Download Activities and Findings - LIGO

Activities and Findings
Research and Education Activities
Overview
This award (PHY-9801158) is a continuation of Award PHY-9700601. PHY-9700601
was a single year award that initiated the LIGO Laboratory Advanced R&D Program. It
was proposed as a five-year program, 1997 through 2001. During formulation of the
proposal, the collaborative activity leading to the LIGO Scientific Collaboration (LSC)
was also initiated. PHY-9700601 was granted with the multi-year award dependent upon
formulation of an LSC multi-year collaborative program and upon favorable review of a
multi-year proposal submitted in coordination with the LSC program. That multi-year
proposal was submitted and was favorably reviewed. As a result PHY-9801158 was
awarded in 1998. Amendment 3 dated July 13, 2001 awarded the final funding increment
bringing the total to $8.900,000 and extending the period of performance to August 31,
2002. A no cost extension was requested and granted internally on August 1, 2002 to
complete the work authorized under PHY-9801158. This extended the period of
performance to August 31, 2003.
The purpose of the LIGO Advanced R&D Program is to investigate and develop
promising techniques suitable for advanced gravitational-wave detectors. This is an
ongoing process and continuing effort is currently funded under NSF Cooperative
Agreement No PHY-0107417 for the LIGO Operations and Advanced R&D. This report
describes activities and results obtained under PHY-9801158. It is worth noting that
operation of the detector at Livingston has been affected somewhat by unexpectedly high
levels of seismic noise from human related activities and microseism. In addition
absorption characteristics of some of the optical components at both sites are not all at
design levels resulting in non-optimal thermal gradients and optical characteristics.
Techniques developed under this Grant for advanced detectors are being installed into the
initial LIGO detectors to improve performance and meet the science requirements.
Our major research activities have been organized in several distinct tasks. These are:





Stochastic Noise (STO) - research related to reducing sources of stochastic noise
limiting advanced LIGO detectors.
Seismic Attenuation Systems (SAS) - research related to reducing the influence of
seismically induced disturbances in advanced LIGO detectors.
Thermal Noise interferometer (TNI) - research related to understanding the
physics origin of thermal noise.
Suspension Fibers (FIB) - research related to understanding mechanical losses and
the technology of forming and attaching test mass suspension fibers.
Advanced Lasers (LAS) - research on the development of lasers of sufficiently
high power and performance to meet the requirements for advanced LIGO
sensing noise.





Sapphire (SAP) - research related to developing sufficiently large and optically
suitable sapphire test mass materials essential to reducing internal thermal noise
and radiation pressure fluctuation noise in advanced LIGO detectors.
Advanced Photodetectors (PDT) - research related to developing suitable very
high power photodetectors for use in the gravitational wave sensing light port of
advanced interferometers.
Active Thermal Compensation (ATC) - research related to active correction of
thermally induced distortions of resonant cavity optics in advanced LIGO
detectors.
Resonant Sideband Extraction (RSE) - research related to the study of advanced
signal tuned interferometer configurations for use in advanced LIGO detectors.
Advanced Control Systems (CTR) - research related to the development of robust,
low-noise control systems suitable for advanced LIGO detectors.
Resonant Sideband Extraction (RSE)
The current design for Advanced LIGO detectors includes gravitational wave signal
recycling. This configuration allows a modification in the frequency response of the
interferometer and offers the possibility of a tailored response for a targeted source. The
increase in the complexity of the interferometer, both in terms of the optical and controls
systems, is significant, and has led to an exploration of a variety of readout and control
schemes in modeling and in tabletop experiments.
A demonstration of one candidate scheme for Resonant Sideband Extraction (RSE) was
completed at Caltech and previously reported during 2001. The purpose of this
experiment was to successfully design and implement a control scheme for a detuned
RSE interferometer, an interferometer configuration being considered for Advanced
LIGO. There are two main problems for detuned RSE. The first is simply where to derive
a signal to control the additional mirror. The second relates to how the off-center locking
of most of the cavities in detuned RSE affects the control for all degrees of freedom, and
how this can be dealt with.
The method proposed to control all five degrees of freedom involved adding a single RF
sideband, in addition to the phase modulated RF sidebands already used in initial LIGO.
Synchronous demodulation at two of the beat frequencies (PM sideband - carrier and PM
sideband - single sideband) generates a control matrix that is largely diagonal, except for
the control of the beamsplitter. The beamsplitter is coupled to the arm-cavity commonmode signal, where a gain hierarchy can be used to effectively diagonalize the
interferometer signals (similar to the power recycling/arm cavity coupling in initial
LIGO).
An additional complication in detuned RSE is that demodulation phase must be more
carefully chosen. The off-resonant nature of the signal cavity, by which detuning is
accomplished, affects the resonance of all RF sidebands in the interferometer.
Differential resonance for the RF sidebands can generate large DC offsets, which can
couple many noise sources into the signals controlling the mirrors. Proper choice of
demodulation phase can be used to null the DC offsets, but this in turn affects the control
matrix, so care must be taken in the design
A prototype of RSE was implemented on an optical table in a Caltech LIGO lab.
Broadband RSE (all cavities on resonance) was demonstrated earlier this year, and
detuned RSE, a much more complicated interferometer, was successfully locked in
September. The method of acquiring lock involves locking successively more
complicated subsets of the RSE interferometer, and doing specific calibrations at each
step of the way. Lock is very robust, and the interferometer can remain locked for hours.
This is critical to the measurement of the control matrix, and clean measurements of the
RSE detuned gravity wave transfer function. These measurements have been made, and
agreement with a model used to design the interferometer is good. Predictions for
demodulation phase shifts relative to simpler configurations, generated by the model, also
match up well.
Sapphire Advanced R&D (SAP)
[Red is from 2001 Report] The use of sapphire as a core optics material offers
significant promise for improving the sensitivity of gravitational wave detectors.
Compared to fused silica, sapphire has higher mechanical Q, speed of sound, and density,
all of which contribute to a significant reduction in the internal thermal noise. The
increased density also lessens noise due to the radiation pressure of the laser light.
Reduction of these two noise sources is essential to achieving the desired sensitivity of
the Advanced LIGO detector.
Sapphire from two different commercial sources is being studied: Crystal Systems, Inc.,
which grows sapphire along the crystal m-axis is the recognized leading commercial
supplier of large high-quality sapphire blanks. The Shanghai Institute of Optical
Materials (SIOM) grows sapphire on the c-axis, and we have included them in our study
as c-axis sapphire may have advantages with respect to birefringence.
Absorption
Measurements of the optical absorption of 1.06-m [what is this supposed to be?] laser
light, which will result in heating and deformation of the optic, show absorption in the
range of 40 - 120 ppm/cm. To date, no correlation has been found between absorption
and sapphire starting material or location within the boule. The effort is focusing on the
effect of the annealing process on absorption. A series of samples is being studied to
examine the effect of annealing temperature, atmosphere and duration on sapphire
absorption. Preliminary results indicate absorption as low as 20 ppm/cm.
Optical homogeneity
Measurements of optical homogeneity of sapphire samples indicate peak-to-valley
variations in optical homogeneity of the order of one-quarter wavelength. This is
acceptable for correction by spot polishing to the requirements of 10 nm RMS, through
an iterative process of computer-controlled localized abrasion and metrology. The
samples are now at the polishing vendors polishing tests have begun.
Coating and bulk birefringence
The presence of birefringence in the optics bulk or coatings can cause unacceptable
power degradation in the polarization sensitive interferometer. Instrumentation has been
developed at Caltech to measure and map birefringence in optics by forming a
polarization sensitive resonant cavity with the optic under test. Coated one and three-inch
samples have arrived at Caltech for this test, oriented along both the m- and c- optical
axes. Preliminary tests indicate that birefringence of the substrates and coatings of both cand m-axis sapphire meets the Advanced LIGO requirements. Further work remains in
mapping the birefringence of these samples over large areas.
Sapphire development has also proceeded at SIOM. The work has been focused on
growth technology, with improvements in oven design, and the annealing processes.
Several test samples have been grown and have been evaluated under the tests described
above. We have found comparable absorption and optical homogeneity to the Crystal
Systems test pieces. We are continuing to interact with SIOM to find out how to lower
the absorption and variation in homogeneity and also how to grow crystals large enough
for Advanced LIGO optics.
Mechanical Loss Studies
Instrumentation has been built at Caltech, Stanford, and Glasgow to measure the Q of
sapphire optics by monitoring the ring down of the optics internal resonances. Q's of a
few times 108 have now been measured for a number of sapphire samples. Studies are
now underway to study the effect of bonding, attachments, and coatings on the Q.
An aggressive program to explore the mechanical losses due to optical coatings on
substrates is underway, after conclusive evidence for such an effect was developed in a
number of labs in the LIGO Scientific Collaboration. Contracts have been issued to
selected coating vendors for a series of coating runs directed towards lower losses.
Stochastic Noise (STO)
[Red is from 2001 Report] Development of approach selected for the Advanced LIGO
isolation system has been a collaborative effort by scientists from Stanford, JILA, LSU,
and MIT. The point of departure was the draft requirements document and the experience
base of the team members in active isolation at JILA, Stanford, and MIT. We based the
design on several fundamental notions:
 The use of relatively high fundamental solid-body mode frequencies (order of 5
Hz); this eases installation of the seismic system and the optical components,
reduces temperature and other material property sensitivities, reduces the
importance of stray forces on the system (e.g., from cables), and does not
compromise the servo design.
 The goal of using high-gain servo systems to 'slave' the optics platform to inertial
sensors, rather than to use passive mechanical filtering. All solid body modes are
sensed and controlled.
 The goal of flexibility in operational modes and the ability to compensate for
shortcomings in the seismic isolation or other subsystem design after installation.

A mechanical design motivated by servo-system design constraints: high internal
mechanical modes, a compact design, and near collocation of sensors and
actuators.
 A use of commercial sensors and actuators where possible. Several sensors are
used to obtain the best sensitivity in each frequency band.
A prototype was designed to test the fundamental ideas of the design, with a
concentration on those points presenting the greatest challenges: to robustly demonstrate
the required isolation in six degrees of freedom in each of two cascaded stages, with a
topologically complete servo design. It was decided neither to build a full-size system nor
to attempt to build a system that was compatible with the very strict vacuum requirements
of LIGO. A rapid timetable was established to allow the design to be considered with
prototype results in hand. Six months were allocated from the start of design to delivery
of the prototype, with testing to follow. We met this schedule.
The team distributed the design responsibility among the institutions, with the MIT LIGO
Laboratory taking two major activities: the design and realization of the servo loops per
se, and the preparation of a vacuum chamber and associated wiring for the prototype. An
existing vacuum system was suitable, minimizing the investment required.
The prototype served its function well, both to confirm the design philosophy, and to
illustrate the flexibility of the approach:
 The system was easy to assemble and to install in the vacuum system; it was lifted
into place as a complete unit with clamps on the stages, and remained aligned to
the desired sub-mm precision.
 The first servo loops were successfully closed within a day of the installation, and
the sensors, actuators, and real-time servo system all functioned as anticipated.
 Shortly after the first tests, an unanticipated coupling was observed. Transfer
functions were measured using the actuators and sensors of the system with the
servo controller as the measurement analyzer, and a cross coupling due to a finite
distance between sensors and actuators was identified as the difficulty. Additional
terms in the servo-control actuator-sensor matrix were added, and the coupling
was eliminated.
 A second-order effect of horizontal-tilt coupling had been anticipated, and
similarly corrected based on geometric models of the system. No physical
changes to the isolation system have been made since it was first installed. All
'changes' were performed via the actuator-sensor matrix, demonstrating the
flexibility of the system and the relative insensitivity to the basic mechanical
design.
The system functions robustly at the design unity-gain-frequencies, for all 12 degrees-offreedom, with all 30 sensors and 12 actuators. The noise performance does not meet the
Advanced LIGO requirements, partially due to the noisy environment at the MIT
Laboratories and also due to a decision to employ low-cost high-noise position sensors in
this prototype. The system was shipped to our collaborators at Stanford University where
characterization continued and where it was integrated into a larger experiment with two
such isolation systems.
In parallel, the LIGO Advanced System Test Bed Interferometer (LASTI) infrastructure
is being assembled (with LIGO Laboratory Operating funds) to prepare for the suite of
tests of Suspensions, Isolation Systems, and the Laser and Input Optics.
Suspension Fiber Research (FIB)
[Red is from 2001 Report] We have started research into the production and use of
fused silica ribbons for Advanced LIGO. Ribbons are in production using the same
automated glass-working lathe used to make fibers. The thermal characteristics of ribbons
are different enough from fibers that reproducibility is
not yet adequate for Advanced LIGO suspensions. Also, the breaking strength of the
ribbons so far is much lower than that of fibers and has no safety margin. Work continues
to improve ribbon manufacture.
The ribbon design includes twists in the ribbon in order to prevent buckling in the
suspension pendulum mode. The static and dynamic elasticity of twisted ribbons is not
yet fully understood. Analytic and numerical analysis of twisted ribbons has been
initiated to understand the strength, mode structure, and buckling of twisted ribbons.
Further calculations of non-linear thermoelasticity were carried out. Experimental
measurements failed to confirm the theory. A second round of effort is underway.
Bonding Studies
The strength and aging characteristics of hydroxy-catalysis bonds between fused silica
and fused silica in have been studied in collaboration with Stanford and Glasgow. Bonds
that are aggressively cleaned (by several diverse techniques) prior to bonding are about
twice as strong as uncleaned bonds. The strength of bonds supporting suspended loads
are under study. The shear strength of two silica/sapphire bonds supplied by Sheila
Rowan at Stanford have been measured and found to be somewhat less strong than
silica/silica bonds but still adequate for Advanced LIGO.
Advanced Laser R&D (LAS)
The Advanced LIGO concept calls for significantly lower sensing noise, brought about
by better optics and higher-power lasers. From the initial LIGO 10-W laser source, an
increase to 100-200 W is required to reach the shot noise limited design sensitivity of the
Advanced LIGO detectors. The goal of the Higher Power Laser Advanced R&D effort is
to investigate 100-Watt class lasers in sufficient detail to identify a configuration that
would be ready for installation in Advanced LIGO. We have continued our collaboration
with the Stanford University group, GEO-Hannover and ACIGA to build and test one or
more 100-W Nd:YAG lasers and perform experiments to develop an understanding of the
issues concerning application to LIGO. This involves developing the lasers and
measuring, modeling, and understanding their characteristics including frequency noise,
intensity noise, pointing stability, and beam quality. Stanford and ACIGA are building
high power lasers, while LIGO is defining the laser noise and performance requirements
and is assisting both groups in laser testing. GEO-Hannover is taking on the
responsibility for the Advanced LIGO subsystem, with the LIGO Lab establishing
interfaces and collaborating in the research.
Thermal Noise Interferometer (TNI)
The principal objective of the Thermal Noise Interferometer (TNI) is to better understand
sources of thermal noise in interferometric gravitational wave detectors. The Advanced
LIGO concept faces several challenges from thermal noise. Toward this goal we are
taking the direct approach of measuring displacement noise in a suspended
interferometer, and using these measurements to validate and extend thermal noise
models. Of particular interest is non-Gaussian noise, which is very difficult to model
theoretically.
Seismic Attenuation System (SAS)
The LIGO Laboratory pursued two approaches to seismic isolation for Advanced LIGO. While the
design we will carry forward uses servosystems to 'slave' a platform to inertial sensors, the
alternative we studied has considerable continuing attraction to the field. The final development
phase of the SAS system is now part of a LIGO-TAMA collaborative effort in which LIGO is
transferring to and supporting TAMA's adoption of the SAS system for a use in the TAMA 300
interferometer in Tokyo.
The SAS strategy uses a chain of mechanical filters that passively and progressively attenuate
the disturbances in all degrees of freedom. Each filter, essentially a six-degree-of-freedom
mechanical oscillator, has the well-known 1/ƒ2 asymptotic transmission above its resonance
frequencies. A suitable isolation at low frequency is obtained by lowering the filter's resonance
frequencies well below 1 Hz. This is not a trivial task in the vertical direction and it is achieved by
means of Geometric Anti Springs. The rms noise reduction is achieved with inertial damping
applied on the first stage, sensing and reacting on the recoil of the filter chain.
A complete, full-scale prototype SAS chain which would be compatible with the Observatory test
mass vacuum chambers has been built and tested, including inertial damping and controls. This
inverted pendulum (IP) has been successfully tuned down to 13 mHz.
In order to carry out a field test and fully validate the approach, the SAS design has been adapted
to the TAMA requirement, and a prototype TAMA-SAS attenuation chain has been built and
tested. This dual use test combined the LIGO program requirement for a full test with TAMA
requirements. The construction of the first two TAMA-SAS towers, complete with mirror
suspensions and controls, is complete. They were installed in a test 3-m interferometer in early
2001. If successful, four more units would be built by TAMA and installed in TAMA-300 in 2001.
To guarantee that the passive attenuators are free of excess noise, a facility to study the creep
and creak phenomena is nearing completion. Previous measurements indicate that the design
should be effectively free of these noise sources. Preliminary results on acoustic emission on
highly stressed blades are under analysis.
As part of the overall approach, a preliminary study and feasibility test has been started on AC
electrostatic actuators for the interferometer core optics. Using electro-optic transducers coupled
to a resonant circuit, we can drive a comb capacitor facing the mirror. This design eliminates:


the quality-factor-spoiling magnets from the mirror surface;
the effect of stray charge coupled to DC electric fields and, at the same time,

the cabling to the last stage of the suspension and all the excess noise related to it.
Advanced Photodetector Development (PDT)
In this period significant new Advanced LIGO photodetector requirements came to light,
motivated by new interferometer optical modeling and simulation results. Due to the high
circulating power expected in Advanced LIGO, classical radiation-pressure recoil of the test
masses places a stringent constraint on laser intensity fluctuations. To achieve the necessary
stability, the laser power control system will need to detect approximately one watt of sampled
power without excess noise (above theoretical shot noise), distortion, or damage. As a result we
have begun researching large-area diodes optimized for low frequency operation, in addition to
the high-frequency diodes required for interferometer phase detection. A tentative proposal to
use a fringe-offset DC readout for the strain degree of freedom adds further motivation to this new
initiative. Progress on high-frequency, high-power RF photodiodes for interferometric phase
detection (a collaboration between MIT and Stanford) was slowed by failure of a primary MBE
facility at Stanford, but is now regaining momentum. A set of prototype back-illuminated devices
was prepared by graduate student David Jackrel at Stanford and brought to the MIT detector lab
for optical and RF testing in March. Preliminary results indicate very low junction capacitance per
unit area (indicative of good RF response). Excellent quantum efficiency may also have been
achieved, although, for ease of handling, the prototypes were built on relatively thick and thus
lossy substrates. The observed external quantum efficiency was consistent with the measured
transmission of the substrate, indicating low intrinsic losses.
Excess dark current and some hysteresis remain problematic in these test devices. We believe
these features are attributable to lattice-mismatch dislocations which provide charge
accumulation and transfer sites, and which are not adequately mitigated by graded buffering of
the compound. Dislocations were observed penetrating to the active junction area in scanningelectron micrographs of the test devices. We believe they can be largely eliminated by going to a
quaternary device composition, in which it is possible to tune the bandgap (to match our
wavelength) and the lattice spacing (to match the substrate) independently. The Stanford MBE
facility has recently been retrofitted to enable quaternary device fabrication and we expect to
fabricate some new devices of this design this summer.
Active Thermal Compensation (ATC)
Spatially inhomogeneous and nonaxisymmetric distortions are expected to arise from
local variability in absorption and geometric mode asymmetries. A more general
actuation scheme is under development to correct such nonaxisymmetric distortions, and
to provide 'bootstrap' preheating to enforce cavity stability during cold startup. This
system involves a scanned mid-infrared laser beam, which selectively heats localized
zones of the optic. A theoretical model for the influence kernel linking the local beam
heat to its effect on optical mode amplitudes has been developed. Simultaneously, a
prototype of this system involving a 10-W CO2 laser, galvanometer mirror scanners and
pyroelectric detectors has been assembled around the test optics and wavefront readout
system. Using a fused silica target in vacuum, the magnitude and spatiotemporal
evolution of the optical path after stepwise introduction of the heating laser beam shows
excellent agreement with the model. Tests are now underway using a sapphire target.
An additional direction for the research is in the estimation of the thermophysical
properties of sapphire, which are critical to our calculations of the thermal noise in
sapphire test masses. The absorption of laser light on the surface and in an optic induces
temperature gradients within the optic. A nonzero coefficient of thermal expansion
translates these temperature gradients into surface expansions (thermoelastic
deformation), adding a spatially dependent phase lag on both reflected and transmitted
beams. In addition, a temperature dependence in the substrate's index of refraction
insures that transmitted light will suffer an added phase distortion (termed 'thermal
lensing'). We have developed a technique to measure thermal conductivity, thermal
expansion coefficient, and index derivative (dn/dT) in Sapphire and Fused Silica through
examining the phase distortions induced by an absorbed pump beam on a probe beam
resolved both spatially and temporally on a Shack-Hartmann wave front sensor. The
initial results are interesting: they give close but somewhat different values for these
parameters than other kinds of tests, and with apparently small error bars. Further
measurements are underway.
The initial prototyping of the two schemes for thermal compensation concluded this year
and resulted in the PhD thesis of a LIGO student1. The lens formed in the substrates due
to the absorption of the laser light in the substrate make the interferometer sensitive to the
power level. Including a thermal compensation system allows the interferometer to be
used with a wide range of input powers, allowing e.g., better low frequency sensitivity
with a reduction in the power. It also allows a trade to be made with the material
properties of the substrate; this is useful for sapphire, and necessary in the fallback for
fused silica.
The basic approach for compensation is to add a complementary additional heat source,
so that the sum of the laser and compensation heating leads to a uniform optical path. In
one technique, a circular heater adds heat to the edge of the optic. In this way, the
scattering effect of lensing can be reduced (in experiments and models that show
excellent agreement) by more than a factor of 50; see Figure 1, upper plots. This is a
very effective approach for the case of uniform absorption, which is expected to
dominate.
Figure 1 Thermal compensation demonstration results. Top left: The contour map for a
uniform absorption of a Gaussian beam. Top right: The residual deformation after
compensation with a ring heater. Bottom left: the distortion due to a ‘point absorber’
(mimicked by a small probe laser beam); Bottom right: the map after compensation with
a scanned compensation beam.
In the second approach, a scanning laser beam is played on the substrate and the dwell
time and/or intensity can be modulated to deposit heat in a pattern optimized to
compensate for a specific defect, for example a volume of higher thermal absorption. As
shown in the lower plots in Figure 1, an additional suppression of a factor of eight can be
achieved for this example of point defect.
Input Optics
R. Lawrence, “Active Wavefront Correction in Laser Interferometric Gravitational Wave Detectors,”
MIT Ph.D Thesis, 2002, P030001-00-R
1
The University of Florida and their collaborators made progress on the challenges in the
Input Optics subsystem. A novel Faraday Isolator design was developed which uses a
pair of crystals in a compensation technique to deliver high isolation at high powers. In
this design shown in Figure 2, two 22.5° Faraday rotators and a reciprocal quartz
polarization rotator placed between them replace the traditional single crystal 45°
Faraday rotator. In such a configuration, polarization distortions that a beam experiences
while passing the first rotator will be compensated in the second. Tests to the maximum
power available are encouraging.
Figure 2 Top: compensated Faraday isolator design. Bottom: isolation for a
conventional and the compensated designs.
We favorably reviewed the Input Optics Design Requirements prepared by the group at
the University of Florida, and the group was given approval to proceed to the preliminary
design.
Systems and Interferometer Sensing and Control
We refined the baseline design and conducted a System Design Requirements Review. A
number of subsystem requirements and trade studies were concluded. We initiated a
study of the data readout approach for the signal-recycled interferometer. The preliminary
result is that the DC readout (in contrast to the traditional RF-modulation technique)
appears to take advantage of the coupling that exists in a signal-recycled interferometer
between the shot-noise fluctuations and the photon pressure on the test masses. We have
also been working with industry to develop a low noise Digital-to-Analog Converter
(DAC). Test results on the first prototypes should be available before the end of this year.
Findings
Sapphire Advanced R&D (SAP)
We have made significant progress in producing and characterizing sapphire as the
preferred test mass material for Advanced LIGO. Our industrial partner fabricated fullsized boules (see Figure 3) that will now be polished for a more thorough
characterization.
Figure 3 Sapphire substrate pathfinder. This piece, fabricated by Crystal Systems, Inc.,
is the full size of an Advanced LIGO test mass, 32 cm dia, 40kg mass. (Courtesy Insaco)
To address absorption in the substrate of the 1-micron laser light, annealing processes
were refined in collaboration with Stanford, resulting in promising reductions. Industrial
partners successfully pursued approaches to compensating for inhomogeneity. The notion
is to polish a surface, which has features complementing transmission defects, on the
anti-reflection side of the optic using two different approaches. One (Goodrich) involves
a small rotary abrasive tip and an x-y table; the other (CSIRO) uses an ion-milling
technique. Both can bring the net optical path seen by the light to an acceptable level. In
parallel, manufacturers were able to produce material with improved homogeneity.
[Recent results in the Thermal Noise Interferometer (TNI) facility at Caltech indicate
that we have reached the thermal noise floor for silica test masses, and we are proceeding
with these measurements.]
Optical Coatings (OPT)
One important measure of the optical coatings is the optical absorption. Acceptable (subppm) losses have been demonstrated this year with conventional coatings by several
vendors.
We pursued a strong LSC/LIGO Laboratory program this year to identify the magnitude
and source of coating mechanical losses, and to improve the model of the coating thermal
noise. The mechanical losses lead to thermal noise; the coating is an important
contributor due to the geometry of the test mass, coating, and laser beam, There is a
limited choice of materials and of processes which lead to both low mechanical and low
optical losses. We are executing a program to identify the source of loss and to explore
alternative coating materials and processes that meet the combined optical and
mechanical requirements.
We can report significant progress: We were able to demonstrate the source of the
mechanical losses in the coating. The high-index tantalum material, rather than the lowindex material or interfaces, is responsible. We are now pursuing alternative coating
materials with several vendors with incremental progress in reducing losses.
Stochastic Noise (STO)
Seismic Isolation
Recently the seismic isolation team has focused on pre-isolator development for initial
LIGO . This advance implementation of the pre-isolator is an important near-term aid for
the Livingston interferometeras well as a significant step forward for the Adv LIGO
seismic isolation system. A photograph of the hydraulic-actuator variant is shown in
Figure 4.
Figure 4 Hydraulic pre-isolator (HEPI) vertical isolator. On the left, the vertical
actuator is shown; differential pressure in the bellows exerts force on the septum in the
middle, which is carried to the load via the pyramidal flex joint at the top. On the right,
the actuator is shown as installed at the MIT LASTI testbed.
A second-generation active isolation system prototype was designed by the LIGO
Scientific Collaboration (LSC) team and fabricated by the LIGO Laboratory. It is
currently being commissioned at the Stanford Engineering Test facility. A photograph of
the prototype is shown in Figure 5. This technology demonstrator will be used to (a)
inform the development of the full-scale LASTI seismic systems for the HAM and BSC
chambers, which will be developed this coming fiscal year and (b) serve as a controls test
bed for the active isolation systems. Initial testing of the demonstrator show that a key
measure of intrinsic mechanical alignment, the coupling from a requested horizontal
actuation to an accidental tilt of the platform, is very low, which will ease the low
frequency controls design. Other measurements indicate that the first internal mechanical
resonance, which will limit the maximum control loop bandwidth, is roughly 200 Hz,
compatible with the design goal of 50 Hz for the loop bandwidth.
Figure 5 Photograph of the prototype at the Engineering Test Facility (ETF), at
Stanford, of the in-vacuum seismic isolation system. The trapezoidal springs which
support the outer and inner stages can be seen; the cavity at the lower left is one of six (3
outer, 3 inner) cavities that receive a plug-in unit containing sensors and actuators
Testing and control law development will continue. A request for bid for the next
generation prototype is in preparation and will be issued in 2003.
Suspensions
An all-metal test mass quadruple suspension prototype was developed at the University
of Glasgow GEO lab and then sent to MIT for testing. All of the solid body modes were
identified, and the model for the suspensions developed at Caltech, Stanford, and
Glasgow was refined. Further trade studies on the lengths and masses were made based
on the updated model. A challenge in the design is to damp the solid-body modes of the
suspension without introducing excess noise in the gravitational-wave band (10 Hz and
higher). Several approaches are being followed: using passive eddy-current damping,
development of a miniaturized interferometric sensor, and an approach using a split
feedback system has been developed in VIRGO2.
An analysis of the thermal noise of tapered fused silica fibers at Caltech showed that this
is an attractive alternative to ribbons for ease of fabrication and ultimate thermal noise
performance. Some first samples have been fabricated for tests. Development of ribbons
continued at Glasgow as the baseline design. Refinement of the attachment technique of
the fused silica suspension fibers to the masses, using hydroxy-catalysis bonding, to
sapphire (for the test mass) and high-density glasses (candidate for the penultimate mass)
was made with good success.
We have completed the design and fabrication of the first prototype auxiliary optics
suspensions; see Figure 6. This suspension design carries the mode-cleaner optics, and
will first be exercised at Caltech to check the solid body modes and damping
characteristics, and then transferred to the MIT LASTI facility to look at installation and
control issues.
2
VIRGO, French-Italian gravitational-wave detector and consortium, http://www.virgo.infn.it/
Figure 6 Photograph of the prototype of a triple suspension design for the Mode Cleaner
mirrors. The dummy optics is made of aluminum with holes bored to match mass and
inertia for the final silica optics. The prototype has coil actuators on all three levels,
identifiable as white ceramic cylinders.
A significant step in 2002 was the installation of the complete set of triple-pendulum
fused-silica fiber suspensions in the GEO-600 interferometer by the GEO project. The
Advanced LIGO suspension design is directly derived from the GEO-600 design, and the
test of fabrication, installation, and now ultimately performance of the working design
will be invaluable for refining the Advanced LIGO design.
Advanced Lasers (LAS)
Programs to develop 200 W laser sources continued at Adelaide, Stanford, and Hannover.
During the past year, each group has built up a prototype high-power head: an injectionlocked end-pumped rod design from Hanover, an injection-locked stable-unstable slab in
Adelaide, and a slab amplifier at Stanford. The near-term goal is to make a selection
based on a set of criteria developed at Hannover, one of which is to produce 100 W by
February 2003. Greater than 90 W have been produced in several designs, although not in
the final configurations; see Figure 7 for an example output curve for a linear resonator
using the Hannover approach.
Figure 7 Laser Zentrum Hannover early prototype of a high-power laser head in a
linear cavity configuration (at left). The final configuration is a ring-resonator. At right,
the power output of the system as a function of the pump light input power; the system
approached the initial goal of 100 W.
Active Thermal Compensation (ATC)
The initial prototyping of the two schemes for thermal compensation concluded this year
and resulted in the PhD thesis of a LIGO student3. The lens formed in the substrates due
to the absorption of the laser light in the substrate make the interferometer sensitive to the
power level. Including a thermal compensation system allows the interferometer to be
used with a wide range of input powers, allowing e.g., better low frequency sensitivity
with a reduction in the power. It also allows a trade to be made with the material
properties of the substrate; this is useful for sapphire, and necessary in the fallback for
fused silica.
The basic approach for compensation is to add a complementary additional heat source,
so that the sum of the laser and compensation heating leads to a uniform optical path. In
one technique, a circular heater adds heat to the edge of the optic. In this way, the
scattering effect of lensing can be reduced (in experiments and models that show
excellent agreement) by more than a factor of 50; see Figure 1, upper plots. This is a
R. Lawrence, “Active Wavefront Correction in Laser Interferometric Gravitational Wave Detectors,”
MIT Ph.D Thesis, 2002, P030001-00-R
3
very effective approach for the case of uniform absorption, which is expected to
dominate.
Figure 8 Thermal compensation demonstration results. Top left: The contour map for a
uniform absorption of a Gaussian beam. Top right: The residual deformation after
compensation with a ring heater. Bottom left: the distortion due to a ‘point absorber’
(mimicked by a small probe laser beam); Bottom right: the map after compensation with
a scanned compensation beam.
In the second approach, a scanning laser beam is played on the substrate and the dwell
time and/or intensity can be modulated to deposit heat in a pattern optimized to
compensate for a specific defect, for example a volume of higher thermal absorption. As
shown in the lower plots in Figure 1, an additional suppression of a factor of eight can be
achieved for this example of point defect.
Input Optics
The University of Florida and their collaborators made progress on the challenges in the
Input Optics subsystem. A novel Faraday Isolator design was developed which uses a
pair of crystals in a compensation technique to deliver high isolation at high powers. In
this design shown in Figure 2, two 22.5° Faraday rotators and a reciprocal quartz
polarization rotator placed between them replace the traditional single crystal 45°
Faraday rotator. In such a configuration, polarization distortions that a beam experiences
while passing the first rotator will be compensated in the second. Tests to the maximum
power available are encouraging.
Figure 9 Top: compensated Faraday isolator design. Bottom: isolation for a
conventional and the compensated designs.
We favorably reviewed the Input Optics Design Requirements prepared by the group at
the University of Florida, and the group was given approval to proceed to the preliminary
design.
Systems and Interferometer Sensing and Control
We refined the baseline design and conducted a System Design Requirements Review. A
number of subsystem requirements and trade studies were concluded. We initiated a
study of the data readout approach for the signal-recycled interferometer. The preliminary
result is that the DC readout (in contrast to the traditional RF-modulation technique)
appears to take advantage of the coupling that exists in a signal-recycled interferometer
between the shot-noise fluctuations and the photon pressure on the test masses. We have
also been working with industry to develop a low noise Digital-to-Analog Converter
(DAC). Test results on the first prototypes should be available before the end of this year.
[Most of the following is based on old material—needs
updating]
Training and Development
Resonant Sideband Extraction (RSE)
This research, concluded in 2001, formed the doctoral thesis research for graduate student
James Mason. Mason presented a talk at the third Edoardo Amaldi Conference held at
Caltech on July 14, 1999.
RSE related SURF projects involved students Brian Kappus, Lisa Goggin, and Ted Jou.
Core Optics Components (OPT)
An Optics related SURF project involved the conversion of LIGO optical metrology data,
undertaken by Martin Stadnik, Warsaw University of Technology [WUT], under the
mentorship of Garilynn Billingsley, LIGO Project.
Stochastic Noise Sources (STO)
The work is the basis for PhD theses for two students, Jamie Rollins and Joshua Phinney.
Other graduate students and undergraduate students are also participating as part of their
training as experimental physicists.
Work on active seismic isolation systems provided partial thesis material for two
Stanford graduate students (Corwin Hardham and Wenshang Hua) through collaborative
work and multiple visits.
Fiber and Sapphire Research (FIB)
John Johnson was a SURF student in 1999 who rejoined this activity as a technical
trainee during the first half of 2000.
Thermal Noise Interferometer (TNI)
The experiment was the basis for the thesis of Caltech graduate student (Shanti Rao) and
supported several undergraduate students. Shanti Rao received his PhD degree during
2003.
Seismic Attenuation System (SAS)
This research is hosting graduate students working on external University PhD theses at Caltech.
Akiteru Takamori
Alessandro Bertolini
Frederick Seve
University of Tokyo
University of Pisa
INSA of Lyon
MOU with University of Tokyo
MOU with University of Pisa
MOU with INSA-Lyon
Hosted undergraduate student performing work study at Caltech
Nicolas Viboud,
INSA Lyon
Undergraduate
In addition, this work is providing several mechanical problems to INSA mechanical engineering
students. The problems, pertinent to SAS, are elaborated by five groups of two students each
over the course of six. SAS scientists follow the hands-on effort, and the students prepare reports
that will be critiqued and graded.
Hosted summer students:
Susha Parameswaran
Brett Maune
Lisa Kaltenegger
James Donald
Xiaowan (Soy) Chen
David Akhavan
Chenyang Wang
Hareem Tariq
Cambridge Univ., UK
Univ. of Missouri-Rolla
Univ. of Tech. Graz
U.C. Berkeley
Rensselaer
Berkeley
Caltech
King's College, London
Undergraduate
Undergraduate
Undergraduate
Undergraduate
Undergraduate
Undergraduate
Undergraduate
Undergraduate
Advanced Photodetector Development (PDT)
Two summer undergraduate research assistants, Patrick Thomas (Wheaton College) and
Andrew Singleton (MIT), performed the upgrade and characterization of the new detector
back scattering apparatus under M. Zucker's supervision.
Active Thermal Compensation (ATC)
The research was the basis for the PhD thesis of MIT student Ryan Lawrence.
An undergraduate research assistant, Phil Marfuta (MIT), assisted graduate student Ryan
Lawrence and the work was his senior thesis topic.
Education and Outreach
Outreach is an ongoing emphasis at LIGO for people directly involved in the Advanced
R&D tasks as well as supporting personnel who are funded under the cooperative
agreements for operations and continuing R&D. Operating funds (PHY-0107417) have
been specifically identified for a coordinator and program costs at each observatory site.
In addition, LIGO is anticipating $5 million in funding over three years from the NSF
beginning next year (FY 2004) for an outreach center and program proposed in
collaboration with the Colleges of Education and Science at Southern University at Baton
Rouge (SUBR, the nation’s only HBCU system), the Louisiana Systemic Initiative
(LaSIP) headed by the Louisiana Board of Regents, and the Exploratorium of San
Francisco, California.
Examples of previous Outreach Activities include:
David Shoemaker, Sam Richman, Ken Mason, Peter Fritschel, Mike Zucker and others
have prepared and presented interactive talks for primary classes in schools throughout
Massachusetts. A typical presentation involved some discussion of our principal research
interests at a level that gave the students a sense of what it is to be a working scientist and
conveyed the excitement and sense of purpose of the vocation. Visual aids included
photos of cataclysmic astrophysical events and of the LIGO and MIT Laboratories.
Optical polarizers, half-wave plates, and various objects with polarization properties were
distributed to the students, and their properties communicated through a combination of
play and directed experiments.
David Berne began the fused silica fiber strength and automated lathe development tasks
as a 1999 SURF (Summer Undergraduate Research Fellows) project at Caltech.
John Johnson started the silicate bond strength experiments as a 1999 SURF project at
Caltech.
Phil Willems presented nonlinear thermo-elasticity as a lecture at the University of
Glasgow on April 14, 2000.
Eric Black gave a LIGO talk at Blair High School in Pasadena and spent the morning
talking to students about science.
Providing several mechanical problems to INSA mechanical engineering students. The
problems, pertinent to SAS, are elaborated by five groups of two students each over the
course of six. SAS scientists follow the hands-on effort, and the students prepare reports
that will be critiqued and graded.
An undergraduate research assistant, Robert Bennett (MIT), assisted graduate student
Ryan Lawrence with construction and software development for an experimental test
stand. He is now working on the theoretical and numerical analysis of the directed-beam
heating effect on wave front and modal distortion as his senior thesis topic.
Ryan Lawrence performed in-school science demonstrations for elementary and
secondary school students and led laboratory demonstration tours for visiting school
groups in the MIT facilities.
Journal Publications:
Book(s) of other one-time publications(s):
Other Specific Products:
Internet Dissemination:
Caltech LIGO site:http://www.ligo.caltech.edu/
MIT Group site: http://space.mit.edu/LIGO/
Seismic Attenuation System (SAS) web page: http://www.ligo.caltech.edu/~citsas/
Contributions:
Contributions within Discipline:
Results from the Resonant Sideband Extraction (RSE) research have contributed directly
to the adoption of a baseline readout system for the Advance LIGO detector, by the LIGO
Science Collaboration (LSC) Advanced Interferometer Configuration (AIC) Working
Group.
The characterization of the optical substrates and coatings, and the metrology needed, are
central ingredients in the planning and design of the next generation interferometer.
The work on active seismic isolation systems has led to a conceptual design for the
Advanced LIGO detector and a second-generation prototype design. In addition, preisolators are now being fabricated for the Livingston based on the R&D. These preisolators are required to address the higher than anticipated noise from human activities
in the area as well as micro-seism.
The overall test program for mechanical systems at the MIT LIGO LASTI installation
provides the structure and the pacing for much of the LSC Advanced LIGO development
schedule.
The nonlinear thermoelastic damping research with fibers is leading to new approaches
for lower thermal noise in suspensions.
The thermal noise research program contributes to our overall understanding of precision
interferometer design and is necessary for constructing advanced gravitational wave
detectors.
The techniques with low natural frequencies being developed in the Seismic Attenuation
System (SAS) research has direct applications for the TAMA Japanese prototype, and is
also being used in smaller scale experiments in a number of laboratories.
The advanced photodetector (PDT) is a critical component of the advanced LIGO
upgrade, and performance characteristics will drive requirements for other subsystems.
The development program has already provided devices with promise for next generation
systems.
The successful modeling and experimental demonstration of radiative compensation for
optics represents extremely important validation for the core advanced LIGO concept.
Estimates of thermophysical properties help in choosing the substrate material and in
making estimates of the performance of the complete system.
Contributions to Other Disciplines:
The development of large, optically homogeneous sapphire substrates will be useful for
the development of increased window sizes for defense applications.
The development of a reliable, high-power, ultra-stable Nd:YAG lasers benefits
researchers beyond our collaboration (for, example, those working on space-based GW
detection systems) and outside of the gravitational wave community for communication
and also high-precision manufacturing.
X-ray telescopes were a source for our optical technology, and the results of our secondgeneration effort will be useful for the follow-up X-ray satellites. The metrology is also
of interest for defense applications.
The next generation of lithography processes for integrated circuits requires extreme
stability of the platform. The active isolation technology developed in our collaboration is
being applied to this problem.
Passive attenuators are of interest for isolating precision instrumentation like electron and
tunneling microscopes, precision crystal growing kilns, etc.
The InGaAs photodetector technology developed for advanced LIGO can be expected to
have applications in optical telecommunications, precision measurement, and laser
physics.
Improved and alternative methods for thermophysical property measurements have wide
applications in materials science. The directed-beam thermal actuator may have
applications for optical metrology, medical lasers or remote sensing.
Contributions to Education and Human Resources:
The Advanced R&D Grant has supported at least partially an average of six graduate
students at Caltech and a similar number at MIT.
Special Requirements for Annual Project Report:
Unobligated funds: NA (The effort supported by this Grant is complete.)