katrin dahl for the aei 10 m prototype team

Katrin Dahl for the AEI 10 m Prototype teamSeptember 2009 –AEI seminar

Suspension Platform Interferometer for the AEI 10 m Prototype Interferometer

Introductory talk

AEI 10m Prototype

2

Outline

1. AEI 10 m Prototype Interferometer– > Why is an SPI needed?

2. Relative distance measurement experiments

3. Experimental setup design

4. Test setup

5. Future plans

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The AEI 10 m Prototype

Goals:

• Train people for GEO600

• Prove new techniques (PSL, digital control system, Khalili cavity…)

• Provide ultra low displacement noise testing environment– To probe at (and later go beyond) the SQL– Entanglement of macroscopic test masses– For geodesy/LISA related experiments – ...

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Noise spectrum

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Vacuum system

Tubes:1.5 m diameter

Volume ca. 100 m3 22 t stainless steel

Tanks:3 m diameter, 3.4 m tall

Tank centers separated by 11.65m

Roughing: One 170 l/s screw pump Main pumps: Two 2000 l/s turbo-molecular pumps Backing and differential pumping: Two scroll pumps

10-6 mbar after about 12 hours , 10-7 mbar after about 2 weeks of pumping

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Seismic Attenuation System

• One SAS per vacuum tank, optical table goes on top of SAS

• Improved version of HAM-SAS

• Resonance frequency around 0.1 Hz

• Up to 80 dB attenuationin both vertical and

horizontal directions

• Angstrom residual motion above 1 Hz

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Why a Suspension Platform Interferometer?

• Ease lock acquisition of cavities by reducing residual test mass motion

• Reduction of burden to actuators on the mirrors

• Testbed for GRACE follow-on and LISA related experiments Sets requirements on SPI

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Horizontal table actuation

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Vertical table actuation

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Vertical table actuation

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A little bit of history

LIGO-T870001-00-R

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Suspension Point Interferometer

• monitors differential motion of the suspension points of input and end test masses

LIGO-T070209-00-Z

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Suspension Point Interferometer

LIGO-T070209-00-Z

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Digital Interferometry

LIGO-T080139-00-I

Advantages:

• no specific lock point• continuous sensing• track the position of mirrors over many µm

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Stabilised metrology testbed

K Numata, J Camp, Proc. of SPIE Vol. 6265

around 1 m

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Stabilised metrology testbed

K Numata, J Camp, Proc. of SPIE Vol. 6265

Control bandwidth 10 Hz

Yaw angle motion of 20 nrad/sqrt(Hz) at 10 mHz leading to about 50 times worse result for only one controlled degree of freedom

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THE AEI SPI

• Requirements:– No specific lock point– Control bandwidth 100 Hz– 100 pm/sqrt(Hz) and 10 nrad/sqrt(Hz) @ 10 mHz

• Heterodyne Mach-Zehnder interferometry– Suits our needs best– In-house knowledge ....Thanks LTP/LISA folks!

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Heterodyne Mach-Zehnder IFO

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Optical layout

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Measurement bench

• Beam height 45 mm• Overall height below 65 mm

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Phase determination

Phase is extracted from heterodyne signal by use of an hardware Phasemeter based on FPGA chips

1. Preamplifier and A/D conversion– Photocurrent converted to voltage– Digitising signals– results in time series

2. Single bin discrete Fourier transform – Fourier transform at only one frequency– complex amplitude of PD signal at fhet

3. Signal combination of each QPD quadrant leads to phase, DC, Differential Wavefront Sensing (DWS) and contrast information

Illustration of DWS

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Choice of parameters

• Due to the arm length mismatch (20 m optical path length) a highly stabilised laser is necessary:

• 280 Hz/sqrt(Hz) @ 10 mHz for green light (532 nm) or• 140 Hz/sqrt(Hz) @ 10 mHz for IR light (1064 nm)• decision made for 1064 nm

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Iodine stabilised Nd:YAG laser

Michael Tröbs

outputpower: 1 W

Stabilisation via Modulation Transfer Spectroscopy

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• According to the arm length mismatch (20 m optical path length) a highly stabilised laser is necessary:


• Control bandwidth 100 Hz heterodyne frequency around 20 kHz new phasemeter interface needed

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Phasemeter Interface

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• According to the arm length mismatch (20 m optical path length) a highly stabilised laser is necessary:


• Control bandwidth 100 Hz heterodyne frequency around 20 kHz

• Thermal drifts requires components to be monolithically bonded to plate with low CTE (ClearCeram, CTE=0.4*10-7/K)

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UV curing epoxy

• Advantage: almost infinite alignment time• Optocast 3553-LV-UTF

– CTE = 55 PPM/°C, viscosity @ 25 °C = 500 cps– cps = centipoise, water = 1 cps, castrol oil = 1,000 cps, honey = 10,000 cps,

ketchup = 50,000 cps

• Disadvantage: layer thickness is not reproducible, e.g. 10 µm, 98 µm, 79 µm, 70 µm, 37 µm misalignment in pitch

• stick to hydroxide-catalysis bonding technique

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Optical layout

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Expected transversal signalsco

ntra

stDC

Phas

e di

ffere

nce

[rad

]DW

S [r

ad]

-2 -1 0 1 2Transveral displacement of MW1 or MS1 [mm]




Red curve: PDCW1Black curve: PDCS1

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Expected longitudinal signalsco

ntra

stDC

Phas

e di

ffere

nce

[rad

]DW

S [r

ad]

-2 -1 0 1 2Longitudinal displacement of MW1 or MS1 [mm]





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Expected rotational signalsco

ntra

stDC

Phas

e di

ffere

nce

[rad

]DW

S [r

ad]

-10 -5 0 5 10Rotation of MW1 or MS1 [mdeg]





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Test setup

• Use of vacuum compatible components (free of grease)

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Longitudinal displacement

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Pitch

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Yaw

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Blind test

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Blind test

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Next steps

• Stabilisation loops– Amplitude stabilisation @ 20 kHz– Optical pathlenght difference stabilisation

• Bond optics– Build calibrated QPD

• Use CDS via phasemeter interface• Install final setup inside vacuum envelope• Calibrate signals• Table actuation• Reach design sensitivity

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THE END

katrin dahl for the aei 10 m prototype team

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