Download WP3 group report (including roadmap text)

5th Annual Report on GWA-WG3
A European strategy for future detectors
V. Fafone3, R. Flaminio2, A. Freise4, S. Hild4, H. Lück5, M. Punturo6-12, S. Rowan7,
G. Losurdo8, P. Rapagnani1, J. Van den Brand9, B. Sathyaprakash10, B. Mours11
1
INFN, Roma1, Italy
Laboratoire des Materiaux Avancés, Lyon, France
3
INFN Roma 2 (Tor Vergata), Italy
4
University of Birmingham, Birmingham, UK
5
Max-Planck-Institut für Gravitationsphysik, AEI, Hannover, Germany
6
INFN, Perugia, Italy
7
University of Glasgow, UK
8
INFN, Firenze, Italy
9
NIKHEF, Amsterdam, The Netherland
10
University of Cardiff, UK
11
LAPP-CNRS, Annecy, France
12
EGO, Italy
2
Coordinators: H. Lueck (University of Hannover) and M. Punturo (INFN, Perugia)
1. Introduction
The N5-WP3 dedicated the last 12 months mainly to the start-up of the design of a 3rd
generation gravitational wave observatory. WP3 has been the environment where the FP7
proposal “ET: Einstein Telescope” has been developed and submitted to the European
Commission. Since this proposal has been successful (and the ET project began in the
summer 2008), WP3 started a merging program to help the structuring of this new European
initiative.
This path had two milestones in 2008: the GWDAW 2008 - VESF meeting and the Joint ETILIAS_GWA general meeting. The first meeting, usually devoted to the technology
refinements for the second generation of gravitational wave detectors (due to the influence
and the partial support of the ILIAS-WG3 group) has been devoted to the preliminary R&D
activities for the 3rd generation observatories. Several aspects of the 3rd generation GW
detectors have been discussed in this meeting:
 Science case
 Possibilities
 Technologies & topologies
 Roadmap
Technologies for the 3rd generation GW detectors will be described in the paragraph 2.2 and
the roadmap aspects will be described in the paragraph 2.3.
The second milestone in the last 12 months has been the organization of the Joint meeting
between ILIAS-N5, ILIAS-JR3 (the last ILIAS-GWA annual meeting) and ET (the first ET
annual meeting). This event, hosted by EGO, has been really successful and 116 scientists
participated to the event, mainly organized and supported by ILIAS-WP3. The official web
site of the event is here:
http://agenda.infn.it/conferenceDisplay.py?confId=811
The distribution of the scientist participating to the meeting is reported in the following graph.
Figure 1- Distribution of the participants to the ILIAS-ET joint meeting
In this meeting the organization, the technological aspects and the science goals discussed in
the last years in the ILIAS-N5 and JR3 (but mainly in the N5-WP3) have been applied to the
Einstein Telescope project.
2. Status of tasks
2.1. Design of the 2nd generation GW detectors
2.1.1. Conceptual design for sensitivity improvements in GEO HF
GEO HF is a program including the upgrade of GEO600 and experiments aimed at techniques
that will be employed beyond the second generation of gravitational wave detectors.
The conceptual design for the GEO600 has been discussed in WP3.
Also the interleaving of the world-wide commissioning and data taking activities to ensure
full time coverage by at least one operating detector upgrade has been supported by WP3.
The upgrade of GEO600 will consist of two subsequent steps starting after the current data
taking effort, called Astrowatch. With the beginning of the upgrades GEO600 will be called
GEO HF (High Frequency) because the focus of sensitivity improvements will be in the
frequency range above 500Hz.
1) Changing the optical readout
a) Converting from heterodyne (RF) readout to self-homodyne (DC) readout
b) Changing from in-air to in-vacuum readout
c) Implementation of an output Mode Cleaner
d) Injection of squeezed light through the output port
e) Changing to Tuned Signal Recycling
2) Increasing the optical power in the interferometer
a) Remove attenuator from interferometer input
b) Increasing the laser power by a factor of three
c) Exchange mode cleaner mirrors to enhance Mode Cleaner throughput by factor of two
2.1.1.1 GEO600 is limited by shot noise for signal frequencies above 500Hz. Improving the
sensitivity in this frequency range can hence only be achieved by lowering the shot noise.
The classical method is an increase of the laser power circulating in the interferometer.
Changing the optical readout can also yield an improvement through the reduction of shot
noise. Changes in the readout of GEO HF are foreseen for the later ¾ of 2009. Enhancing
the power circulating in the Power Recycling cavity will be started in 2010.
2.1.1.1.a Currently all gravitational-wave detectors are using so called Schnupp
modulation or frontal modulation together with a slight asymmetry in the arm length.
This technique has a very similar effect as modulating the differential arm length at
the (RF = radio frequency) modulation frequency and provides an error signal even if
the interferometer operates exactly on a dark fringe. The disadvantage is contributions
of quantum fluctuations to the shot noise from two different frequency ranges at twice
the modulation frequency and at low (detection band) frequencies. In contrast to this
the interferometer can be operated slightly off the dark fringe without an asymmetry in
the arms giving a weak dependence of the light power at the output on the arm length
difference (DC readout). This way only quantum fluctuations in the detection band
contribute to the shot noise. Only the excellent stability of the laser power which could
be achieved in the last decade enabled the usage of this technique. All large scale
gravitational-wave detectors will change to this detection scheme within the upgrade
processes.
2.1.1.1.b Placing the photodiode, which detects the main signal in vacuum reduces the
beam jitter on the diode caused by fluctuations of the refractive index of the air. Beam
position jitter on the photodiode is converted into a fake signal through lateral
detection efficiency inhomogeneities of the photodiode. The in-vacuum readout
together with the Output Mode Cleaner (OMC, see 2.1.1.1.3) will be done in an
additional vacuum tank which will be added to the vacuum system.
2.1.1.1.c As the shot noise depends on the overall light power detected by the photo
diode it is beneficial to remove all light from the output beam that does not carry
signal. Typically the signal is only contained in the TEM00 mode, which is the mode
resonantly enhanced in the interferometer itself. In GEO HF an output Mode Cleaner
will be inserted in front of the main photo diode to remove all higher order mode
contributions.
2.1.1.1.d The shot noise occurring at the dark port of an interferometer originates from
quantum fluctuations entering from that port and being reflected by the interferometer
to the photo diode. Substituting these fluctuations with a non-classical light field with
less fluctuation can reduce the shot noise and hence improve the sensitivity. Squeezing
levels of up to 11dB and squeezing down to frequencies of 1Hz have separately been
demonstrated in the labs at the AEI in Hannover. Both achievements will be
implemented in the Squeezing source for GEO HF and aim at a sensitivity
improvement of a factor of two.
2.1.1.1.e Only if the Signal Recycling cavity is tuned to the resonance of the laser light
frequency, squeezed light injection will improve the sensitivity over the full detection
band of GEO HF. If the Signal Recycling cavity is tuned off resonance the dispersion
will cause additional noise outside the tuning frequency and hence limit the usefulness
of the squeezed light. GEO HF will therefore be operated with a ‘tuned’ Signal
Recycling cavity.
2.1.1.2 The techniques described above reduce the shot noise but do not increase the signal.
The GW signal can be increased by increasing the light power inside the interferometer.
Starting in 2010 GEO HF will aim to increase the light power insider the interferometer
by a factor of ten. To achieve this three different steps will be taken.
2.1.1.2.a Due to instability problems when operating the detector at full laser power
GEO600 currently operates at a reduced input power of 6W where the laser yould
deliver 11W. The instabilities are partly due to scattering laser light to optical sensors
of the mirror suspension. The sensing system will be changed and the instabilities will
be removed. Then GEO HF will operate with unattenuated laser power.
2.1.1.2.b The 11W laser system will be exchanged for a 35W system developed in the
GEO collaboration.
2.1.1.2.c Due to absorption of the mirrors the two successive input Mode Cleaner
currently only transmit about 50% of the laser light to the input of the interferometer.
Reducing the finesse of the Mode Cleaner and also using mirrors with fewer losses
will increase the transmission to about 90%.
With the increase of the light power in the interferometer from currently to 3.5kW to 30kW
the shot noise contribution will be reduced by another factor of about 3.
2.1.2. Support for the advanced Virgo project: contribution to the
realization of the technical design
WP3 aided the technical design of the advanced Virgo detector, mainly through the support of
the University of Birmingham scientists (A.Freise and S.Hild) participating to this effort. The
documentation describing the preliminary design of the advanced Virgo detector is reported
here: http://wwwcascina.virgo.infn.it/advirgo/, meanwhile here we report a short summary of
the activity performed in the last year, in this task, with the WG3 help.
Freise and Hild have mainly contributed to the design of the optical configuration of the
interferometer. Freise is the subsystem manager of the OSD (Optical Simulation and Design)
subsystem. So far, OSD has been committed on three main taks:
- Determining the configuration of the recycling cavities and exploring the possibility to
implement a non-degenerate cavity design in Advanced Virgo
- Determining the test masses radius of curvature and, consequently, the beam
geometry, with the aim of enlarging the spot size in the input test masses and,
therefore, reducing the thermal noise contribution
- Choosing the arm cavity finesse
The internal reports documenting such activities are available on the following page:
http://wwwcascina.virgo.infn.it/advirgo/technical.html
but here we present a short description.
2.1.2.1. Optical System Design for Advanced Virgo
Advanced Virgo will feature several advanced technologies in order to achieve a 10 times
increased sensitivity. The main differences in the optical configurations of initial and
Advanced Virgo will be a) the application of signal recycling, b) a near symmetric beam
geometry inside the arm cavities and c) the potential implementation of non-degenerate
recycling cavities. Signal recycling can be used to optimize and reduce the quantum noise
contribution (made of photon shot noise at high frequencies and photon radiation pressure
noise at low frequencies). The near-symmetric beam geometry inside the arm cavities will
allow us to achieve minimal coating Brownian noise. While these two techniques directly
influence the Advanced Virgo sensitivity, the implementation of non-degenerate recycling
cavities is (in first order) not motivated by sensitivity reasons, but will make the overall
instrument performance much more insensitive to unavoidable technical imperfections and
potentially speed up the commissioning process by a significant amount.
Figure 2 - Fundamental noise contributions of Advanced Virgo. Over the full detection band the
sensitivity is either limited by Quantum noise or Coating Brownian noise. The actual level of these two
noises strongly depend on the optical configuration and optical optical design. The quantum noise can be
shaped by means of the signal recycling technique. The coating Brownian noise depends on the chosen
beam sizes at the main test masses.
2.1.2.2. Signal Recycling and Sensitivity Optimisation
The variation of the two signal recycling parameters, detuning and bandwidth (together with
the circulating optical power), offers the possibility to optimise the Advanced Virgo
sensitivity for different figures of merit, such as the binary neutron star (BNS) inspiral range.
The sensitivity range available by such an optimisation is limited by the boundaries given
from other fundamental noise limits: At low frequencies the boundary is given by the level of
gravity gradient noise, while in the mid and high frequency range coating Brownian noise
restricts the achievable sensitivity. The detector configuration optimised for binary neutron
star inspiral range was chosen to be the Advanced Virgo reference configuration (see Figure
X1) featuring a signal recycling detuning of 0.15 rad and a signal recycling mirror
transmittance of 11%. The resulting BNS star inspiral range amounts to about 150 Mpc.
2.1.2.3. Arm cavity geometry
The size and shape of the laser beam inside the interferometer is defined by the surface shape
of the cavity mirrors: the beam sizes at the IM and EM as well as the position of the cavity
waist are determined by only two parameters, the radii of curvature (ROC) of IM and EM.
Since inside the two Fabry-Perot cavities of the Michelson interferometer the GW interacts
with the laser light, creating signal sidebands, the two arm cavities can be seen as the heart of
the Advanced Virgo detector. The characteristics of the arm cavities have not only a high
impact on the detector sensitivity and bandwidth, but also on the overall detector
performance. Therefore a thorough design of the ROCs, taking all relevant aspects into
account, is of high importance.
Figure 3 – The lower subplot shows the mode-non-degeneracy in the arm cavities. Areas of the same color
represent beam size combinations with identical cavity stability. As indicated by the dashed arrows one
can move along a color boundary and keep the cavity stability at the same level. The upper subplot shows
the achievable sensitivity, i.e. the BNS-range for AdV depending on the beam sizes. Again areas of same
color indicate identical sensitivity. The orientation of the terraces for the non-degeneracy and the
sensitivity is found to be different. This is why it is possible to improve the sensitivity, while keeping the
cavity stability constant by following the black arrows in the upper subplot towards the upper left corner.
In order to find the optimal ROC values a trade off analysis needs to be performed taking into account
the following aspects: While the coating Brownian noise goes down with increased beam sizes at the
test masses, the clipping losses and the cavity stability (measured by the mode non-degeneracy) favour
smaller beam sizes.
Since, for identical beam sizes the coating Brownian noise is slightly higher at the end mirror
compared to the input mirror (caused by different coating thicknesses, i.e. reflectivities), it is in the
end favorable to implement a near-symmetric beam geometry, featuring a slightly smaller beam size at
the input mirror. The current reference design features beam radii of 5.5 and 6.5cm for the input and
end mirror, respectively.
2.1.2.4. Non-degenerate Recycling Cavities
Deformations of the RF sidebands inside the power recycling cavity have been widely observed in
initial Virgo and LIGO and this has been one of the main problems encountered during the
commissioning of the two detectors. Due to these deformations, the LIGO wavefront sensors, used for
the global alignment of the interferometer, were not correctly working without a thermal compensation
system. Furthermore, a reduction of the RF sidebands recycling gain was observed. A similar decrease
of the recycling gain and an unbalancing between the upper and lower sidebands have been observed
in Virgo, yielding a long delay in establishment of a robust lock acquisition procedure and of a global
alignment strategy.
Figure 4 – Marginally stable recycling cavities (upper left plot) have been used in initial Virgo. Currently,
several options for so-called non-degenerate recycling cavities (upper right plot) are discussed for
Advanced Virgo. While non-degenerate recycling cavities strongly improve the detector performance
regarding thermal distortions and optics imperfections, they come on the cost of additional hardware and
complexity. The lower plot shows one potential implementation of non-degenerate recycling cavities. The
main difficulties arise from astigmatism in the folded beam path and the necessity of suspending
additional optics in the beam-splitter tower and on the injection bench
.
In order to avoid these problems the implementation of so-called non-degenerate recycling
cavities is currently considered for Advanced Virgo. By introducing additional focussing
elements inside the recycling cavities (see upper right plot of Figure X3) it is possible to
increase the transversal mode spacing to be much larger than the line width of the recycling
cavity. In such kind of cavities the high-order modes cannot simultaneously build-up when
the fundamental mode is resonant. However, the realization of non-degenerate recycling
cavities does not only imposes serious constraints to various subsystems due to installation of
additional large-size suspended optical elements (PRM2, PRM3, SRM2, SRM3), but also
requires a very careful optical design: For instance the astigmatism caused from the off-axis
telescopes needs be evaluated and a complete tolerancing analysis needs to be performed in
order to obtain a realistic and reliable optical design for Advanced Virgo.
2.2. Evaluation of the major technologies and topologies of a 3rd generation detector
A scenario of the technologies under study for 3rd generation GW detectors has been depicted
both at the GWDAW 2008 - VESF meeting and the Joint ET-ILIAS_GWA general meeting.
The reduction of the seismic noises (direct seismic and gravity gradient noise) could be
performed through the realization of the 3rd generation observatory in an underground site.
The improvement due to an underground site has been found to be related mainly to the
reduction of the seismic noise, meanwhile the role of the geometry experimental hall (the
“cavern”), believed relevant, has been demonstrated to be negligible (see
https://indico.pi.infn.it/getFile.py/access?contribId=45&sessionId=10&resId=0&am
p;materialId=slides&confId=225). Even in this case the suspension system should permit
a seismic noise filtering down to one hertz. This requires long suspension chains and low
noise cryogenic systems.
The evaluation of the cryogenic system in a 3rd generation detector has been performed during
all the WP3 life. At the GWDAW 2008 - VESF meeting, the results have been summarized in
the following talk:
(https://indico.pi.infn.it/getFile.py/access?contribId=39&sessionId=5&resId=0&am
p;materialId=slides&confId=225) that reports the result of an intense ILIAS-JR3 activity.
The system could be based on low noise cryo-coolers, as shown in the Figure 5 and Figure 6.
Figure 5 - Possible cooling scheme for a seismic filtering suspension
The possible cooling scheme of the payload is reported in the same talk and shown in Figure
7.
Figure 6 - Vibration free cryostat
The selection of the payload material is driven by thermal, optical and noise issues. Silicon
seems the best choice for both the suspension wires and mirror substrate; an interesting
evaluation of the performances of silicon as substrate material is reported here:
https://indico.pi.infn.it/getFile.py/access?contribId=33&sessionId=4&resId=0&amp
;materialId=slides&confId=225
where the effects of both the production technique (see Figure 8) and the doping
concentration have been investigated.
Also the usage of Silicon as suspension fiber or ribbon has been investigated. The loss angle
in ribbons is shown in Figure 10; the mechanical loss (and therefore expected thermal noise)
of small silicon flexures has been observed to decrease with temperature; No obvious
dissipation peaks were observed, at the levels of loss measured, in contrast to bulk samples
(Reid et al., Alshourbagy et al., Martin et al.).
The properties of the Silicate bonding, used to assemble the cryogenic payload, have been
investigated here:
https://indico.pi.infn.it/getFile.py/access?contribId=104&sessionId=5&resId=0&am
p;materialId=slides&confId=225
In the same talks are reported part of the studies performed on the behavior of the dielectric
coatings at low temperature, looking also for new material respect to the standard Ta2O5.
Huge effort is still needed in this direction, but some new idea could produce a revolution in
this field. In fact, it is investigated the possibility to realize high reflectivity mirrors without
multilayer coatings
(see
https://indico.pi.infn.it/getFile.py/access?contribId=31&sessionId=4&resId=0&amp
;materialId=slides&confId=225).
In fact, with the technology of the grating waveguide it is possible to reach high reflectivity
with a single (but complexly structured) layer of dielectric coating (see Figure 11).
Figure 7 - Scheme of the cryogenic payload
Figure 8 - Q of the Silicon: Czochralski vs floating zone production (Nawrodt, 05/08)
Figure 9 - Q of the Silicon: doping effect (Nawrodt, 05/08)
Figure 10 - Loss angle in small silicon flexures (Rowan, 05/08)
Figure 11 - SEM images of trapeziodal Tantala-Silica grating waveguide coatings (Friedrich, 05/08)
Two methods of producing a highly reflective coating have been compared:
- putting the grating atop of a highly reflective stack of dielectric layers
- over-coating a grating with a highly reflective stack of dielectric layers.
The latter method results in lower losses due to grating errors.
Reference: T. Clausnitzer, E.-B. Kley, A. Tünnermann, A. Bunkowski, O. Burmeister, K.
Danzmann, R. Schnabel, S. Gliech, A. Duparré
Ultra low-loss low-efficiency diffraction gratings
Optics Express 13 (2005) 4370 - 4378
from: http://www.opticsinfobase.org/DirectPDFAccess/2EB438D6-BDB9-137ECE0F8D8E3D481202_84315.pdf?da=1&id=84315&seq=0&CFID=25619849&CFTOKEN=9
6598905
Very high power lasers are another crucial ingredient of the 3rd generation GW detectors.
Lasers for the 2nd generation GW detector are able to supply about 200W, at =1064nm, in a
low noise condition. The lasers for 3rd generation GW detectors will need to deliver about
1kW power, probably in a different wavelength (1550 nm) where Silicon is transparent. The
studies for this new technology are summarized in the following document:
http://agenda.infn.it/contributionDisplay.py?contribId=26&sessionId=10&confId=811
where a 5 year path to bring current lasers at 1kW (@ 1064nm) and 1550nm laser at 50W is
presented.
A further technology, that could improve the sensitivity in the high frequency range, is the
usage of the squeezed light status in the GW detectors. A review of the result in this field is
reported here:
http://agenda.infn.it/contributionDisplay.py?contribId=27&sessionId=10&confId=811
As shown in Figure 12, it is currently possible to reduce (at least in a bench experiment) the
noise in a wide range of frequency. In the next future this technology will be tested in the
GEO experiment.
Figure 12 - Noise reduction due to Squeezing down to frequencies of 1 Hz (Khalaidovski 11/08)
10dB of squeezing reached.
(More than 11dB have been achieved by now (private communication))
http://arxiv.org/pdf/0706.1431v1
"Observation of squeezed light with 10dB quantum noise reduction"
H. Vahlbruch, M. Mehmet, N. Lastzka, B. Hage, S. Chelkowski, A. Franzen, S. Gossler, K. Danzmann, and R.
Schnabel,
Phys. Rev. Lett. 100, 033602 (2008)
2.3. Strategy design for the evolution of the GW experimental research in Europe
including the 2nd and 3rd generation: Roadmap over a 15 years time scale
N5-WP3 had a fundamental role in the design of the evolution of the GW experimental
research in Europe. In fact N5_WP3 group constitutes the core of the working group that in
ASPERA (an ERA-NET devoted to the coordination of the national funding agencies in the
Astro-physics research: http://www.aspera-eu.org/) realized the gravitational wave section of
the European Astro-physics roadmap:
http://www.aspera-eu.org/images/stories/roadmap/aspera_roadmap.pdf
Furthermore, since the detection of gravitational wave needs a world-wide collaboration, N5WP3 is playing a relevant role also in the realization of the world-wide roadmap coordinated
by the GWIC (Gravitational Wave International Committee) composed by the representatives
of all the experiments involved, in the World, in the gravitational wave search. In fact,
members of the N5-WP3 are also members of the GWIC and the GWDAW 2008 - VESF
meeting hosted one of the GWIC meetings devoted to the GW roadmap:
https://indico.pi.infn.it/sessionDisplay.py?sessionId=12&slotId=0&confId=225#2008-05-16
The road-mapping activity is summarized by the documentation hereafter included, extracted
by the ASPERA astrophysics roadmap.
The conclusive scenario of the evolution of the gravitational wave detectors in Europe (and in
the World) is reported in the Figure 13.
Figure 13 - Timeline of current detector operation and planned detector upgrades. The solid lines for the
existing detectors indicate data taking times. In the regions of dotted lines the mode of operation is not yet
defined. In the scenario shown, LISA will be launched in 2018 and start data taking in 2020 for a duration
of at least 5 years. Limited by the supply of consumables this period may be extended up to 10 years. The
3rd generation plans start with a 3 year design study in 2008, followed by a 4 year preparatory
construction phase. Site preparation, construction and commissioning will last for 7 years and allow data
taking from 2022 onwards.
3. Conclusions
The activities of N5_WG3 resulted in the successful proposal for a 3rd generation gravitational
wave detector design study under FP7. The next 15 years scenario has been depicted by WG3
within the ASPERA framework. The medium term upgrade of the current detectors has been
supported in the advanced Virgo and GEO-HF frameworks.
4. List of publications and conference proceedings
http://arxiv.org/pdf/0706.1431v1
"Observation of squeezed light with 10dB quantum noise reduction"
H. Vahlbruch, M. Mehmet, N. Lastzka, B. Hage, S. Chelkowski, A. Franzen, S. Gossler, K. Danzmann, and R.
Schnabel,
Phys. Rev. Lett. 100, 033602 (2008)
Using the etalon effect for in situ balancing of the Advanced Virgo arm
cavities Hild, S.; Freise, A.; Mantovani, M.; Chelkowski, S.; Degallaix, J.;
Schilling, R.
Classical and Quantum Gravity, Volume 26, Issue 2, pp. 025005 (2009).
Prospects of higher-order Laguerre Gauss modes in future gravitational wave
detectors Chelkowski, Simon; Hild, Stefan; Freise, Andreas Submitted, preprint
at: http://adsabs.harvard.edu/abs/2009arXiv0901.4931C
Pushing towards the ET sensitivity using 'conventional' technology Hild,
Stefan; Chelkowski, Simon; Freise, Andreas Technical note:
http://adsabs.harvard.edu/abs/2008arXiv0810.0604H
Triple Michelson Interferometer for a Third-Generation Gravitational Wave
Detector Freise, A.; Chelkowski, S.; Hild, S.; Del Pozzo, W.; Perreca, A.;
Vecchio, A.
Submitted, preprint at: http://adsabs.harvard.edu/abs/2008arXiv0804.1036F