High-Power Stabilized Lasers and Optics of GW Detectors

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High-Power Stabilized Lasers and Optics of GW
Detectors

• Rick Savage
• LIGO Hanford Observatory

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Overview

• In general, I will discuss 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 as will be seen in subsequent talks in
  this session.
• Lasers
• Initial LIGO - 10 watts
• Requirements, performance, technical issues
• Advanced LIGO 200 watts
• Concept, status
• Optics
• Initial LIGO core optics test masses
• Requirements, performance, technical issues
• Advanced LIGO
• Plans

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GW detector laser and optics
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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 PSL hardware

• 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

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Concept for Advanced LIGO laser

• Being developed by GEO/LZH
• Injection-locked, end-pumped slave lasers
• 180 W output with 1200 W of pump light

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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 Df/f 3 x 10-22

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Power stabilization

• Sensors located before and after suspended
  modecleaner
• Current shunt actuator controlling amplifier pump
  diode current
• Pre-modecleaner for

RIN measured upstream of MC
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RIN at 20-30 MHz

• Describe requirement
• Give formula for filtering by PMC ala T. Ralph
  (from old CCD)
• PMC parameters
• Photo of optically contacted PMC

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Tidal Compensation
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Overall experience with LIGO I PSL

• Reliability
• Long locks
• Pmc problems
• Laser problems
• Ref cav performance

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Core Optics Test Masses

• Core optics requirements for initial and advanced
  ligo
• Coating requirements
• Q factor
• Scattering/ absorption, etc.
• Thermal noise internal modes noise due to
  coatings

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LIGO I core optics

• Surface uniformity lt 1 nm rms
• Scatter lt 50 ppm
• Absorption lt 2 ppm
• ROC matched lt 3
• Internal mode Qs gt 2 x 106

Caltech data
CSIRO data
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Advanced LIGO core optics
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Preparation and installation challenges

• Photos of cleaning and installation
• Description of problems with etching coatings
  during cleaning.

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Practical issues

• Anamolous absorption
• Vacuum incursions very costly time and risky.
• Need to make remote measuements due to water
  absorption in spring seats

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Thermal compensation system
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Next-generation TCS

• Design utilizes a fused silica suspended
  compensation plate
• Actuation by a scanned CO2 laser (Small scale
  asymmetric correction) and nichrome heater ring
  (Large scale symmetric correction)
• No direct actuation on ITMs for improved noise
  reduction, simplicity and lower power (Sapphire)

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Kilometer-scale Fabry-Perot cavities

• Free spectral range 37.5 kHz
• Plot of H_w(f) and H_L(f)

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G-factor measurements
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• Pre-stabilized laser
• MOPA source
• Frequency reference cavity
• Pre-modecleaner cavity
• Electro-optics modulators for stabilization and
  locking to cavities

• Core optics
• Optical levers
• Wavefront sensors
• Output beams
• modematching telescopes
• Periscopes
• Photodetectors

• 15-m modecleaner cavity
• Wavefront sensors and piezo-controlled input
  pointing
• Faraday isolator
• Mode-matching telescope

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Pre-stabilized laser

• Laser source and ancillary optical components and
  feedback control loops necessary to provide
  frequency and amplitude stabilized light to the
  interferometers (input optics subsystem).
• Requirements
• 10 watts of stabilized light (first generation)
• Frequency pre-stabilization to the ?? Level (10
  Hz to 100 kHz)
• Power stabilization to the ?? level (10 Hz to 100
  kHz).
• Power stabilization at GW detection modulation
  freq. (20-30 MHz).
• Availability long (10s to 100s of hours
  continuous operation without loss of lock).
• Insert schematic of PSL/photos

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LIGO 10-W laser

• Master oscillator power amplifier configuration
• Developed under contract with Lightwave
  Electronics (model 126MOPA)
• Oscillator Non-planar ring oscillator
• Monolithic design
• Free-running frequency stability
• Free-running RIN
• Power amplifier
• Four-rod, double-pass

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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,
  Hw(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/p) 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.
ITMx TCS annulus heating ? decrease in ROC
(increase in curvature)
R 14337 m
R 14096 m
Assume metrology value for RETMx 7260
m Metrology value for ITMx 14240 m
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To investigate heating via 1 mm light

• Lock ifo. for gt 2 hours w/o TCS Plaser 2 W
• Break full lock (t 0) and quickly lock a single
  arm.
• Misalign input beam (MMT3) in yaw
• Measure temporal evolution of Hw(s)
• Note 1mm light heats both ETM and ITM
• H1 Xarm dataFeb. 18, 2005

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Yarm measurement Feb. 19, 2005
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Comparison with model Phil Willems

• Time-dependent model based on Hello-Vinet
  formalism (J. Phys. France 51(1990) 2243-2261)
• Free parameters cold radius of curvature and
  power absorbed
• Fits by eye (,- 20)

Xarmbulk absorption76 mW
Xarmsurface absorption33 mW
DR 370 m
DR 320 m
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Comparison with model - Yarm

• Phil Willems time-dependent Hello-Vinet model

Yarmsurface absorption25 mW
Yarmbulk absorption50 mW
DR 250 m
DR 190 m
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Calibration using TCS heaing results

• TCS calibrationXarm 220m / 37mW 5.9
  m/mWYarm 190m / 45mW 4.2 m/mW
• Surface (not bulk) absorption
• 1064 nm heatingXarm 293m / 5.9 m/mW
  49mWYarm 177m / 4.2 m/mW 42 mWAssumes all
  heating on surface and no absorption in ETMs
• Surface-equivalent, ITM-onlyabsorption
  calibration

14.5 km
D 220 m
14.28 km
13.9 km
D 190 m
13.71 km
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Issues cold curvature differences

• Cold values from 1064 nm meas.ITMX 14.226
  km difference 50 mITMY 13.615
  km difference 100m
• Systematic errors?
• Alignment drifts sampling different areas of TM
  surfaces
• More complex, time-dependent behavior of surface
  distortions?
• Phil Willems studying with time-dependent model
  of surface distortions
• g factor measurements and reduced data available
  inLIGO-T050030-00-W