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

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FSI Overview 4

FSI: Length MeasurementGLI

Ref

GLI

Ref

GradientD

L

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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

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FSI Overview 6

FSI: Thermal Drift CancellationGLI

Ref

True Gradient

Measured G

radient with

Laser Tuning U

p

Measured Gradient with Laser Tuning Down

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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

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FSI Overview 8

FSI: ATLAS Implementation

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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.

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FSI Overview 10

FSI: ATLAS Resolution

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FSI Overview 11

1m

FSI: ATLAS Resolution

Stage is kept stationary

RMS 3D Scatter

< 1 m

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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

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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

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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

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FSI Overview 15

Results of Demodulation

Demodulation of modulated laser does not effect interferometer signal

Both signals have same frequency !!

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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

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FSI Overview 17

Raw Data Reconstructed Interferometer Signal

Software Phase Extraction Telecoms laser tunes linearly Extract phase with software “phase-stepping”

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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

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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

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FSI Overview 20

EDFA System

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FSI Overview 21

Quill Collimation Refractive

Reflective

Fibre end

RetroreflectorCollimation lens

Retroreflector

Reflective, off-axis paraboloid

Fibre

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FSI Overview 22

Commercial Collimation

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FSI Overview 23

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FSI Overview 24

Custom Collimation

0.8% Return for on-axis Retro

0.04% return for 7mm off axis retro

Fibre Retro

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FSI Overview 25

Reference Interferometer 1

Michelson style interferometer

Long Arm

Short Arm

Beam Splitter

Photodetector

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FSI Overview 26

Reference Interferometer 2

GLI Style interferometer

Retroreflector

Collimatior

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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