Diffused light Studies

B. Canuel – ILIAS WGI meeting 10/07/08

Diffused light Studies

Outline...

•Measures of diffused light performed in the EGO optics lab

•Mitigation work on the different virgo benches (EDB, EIB, end benches)

•How it is recoupled: noise simulations and projections for EIB and EDB

•Results on sensibility

•Conclusions


Measures of diffused light performed in the EGO optics lab


Measures of diffused light performed in the optics lab

Beam dump

750mW@1064nm

PM@45°

sample

diaph

L=500mm

box

Beam dump

CCD

stage

Diffused light measured @45° at 5cm from the sample with a PM of 1cm2

Homogeneity evaluated by moving the sample placed on a translation stageImage taken with a CCD camera @10cm.

w0=1,2mm

Derive a value of total diffused light Cross-check with LMA characterization for mirrors: quite good agreement

Make comparisons between very differents elements

Experimental setup:


Mirrors Tests

total scattering

Typical used mirror from CVI:

(1”,HR@45°)

6-7 W 1200-1400ppm

Brand new from CVI:(1”,HR@45°) 3-3,5 W 600-750ppm

Fichou HR miror(3”,spare of M1 detection)

1-1,6 W 200-340ppm

First tested with the setup: a few typical commercial and customs mirrors

CCD image Light measured


Mirrors Tests

HR quadrant alignment miror Laseroptik: 0,8-1,2W 150-250ppm

Mirror from SILO:(2”,HR@45°)

End bench spare M20,4-0,75 W 80-150ppm

CVI substrate (fused silica), coated by LMA (2”,HR@45°)

Tested by LMA@ 45 ppm:300-500nW 60-100ppm

General optics super polished, coted by LMA(2”,HR@45°)tested by LMA @ 30 ppm

70-210 nW 15-45ppm

Gain *8

Gain *24

Gain *24

Gain *8


Mirrors tests:

Huge difference between the different tested mirors...

Commercial mirrors widly used on virgo benches really worse by one order of magnitude than customs from SILO or coated substrates from LMA and 2 orders of magnitude from superpolished ones.

Typical used mirror from CVI (1”,HR@45°): 1400 ppm

General optics super polished, coated by LMA (2”,HR@45°): 15-45 ppm

According to LMA values of a few ppm are reached on some superpolished...


Lenses tests

Optosigma plano convex 200mm 0,4-1uW 80-200ppm

Optosigma plano convex 300mm 120-300 nW 25-60ppm

CVI biconvex 200mm 70-200 nW 15-40ppm

CVI biconvex 500mm 150-200 nW 30-40ppm


Lenses tests:

Values (quite) homogenous with comparison with mirors

Ppm values closed to very good mirors 15-200ppm

But lots of local defaults (dots ?)


“standard” LAPP model Reduced beam size of around 100 m

LAPP model with polarizing cube10-12 W 2000-2500ppm

270-310W 55000-65000 ppm

Photodiodes and quadrants

Additional problem: beam size on these photodiodes increasing the recoupling ??

window

photodiode


Photodiodes and quadrants

2,5-4W 500-800ppm

NIKHEF quadrant model used for alignment

Low optical quality of the window (dots)

Surface quite homogenous

Quite good in comparison with photodiodes

New BMS quadrant photodiodes (UDT sensors)

10 W 2000ppm 100 W 20000ppm

Bright and very reflectingWires !


Absorbing glass70-150 nW 15-30ppm

Razor blade: medium power up to 5W

New: 40-50 W

Damaged: 80-100 W

10000 ppm

20000ppm

Very directive in the incident plane

High power from opto-sigma,up to 30W,2 inches aperture

Home made beam trap

Damage threshold1,8W@w0=1mm5,6W@w0=2,5mm

1,4 mW detected @5cm

Beam dump studies

Need good HP Beam dump..


Mitigation work on the different benches (EDB, EIB, end benches)


Tools used for hunting diffused light

Shaker placed on the different benches to introduce seismic white noise or lines.

Mesures with episensensor placed on the benches (3 axis seimic sensor 0,2-200Hz).

Used the “radio dark fringe” to perform tapping test

Realized several beam dump with absorbing glass (for big and powerful beams with two absorbing glasses glued with a small angle)

We can see if the level of seismic noise reintroduced by diffused light on the dark fringe is improved by some action on the benches ...


External detection bench

EDBPZT accelerometer

episensor

shaker

- B1s beam dump- mirror M50, M52, M1p1- lens L52- B5 polarizer cube

Critical elements identified by listening to the dark fringe:


Improvements on EDB

B1s beam dump (razor blade type) replaced by lens (f=-0.05) + beam trap (absorbing glass).

Diode mounted on top of the B5 polarizer cube removed

All the other optics were un-mounted and cleaned, mounts replaced with stiffer ones.

(This model of diode scatters a lot of light)


Improvements on EDB

New tapping test performed:

Clear improvement for all of them apart from M1p1 (too thin mirror, too be replaced…)

no more visible Up-converted noise.

quiet conditions

noise injection before modifications

noise injection after modifications

Improvement on the dark fringe …


EIB investigations

ShakerIn horizontal position

episensor


EIB Upconverted seismic noise

quiet conditions

noise injectionwith shakerAmplitude *2

noise injectionwith shaker

Situation by exiting the bench whith noise around 10-20 Hz using the shaker...


EIB investigations

Most critical :B2_M1, B2_M1

Actual source of the light: reflection on B2_L1, B2_L2

B2_M1 and B2_M1 cleaned,B2_L1and B2_L2 misaligned and reflections dumped with absorbing glassLenses of Q21 and Q22 telescope misaligned : no visible effect


Improvements on EIB

Improvement on the dark fringe …

Up-converted noisemuch reduced

quiet conditions



No more obvious source of scattered light (also tested: replacement of standard beam dumps by home made absorbing glass with no effect)


NEB investigations

episensor

shaker Identified at first Check up:

very small clipping, reflection of quadrant not well dumped..

Biggest contributions:splitter just before the 2 photodiodes

photodiodes themselves

The group splitter + photodiode is moved by 50 cm upstream, some absorbing glass is glued on the BSto dump the reflection on photodiodes. Beamsize: 200 to 600 m. Lowered backscattering rate


Results of the firts check up of NEB

quiet conditions



Second check up....


NEB second check

episensor

shakerOther problems found:

5 or more secondary beams visible after M2 but still very close from the main beam. They have different sizes and angles

(suspect the NE second face, the doublet multi reflections, the second faces of M1 and M2)

some clipping on NEB_M_Flip

NEB_M_Flip beam dump replaced by absorbingGlass.


Results of the second check up...

Comparison of noise level (while injecting noise) before and after these modifications...


Remaining noise...

Shaking amplitude multiplied by a factor 2...Noise with beam dump placed at different places

Black: no beam dumpRed: after M3Green: after M2Blue :after M1Purple: before M1

Limited by some element between M2 and M3Close to the noise probably due to the lentone


WEB investigations

Same work as the NEB performed:

- moved the diodes upstream by 50cm (beam size is now 800um instead of 200um)

- absorbing glass placed on diode reflections

-dumped various secondary beams

-misaligned a bit more the quadrants.

-Multiple beams after M2 correctly dumped by absorbing glass.


Noise simulations and projections for all the benches(Thanks to Irene Fiori)


Effect of back scattered light on the dark fringe...

Backscattered light creates noise by seismic movement of the benches:How can we model this recoupling ?

Contribution to dark fringe of a parasitic beam that is delayed by a scattering surface moving by d(t) and then recombines in the ITF:

))(22sin()( gtdGt

d(t) = bench displacement, given by the episensor on the benchg = 1 – 1.6, adjusted to best batch the upconv. bumps (accounts for a scattering surfaces that moves up to 60% more than the bench)

G = adjusted to best match the sensitivity measured [h/sqrt(Hz)]

Values of G determined by seismic injection can be used to:•Caracterize the diffused light mitigation on the bench•Compare the recoupling rates of the different benches•Make the projection of quiet seismic noise on the benches on the dark fringe...


Noise simulations and projection:EDB

Evaluation of recoupling factor before the work by injecting a line @ 27 Hz...


Evaluation of recoupling factor before the work by injecting 10- 20 Hz white noise


Quite good agreement...


Evaluation of recoupling factor after the work by injecting 10- 20 Hz white noise

We see at least the improvementis a factor 6....


EDB white 10-20 HZ, after works (april 18)Simulation



Projection of quiet bench displacement on the dark fringe before and after the mitigation

The level of diffused light introduced by the movement of the bench is now under the design


Noise simulations and projection:EIB

Evaluation of recoupling factor before and after the work by the noise injections

3,8e-20 1,2e-20

Improvement more than a factor 3...


Projection of quiet bench displacement on the dark fringe before and after

Noise simulations and projection:EIB

Good improvement close to the design sensitivity ...


G = 9 x 10-20 WEB G = 2 x 10-20

G = 2,6 x 10-20 G ~ 0,9-1,4 x 10-20 NEB

Evaluation/results of the improvement for the terminal benches

Noise simulations and projection: terminal benches


Situation of projected noise after mitigation

Main contributions of the WEB and NEB, then comes the EIB and the EDB:

Major noise contribution is associated to the bench mechanical resonances (10-20Hz).

Required for Virgo+:•Reduce the bench motion at the resonances (factor 5 is needed) for NEB, WEB and EIB

Required for Virgo++/Adv Virgo:•A factor 100 is required around the resonnances and 10 in the range of 10-100Hz


To go further: passive damping of vibration of the benches

Effect of placing a resonator on the benches to selectively damp the vibrations (around the resonance of the benches)

Effect of the resonnator for the NEB

Tested on this bench: improvement by a factor of 3...An other factor 2 needed to reach V+ specs (fine tuning to be done...)

•To be tested on all the benches....•Other resonnances to damp... (55Hz...)

55 Hz


To go further: Placing the benches “on the ground”?

Example of WEB...Seismic noise on the ground is much smaller!

Good improvement. Remaining bumps are due to AC machines the cooling fans of the turbo pump...To reach the AdV design, some damping seems necessary...


To go further: Using active isolation

•Active Feedback Vibration Control system provides fast response and control of vibrations •Vibration Isolation for all six translational and rotational modes of vibration •Ultra low natural frequency < 1 Hz •No low frequency resonance •Isolation beginning at 0.7 Hz

Kinetics system mod. 8001

Isolation Efficiency @ 10Hz 99%


To go further: Using active isolation

Good improvement. Results close to the AdV design. It seems that the 0,7Hz resonnance of the platform is much spoiling the isolation.Long term stability should be evaluated.


Results of the improvements


Resulting improvements

Clear gain on sensibility for frequencies under 100 -150 Hz

Reduced up-conversion by back scattering


Resulting improvements (2)

improvement on quadrant demodulated signals after cleaning optics on their beam path

improvements in longitudinal error signals during diffused light mitigation.


Conclusion

We are thinking about setting up a permanent test facility...

Orders of magnitude identified between elements, the worse are not the most obvious ones (lenses vs mirors).

Lower the diffused light rate on the optics side

•We should be very careful using standard HR mirors(replace some critical mirors ?)

•Take care about cleanless!•Precise effect of incident angle on backscattering....•Design High power beam dump...

Lower the seismic exitation of the benchesShort timescale: reduce the benches resonnances by resonnant damper of using stiffer benches structures.

Longer Timescale: study the use of active damped benches or passive systems in combination with external control of benches absolute position.

Documents

Diffused light Studies