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

Gain 8000 L  Laser: Nd:YAG (infrared, l =1064 nm) 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)

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

Mount for travel Final cavity

Beam pipe Bellow ‘laser axis’

Beam pipe & laser tube inside cavity Holes for vacuum conductivity

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

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

cavity nlaser zoom Mirror Laser P-diode laser glan Data (oscillo) fit 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

Beam intensity after cavity (Gaussian in principle) scan measure) x y

Zoom : Slope not symetric bump

circular polarisation 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 …

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

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

f Polar vertical Calibration of the Quartz plate l/4 p-diode I2 10 mW YAG Laser l/4 Wollaston cube p-diode I2 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

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

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

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

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

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%

Pate auto-calibration Results (prel.) De/mm c2 Dne Dno < 0.1% < 50 nm/150 mm f 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

Conclusions Mechanics: Feedback and cavity gain Laser polarisation 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

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

Cavité de test au LAL: Schéma optique 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

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