LIGO Advanced Research and Development
David Shoemaker NSF Annual Review of LIGO 17 November 2003
LIGO Laboratory
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LIGO mission: detect gravitational waves and initiate GW astronomy Commissioning talk shows considerable progress toward initial LIGO planned performance and operation Direct detection of gravitational waves plausible and eagerly awaited How do we move from the sensitivity of initial LIGO to an instrument which regularly makes astrophysical measurements of gravitational waves?
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Advanced LIGO
Requirements for the next detector in the LIGO infrastructure
Should have assured detection of known sources Should be at the limits of reasonable extrapolations of detector physics and technologies Must be a realizable, practical, reliable instrument Should come into existence neither too early nor too late
Advanced LIGO
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Initial and Advanced LIGO
Factor 10 better amplitude sensitivity
(Reach)3 = rate
Factor 4 lower frequency bound NS Binaries: for three interferometers,
Initial LIGO: ~20 Mpc Adv LIGO: ~350 Mpc
BH Binaries:
Initial LIGO: 10 Mo, 100 Mpc Adv LIGO : 50 Mo, z=2
Stochastic background:
Initial LIGO: ~3e-6 Adv LIGO ~3e-9
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Anatomy of the projected Adv LIGO detector performance
10-21
Newtonian background, estimate for LIGO sites
Strain Noise, h(f) /Hz1/2
Seismic `cutoff' at 10 Hz Suspension thermal noise Test mass thermal noise Unified quantum noise dominates at most frequencies for full power, broadband tuning
10 10-22
-22
Initial LIGO
10 10-23
-23
Advanced LIGO
10 10-24
-24
10 Hz
LIGO Laboratory
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1
10 Frequency (Hz)
100 Hz
5
2
1 kHz
10
3
Advanced LIGO's Fabry-Perot Michelson Interferometer is flexible can tailor to what we learn before and after we bring it on line, to the limits of this topology
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Limits to the performance
Two basic challenges:
Sensing the motion of the test masses with the required precision; ideally limited by quantum effects Reducing undesired motion of the test masses which can mask the gravitational wave; intrinsic thermal motion a fundamental limit, seismic noise an obvious difficulty
Many `merely technical' challenges
Defects in the sensing system which give an excess above the quantum noise Control system sensors, dynamic range, actuators, etc. Work hard on these challenges to make system reliable, ease commissioning, improve statistics of noise, availability
LIGO Laboratory
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Sensing for initial LIGO
Shot-noise limited counting statistics of photons (or photodiode current)
Precision improves with (laser power)1/2 until....
1 hc 1 h( f ) = 2 F L 8 Pbs Tifo ( s , f )
2 F 2hPbs Tifo ( s , f ) h( f ) = ML 3c f2
Transfer of momentum from photons to test masses starts to dominate
1/f 2 spectrum (inertia of test masses) Gives `standard quantum limit'
Initial LIGO power recycled interferometer layout
Michelson for sensing strain Fabry-Perot arms to increase interaction time Power recycling mirror to increase circulating power ....still far from standard quantum limit
Power on beamsplitter Pbs = Plaser * Grecycling
Test Mass M Arms of length L Cavity finesse F
Laser
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Sensing for Advanced LIGO
Build on initial LIGO layout
retain Fabry-Perot cavities, power recycling
Increase the laser power to a practical limit to lower shot noise
Laser power require TEM00, stability in frequency and intensity Absorption in optics state-of-the-art substrates and coatings, compensation system to correct for focussing ~180 W input power is the practical optimum for Advanced LIGO Leads to ~0.8 MW in cavities (6cm radius beams, though) Significant motion due to photon pressure quantum limited!
Modify optical layout: Add signal recycling mirror
Gives resonance for signal frequencies can be used to optimize response Couples photon shot noise and backreaction some squeezing of light
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Laser
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Stray forces on test masses
Most Important: Make the interferometer long!
Scaling of thermal noise, seismic, technical Cross-coupling from vertical to horizontal 4km not far from ideal
Thermal noise
kT of noise per mode Coupling to motion according to fluctuation-dissipation theorem Gather the energy into a narrow band via low mechanical losses, place resonances outside of measurement band by choosing the right geometry
Initial LIGO: fused silica substrates, attachments made to limit increases in loss, steel suspension wire Seismic Noise
Due to seismic activity, oceans, winds, and people
Initial LIGO: cascaded lossy oscillators, analog of multipole low-pass filter and now also an active pre-isolator in preparation
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Managing Stray forces in Advanced LIGO
Seismic Isolation: use servo-control techniques and low-noise seismometers to `slave' optics platform to inertial space
Decreases motion in the gravitational-wave band to a negligible level Decreases motion in `controls' band, moving forces away from test mass
Suspension thermal noise: all-silica fiber construction
Intrinsically low-loss material Welded and `contacted' construction also very low loss
Substrate thermal noise: use monolithic Sapphire
High Young's modulus Low mechanical loss (fallback: very low-loss silica)
Optical coating thermal noise: develop low-loss materials and techniques
Area of active development
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Design features
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
QUAD SILICA SUSPENSION
180 W LASER, MODULATION SYSTEM
PRM BS ITM ETM SRM PD
Power Recycling Mirror Beam Splitter Input Test Mass End Test Mass Signal Recycling Mirror Photodiode
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Laser
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
QUAD SILICA SUSPENSION
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Pre-stabilized Laser
Require the maximum power compatible with optical materials
1999 White Paper: 180 W at output of laser, leads to 830 kW in cavities Continue with Nd:YAG, 1064 nm Three approaches studied by LSC collaboration stable/unstable slab oscillator (Adelaide), slab amplifier (Stanford), end-pumped rod oscillator (Laser Zentrum Hannover (LZH)); evaluation concludes that all three look feasible Choose the end-pumped rod oscillator, injection locked to an NPRO 2003: Prototyping well advanced of Slave system has developed 114 W, 87 W single frequency, M2 1.1, polarization 100:1
output
f QR f
NPRO EOM
FI BP FI
f QR
HR@1064 HT@808
f 2f f
YAG / Nd:YAG / YAG 3x 7x40x7
modemaching optics
BP
YAG / Nd:YAG 3x2x6
f
High Power Slave
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20 W Master 13
Pre-stabilized laser
Overall subsystem system design similar to initial LIGO
Frequency stabilization to fixed reference cavity, 10 Hz/Hz1/2 at 10 Hz required (10 Hz/Hz1/2 at 12 Hz seen in initial LIGO) Intensity stabilization to 2x10-9 P/P at 10 Hz required 2003: 1x10-8 at 10 Hz demonstrated
Max Planck Institute, Hannover leading the Pre-stabilized laser development
Close interaction with Laser Zentrum Hannover Experience with GEO-600 laser, reliability, packaging German GEO Group contributing laser to Advanced LIGO
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Input Optics, Modulation
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
QUAD SILICA SUSPENSION
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Input Optics
Provides phase modulation for length, angle control (Pound-Drever-Hall) Stabilizes beam position, frequency with suspended mode-cleaner cavity Matches into main optics (6 cm beam) with suspended telescope 1999 White Paper: Design similar to initial LIGO but 20x higher power
Challenges:
Modulators Faraday Isolators
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Input Optics
University of Florida leading development effort
As for initial LIGO 2003: LIGO Lab developing controls, suspensions (see later...)
2003: Faraday isolator from IAP-Nizhny Novgorod
thermal birefringence compensated Ok to 80 W more powerful test laser to be installed at Livingston for further tests
-20 -25 Isolation Ratio (dB opti cal) -40 -45 -50 -55 0 20 40 Compensa Design ted 60 80 100 -30 -35 Conventional FI
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Laser Power (W)
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Test Masses
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
QUAD SILICA SUSPENSION
200 W LASER, MODULATION SYSTEM
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Test Masses / Core Optics
Absolutely central mechanical and optical element in the detector
830 kW; <1ppm loss; <20ppm scatter 2x108 Q; 40 kg; 32 cm dia
1999 White Paper: Sapphire as test mass/core optic material; development program launched Low mechanical loss, high Young's modulus, high thermal conductivity all desirable attributes of sapphire Fused silica remains a viable fallback option Significant progress in program
Industrial cooperation Characterization by very active LSC working group
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Full-size Advanced LIGO sapphire substrate
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LIGO Laboratory
Sapphire Core Optics
Fabrication of Sapphire: Bulk Homogeneity: requirement met
Sapphire as delivered has 50 nm-rms distortion Goodrich 10 nm-rms compensation polish
Q, millions
Best Measured 250 200 150 100 50 0 14 16 18 20 frequency , kHz 22 24 Qs for Both Sapphires
Full-size Advanced LIGO boules grown (Crystal Systems); 31.4 x 13 cm
Polishing technology:
CSIRO has polished a 15 cm diam sapphire piece: 1.0 nm-rms uniformity over central 120 mm (requirement is 0.75 nm)
2003: Mechanical losses: requirement met
Highest Q measured at <250 million Program to identify possible anisotropies in losses well underway: finite-element modeling with Q measurements of many modes
2003: Bulk Absorption:
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Measured; uniformity needs work LIGO Laboratory Average level ~60 ppm, 40 ppm desired
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Backup: Fused Silica
Alternative test mass material Familiar; fabrication, polishing, coating processes well refined Disadvantages:
Overall thermal noise may be higher noise Thermal signature not as well suited to Adv LIGO Lower Young's modulus leads to higher coating thermal noise More expensive (!)
10-22 Sapphire thermoelastic noise Silica brownian noise 10-24
Strain, Hz-1/2
10-23
101
102 Frequency, Hz
103
Development program to reduce mechanical losses, understand frequency dependence
Annealing proven on small samples, needs larger sample tests and optical post-metrology
strong backup reduction in sensitivity would be minimal for current parameters
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Test Mass downselect
Remaining tests/models:
Absorption in second sample of sapphire Scattering tests (inclusions) Q tests of other sapphire samples (with polished barrel) Annealing of small samples of both sapphire (absorption) and silica (mechanical losses) Models of interferometer performance with absorption maps
April 2004 for evaluation
Set to match suspension development plan Could lead to requiring further actions Believe we are close to adopting sapphire
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Mirror coatings
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
COATINGS
QUAD SILICA SUSPENSION
200 W LASER, MODULATION SYSTEM
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Test Mass Coatings
BNS Range vs Y coat = 10 Coating loss for = 5 5*10-5 -5
200 200
B in a r y N e u t r o n S t a r In s p ir a l D is t a n c e ( M p c )
Binary Inspiral Range, Mpc
Optical absorption (~0.5 ppm) requirements met by (good) conventional coatings R&D mid-2000: Thermal noise due to coating mechanical loss recognized; LSC program put in motion to develop low-loss coatings
Series of coating runs materials, thickness, annealing, vendors Measurements on a variety of samples
210
190
Sapphire, Q 200, 60 *106
180 180 170
160 160 150
Silica 200 million Q
140 140 130
Ta2O5 identified as principal source of loss
Typical good coating =3-5e-4
Silica 130 Q Silica, million Q Sapphire 200 million Q 200, 130 *106
Sapphire 60 million Q
120 120 10 10
100 100
1000 1000
Test coatings show somewhat reduced loss
Alumina/Tantala Doped Silica/Tantala Best (one sample) to date: =8e-5; 2e-4 reproducible
Coating Young's modulus, GPa
Coating Young's modulus (GPa)
Need ~5x reduction in loss to reduce current ~20% compromise to a negligible level
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Direct measurement
Thermal Noise Interferometer (TNI) designed to measure coating and substrate thermal noise Presently set up with fused silica substrates with conventional coatings 2003: Recent results appear to show confirmation of models for anticipated coating losses; similar confirmation from Japanese experiment Sapphire substrates for measurement of thermoelastic noise ready
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Thermal Compensation
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
COATINGS
QUAD SILICA SUSPENSION
200 W LASER, MODULATION SYSTEM
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Active Thermal Compensation
1999 White Paper: Need recognized, concept laid out Removes excess `focus' due to absorption in coating, substrate Allows optics to be used at all input powers Initial R&D successfully completed
Quasi-static ring-shaped additional heating Scan to complement irregular absorption Optical path distortion
ITM Compensation Plates
PRM SRM
ITM
Shielded ring compensator test
Sophisticated thermal model (`Melody') developed to calculate needs and solution 2003: Gingin facility (ACIGA) readying tests with Lab suspensions, optics 2003: Application to initial LIGO in preparation
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20 nm
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5
mm
10
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Seismic Isolation
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
COATINGS
QUAD SILICA SUSPENSION
200 W LASER, MODULATION SYSTEM
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Isolation: Requirements
1999 White Paper: Render seismic noise a negligible limitation to GW searches
Newtonian background will dominate for frequencies less than ~15 Hz Suspension and isolation contribute to attenuation
h(f) / Hz1/2 Optical noise Int. thermal Susp. thermal Total noise 10
-22
10
-23
1999 White Paper: Reduce or eliminate actuation on test masses
Actuation source of direct noise, also increases thermal noise Acquisition challenge greatly reduced In-lock (detection mode) control system challenge is also reduced
10
-24
10
-25
10
0
10
1
10 f / Hz
2
10
3
Seismic contribution
Newtonian background
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Isolation: multi-stage solution
Choose an active approach:
high-gain servo systems, two stages of 6 degree-of-freedom each External hydraulic actuator pre-isolator Allows extensive tuning of system after installation, operational modes Dynamics decoupled from suspension systems
Lead at LSU 2003: External pre-isolator Prototypes in test and evaluation at MIT
early deployment at Livingston in order to reduce the cultural noise for initial LIGO System performance meets initial needs, exceeds Advanced LIGO requirements
2003: Stanford Engineering Test Facility Prototype fabricated, in test
First measurements indicate excellent actuatorstructure alignment, rigidity
2003: Vendor chosen for final Prototypes
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Suspension
40 KG SAPPHIRE TEST MASSES
ACTIVE ISOLATION
COATINGS
QUAD SILICA SUSPENSION
200 W LASER, MODULATION SYSTEM
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Suspensions: Test Mass Quads
1999 White Paper: Adopt GEO600 monolithic suspension assembly Requirements:
minimize suspension thermal noise Complement seismic isolation Provide actuation hierarchy
Quadruple pendulum design chosen
Fused silica fibers, bonded to test mass Leaf springs (VIRGO origin) for vertical compliance
Success of GEO600 a significant comfort
All fused silica suspensions installed Ultimately tests to ~12x Adv LIGO at 40 Hz
2003: PPARC funding approved!
significant financial, technical contribution; quad suspensions, electronics, and some sapphire substrates
U Glasgow, Birmingham, Rutherford Quad lead in UK LIGO Laboratory
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