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.