Sebastian Steinlechner for the Glasgow Sagnac Speed Meter Team

LVC Meeting Pasadena, March 2015

LIGO-DCC: G1500358

Status of the Glasgow Sagnac Speed Meter Experiment

What is the Sagnac Speed Meter Experiment?

The Glasgow Sagnac Speed Meter experiment is an ERC funded project with

three major goals:

1. Create an ultra-low noise speed meter testbed which is dominated by radiation

pressure noise

2. Demonstrate the back-action noise cancellation of the Sagnac topology

3. Explore speed meter technology for future GW detectors, such as ET

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How will we reach these goals?

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• Show that Sagnac

configuration can beat the

equivalent Michelson

configuration

• Need low-mass optics and

high laser powers so that

Michelson would be

backaction-noise limited

• Aim for 2-3x better

sensitivity between 100Hz

and 1kHz

• Assume Michelson is

understood well enough

– Won’t actually build it

– Go straight for Sagnac

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Conceptual approach

• In-vacuum operation, passive

multi-stage seismic pre-isolation

• Triangular arm cavities with

monolithically suspended mirrors

• One gram ITMs, 100g ETMs

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Target displacement sensitivity:

better than 10-18 m/√Hz at 1kHz

• Approx. 2.8m cavity round trip length,

20ppm – 30ppm loss per round trip

• Approx. 1kW of intra-cavity power

• Large laser beam spots to reduce

coating Brownian thermal noise

• In vacuum suspended balanced

homodyne detector

ITM ETMs

BHD

Work ongoing on all fronts

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Quantum-noise modelling

• Model influence of imperfections on

QRPN reduction

• Analytical model based on two-photon

transfer matrix S. Danilishin et al.: Quantum noise of non-ideal

Sagnac speed meter interferometer with

asymmetries, arXiv:1412.0931, accepted by NJP

• Asymmetries cause Michelson-like

behaviour, reintroducing back-action

noise

• Can now simulate and reproduce the QN

behaviour in Finesse and Optickle models

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Results from quantum-noise modelling

• Lossless Sagnac has 1/𝑓

slope at low frequencies

• Even small loss values

introduce Michelson-type

1/𝑓2 slope again

• Important quantity here is

𝑇𝐿𝑜𝑠𝑠/𝑇𝐼𝑇𝑀

• We will have 𝑇𝐼𝑇𝑀 ≈700ppm, so can’t afford

much more than

𝑇𝐿𝑜𝑠𝑠 ≈ 30ppm

• For Einstein Telescope,

𝑇𝐼𝑇𝑀 will be on the few-

percent level, makes loss

requirements much more

lenient

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Reducing arm-cavity finesse?

• It’s the small arm-cavity bandwidth that makes our experiment

susceptible to small loss

• Could we reduce ITM reflectivity and “just use more laser power”?

• Turns out there is an optimal ratio of power to cavity linewidth

• Decreasing linewidth by x10 needs x104 more laser power

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More results from quantum-noise modelling

• Beam-splitter

imbalance again

gives Michelson-

like QN

• This time, 1/𝑓

slope from

decreasing Sagnac

signal transfer

combines with

1/𝑓2 RPN curve to

give 1/𝑓3

• Need beam-

splitter balanced

to within 0.1%

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Bridge installation

• Bridge structure on top of seismic

isolation stack rigidly connects

breadboards inside the two vacuum

tanks for LF stability

• Filled with Silastic rubber compound to

dampen resonances

• Test assembly worked, but…

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Cleanliness issues

• RGA spectrum shows lots of long-chained hydrocarbons

• Probably residue from cutting fluid? (Anyone good at analysing RGA data?)

• Breadboards will get baked, hopefully removing the issue

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Work on suspensions

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Work on suspensions

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Auxiliary suspensions

• Input beam

steering

• Small Sagnac

• Double pendulum

• No vertical stage

• Compact design

• Coil actuation on

upper mass

Work on suspensions

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ETM suspensions

• 100g mirror

mass

• AEI prototype

design

• Triple

pendulum

• Monolithic last

stage

• Fast ESD

actuation

Work on suspensions

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1g suspensions

• Similar to 100g

suspensions, but

scaled down

• Work in progress ?

Parts for auxiliary suspensions arrived

From design to reality

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Auxiliary suspensions assembly

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One-gram suspensions

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• Extensive modelling underway

• Investigating and optimising parameters such as

– Mirror size and geometry

– Suspension options (number of fibres, attachment points)

– Fibre diameter and length

• Identified possible parameter

set giving us 100Hz to 1kHz

window

Meet the Scientists

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Daniela Pascucci

Stefan Danilishin

Sebastian Steinlechner

Stefan Hild

Andreas Gläfke

Christian Gräf

Jennifer Wright

Alasdair Houston

Jan-Simon Hennig

Russell Jones

Sean Leavey

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Thanks for listening!