LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo)
Interferometer design of LCGTInterferometer design of LCGT
Masaki Ando Department of Physics, University of Tokyo
the LCGT collaboration
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 2
OutlineOutline
1. LCGT interferometer overview
2. LCGT optical layout
3. Main interferometer design
4. Input and output optics
5. Observation system
6. Summary
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 3
Interferometer overview
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 4
Overview (1) Overview (1)
Interferometer overview
Two nearby interferometers As independent as possible Center rooms: separated 3km vacuum tube: common
Main interferometerBroad band RSE with power recyclingArm cavity Finesse : 1550Power Recycling Gain : 11Signal band Gain : 15Stored Power : 771kWSignal band : 230Hz
Input opticsTwo mode cleaners Baseline lengths : 10m and 180mModulators for IFO controlLaser stabilization
Output opticsOutput mode cleaner Round trip length : 70mmMultiple InGaAs photo diodes
Observation system
Monitor and organize the whole detector system Automatic lock-acquisition Automatic interferometer adjustment Monitor and diagnosis
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 5
Overview (2) Overview (2)
LCGT interferometerMain IFO : 3km IFO Input optics : Two MCs, Modulators, MMTOutput optics: OMC, Photo detectors
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 6
Interferometer layout
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 7
Interferometer layout (1) Interferometer layout (1)
Layout of two interferometers
Center room for IFO-1
Center room for IFO-2
3km arm
3km
arm
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 8
Interferometer layout (2) Interferometer layout (2)
Optical layout of two interferometers
Two interferometers are placed in separated center rooms make them as independent as possible
Should be careful on Cross-coupling between interferometers Scattered-light from vacuum tube
Combined at steering tanksVacuum tube diameter: 1000 mmBeam separation: 600 mmEach main mirror is placed in an independent vacuum tanksMirrors are separated ~30m
3km vacuum tubes are used commonly by two IFOs
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 9
Interferometer layout (3) Interferometer layout (3)
Cross-coupling estimation
Scattered-light noise
D
Scatter
D
Scatter
Scatter
Refraction
1ppm scattering Cross-coupling < 10-10
0 10 20 30 40 50 6010–120
10–100
10–80
10–60
10–40
10–20
100
Be
am
–p
ow
er
Ra
tio
Separation [cm]
Direct beam
Scattered lightCase1 Refraction- Scattering
Case2 Scattering- Scattering
Scattered-light noise: Scattered-light on mirror surface reflected by inner surface of the vacuum tube Vacuum tube is shaken by seismic motion Re-enter to the main beam by scatteringReduced by baffles in the vacuum
tube
Vacuum tube
One-reflection case
Two-reflection case
Mirror Mirror
Baffle
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 10
Interferometer layout (3) Interferometer layout (3)
Baffle for LCGTSpecification of LCGT
Tube diameter 1000mmMirror diameter 250mmMirror distance 600mmMirror offset 340mmSeismic motion 1 x 10-11m/Hz1/2 @30Hz
Estimated scattered light noise4.2 x 10-21m/Hz1/2 @30Hz (Safety factor=7)
1/10
45
(for one mirror)
Integration
Each reflection
Baffle designNumber: 45x2 for one armHeight: 35~50mmSurface: DLC coatings
Diamond-Like Carbon (DLC) coatingsSurface roughness: Ra=20nmOutgassing: lower than SS316 with bakingReflectivity: 5% for p-polarization at 45deg.
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 11
Main interferometer
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 12
Main interferometer (1) Main interferometer (1)
Optical readout noise
100 101 102 103 104
10–24
10–22
10–20
10–18
No
ise
lev
el
[1
/Hz1/
2 ]
Frequency [Hz]
Shot noise
Mirror thermal
Seism
ic no
ise
Suspension thermal
Radiation pressure
TAMAnoise level
Parm=771kW fsig=230 HzLCGT noise budget
LCGT sensitivity : Mostly limited by quantum noises (Radiation pressure noise and Shot noise)
Design the interferometer optical parameters
Shot noise Phase fluctuation of light Proportional to P -1/2
Radiation pressure noise Amplitude fluctuation of light Proportional to P 1/2
Noise level depends on Laser power in IFO Signal band of IFO
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 13
Main interferometer (2) Main interferometer (2)
Interferometer optical configuration
Ideally, possible to realize same power and signal BW with any config.
Power : cavity finesse, PRMSignal BW : cavity finesse, SRM
Realistic constraint Loss in optics and interference Simplicity of control system Thermal problem in optics
Largepower on BS
LargeFinesseof cavities
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 14
Main interferometer (3) Main interferometer (3)
LCGT optical configuration
Resonant-sideband extraction with power recycling
Front mirror
Front mirror
Power recycling mirror
Fabry-Perot cavity
Fabry-P
ero
t cavity
End mirror End mirror
Photo detector
Beam splitter
Signal-extraction mirror
High-finesse arm cavitiesPRM between BS and laser sourceSEM at the detection port
Power recycling PRM+FM Increase effective finesse Increase power in cavities by Power-recycling gain (PRG) Resonant-sideband extraction SEM+FM Decrease effective finesse for signals Increase signal band by Signal-band gain (SBG)
Finesse x 2/ = Light bounce number in a FP cavity
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 15
Main interferometer (4) Main interferometer (4)
Merit of RSEHigh-finesse cavity and moderate PRGEasier to realize high power in cavities
Smaller transmission light in optics
Flexible optimization for GW sources Independent adjustment of
power in cavities and signal bandNarrow-band observation (optional)
Main reason for LCGT
Absorption in sapphire substratesHeat absorption :
20ppm/cm x 15 cm = 300 ppmCooling power : 1W for each mirror
Laser power on BS should be less than ~1kW (safety factor 3)
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 16
104 105 106101
102
103
Sig
na
l b
an
dw
idth
[H
z]
Power on end mirrors [W]
50
50
75
75
75
100
150
175 200
100 15
0
100
150
175
200
150
175
125
125
125
122
246
991
495
2488
FPMI (without PR, RSE)
RFP
MI (m
aximum
PR
G)
225
Main interferometer (5) Main interferometer (5)
Optical parameter selection
Optimize for 1.4Msolar NS inspiralRealistic parameters
Observable range : 185Mpc SNR = 10Single interferometerOptimal direction and polarization
Arm cavity Finesse : 1550Power Recycling Gain : 11Signal Band Gain : 15Stored Power : 771kWSignal band : 230Hz
Observable range [Mpc]
Calculate observable range as a function of Light power in cavities Signal bandwidth
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 17
Interferometer control (1) Interferometer control (1)
Interferometer length control
L+ L - l p l - l s
VB 1 0 -4x10-3 0 3x10-3
VD0 1 0 1x10-3 0
VP2x10-3 0 1 0 0.7
VD0 1x10-3 0 1 0
VD-3x10-4 0 -1.3 0 1
5degrees of freedom to be controlled Common arm length: L+ = L1 +
L2
Differential arm length: L+ = L1 - L2
PRC length: l P = l 1 + l 2
Michelson length: l - = l 1 + l 2
SEC length: l s = l 1 + l 2
Signal extraction
Two modulations on the input light Phase modulation Amplitude modulationDemodulate the output of photo-detectors at three signal port (Dark, Bright, Pick-off)
Independent signals
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 18
Interferometer control (2)Interferometer control (2)
Interferometer length control (contd.)
Signal feedback
Common arm length: L+ Input optics, mirrors (~0.1Hz)Differential arm length: L- Mirrors (~1kHz)PRC length: l P PRM (~1kHz)Michelson length: l - BS (~10Hz)SEC length: l s SEM (~1kHz)
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 19
Interferometer control (3) Interferometer control (3)
Lock acquisition
Linear error signals are obtained only around the operational point of IFO
How to acquire the operational condition at first ‘Lock acquisition’ problem
Experimental results - successful lock acquisition and operationHigh finesse cavity:
20m prototype interferometer at Kamioka Finesse : 25,000 comparable light-storage time with LCGTPower recycling on FPMI: Prototype IFOs (3m at UT, 20m at NAO, 40m at Caltech) Current interferometric detectors (LIGO, VIRGO, TAMA) >10 PRG has been realizedRSE (or signal recycling): Prototype IFOs (4m at NAO, 10m at Glasgow, 40m at Caltech)LCGT environment
Kamioka site : very small seismic vibration (10-2-10-3 of the TAMA site) Low-frequency vibration isolation system (SAS)Suspension-point interferometer
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 20
Input and output optics
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 21
Input and output optics (1) Input and output optics (1)
Input optics1st MC Baseline length : 10m Finesse : 1700 Reject RF noises
2nd MC Baseline length : 180m Finesse: 1000 Reject wavefront distortions caused by modulators, etc. Reject beam jitters Transmits RF sidebands used for IFO control
Modulators Mach-Zehnder interferometer for phase and amplitude modulation Mode-matching telescopes (MMT) Change mode-profile to match IFOs Reflective-type to avoid scattered light
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 22
Input and output optics (2) Input and output optics (2)
Frequency stabilization
Multi-stage stabilization
L+ signal 2nd MC2nd MC 1st MC1st MC Laser source
Stability requirement10-8 Hz/Hz1/2 at 6Hz (Safety factor:10, CMRR: 100)
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 23
Input and output optics (3) Input and output optics (3)
Output optics
Output Mode Cleaner (OMC) Reject junk light before photo detector Suppression ratio : >16dB Short fixed-mirror cavity Round-trip length : 70mm Finesse 14 Transmits both carrier and RF sidebandsStability requirement
15 Hz/Hz1/2 at 100Hzwill be possible to achieve with vibration isolation in a vacuum tankPhoto detector
Receive a few watt power Have high quantum efficiency High response speed to detect RF signals
Use multiple (4-32) InGaAs photo diodes
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 24
Observation system
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 25
Observation system (1) Observation system (1)
Observation system
Monitor the detector conditionKeep high duty cycle operation and best sensitivity of the detector
Automatic lock-acquisitionAutomatic interferometer adjustmentMonitor and diagnosis
Master-system (intelligent switching) Supervise whole detector system Self-switching sub-system Stand-alone control of subsystem Laser system, MC system, Seismic isolation system, etc.Control circuits Switched and adjusted by master system
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 26
Observation system (2) Observation system (2)
Automatic operation in TAMA-DT9 Master-system (intelligent switching) PC + PCI boards + LabVIEW 64ch digital I/O 16ch A/D + 24ch D/A Lock acquisition Auto adjustment Interface with external system
Self-switching sub-system Logic circuit + Embedded micro processor Laser source control MC control Laser intensity stabilization Beam axes control
Thu SunTueMon Fri SatWed
Observation (continuous over 10min)
Date and Time in JST (UTC +9 hours) 29
6
17
24
313029
22
15
11
Nov. 28, 2003
54
12
16
23 25
18
7
30
Dec. 1
8
19
26 27
20
139
2 3
10 14
21
28
Jan. 1 32 4
8 95 106 7 11
Crewless operation3 times of continuous operations longer than 24h
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 27
Summary
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 28
Summary Summary
Interferometer conceptual design
Two nearby interferometers As independent as possible Center rooms: separated 3km vacuum tube: common
Main interferometerBroad band RSE with power recyclingArm cavity Finesse : 1550Power Recycling Gain : 11Signal Band Gain : 15Stored Power : 771kWSignal band : 230Hz
Input opticsTwo mode cleaners Baseline lengths : 10m and 180mModulators for IFO controlLaser stabilization
Output opticsOutput mode cleaner Round trip length : 70mmMultiple InGaAs photo diodes
Observation system
Monitor and organize the whole detector system Automatic lock-acquisition Automatic interferometer adjustment Monitor and diagnosis
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 29
End
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 30
Comparison of detectors Comparison of detectors
Comparison of next generation GW detectors
2 detectors (3km) (2 nearby detectors)
Long baselineBetter seismic attenuation system Underground site
Low-mechanical-loss mirrors and suspensionsCryogenic (20k)
High-power laser sourceLow-loss opticsBroad-band RSE config.
LCGT (JPN)
Scale
Seismic noise reduction
Thermal noisereduction
Quantum noisereduction
3 detectors (4km) (2 nearby, 1 separated)
Long baselineBetter seismic attenuation system Suburban site
Low-mechanical-loss mirrors and suspensionsFlat-top beam
High-power laser sourceLow-loss opticsDetuned RSE config.
Advanced LIGO (USA)
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 31
Main parameters Main parameters
Detector parameters
Laser Nd:YAG laser (1064nm)
Injection lock + MOPAPower : 150 W
Main Interferometer Broad band RSE configuration
Baseline length : 3kmBeam Radius : 3-5cmArm cavity Finesse : 1550Power Recycling Gain : 11Signal Band Gain : 15Stored Power : 771kWSignal band : 230Hz
Vacuum systemBeam duct diameter : 100cmPressure : 10-9 Torr
MirrorSapphire substrate + mirror coatingDiameter : 25cmThickness : 15cmMass : 30 kgAbsorption Loss : 20ppm/cmTemperature : 20 KQ = 108
Loss of coating : 10-4
Final SuspensionSuspension + heat link with 4 Sapphire fibersSuspension length : 40cmFiber diameter : 1.5mmTemperature : 16KQ of final suspension : 108
LCGT Technical Advisory Committe (August 23, 2005, ICRR, Tokyo) 32
Broadband vs NarrowbandBroadband vs Narrowband
Detuned interferometer caseDetuned RSE configuration SNR increase 23% Event rate increase 86%
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