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

2. 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 =  laser/cavity feedback
Done by changing the laser frequency
 Laser: Nd:YAG (infrared, l =1064 nm)

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

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: rotation 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. 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

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

11. Zoom :
Slope not symetric
bump

12. 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 …

13. 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

14. 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)

15. 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

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

22. 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

23. 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

24. Cavité de test au LAL: Schéma optique4
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