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Lasers and Optics ofGravitational Wave Detectors

Rick SavageLIGO Hanford Observatory

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GW detector – laser and optics

Laser

end test mass

4 km (2 km) Fabry-Perotarm cavity

recyclingmirror input test mass

beam splitter

Power RecycledMichelsonInterferometerwith Fabry-PerotArm Cavities

Power RecycledMichelsonInterferometerwith Fabry-PerotArm Cavities

signal

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Closer look - more lasers and optics

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Pre-Stabilized Laser System

Laser source

Frequencypre-stabilizationand actuator forfurther stab.

Compensation for Earth tides

Power stab. inGW band

Power stab. at modulation freq.(~ 25 MHz)

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Initial LIGO 10-W laser Master Oscillator Power Amplifier

configuration (vs. injection-locked oscillator)

Lightwave Model 126 non-planar ring oscillator (Innolight)

Double-pass, four-stage amplifier» Four rods - 160 watts of laser diode pump

power

10 watts in TEM00 mode

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LIGO I PSL performance

Running continuously since Dec. 1998 on Hanford 2k interferometer

Maximum output power has dropped to ~ 6 watts

Replacement of amplifier pump diode bars had restored performance in other units

Servo systems maintain lock indefinitely (weeks - months)

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Frequency stabilization Three nested control loops

» 20-cm fixed reference cavity

» 12-m suspended modecleaner

» 4-km suspended arm cavity Ultimate goal: f/f ~ 3 x 10-22

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Power stabilization In-band (40 Hz – 7 kHz) RIN

» Sensors located before and after suspended modecleaner

» Current shunt actuator - amp. pump diode current

3e-8/rtHz

RIN at 25 MHz mod. freq.» Passive filtering in 3-mirror

triangular ring cavity (PMC)

» Bandwidth (FWHM) ~ 3.2 MHz

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Earth Tide Compensation Up to 200 m over 4 km Prediction applied to ref.

cav. temp. (open loop) End test mass stack

fine actuators relieveuncompensated residual

100m

prediction residual

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Concept for Advanced LIGO laser

10 x 30W@ 808 nm

Master

Slave II

diode-boxes

power supplys contro lPC

2 x 30W@ 808 nm

output

Slave I

10 x 30W@ 808 nm

10 x 30W@ 808 nm

10 x 30W@ 808 nm

Pound-Drever-Hall Locking - Electronicoscilla tor, m ixer, phase-splitter, servo

Being developed by GEO/LZH

Injection-locked, end-pumped slave lasers

180 W output with 1200 W of pump light

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Brassboard Performance LZH/MPI Hannover Integrated front end based on

GEO 600 laser – 12-14 watts High-power slave – 195 watts

M2 < 1.15

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Concept for Advanced LIGO PSL

m ed iump ow ersta ge

hig hp ow ersta ge

susp end e dm od e cle a ner

NPR O

refe re nceca vity

AOM

tid al fee d b ackD ia gno stic

sp atia lfi lte r

ca vity

I 4

I 1

I 2

I 3

I 5

1 2 3

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FS S- A2

FS S

ILS2

ILS1 PS S2

PS S3

FS S- A1

PM C 1

PM C 2

PS S1

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Core Optics – Test Masses Low-absorption fused silica substrates

» 25 cm dia. x 10 cm thick, 10 kg Low-loss ion beam coatings Suspended from single loop of music wire (0.3 mm) Rare-earth magnets glued to face and side for

orientation actuation Internal mode Qs > 2e6

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LIGO I core optics

Caltech data RITM ~ 14 km (sagitta ~ 0.6 ) ; RETM ~ 8 km

Surface uniformity ~ /100 over 20 cm. dia. (~ 1 nm rms) “Super-polished” – micro-roughness < 1 Angstrom Scatter (diffuse and aperture diffraction) < 30 ppm Substrate absorption < 4 ppm/cm Coating absorption < 0.5 ppm

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Adv. LIGO Core Optics LIGO recently chose fused silica over sapphire

» Familiarity and experience with polishing, coating, suspending, thermally compensating, etc. – less perceived risk

Other projects (e.g. LCGT) still pursuing sapphire test masses Thermal noise in coatings expected to be greatest challenge

sapphirefused silica38 cm dia., 15.4 cm thick, 38 kg

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Processing, Installation and Alignment

Experience indicatesthat processing andhandling may besource of optical loss

gluingvacuum bakingwet cleaningsuspendingbalancingtransporting

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Thermal Issues Circulating power in arm cavities

» ~ 25 kW for initial LIGO

» ~ 600 kW for adv. LIGO Substrate bulk absorption

» ~ 4 ppm/cm for initial LIGO

» ~ 0.5 ppm/cm ($) for adv. LIGO Coating absorption

» ~ 0.5 ppm for initial & adv. LIGO Thermo-optic coefficient

» dn/dT ~ 8.7 ppm/degK Thermal expansion coefficient

» 0.55 ppm/degK “Cold” radius of curvature of

optics adjusted for expected “hot” state

radius

dept

h

Surface absorption

Bulk absorption

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Thermal compensation system

CO2 Laser

?

Over-heat Correction

Inhomogeneous Correction

Under-heat Correction

ZnSe Viewport ITM

PRM

SRM

ITM

ITM

Compensation Plates

Adv. LIGOconcept

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Coating vs. substrate absorption

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0x 10

-6

radius (m)

OP

D (

m)

Optical Path Difference in Transmission for 1 W absorbtion

CoatingSubstrate

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0x 10

-7

radius (m)

OP

D (

m)

Surface Distortion in for 1 W absorbtion

CoatingSubstrate

Optical path difference Surface distortion

OPD almost same for same amount of power absorbed in coating or substrate Power absorbed in coating causes ~ 3 times more surface distortion than same

power absorbed in bulk

coating

substratecoating

substrate

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Summary LIGO utilizes 10-W solid state lasers

» Relative frequency stability ~ 10-21/rtHz

» Relative power stability ~ 10-8/rtHz

» Advanced LIGO lasers: similar requirements at 200 watt power level LIGO test masses (mirrors) 25 cm dia., 10 cm thick fused silica

» Surface uniformity ~ /100 p-v (1 nm rms) over 20 cm diameter

» Coating absorption < 1 ppm, bulk absorption ~ few ppm/cm

» Active thermal compensation required to match curvatures of optics

» Non-invasive measurement techniques required for characterizing performance of optics

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Anomalous absorption in H1 ifo.

ITMY

ITMX

Negative values imply annulusheating

Significantly more absorption in BS/ITMX than in ITMY

How to identify absorption site?

TCS power is absorbedin HR coatings of ITMs

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Need for remote diagnostics

Water absorption in viton spring seats makes vacuum incursions very costly.

» Even with dry air purge, experience indicatesthat 1-2 weeks pumping required per 8 hours vented before beam tubes can be exposed to chambers

Development of remote diagnostics to determine which optics responsible of excess absorption

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Spot size measurements

ITMX

ITMY

BeamView CCD cameras in ghost beams from AR coatings

Lock ifo. w/o TCS heating Measure spot size changes as ifo.

cools from full lock state Curvature change in ITMX path

about twice that in ITMY path

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Arm cavity g factor changes Again, lock full ifo. w/o TCS heating, break lock, lock single arm and

measure arm cavity g factor at precise intervals after breaking lock g factor change in Xarm larger than Yarm by factor of ~ 1.6 Calibrate with TCS (ITM-only surface absorption)

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Results and options Beamsplitter not significant

absorber ITMX is a significant absorber

~ 25 mW/watt incident ITMY absorption also high

~ 10 mW/watt incident» Factor of ~5 greater than

absorption in H2 or L1 ITMs Options

» Try to clean ITMX in situ» Replace ITMX» Higher power TCS system

30-watt TCS laser was tested Eventually ITMX was replaced

and ITMY was cleaned in-situITM bulk

ITM surface

ET

M s

urfa

ce

From analysis by K. Kawabe

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Simple question:“For a resonant optical cavity, can the Pound-Drever-Hall locking signal distinguish between frequency and length variations?”

i.e. does

Of course!

Or does it?

Origin of G-factor measurement technique

L

L

f

f

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High-frequency response of optical cavities

Dynamic resonance of light in Fabry-Perot cavities(Rakhmanov, Savage, Reitze, Tanner 2002 Phys. Lett. A, 305 239).

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High frequency length response

1FSR 2FSR

LIGO band

Peaks in length response at multiples of FSR suggest searches for GWs at high frequencies.

HF response to GWs different than length response

Different antenna pattern, but still enhancement in sensitivity

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High frequency response to GWs Long wavelength approximation not

valid in this regime Antenna pattern becomes a function

of source frequency as well as sky location and polarization

All-sky-averaged response about a factor of 5 lower than at low freq.

Significant sensitivity near multiples of 37.5 kHz (arm cavity FSR)

Movie (by H. Elliott): Antenna pattern for one sourcepolarization as source frequency sweeps from 22 to 36 kHz

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G-factor Measurement Technique

Dynamic resonance of light in Fabry-Perot cavities (Rakhmanov, Savage, Reitze, Tanner 2002 Phys. Lett. A, 305

239). Laser frequency to

PDH signal transfer function, H(s), has cusps at multiples of FSR and features at freqs. related to the phase modulation sidebands.

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Misaligned cavity

Features appear at frequencies related to higher-order transverse modes. Transverse mode spacing: ftm = f01- f00 = (ffsr/ acos (g1g2)1/2

g1,2 = 1 - L/R1,2

Infer mirror curvature changes from transverse mode spacing freq. changes. This technique proposed by F. Bondu, Aug. 2002.

Rakhmanov, Debieu, Bondu, Savage, Class. Quantum Grav. 21 (2004)

S487-S492.

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H1 data – Sept. 23, 2003

• Lock a single arm

• Mis-align input beam (MMT3) in yaw

• Drive VCO test input (laser freq.)

• Measure TF to ASPD Qmon or Imon signal

• Focus on phase of feature near 63 kHz

2ffsr- ftm

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Data and (lsqcurvefit) fits.

Assume metrology value for RETMx = 7260 mMetrology value for ITMx = 14240 m

ITMx TCS annulus heating decrease in ROC (increase in curvature)

R = 14337 m R = 14096 m