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%