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LPOL-cavity

LPOL-cavity. Mechanics Test cavity (pb of the gain) Optics (laser polarisation) Electronics & DAQ  Z. Zhang’s talk. Gain  8000. Fabry-Perot cavity: principle. e beam. L. Polar. Circ. Polar. Lin. When n Laser = n 0  c/2L  r e sonance.

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LPOL-cavity

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  1. LPOL-cavity • Mechanics • Test cavity (pb of the gain) • Optics (laser polarisation) • Electronics & DAQ  Z. Zhang’s talk

  2. Gain 8000 Fabry-Perot cavity: principle e beam L Polar. Circ. Polar. Lin. When nLaser =n0c/2Lresonance • But :Dn/nLaser = 10-11 laser/cavity feedback • Done by changing the laser frequency • Laser: Nd:YAG (infrared, l =1064 nm)

  3. inside laser Ionic pumps bellow Beam pipe laser Mirror mount Mirror mount amortisseurs

  4. Mount for travel Final cavity

  5. Beam pipe Bellow ‘laser axis’

  6. Beam pipe & laser tube inside cavity Holes for vacuum conductivity

  7. Mirror mounts: On the optical table & isolated from cavity & beam pipe rotation cavity mirror bellow Orientations (‘gimbolt’): ‘plan/line/point’ system

  8. Test cavity at orsay Vacuum pump Motorised mirrors Mirror mounts CCD Laser ND:YAG Optcal room Temperature:  0.5o Photodiode  feedback (Saclay)

  9. P-diode laser glan cavity nlaser Data (oscillo) 2Hz & 10V pic-pic Ramp fit V Intensity reflected zoom Intensity transmited Dnlaser=75MHz (nlaser=3.108MHz) t(oscillo)/s t(oscillo)/s 200 ms 100 ms Mirror Laser Under investigation  gain cavity test  2000/8000

  10. Beam intensity after cavity (Gaussian in principle) Intensity (beam scan measure) x y

  11. Zoom : Slope not symetric bump

  12. Ellipsometry (`classic’) : Quarter wave plate cavity degree of circular polarisation after cavity • such :(I1-I2)/(I1+I2) = • Quart wave plate is the most sensitive element … :- • Choice & calibration importants for a per mill precision measurement …

  13. f 50 kHz 12 bits ADC Linear polarised light p-diode:I1 10 mW YAG Laser l/4 Glan Thomson p-diode:I0 I1/I0 <0.25% I1/volts 2% In practice … f/deg choice l/4anti-reflec. coated f In principle n2  1.90, d2  50 nm n1  1.36, d1  238 nm Quartz, <n>  1.54, d  150 m

  14. R/% -20 nm • Reflection coef. at normal incidence • Choice of an uncoated l/4plate • But model required … d2 d1 +20 nm Transmitted field Depends on thickness & optical indices no & ne. Quartz = Anisotropic uniaxial medium  4 directions for the field E (2GO & 2BACK)

  15. Polar vertical Calibration of the Quartz plate p-diode I1 Polar vertical f 10 mW YAG Laser l/4 Wollaston cube p-diodeI2 Glan Thomson Polar elliptic p-diode I0 Polar horizontal • Performances Wollaston & Glan Thomson : 10-5 (verified) • Measurements of I1/I0 et I2/I0 as function of f for différent incident angles  fit  no, ne & thickness

  16. Photodiode readout • Sequence of measurements(ADC 12bits, 50kHz, [-50mV,50mV] range) • Laser off (beam shutter) 10k-20k evts/angle • Pedestals of the 3 diodes = br0, br1, br2 • Laser on : 10k-20k evts/angle • Int. for the 3 diodes: • I0=Int1-br0, I1=Int1-br1, I2=Int2-br2 • I1/I0, I2/I0 recorded evt by evt to compensate for laser variations

  17. 70 mV 15 mV 12 mV Fixed angle DT= 0.2o 1.4 mV pedestals 26 h

  18. 20 min periode 100% correlated with Temperature Same plots For 2h

  19. Diode 0 Glan Thomson Wedge plate Diode 1 & 2 Wollaston L/4: 6 m-metric screws

  20. Plans • We checked that Temp. variations come from pdiode analog electronics • Long term variations (24h periode) not understood… • Use Temperature stable preamps for photodiodes(now fast preamps of feedback pdiode are used…) • Other solution: analog switching  same preamp for the 3 diodes Precision better than 0.1%

  21. c2 < 50 nm/150 mm c2 De/mm < 0.1% c2 Dno < 0.1% Dne f Results (prel.) f Pate auto-calibration by Interferometry •  Laser polar controled at 0.1% level for HERA already • no(T) & ne(T) for ND:YAG in handbooks are computed !  mesurement at the per mill level

  22. Conclusions • Mechanics: • cavity arrived beg. july at Orsay • August-sept: vacuum tests • Optical mounts and cavity mirror mounts • Being done in LAL workshop (finished in sept.) • Feedback and cavity gain • Still low, investigations being done & wait the final system for more more tests (mirror alignment syst. variations) • Laser polarisation • Per mill level almost reached after 1 year of work… • Test of the final setup: start in september

  23. Feedback nL nL-930kHz nL+930kHz  4MHz/Volt (nL =3.108 MHz) YAG laser Glan Cavity nC=nc/(2L) piezo gene Reflected signal Ramp + sin 930 kHz Photodiode Interference between central & side bandes Servo (analog elec) X Correction signal (closed loop=ramp off) V nL – nC whennLnC

  24. 4 mirrors Cavité de test au LAL: Schéma optique Signal refléchi feedback lens Pockels cell Glan thomson lens l/2 plate Faraday isolator laser hublot

  25. Implémentation à HERA et `électronique’ • Laser et éléments optiques sont près de la cavité s  fini En cours de réalisation

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