high-power stabilized lasers and optics of gw detectors

1LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection

High-Power Stabilized Lasers and Optics of GW Detectors

Rick SavageLIGO Hanford Observatory


Overview In general, issues and hardware solutions from a LIGO perspective

because of familiarity.» Other GW interferometers (GEO, LCGT, TAMA, Virgo) face similar

issues and have developed their own solutions Lasers

» Initial LIGO - ~10 watts– Requirements, performance

» Advanced LIGO ~ 200 watts– Concept, status

Optics» Initial and advanced LIGO core optics – test masses

– Requirements, performance

» Excess absorption in H1 interferometer optics– Efforts to identify absorption site


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


Closer look - more lasers and optics


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)


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


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)


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


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


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


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


Brassboard Performance LZH/MPI Hannover Integrated front end based on

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

M2 < 1.15


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

4

FS S- A2

FS S

ILS2

ILS1 PS S2

PS S3

FS S- A1

PM C 1

PM C 2

PS S1


Core Optics – Test Masses Low-absorption fused silica substrates

» 25 cm dia. x 10 cm thick, 20 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


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


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


Processing, Installation and Alignment

Experience indicatesthat processing andhandling may besource of optical loss

gluingvacuum bakingwet cleaningsuspendingbalancingtransporting


Thermal Issues Circulating power in arm cavities

» ~ 25 kW for inital 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


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 more surface distortion than same power

absorbed in bulk


Thermal compensation system

CO2 Laser

?

Over-heat Correction

Inhomogeneous Correction

Under-heat Correction

ZnSe Viewport ITM

PRM

SRM

ITM

ITM

Compensation Plates

Adv. LIGOconcept


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


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 develop which optics responsible of excess absorption


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


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)


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 presently

being tested

ITM bulk

ITM surface

ET

M s

urfa

ce


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.


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.


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


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

high-power stabilized lasers and optics of gw detectors

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