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Locking tests with TCS

Locking tests with TCS. WG1 at Birmingham. July, 10 th -11 th 2008. Enrico Campagna on the behalf of the Locking/TCS group. European Gravitational Observatory. Universita` di Urbino. INFN Sez. Firenze. Summary.

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Locking tests with TCS

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  1. Locking tests with TCS WG1 at Birmingham July, 10th-11th 2008 Enrico Campagna on the behalf of the Locking/TCS group European Gravitational Observatory Universita` di Urbino INFN Sez. Firenze

  2. Summary A description of all the tests is in EGO working area under Virgo/locking g Activities g TCS • Thermal compensation system in Virgo • Why we need it • An overview • Tests • Step 12 • 10 mW • Higher powers • Etalon effect, Newton rings • Trying to balance the SBs • with locking/alignment signals • with input power • Step 8 • 1.3 W and up to 4.4 W • Scanning OMC temperature • Lock acquisition • 700 mW from the beginning • Various power form step 6 • Investigation of 11 Hz loop instabilities • Single free swinging cavity • Changing TCS annulus shape • Future • Conclusions

  3. TCS in Virgo: why we need it • Absorbed laser power in the input optics • 14 ppm in the WI • 5 ppm in the NI • Evident effect during the lock acquisition • After reaching the DF we have to wait 10-15 min for the ITF to thermalize • Big transients on sidebands powers (great asymmetry) • The cleaning with first contact polymer had no effect • The transient remained pretty much the same • Slight increase of mean SB power • TCS should • Give the optics a constant time behavior • make the locking signal more stationary • make the lock acquisition faster

  4. TCS in Virgo: an overview • 25 W CO2 laser (only on WI) • 3-4 % on power meter gph.diode • 2 telescopes L1-L2 and L3-L5 • Axicon: double conic lense • Ext. thermal camera

  5. Step 12: 10 mW • With 10 mW we already have a strong effect on SB balancing. • NOT EXPECTED, but observed many times, in subsequent tests also • Opposite to the std transient • Due to central spot? • Phase camera: clear effect on central area (movies) • Towards a flatter shape • MORE INVESTIGATION NEEDED

  6. Step 12: higher powers • Up to some hundreds mW no significant differences • 700 mW gives encouraging effects • SBs unbalancing as in a time-reversed std locking acquisition transient • Indicates a possible reduction of thermal lensing • 5 mins transient for each step in powers • B5_2f_ACq is decreasing a lot even if mean SB power is increasing • Decrease in MICH offset g B5_DC increasing • B1_DC/B1p_DC increases indicating a cleaner matching of the SBs in the OMC • West cavity finesse is significantly growing • More than 1 W • Upper SB starts going up • 11 Hz instabilities (see later)

  7. Step 12: cooling down and etalon effect • Switching TCS off after the test • The same 5-10 min fast trends • SBs start from very low values and then slowly increase • g long trend in the optical gain of NE, WE and BS • correlated quite well with the horizon • Clear slow trends on B5_2f_ACq and on Gc_Driving_PRCL_NE-WE • WI heating g change in thickness g change in etalon g finesse asymmetry g pure common modes couple to differential g PRCL enters differently the df g gain beta changes • but PRCL noise not limiting the sensitivity • The same happens with B5 frequency noise coupling

  8. More on etalon: Newton rings • From mirror temperature measurement (resonant mode technique) rTM=0.3 K is not enough to explain the etalon fringes • From FE symulation (M.Punturo) • Locally rT of few degrees • Differential radial reflectivity • Time dependent transmittivity • TCS with current configuration will never get rid of thermal transient • Restyle it to allow for a central spot to heat as the Nd:Yag laser while unlocked

  9. Step 12: trying to balance the SBs with locking and alignment signals • With locking signals • MICH offset has little effect and fast brings B5_DC to very low values (the std way of balancing the SBs is not working) • B2_3f phase and B5 phase have no effect • With TCS off, B2_8MHz phase tuned to move B2_8MHz_ACq to 0 • MICH offset goes to 0 • It has no effects on B5_DC or B5_2f _ACq • But with TCS on nothing changes • Lowering the PRCL offset • SBs unbalance but • their power goes up • in another test at step 8 has no effect at all • Alignment signal of the input have no effects (at step 8) • MORE TESTS NEEDED

  10. Step 12: trying to balance the SB with input power • Two successful tests performed • Increase TCS power • Increase up to 9.3 W IMC transmitted power in order to balance the SBs • Obvious gain in the B1 shot noise • Around 15% PIMCincrease gives a 7% shot noise reduction • No particular signal worsening detected

  11. Moving the locking offsets at step 12 • Sensitivity comparison with TCS off/on/unb-sb • TCS power-noise budget (E.Tournefier) • From noise measurement • Stating a 1/f • matching 596 Hz line Thanks to Edwige

  12. Step 8: up to 4.4 W • With the same configuration as during the transient (MICH on B5_ACq and PRCL on B2_3f_ACp) • USB reaches very low values (SFP reconstruction fails) • Looking at phase camera: • At about 1.3 W the ITF behaves like a “cold” one • Should correspond to equal input mirror RoCs • Long unlockable period • Only two locking trials • Saturation of PR_zCorr signal but with different behhaviors: • Step 6.5 unlock (25-30 Hz oscillation of PRCL loop) • Step 7.5 unlock (145 Hz oscillation on all the loops) • Not enough to conclude anything

  13. Step 8: scanning OMC temperature • In the normal state the • total SB power in the df is about 27.9 mW • 46% TEM00 • 54% Laguerre • With 1.5 W TCS laser power • 53.1 mW (doubled!) • 87% TEM00 • 13% Laguerre • Going to 2W slightly increases the total power • SBs strongly unbalanced • rotating B2_3f phase (for best P/Q at PRCL 62 Hz line) of 12 degrees it unlocks • Possible explanation: • LSB has a good Gaussian shape ( g high recycling gain) • USB has almost completely a Laguerre shape ( g almost no recycling gain) • Without the new phase camera there is no way to prove it.

  14. Lock acq.: 700 mW from the beginning • Starting with an already heated WI mirror with 700 mW • B5_2f power starts from higher values at the beginning of the lock acquisition and decreases slower than usual. • At the MICH/PRCL mixing now there is no powers step • B2_3f_ACp responds in a different way to MICH displacement • At the MICH PRCL driving matrix reshuffling signals seem to come back to the “normal” configuration • 8MHz more or less the same as before, B2_3f is changed by the TCS • TCS has a clear effect on the ITF working point • It is not clear what really happens • Is it a good thing? • MORE TESTS NEEDED STARTING WITH LOW Nd:Yag POWER

  15. Lock acquisition: with various power from step 6 • Modifying the lock acquisition in order to survive in the old dying high-power state • DARM on B1p • As soon as ITF in df (on B8_ACp) we switch on the TCS at various powers • No evident effects

  16. Investigation on 10-11 Hz instabilities • At step 12 “resonance” appearing on MICH and PRCL loops • Similar instability triggered by BS-marionette reallocation • Beta servo non working well around 10 Hz (different shape from PRCL to B1) • Measurements on PRCL OLTF while moving the MICH driving term to PR show something around 10 Hz • Not well balanced BS marionette re-allocation? • Measure BS TFs Ma=MariogB1, RM=RMassgB1, zGc=z_GcgB1 • Ma/RM gives the filter to be used for reallocation ( but fz<0 ) • zGc/RM actuation unbalancing to be compensated • TO BE IMPLEMENTED =

  17. Single free swinging cavity • We locked the two arms (step 1) and then • We induced several unlock (up to 0.9 W TCS power) • Look at WI free swinging cavity • estimate the elastic deformation (RoCs) of the WI HR-coating surface from measuring higher order mode (TEM02) frequency shift (R.Day/B.Swinkels) • Main peak identification • Sub-peak identification • Frequency shift • Compare the result with what is expected (0.3 % shift) • Slight shift detected (about 5 time less then expected)

  18. Unlockable period after single cavity tests • 5 unlocks at step 5.5 • Pretty much at the moment of reaching the dark fringe • Similar behaviors • Unlock due to B5_d2_ACp (SSFS) fast oscillation • B5_d2_ACp dirty since some seconds (except #2) • #4 very dirty DARM • 2 unlocks at step 6.5 (#5 and #8) • 30 Hz DARM oscillation • B5_d2_ACp saturation

  19. Changing the TCS laser shape • Std rIN=5 cm, rOUT=20 cm • increase of about 40% the rIN • same laser power • No visible effect on transient signals Not the actual shape Not the actual shape DL3-Axiincreased DL3-Axidecreased

  20. Conclusions • To be understood: • Origin of 10 mW effect • if it is a physical problem on the system should ABSOLUTELY be solved (we want to perform accurate locking test with low power) • What signal to control SB balancing (alignment…) • Free swinging cavity null result • To be done: • Central spot TCS for avoiding Newton rings • Tests with low Nd:Yag power • Phase camera 1 installation to understand the actual SB behavior • New BS marionette re-allocation filter implementation • Trying to survive in the old dying high power state • Tests varying the annulus shape

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