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Monitored using Geodetic Grids of length measurement interferometers Approx 800 Grid Line...
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Transcript of Monitored using Geodetic Grids of length measurement interferometers Approx 800 Grid Line...
significant signal degradationin the presence of
measurements using FSIPrecise distance
P. A. Coe, A. Mitra, D. F. Howell & R. B. Nickerson
ATLAS Group Particle Physics University of Oxford
O verviewW hat is Frequency Scanning Interferom etry?
H ow is FS I used to m easure d istance?
W hat effects degrade the signals?
H ow can the effects be dealt w ith?
Outlook and future work.
W hy is FS I needed inside a partic le tracker?
emi onductor rackerS c T
ATLAS
Monitored using GeodeticGrids of length measurementinterferometers
Approx 800Grid Line Interferometers
Lasers
ReferenceI nterferom eters
SplitterTree
M ainD elivery
F ibre
Beam -S plitter
RadHard
Fibres
S CT S TRUCTURE
J ewel J ewelGLIPhotodetector
Retroreflector
To other GLI s
Interferometer design means returnsAre only -70 dB to -90 dB!
Reference Phase Measurem ent
Carre algorithm (110 steps)O
M odulate Long R eference Interferom eterusing PZT m ounted m irror
PZT voltage
PZT
12.5 m mDiam eter Mirror
LONG REFERENCE I NTERFEROMETER
25 m m Diam eter
Mirror
Non- polarising beam - splitter cube
PHOTODI ODEwith PI NHOLE
980 1000 1020 1040 1060 1080
-4
-3
-2
-1
0
1
2
3
4 E
xtr
act
ed
Ph
ase
/ r
ad
ian
s
Phase stepping cycle
(S ingle Laser)
Phase unwrapping
-800 -600
0
500
1000
1500
2000
2500
Inte
nsity
Laser Freq Shift / a .u.
To measure GLI phase shift…
(Reference phase)
How is sinusoidal fit chosen?
Minimise Fit Quality
SearchFor minimausing MINUIT
Think modulo 2
Signal degrading effects in ATLAS
• Low signals – 800 hungry interferometers to be fed
• Potential for reflections off many surfaces inside ATLAS
• Potential for mechanical vibrations (& turbulence)
Low signal power
Low signal power reduces depth of minimum in fit quality
In the presence of spurious reflections…
…other minimaappear
Measurement precision unaffected by spurious reflections
4 spuriousReflectors added
Vibration
More vibration….. Fit collapses!!
To Stop the Fit Collapsing
Redefine the WEIGHT inthe fit quality to penalise decreasein fringe amplitude
New WEIGHT = Fringe amplitude 1/2
Is there a problem at 400 nm = 0.5?or was it just because two subscans are not enough?
Increase peak to peak vibration amplitude to 1.8 m (>2)and use 6 subscans
At 70 Hz
Looks very promising…
CONCLUSIONS
• Initial investigations into the effects of low SNR, spurious reflections and vibrations show that…
• …FSI is a robust technique for precise distance measurements.
• Further work needed to improve linking and investigate data analysis alternatives and their potential benefits, eg FFT.