %
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=> In good alignment conditions the simulated CMRR explains well the measured sensitivity
CMRR
Evolution of the CMRR (arbitrary units) with LA configuration:
C6 (12 Aug) (drift control)
Aug 27th (4 LA loops + drift control)
Aug 31st (10 LA loops)
 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?
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PMC Mechanical
- Triangular zerodur cavity
Vacuum tank
* 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)
74mm
- Vacuum tank, Brewster window
Zerodur
Corundum ½ sphere
0.35m
126mm
actuator
Mirror glued on piezo
Front mirrors
Spider
mirror
piezo
Brewster window
Glued mirrors
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