LPOL-cavity

• Mechanics

• Test cavity (pb of the gain)

• Optics (laser polarisation)

• Electronics & DAQ Z. Zhang’s talk

Fabry-Perot cavity: principle

e beam

Polar.Lin.

Polar.Circ.

When Laser =0 c/2L resonance

L

•But : /Laser = 10-11 laser/cavity feedback

•Done by changing the laser frequency

Laser: Nd:YAG (infrared, =1064 nm)

Gain 8000

Beam pipe bellowIonic pumps

amortisseurs

insidelaser

laser

Mirrormount

Mirrormount

Final cavity Mount for travel

Beam pipe

‘laser axis’

Bellow

Beam pipe & laser tubeinside cavity

Holes for vacuum conductivity

Mirror mounts: On the optical table &isolated from cavity & beam pipe

bellow

cavity

mirror

Orientations (‘gimbolt’): ‘plan/line/point’ system

rotation

Photodiode feedback (Saclay)

Laser ND:YAG

Mirrormounts

Motorised mirrors

CCD

Vacuum pump

Optcal roomTemperature:

0.5o

Test cavity at orsay

V

2Hz & 10V pic-pic Ramp

Intensitytransmited laser=75MHz

t(oscillo)/s

zoom

t(oscillo)/s

Intensity reflected

100 s

laserglancavityP-diode

200 ms

fit

Data (oscillo) laser

gain cavity test 2000/8000MirrorLaser

Under investigation

(laser=3.108MHz)

Beam intensity after cavity (Gaussian in principle)

(beam scanmeasure) x

y

Intensity

Zoom :

bump

Slope not symetric

cavity

•Quart wave plate is the most sensitive element … :-

•Choice & calibration importants for a per mill precision measurement …

Ellipsometry (`classic’) :

• such : (I1-I2)/(I1+I2)

degree ofcircular polarisation

after cavity=

Quarter wave plate

4 anti-reflec. coated

Quartz, <n> 1.54, d 150 m

4

p-diode: I1Linear polarised light 50 kHz12 bits ADC

n1 1.36, d1 238 nmn2 1.90, d2 50 nm

In principle

In practice …

<0.25%

I1/I0

deg

I1/volts

2%

10 mW YAG Laser

Glan Thomson p-diode: I0

choice

• Reflection coef. at normal

incidence

• Choice of an uncoated

4 plate – But model required …

Quartz = Anisotropic uniaxial medium 4 directions for the field E (2 GO & 2 BACK)

Transmitted field

Depends on thickness & optical indices no & ne.

R/%

-20nm

+20nm

d2 d1

10 mW YAG Laser

Wollaston cube

4

p-diode I1

p-diode I2

Glan Thomson

Polar vertical

Calibration of the Quartz plate

p-diode I0

• Performances Wollaston & Glan Thomson : 10-5 (verified)

• Measurements of I1/I0 et I2/I0 as function of for différent

incident angles fit no, ne & thickness

Polar horizontal

Polar vertical

Polar elliptic

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/angleInt. 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

12 mV

15 mV

26 h

70 V

Fixedangle

1.4 V

pedestals

T=0.2o

20 min periode 100% correlated with Temperature

Same plotsFor 2h

Diode 1 & 2

Wollaston

Diode 0

L/4: 6-metricscrews

Glan Thomson Wedge plate

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%

e/m

2

2

2

ne

no

< 0.1%

< 0.1%

< 50 nm/150 m

Results (prel.)

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

Pate auto-calibration

by Interferometry

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

Feedback

YAG laser

piezo

+

X

sin 930 kHzPhotodiode

Servo(analog elec)

Ramp

L L+930kHzL-930kHz

Reflected signal

CavityC=nc/(2L)

V L – C when L C

Glan

Interference between central& side bandes

Correction signal(closed loop=ramp off)

4MHz/Volt

(L =3.108 MHz)

gene

laser

Faraday isolator

/2 plate

lens

Pockels cell

Glan thomson

lens

4mirrors

Cavité de test au LAL: Schéma optique

Signal refléchi feedback

hublot

Implémentation à HERA et `électronique’

•Laser et éléments optiques sont près de la cavité

En cours de réalisation fini

s