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Lasers and Optics of Gravitational Wave Detectors. Rick Savage LIGO Hanford Observatory. Power Recycled Michelson Interferometer with Fabry-Perot Arm Cavities. end test mass. 4 km (2 km) Fabry-Perot arm cavity. recycling mirror. input test mass. Laser. beam splitter. signal. - PowerPoint PPT Presentation
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1
Lasers and Optics ofGravitational Wave Detectors
Rick SavageLIGO Hanford Observatory
2
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
3
Closer look - more lasers and optics
4
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)
5
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)
7
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
8
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
4
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
22
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
23
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
24
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)
25
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
26
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
27
High-frequency response of optical cavities
Dynamic resonance of light in Fabry-Perot cavities(Rakhmanov, Savage, Reitze, Tanner 2002 Phys. Lett. A, 305 239).
28
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
29
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
30
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.
31
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.
32
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