Status of the Glasgow Sagnac Speed Meter · PDF fileSebastian Steinlechner for the Glasgow...
Transcript of Status of the Glasgow Sagnac Speed Meter · PDF fileSebastian Steinlechner for the Glasgow...
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Sebastian Steinlechner for the Glasgow Speed Meter Team LVC Meeting Nice, 2014 LIGO-DCC: G1400228v1
Status of the Glasgow Sagnac Speed Meter Experiment
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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
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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)
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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))
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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!
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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
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Design Principles of the Speed Meter Test Bed
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• 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
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Design Performance of Sagnac Topology
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• 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
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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
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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
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Updated Quantum-Noise Model
• Can simulate asymmetric loss in arm cavities • Turns out to be quite important for our experiment!
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Preliminary Optical Layout
• OptoCad model of speed meter layout reached version 1.0
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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
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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
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Images: Russell Jones
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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
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Mechanical Construction and Seismic Isolation
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• 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
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Installation of Seismic Isolation Stacks
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Installation of the seismic isolation stacks was recently completed
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Simulated Performance of Seismic Isolation
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• 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
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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
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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!
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PhD position available, starting September this year! (EU citizens only) contact Stefan Hild ([email protected])
http://www.speed-meter.eu