Physics Program at the Caltech LIGO 40m Prototype

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Physics Program at the Caltech LIGO 40m Prototype

• Intro to gravity waves
• The LIGO experiment
• Purpose of the 40m
• Progress
• Building renovation
• Vacuum system
• Pre-stabilized laser
• Suspended mode cleaner
• Data acquisition
• System modeling
• Milestones

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Warped space-time Einsteins General Relativity
(1916)
We envision gravity as a curvature of space as a
massive body moves, the curvature changes with it.
Einsteins theory tells us that this information
will be carried by gravitational radiation at the
speed of light.
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Studying strong-field GR with gravitational waves
Compact binary mergers
We can use (weak-field) gravitational waves to
study strong-field general relativity, where GR
gets nonlinear and interesting!
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Why are we so confident? Hulse-Taylor binary
pulsar
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Nature of Gravitational Radiation

• General Relativity predicts
• transverse space-time distortions, freely
  propagating at speed of light
• expressed as a strain (?h ?L/L)
• Conservation laws
• conservation of energy ? no monopole
  radiation
• conservation of momentum ? no dipole radiation
• quadrupole wave (spin 2) ? two polarizations
• plus (?) and cross (?)

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Magnitude of GW strain

• Accelerating charge ? electromagnetic radiation
• Accelerating mass ? gravitational radiation
• Amplitude of the gravitational wave (dimensional
  analysis)
• second derivative
• of mass quadrupole moment
• (non-spherical part of
• kinetic energy)
• G is a small number!
• Need huge mass, relativistic
• velocities, nearby.
• For a binary neutron star pair,
• 10m light-years away, solar masses
• moving at 15 of speed of light

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The LIGO Project

• LIGO Laser Interferometer Gravitational-Wave
  Observatory
• US project to build observatories for
  gravitational waves (GWs)
• to enable an initial detection, then an astronomy
  of GWs
• collaboration by MIT, Caltech other institutions
  participating
• (LIGO Scientific Collaboration, LSC)
• Funded by the US National Science Foundation
  (NSF)
• Observatory characteristics
• Two sites separated by 3000 km
• each site carries 4km vacuum system,
  infrastructure
• each site capable of multiple interferometers
  (IFOs)
• Evolution of interferometers in LIGO
• establishment of a network with other
  interferometers
• A facility for a variety of GW searches
• lifetime of gt20 years
• goal best technology, to achieve fundamental
  noise limits for terrestrial IFOs

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International network
Simultaneously detect signal (within msec)
GEO
Virgo
LIGO
TAMA

• detection confidence
• locate the sources
• verify light speed propagation
• decompose the polarization of gravitational
  waves

AIGO
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How does the LIGO interferometer work?

• The concept is to compare the time it takes light
  to travel in two orthogonal directions transverse
  to the gravitational waves.
• The gravitational wave causes the time difference
  to vary by stretching one arm and compressing the
  other.
• The interference pattern is measured (or the
  fringe is split) to one part in 1010, in order to
  obtain the required sensitivity.

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Initial LIGO sensitivity
LIGO I
AdvLIGO
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LIGO Science Run (S1)Strain Sensitivities
LIGO S1 Run ----------- First Upper Limit
Run Aug Sept 02
Total time with all three IFOs in lock 138
hours Duty cycle 36
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We need Advanced LIGO!

• X10 in sensitivity x1000 volume searched
• LIGO 0.3-3 inspirals/year
• Adv. LIGO 300-3000 inspirals/year
• Factor of ten improvement needed at all
  frequencies

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40m Laboratory Upgrade - Objectives

• Key elements of Advanced LIGO to be prototyped
  elsewhere
• LASTI, MIT full-scale prototyping of Adv.LIGO
  SEI, SUS (low-f)
• TNI, Caltech measure thermal noise in Adv.LIGO
  test masses (mid-f)
• AIGO, Gingin high powered laser, thermal
  effects, control stability
• ETF, Stanford advanced IFO configs (Sagnac),
  lasers, etc

• 40m Primary objective full engineering prototype
  of optics control scheme for a dual recycling
  suspended mass IFO (high-f)
• Minimize transition time to Advanced LIGO at main
  sites
• Control scheme set by LSC/AIC, first test at
  Glasgow 10m

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Arm cavity parameters and LIGO sensitivity
As rITM is increased, Garm is increased, fpol-arm
is decreased.
fpol-arm
We wish to control Garm and fpol-arm
independently to optimize shot noise curve
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The signal recycling mirror
We add a signal recycling mirror (SM) at the
asymmetric output port. This forms a compound
mirror with the input test masses (ITMs) with
reflectivity
with f kls 2pls(fcarrfsig)/c
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Tuning the signal response
By choosing the phase advance of the signal
(fcarrfsig) in the signal recycling cavity, can
get longer (SR) or shorter (RSE) storage of the
signal in the arms
f kls 2pls(fcarrfsig)/c
rCC
rITM
RSE
tuned (narrow band)
RSE
SR
SR
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Using DR to optimize sensitivity
Now we can independently tune hDC and fpolarm to
optimize sensitivity (eg, hug the thermal noise
curve)
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Cavity control

• Pound-Drever (reflection) locking used to control
  lengths of all the optical cavities in LIGO
• Phase modulate incoming laser light, producing
  RF sidebands
• Carrier is resonant in cavity, sidebands are not
• Beats between carrier and sidebands provide
  error signal for cavity length

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Demodulation
Modulated light A cos (wt) B cos (w wm)t
B cos (w wm)t Power A2 AB cos (wmt)
B2 cos (2wmt)
If we multiply the power by our modulation
frequency in an electronic mixer and average over
several cycles, only the AB cos (wmt) term
will remain. But this term is linear in A, which
is the amplitude of the signal in the arms. We
now have a correction signal for our length
controls, and a linearly-sensitive signal for
gravitational waves.
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Advanced LIGO control scheme

• AdvLIGO will use two pairs of
• RF sidebands (9/180 MHz)
• Applied before input MC
• 9 MHz to symmetric port, sensing PRM
• 180 MHz to asymmetric port, sensing SRM
• Demod at 171/189 MHz to sense lPRC, lSRC, lmich,
  insensitive to arms

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40m Laboratory Upgrade More Objectives

• Expose shot noise curve, dip at tuned frequency
• Facility for testing small LIGO innovations
• Laser intensity stabilization
• Small-scale multiple pendula
• Hands-on training of new IFO physicists!
• Public tours (students, DNC media,
  the Duke of York, etc)

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40m Lab Staff

• Alan Weinstein, project leader
• Dennis Ugolini, postdoc
• Steve Vass, master tech and lab manager
• Ben Abbott, electrical engineer
• Osamu Miyakawa, TAMA postdoc
• AdvLIGO engineers/physicists Larry Jones, Jay
  Heefner, Garilynn Billingsley, Janeen Romie, Mike
  Smith, Fred Asiri, Dennis Coyne, Peter King, Rich
  Abbott, Bob Taylor, etc.
• Collaborating institutions (TAMA, CSUDH)
• Summer 2000-2002 sixteen SURF undergraduates

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Building renovation

• Old IFO dismantled, surplus distributed to LIGO,
  LSC
• Wall removed for added lab space
• New optical tables installed at vertex, south
  arm, ends

• Roof repaired, cranes retouched
• Laser safety enclosure installed
• New control room, entrance changing area added

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Vacuum envelope additions

• 12m suspended mode cleaner

• Output optics chamber

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Hardware and electronics

• Acquired and installed new electronics racks,
  crates, power conditioners, 12 cable trays, etc.
• Installation of vacuum ion pumps, control system

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STACIS Active seismic isolation

• One set of 3 for each of 4 test chambers
• 6-dof stiff PZT stack
• Active bandwidth of 0.3-100 Hz,
• 20-30dB of isolation
• passive isolation above 15 Hz.

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Residual Gas Noise Requirement
The plot at left includes the residual gas noise
for a vacuum of 10-6 torr, dominated by water and
nitrogen. At higher pressures the noise becomes
significant at the tuned frequency.
The 40m vacuum system can run as low as 310-7
torr, and has a pressure of 1.310-6 torr in
low-vibration mode (ion pumps only).
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Pre-stabilized laser (PSL)
We use the same 10-watt NdYAG solid-state
infrared laser as the main LIGO sites. The goal
of the PSL system is to provide frequency and
intensity stabilization, with minimal power loss.
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Frequency stabilization servo (FSS)
By adjusting the lasers master oscillator to
keep a fixed cavity in resonance, the FSS reduces
frequency noise to lt 1 Hz.
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Pre-mode cleaner (PMC)
The PMC uses the concept of Guoy phase to remove
all non-TEM00 modes from the main beam, without
reflecting it back to the PSL
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13m Suspended Mode Cleaner

• Provides further frequency noise suppression
• Long enough to transmit RF sidebands
• Optics are suspended

14 hour lock stretch
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Length Sensing and Control (LSC)

• Each optic has five OSEMs (magnet and coil
  assemblies), four on the back, one on the side

• The magnet occludes light from the LED, giving
  position
• Current through the coil creates a magnetic
  field, allowing mirror control

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Digital suspension controls
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Data acquisition system (DAQ)
Anti-aliasing filters
ADCU 64 analog-to-digital channels sampled at
up to 16 kHz
EDCU collects data from EPICS databases
512 GB RAID array Full data for 48 hours
Second trends for 1 month Longer trends
forever
Sun Ultra 10 Frame Broadcaster Fast
ethernet connection Serves data for
diagnostics Connection to CACR
Sun Ultra 60 Frame Builder Collects data
from DCUs Creates frame files Sends
files to RAID array
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Interferometer modeling efforts

• Length and alignment control with Twiddle,
  ModalModel
• Suspensions for 5" test masses modeled using
  Simulink
• Model of lock acquisition dynamics using E2E in
  progress

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Milestones through 2004

• 2Q 2002
• Install cables and seismic stacks, in MC, output
  optics chambers (done)
• Hang and install mode cleaner optics (done)
• Install suspension controllers, LSC, some ASC
  (done except for ASC)
• 4Q 2002
• Hang and install core optics (in progress)
• Complete ASC, ISC, all auxiliary electronics (in
  progress)
• Control system finalized
• 3Q 2003 Core subsystems commissioned, begin
  experiments
• Lock acquisition with all 5 length dof's, 2x6
  angular dof's
• Measure transfer functions, noise
• Inform CDS of required modifications
• 3Q 2004 Next round of experiments.
• DC readout. Multiple pendulum suspensions?

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Summary

• The 40-meter is on schedule to serve as an RSE
  controls/engineering prototype for Advanced LIGO
• Significant progress has been made in several
  subsystems
• Vacuum controls, PSL, DAQ up and running
• Mode cleaner cavity locked with digital
  suspensions, LSC
• Remaining suspended optics, LSC and ASC in coming
  months
• Expect to exercise mode full IFO in summer 2003