licas project: fsi overview richard bingham, edward botcherby, paul coe, john green, grzegorz...
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LiCAS Project: FSI Overview
Richard Bingham, Edward Botcherby, Paul Coe, John Green,Grzegorz Grzelak, Ankush Mitra, John Nixon, Armin Reichold
University of Oxford
Andreas Herty, Wolfgang Liebl, Johannes Prenting, Markus Schloesser Applied Geodesy Group, DESY
1 April 2003
FSI Overview 2
Contents
FSI Concept
ATLAS FSI• Implementation• Results
LiCAS Extensions
1 April 2003
FSI Overview 3
Frequency Scanning Interferometry Interferometric length measurement technique Require precision of 1m over 5m Originally developed for online alignment of the ATLAS SCT tracker
Tunable Laser
Reference Interferometer: L
Measurement Interferometer: D
Change of phase: GLI
Change of phase: Ref
time
IRef
time
IGLI
(Grid Line Interferometer (GLI))
Ref
GLI
L
D
1 April 2003
FSI Overview 4
FSI: Length MeasurementGLI
Ref
GLI
Ref
GradientD
L
1 April 2003
FSI Overview 5
FSI: Thermal Drift Cancellation
Thermal effects add subtle systematic errors to FSI− Nanometre movements can contribute micron errors (
Use two lasers tuning in opposite directions to cancel thermal drift
Expansion ofInterferometer
I
I
1 April 2003
FSI Overview 6
FSI: Thermal Drift CancellationGLI
Ref
True Gradient
Measured G
radient with
Laser Tuning U
p
Measured Gradient with Laser Tuning Down
1 April 2003
FSI Overview 7
FSI: 2-Laser Thermal Drift Cancellation
m mRange
20 30 40 50 60
417.0
417.4
Laser 1 Laser 2 Com bined
Est
imat
ed G
LI le
ngth
/ m
m
Tim e / h r
2 0 3 0 4 0 5 0 6 0
-4 0
-2 0
0
2 0
4 0
6 0
8 0
Tim e / hr
Warm ing
Coo ling
dT
/dt
(k
s)
-1
1 April 2003
FSI Overview 8
FSI: ATLAS Implementation
1 April 2003
FSI Overview 9
FSI: ATLAS Test Grid
6 simultaneous length measurements made between four corners of the square.
+7th interferometer to measure stage position.
Displacements of one corner of the square can then be reconstructed.
1 April 2003
FSI Overview 10
FSI: ATLAS Resolution
1 April 2003
FSI Overview 11
1m
FSI: ATLAS Resolution
Stage is kept stationary
RMS 3D Scatter
< 1 m
1 April 2003
FSI Overview 12
Retro Reflector
ATLAS FSI SystemLaser 1
Laser 2
Reference Interferometer
piezodetector
C-Band Amplifier (1520-1570 nm)
L-Band Amplifier (1572-1630 nm)
Splitter Tree
LiCAS FSI System
1m GLI
Uncollimated Quill
APD
Collimated Quill
5m GLIADC
+AMPS
RAM To PC
f1
f2
Amplitude Modulation @ f1
Amplitude Modulation @ f2
Detectors
Demodulator
@ f1 , 1
Demodulator
@ f2 , 2
Demodulator
@ fn , n
1 April 2003
FSI Overview 13
Laser 1
M1
M2
DetectorLaser 2
Demodulator
@ f1 , 1
Demodulator
@ f2 , 1
wa
vele
ng
th
time
1
2
wa
vele
ng
th
time
0
2
Vo
lts
time
Vo
lts
time
t0 t1
t0 t1
Amplitude Modulation @ f1
Amplitude Modulation @ f2
f1
f2
Two Laser AM Demodulation
Need 2 lasers for drift cancellation Have both lasers present & use
AM demodulation to electronically separate signals
1 April 2003
FSI Overview 14
Vol
ts
Time
15% mod.
15% mod.
Time
Vol
ts
• Amplitude Modulation on FSI fringe
@ 40 & 80 kHz (now) 0.5 & 1MHz (later)
• FSI fringe stored as amplitude on
Carrier (à la AM radio)• Demodulation reproduces FSI Fringes
• High Pass Filter
Two Laser AM Demodulation
1 April 2003
FSI Overview 15
Results of Demodulation
Demodulation of modulated laser does not effect interferometer signal
Both signals have same frequency !!
1 April 2003
FSI Overview 16
Reference Interferometer Phase Extraction
Reference Interferometer is FSI’s “yard-stick”• Must measure interferometer phase precisely
Uses standard technique of Phase-Stepping
Expansion ofInterferometer
Step1: I(true-1.5)Step2: I(true-0.5)Step3: I(true+0.5)Step4: I(true+1.5)
Carré Algorithm
true
Reference Interferometer mirror moved in 4 equal sized steps
1 April 2003
FSI Overview 17
Raw Data Reconstructed Interferometer Signal
Software Phase Extraction Telecoms laser tunes linearly Extract phase with software “phase-stepping”
1 April 2003
FSI Overview 18
FSI Challenges for LiCAS Telecoms wavelength (1520nm – 1640nm)
• Cheap, high quality tuneable lasers− 120nm (15THz) continuous tuning range− 40nm/s (5THz/s) continuous tuning speed
• Use Erbium Doped Fibre Amplifiers− Output power between 4mW and 125mW− Reduced cost− Increased flexibility and reusability
1 April 2003
FSI Overview 19
Erbium Doped Fibre Amplifiers EDFA are optical power amplifiers
• Used to amplify low power tunable laser• Standard equipment for Telecoms
− but will it work for FSI ?
Decay
Signal~1550nm
Pump980nm
4I15/2
4I11/2
4I13/2
Incoming Single Photon
Outgoing Photons
fluor
esce
nce
Wavelength / nm 1530 1610
Single Telecoms Channel
1 April 2003
FSI Overview 20
EDFA System
1 April 2003
FSI Overview 21
Quill Collimation Refractive
Reflective
Fibre end
RetroreflectorCollimation lens
Retroreflector
Reflective, off-axis paraboloid
Fibre
1 April 2003
FSI Overview 22
Commercial Collimation
1 April 2003
FSI Overview 23
1 April 2003
FSI Overview 24
Custom Collimation
0.8% Return for on-axis Retro
0.04% return for 7mm off axis retro
Fibre Retro
1 April 2003
FSI Overview 25
Reference Interferometer 1
Michelson style interferometer
Long Arm
Short Arm
Beam Splitter
Photodetector
1 April 2003
FSI Overview 26
Reference Interferometer 2
GLI Style interferometer
Retroreflector
Collimatior
1 April 2003
FSI Overview 27
Summary FSI provides abosoluted length measurements
Multiple laser system using amplitude modulation
Software phase extraction
Telecommunications wavelengths: cheap, high quality equipment
Erbium Doped Fibre Amplifiers provide scalable power output
Collimation optics used for longest distance
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