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Diffused light Studies

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.

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Diffused light Studies

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  1. 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

  2. Measures of diffused light performed in the EGO optics lab

  3. Measures of diffused light performed in the optics lab Beam dump Experimental setup: 750mW @1064nm box PM @45° CCD diaph Beam dump sample L=500mm stage w0=1,2mm Diffused light measured @45° at 5cm from the sample with a PM of 1cm2 Homogeneity evaluated by moving the sample placed on a translation stage Image taken with a CCD camera @10cm. Make comparisons between very differents elements Derive a value of total diffused light Cross-check with LMA characterization for mirrors: quite good agreement

  4. Mirrors Tests First tested with the setup: a few typical commercial and customs mirrors CCD image Light measured

  5. Mirrors Tests Gain *8 Gain *8 Gain *24 Gain *24

  6. 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...

  7. Lenses tests

  8. Lenses tests: Values (quite) homogenous with comparison with mirors Ppm values closed to very good mirors 15-200ppm But lotsof local defaults (dots ?)

  9. Photodiodes and quadrants “standard” LAPP model Reduced beam size of around 100 mm window Additional problem: beam size on these photodiodes increasing the recoupling ?? photodiode 10-12 mW 2000-2500ppm LAPP model with polarizing cube 55000 -65000 ppm 270-310mW

  10. Photodiodes and quadrants NIKHEF quadrant model used for alignment Low optical quality of the window (dots) Surface quite homogenous 2,5-4mW 500-800ppm Quite good in comparison with photodiodes New BMS quadrant photodiodes (UDT sensors) Bright and very reflecting Wires ! 100 mW 10 mW 2000ppm 20000ppm

  11. Beam dump studies Razor blade: medium power up to 5W New: 40-50 mW Damaged: 80-100 mW 10000 ppm 20000ppm Very directive in the incident plane High power from opto-sigma, up to 30W,2 inches aperture 1,4 mW detected @5cm Absorbing glass Home made beam trap 70-150 nW 15-30ppm Damage threshold 1,8W@w0=1mm 5,6W@w0=2,5mm Need good HP Beam dump..

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

  13. 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). 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 ... 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)

  14. External detection bench EDB PZT accelerometer episensor shaker - B1s beam dump- mirror M50, M52, M1p1- lens L52- B5 polarizer cube Critical elements identified by listening to the dark fringe:

  15. 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 (This model of diode scatters a lot of light) All the other optics were un-mounted and cleaned, mounts replaced with stiffer ones.

  16. Improvements on EDB New tapping test performed: Clear improvement for all of them apart from M1p1 (too thin mirror, too be replaced…) Improvement on the dark fringe … quiet conditions noise injection before modifications noise injection after modifications no more visible Up-converted noise.

  17. EIB investigations episensor Shaker In horizontal position

  18. EIB Upconverted seismic noise Situation by exiting the bench whith noise around 10-20 Hz using the shaker... quiet conditions noise injection with shaker noise injection with shaker Amplitude *2

  19. 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 glass Lenses of Q21 and Q22 telescope misaligned : no visible effect

  20. Improvements on EIB Improvement on the dark fringe … quiet conditions noise injection before modifications noise injection after modifications Up-converted noise much reduced No more obvious source of scattered light (also tested: replacement of standard beam dumps by home made absorbing glass with no effect)

  21. NEB investigations Identified at first Check up: shaker very small clipping, reflection of quadrant not well dumped.. episensor 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 BS to dump the reflection on photodiodes. Beamsize: 200 to 600 mm. Lowered backscattering rate

  22. Results of the firts check up of NEB quiet conditions noise injection before modifications noise injection after modifications Second check up....

  23. NEB second check Other 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) shaker episensor some clipping on NEB_M_Flip NEB_M_Flip beam dump replaced by absorbing Glass.

  24. Results of the second check up... Comparison of noise level(while injecting noise) before and after these modifications...

  25. Remaining noise... Shaking amplitude multiplied by a factor 2... Noise with beam dump placed at different places Black: no beam dump Red: after M3 Green: after M2 Blue :after M1 Purple: before M1 Limited by some element between M2 and M3 Close to the noise probably due to the lentone

  26. 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.

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

  28. 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: d(t) = bench displacement,given by the episensor on the bench g = 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...

  29. Noise simulations and projection:EDB Evaluation of recoupling factor before the work by injecting a line @ 27 Hz...

  30. Noise simulations and projection:EDB Evaluation of recoupling factor before the work by injecting 10- 20 Hz white noise Quite good agreement...

  31. Noise simulations and projection:EDB Evaluation of recoupling factor after the work by injecting 10- 20 Hz white noise Simulation EDB white 10-20 HZ, after works (april 18) We see at least the improvement is a factor 6....

  32. Noise simulations and projection:EDB 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

  33. 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...

  34. Noise simulations and projection:EIB Projection of quiet bench displacement on the dark fringe before and after Good improvement close to the design sensitivity ...

  35. Noise simulations and projection: terminal benches Evaluation/results of the improvement for the terminal benches WEB G = 9 x 10-20 G = 2 x 10-20 G ~ 0,9-1,4 x 10-20 NEB G = 2,6 x 10-20

  36. 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

  37. 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

  38. 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...

  39. To go further: Using active isolation Kinetics system mod. 8001 • 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 Isolation Efficiency @ 10Hz 99%

  40. 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.

  41. Results of the improvements

  42. Resulting improvements Reduced up-conversion by back scattering Clear gain on sensibility for frequencies under 100 -150 Hz

  43. Resulting improvements (2) improvement on quadrant demodulated signals after cleaning optics on their beam path improvements in longitudinal error signals during diffused light mitigation.

  44. Conclusion Lower the diffused light rate on the optics side Orders of magnitude identified between elements, the worse are not the most obvious ones (lenses vs mirors). We are thinking about setting up a permanent test facility... • 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 benches Short 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.

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