status of the glasgow sagnac speed meter · pdf filesebastian steinlechner for the glasgow...

Sebastian Steinlechner for the Glasgow Speed Meter Team LVC Meeting Nice, 2014 LIGO-DCC: G1400228v1

Status of the Glasgow Sagnac Speed Meter Experiment

Why Speed Meters?

•  Second generation of GW detectors will be limited by radiation-pressure noise at low frequencies

•  RPN is back-action noise; a measurement of the test-mass position disturbs the test mass

•  This is because current GW detectors are position meters, and [𝑥 (𝑡), 𝑥 (𝑡↑′ )]≠0

•  However, for momentum/speed 𝑝 (𝑡), [𝑝 (𝑡), 𝑝 (𝑡↑′ )]=0�→ speed meters are back-action noise free

LVC Nice, 2014 S. Steinlechner 2

History of Speed Meters

•  Speed meter concept proposed by Braginsky & Khalili, 1990

•  Idea based around weakly coupled resonators, transforming a position signal in one resonator into a velocity signal in the other

•  Implementation ideas for actual interferometers appeared around the year 2000

•  E.g. sloshing cavity approach by Purdue & Chen (2002)


Sagnac IFO is a Speed Meter

•  Chen (2003): Sagnac IFO is automatically a speed meter •  Sagnac interferometer roundtrip phase:

𝜙↓𝑐𝑤 ∝𝑥↓𝑁 (𝑡)+ 𝑥↓𝐸 (𝑡+𝜏) 𝜙↓𝑐𝑐𝑤 ∝𝑥↓𝐸 (𝑡)+ 𝑥↓𝑁 (𝑡+𝜏) Differential phase is proportional to test-mass speed:

Δ𝜙= 𝑥↓𝑁 (𝑡)− 𝑥↓𝑁 (𝑡+𝜏)−[𝑥↓𝐸 (𝑡)− 𝑥↓𝐸 (𝑡+𝜏)] �≈𝜏(𝑥 ↓𝐸 (𝑡)− 𝑥 ↓𝑁 (𝑡))


Proof-of-Principle Speed Meter Required

•  Unfortunately, work on Sagnac interferometers (Stanford, ANU) stopped before its QND properties were discovered

•  All upcoming detectors are position meters, and all will be limited by RPN •  Investigation of the Sagnac speed meter topology urgently needed!


Goals of the ERC Speed Meter Project

Glasgow Speed Meter project 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


Design Principles of the Speed Meter Test Bed


•  In-vacuum setup, suspended optics •  High laser power acting on low-mass mirrors •  Use large beam spots to reduce coating thermal noise » Michelson IFO (position meter) would be strongly dominated by RPN

Design Performance of Sagnac Topology


•  Sagnac topology gives about 5x higher sensitivity between 100Hz and 1kHz

•  note that it is not required to reach or surpass the SQL, and it is also not one of our goals

Challenging Parameters!

•  Arm cavities with finesse of 10000, for 1kW of circulating power, 2.4m roundtrip •  Less than 10-20ppm loss per round-trip •  1.6g mirrors, monolithic fused silica suspensions •  Sophisticated seismic isolation + double pendulums with one vertical stage •  Beam radius ~1mm at cavity mirrors •  Zero-area configuration for insensitivity against (earth-)rotation •  Balanced Homodyne Detection (in vacuum, suspended, audio-band frequencies) •  Target sensitivity better than 10-18m/√Hz at 1kHz


Updated Quantum-Noise Model

•  Updated quantum-noise model in collaboration with Stefan Danilishin

•  Matrix-based MatLab code

•  Includes loss & imbalance at beam splitter

•  Loss makes RPN cancellation imperfect, 1/f2 slope reintroduced


Updated Quantum-Noise Model

•  Can simulate asymmetric loss in arm cavities •  Turns out to be quite important for our experiment!


Preliminary Optical Layout

•  OptoCad model of speed meter layout reached version 1.0


Closer Look at the Near Tank

•  >15 suspended optics! •  Simple double-stage

pendulums for most of these

•  Large beam splitter for good separation of multiply reflected beams


Suspension Design Started

•  Design for very compact suspensions for the auxiliary/input optics is on its way

•  Using 1” optics inside 5mm steel ring for extra weight and just enough space for the steering magnets


Images: Russell Jones

Balanced Homodyne Detection

•  Sagnac output signal will appear in the phase quadrature

•  Balanced homodyne detection needed •  Table-top detector set up

–  Gain experience at audio-band frequencies

–  Determine noise requirements for in-vacuum, suspended balanced homodyne detection


Mechanical Construction and Seismic Isolation


•  Two vacuum tanks, 1m diameter each

•  Seismic isolation stacks in each tank, consisting of four steel plates (60kg each) sitting on fluorel springs

•  Breadboards sit on a bone-shaped steel structure

•  Steel bridge for further stability

Image: Russell Jones

Installation of Seismic Isolation Stacks


Installation of the seismic isolation stacks was recently completed

Simulated Performance of Seismic Isolation


•  Measured seismic noise in lab with S13 seismometers •  Assuming a crosscoupling of 1:100 from vertical to horizontal noise,

the four rubber stages + 1 vertical stage in the pendulums should be enough to reach the target sensitivity

Outlook

•  As GW detectors become limited by quantum back-action noise, measuring speed instead of position is the way to go

•  Unfortunately, experiments on Sagnac IFOs stopped before its speed meter properties were known

•  In Glasgow, we’re now picking up that work: –  Speed meter test bed:

•  Proof-of-principle experiment •  QND demonstration

–  12m Sagnac IFO: •  Four-mirror cavities •  Control and readout investigation

•  Ultimately, we want to present a design for a full-scale GW detector using Sagnac technology


Summary

•  We want to demonstrate the QND potential of the speed meter topology

•  Exciting challenges, and lots to learn •  Work continuing on all fronts: theory,

simulation, hardware installation

•  We’re always looking for more helping hands, just get in touch with us!


PhD position available, starting September this year! (EU citizens only) contact Stefan Hild ([email protected])

http://www.speed-meter.eu

status of the glasgow sagnac speed meter · pdf filesebastian steinlechner for the glasgow...

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