N5: Gravitational Wave Antenna (GWA)

N5: Gravitational Wave Antenna (GWA)

Coordinator: M. Punturo (INFN and EGO))

Deputy Coordinator: K. Danzmann (MPI/Hannover)

The objective of this network is to increase the level of European coordination in the field of gravitational wave research in order to maximize the return on the large investment already made, to enhance the performance of the existing detectors, and to prepare the future of this discipline in Europe. The network is used to exchange information on subjects of common interest such as commissioning and operation of existing detectors, to foster the development of common methodologies for joint observations, and to support the elaboration of a common strategy for future gravitational wave detectors in Europe.

Several institutions, participating in ILIAS, are active in this network. The participation of contractors in terms of persons-months is given in Table 1a. A single contractor, namely the European Gravitational Observatory (EGO), ensures the financial management of this network.

The GWA network activity is progressing as foreseen. The network includes three Working Groups: WG1 on antenna commissioning and characterization, WG2 on joint operation of antennas, and WG3 defining a European strategy for future detectors. All the working groups are holding meeting on a regular bases. The list of meetings organized since April 1st 2008 is given in Table 3. All the documentation concerning this network is available on the WEB at

N5 – Table 1a: Participants institutes

N5 – Table 1b: Other entities

N5 – Table 2: Deliverables.

N5 – Table 3: Workshops/Meetings.

N5 – Table 4 : Role of participant contractors

Activity of working group #1: Antenna commissioning and characterisation

Working group #1 is devoted to the commissioning and characterization of existing antennas.

After the completion of the Virgo Science Run 1 (May-October 2007), Virgo has undergone a period of commissioning (until May 2008). The improvement in the Virgo sensitivity achieved at the end of this commissioning period has been excellent, approaching the design sensitivity and transforming the Virgo interferometer in the most sensitive gravitational wave detector in the World, in the low frequency regime. Afterwards, the implementation of the first set of Virgo+ upgrades started (laser amplifier to get more power, new electronics, thermal compensation system). Since October 08 Virgo is again in commissioning phase, which will last until the middle of 2009, when a new science run (VSR2) will start.

The GEO600 detector has been operating in the so-called Astrowatch mode since November 2007. In this mode, the GEO team has been aiming to achieve an up-time of the detector of

at least 80% of the time. Any fraction of time available beyond this goal, could be used for experiments, investigations, and improvements of the detector.

From November 2007 to the end of December 2008, the up-time of GEO600 has been 86.5%.

The improvement of the detectors passed through the reduction of the technical noises, affecting the machines in a fraction of the frequency regime. N5-WG1 discussed and analyzed the technical noises in both the detectors.

Control noise is typically the most important technical noise source at the lower end of the detection band of both, VIRGO and GEO. In the latter, a new optical bench has been installed, which is to be used for the alignment of the Michelson interferometer in the future, replacing the existing alignment system at the main detection bench. In the course of the setup of this new system, the sensors have been optimized, with the result of a slightly better sensitivity of the alignment sensors. In Virgo, many efforts have been dedicated to reducing the contribution of longitudinal and angular control noise to the sensitivity. In summary control noise was no more limiting the Virgo measured sensitivity at any frequency, and even below the design sensitivity at many frequencies.

Scattered light from a large variety of sources has continuously been important as a limiting noise source at different frequencies at both, the VIRGO and the GEO, detectors. In some cases it is possible to find a source of scattered light by blocking a laser beam path on its way to a component that is to be investigated for scattered light effects. In other cases however,the component to be investigated, for example a photodiode, is an essential part of the locking system of the interferometer, such that blocking the light on its path to the component is simply not possible without interrupting the lock. To investigate this class of scattered light sources (or even provide means of suppressing such a noise), a device was developed within the GEO group, which transmits all the light incident, but applies a frequency shift to the light, in a way that light being scattered back to the interferometer (and thus passing the device twice) has close to zero light power remaining at the fundamental laser frequency.

The scattered light suppressor was implemented to the main detection port of the GEO detector, with the result that no change in the sensitivity of the GEO detector was observed. Thus we could conclude for the first time, that the GEO sensitivity was not limited from back-scattering of the dark port optical components to the main interferometer. In the Virgo detector scattered light has been mitigated, first by identifying the optical components source of the process, through an intense measuring campaign, inserting extra seismic lines with a shaker placed on the benches. Modification of the optical paths, mirror supports and improvement of the optical quality of the components permitted a huge suppression of the scattering processes.

Other technical issues have been discussed in WG1: mitigation of the thermal effects in Virgo, through the introduction of a thermal compensation system (now operative), new Virgo injection system (now operative), the phase camera (now under commissioning). Furthermore, investigation tools have been exchanged between the Virgo and GEO collaborations; for example a glitch monitoring system (called HACR), developed by GEO scientists has been introduced and adopted in Virgo.

We can state that Working Group 1 has continued to play an important role in exchanging experience between the VIRGO and GEO commissioning teams. Both detectors improved in sensitivity and understanding of the limiting noise sources, and planning of medium- and long term upgrades is well under way.

Activity of working group #2: Joint operation of antennas and network data analysis

Working group #2 is devoted to the joint operation and data analysis of the existing European gravitational wave antenna.

The GWA-WG2 focus on promoting information exchange between gravitational wave data analysts and astronomers has continued into the final year of the project. This year, in addition to supporting ongoing searches, a search for gravitational waves associated with glitches in radio frequency pulses from neutron stars was performed. The results of the analysis stressed the importance of using all available information, from simulations and observations, to improve the sensitivity of the gravitational wave search and provide better interpretation of the results.

The ongoing support for joint analysis between the LIGO Scientific Collaboration, which includes GEO 600, and the Virgo collaboration has helped produced results from the joint analysis of the S5/VSR1 run. Members from both collaborations came together and worked hard to learn about the methods and philosophies of their data analysis approaches. The preliminary results of this work were presented at the GWDAW13 meeting in January, 2009.

To increase interaction between the supernova simulation, neutrino and gravitational wave communities, a workshop was organized in Cascina which brought together representatives from all three communities. The focus of the workshop was on neutrino transport in core-collapse supernovae which affects the simulations for gravitational wave emission from core-collapse supernovae. From the gravitational wave point of view, the discussions from this workshop provided insight into the physics employed by the simulators. There was also discussion and exchange of information on the interpretation of results from the different communities.

Preparation of the science case for future gravitational-wave detectors is being actively pursued, with particular emphasis on third-generation ground-based observatories such as the proposed Einstein gravitational-wave Telescope. The latter will be more sensitive to both lower (down to a few Hz) and higher (up to 10 kHz) frequencies than existing and planned observatories such as Advanced LIGO, Virgo, and future upgrades of GEO600.

Activity of working group #3: A European strategy for future detectors

Working group #3 is devoted to the elaboration of a strategy for the future development of gravitational wave detectors in Europe.

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 cornerstones in 2008: the GWDAW 2008 - VESF meeting and the Joint ET-ILIAS_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

The second cornerstone 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:

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.

In addition, WP3 supported the study of the upgrade of the current detectors toward the advanced generation. In this context the design of GEO-HF and advanced Virgo has been supported.

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.

WP3 aided the technical design of the advanced Virgo detector, mainly through the support of the University of Birmingham scientists participating to this effort. The documentation describing the preliminary design of the advanced Virgo detector is reported here:

The Birmingham University scientists 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 final effort of the WG3 has been the realization of a roadmap depicting the evolution of the gravitational wave experimental search in the next 15 years. This activity has been performed in collaboration with the ASPERA groups and the GWIC organization. In the roadmap, the upgrade of the current interferometers to the 2nd generation will start about the 2011 and should be completed in 2014.The realization of the 3rd generation detectors will start about the 2016-2017.

List of publications and conference proceedings

S Hild, H Grote, J Degallaix, S Chelkowski, K Danzmann, A Freise, M Hewitson, J Hough, H Lück, M Prijatelj, K A Strain, J R Smith, and B Willke,

DC-readout of a signal-recycled gravitational wave detector,

Class. Quantum Grav., accepted for publication (2009)

H Grote et al.,

The Status of GEO 600,

Class. Quantum Grav. 25 114043 (7 June, 2008) 1513-1523

H Lück, J Degallaix, H Grote, M Hewitson, S Hild, B Willke and K Danzmann,

Opto-mechanical frequency shifting of scattered light,

J. Opt. A: Pure Appl. Opt. (2008) 10 085004

The Virgo Collaboration,

Noise studies during the first Virgo science run and after,

Class. Quantum Grav. 25 184003 (2008)

The Virgo Collaboration,

Status of Virgo,

Class Quantum Grav 25 (2008) 114045

The Virgo Collaboration,

Lock acquisition of the Virgo gravitational wave detector,

Astropart. Phys. (2008), 30, 29-38

I.S. Heng:

"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:

Pushing towards the ET sensitivity using 'conventional' technology Hild, Stefan; Chelkowski, Simon; Freise, Andreas Technical note:

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: