th
B.Canuel
2
, J.Degaillaix
3
, A.Freise
9
, H.Grote
3
, H.Heitmann
4
, S.Hild
9
, G.Losurdo
5 , H.Lück 3
,
M.Mantovani
8
, J.Marque
2
, B.Sorazu
10
, B.Swinkels
2
, L.Taffarello
6
, E.Tournefier
1
, G.Vajente
8
,
M.Visco
7 , B.Willke
3
1 CNRS/IN2P3/LAPP, Annecy, France
2
European Gravitational Observatory, Italy
3
Max-Planck-Institut für Gravitationsphysik, AEI, Hannover, Germany
4
CNRS/Observatoire de la Cote d'Azur, France
5
INFN Firenze/Urbino, Firenze, Italy
6
INFN and Universita di Padova, Italy
7
CNR/IFSI and INFN Roma, Roma, Italy
8 INFN Pisa, Pisa, Italy
9
University of Birmingham, UK
10
University of Glasgow, UK
Coordinators: G. Losurdo (INFN, Firenze) and H. Grote (MPI, Hannover)
After the completion of the Virgo Science Run 1, Virgo has undergone a period of commissioning (until May 2008). The sensitivity achieved at the end of this commissioning period is shown in fig. 1. 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 As trowatch 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%.
A typical sensitivity curve of the GEO600 detector during the Astrowatch period is shown in figure 2.

Figure 1: Virgo sensitivity in May 08 (magenta) compared with theVirgo target sensitivity
(green) and the Virgo+ ones (blue and grey corresponding to with/without monolithic payload)
Analysis and comparison of main sources limiting the Virgo and GEO sensitivities:
Control noise - GEO
Control noise is typically the most important technical noise source at the lower end of the detection band of both, VIRGO and GEO.
Figure 2 shows the estimated contribution of the known noise sources to the strain sensitivity output, h, of the GEO600 detector. Control noise does not limit the detector for any frequency above approx. 60Hz, therefore the priority to further lower the contribution of control noise has been rather low for the GEO detector over the last year. However, within this time, a new optical bench has been installed at GEO, 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.
A small but significant reduction of the Michelson alignment feedback noise (up to factor 2 at
45Hz) could be achieved with the new sensors.

Figure2: Noise composition of the GEO600 detector, as of September 2008. The control noise levels are similar to the ones from 2007, with a small improvement of the control noise from the Michelson autoalignment system.
Control noise – Virgo
In the commissioning period between the end of the VSR1 and the Virgo+ shutdown, many efforts have been dedicated to reducing the contribution of longitudinal and angular control noise to the sensitivity (see fig.3, right). In summary control noise was no more limiting the
Virgo measured sensitivity at any frequency, and even below the design sensitivity at many frequencies.
Figure 3: Typical measured projection of longitudinal (left) and angular (right) control noise just before the Virgo+ shutdown. Both kind of control noises are not limiting the sensitivity at any frequency. Longitudinal control noise is below the design sensitivity above 30 Hz, while angular noise is below design above 20 Hz.

The strategy used to reduce the contribution of longitudinal noise focused mainly in
 mitigating the source of error signal noises, mainly due to environmental noises coupling through diffused light.
 improving the performances of the noise subtraction paths: the correction used to control the short degree of freedom are filtered and sent to the differential mode of the cavity end mirrors to cancel the direct coupling of control noise. With careful and precise measurements of the couplings we could obtain a stable reduction of noise coupling of a factor 500-1000.
The strategy used to reduce the contribution of angular noise focuses on similar topics:
 improvement of the sensors noise floor by implementing better electronics
 an extensive campaign of beam centering on all mirrors: indeed the coupling of angular noise scales directly with the distance of the beam from the mirror center
 optimization of the control filters used for the control, obtaining a better trade-off between low noise gain (below 100 mHz) and high frequency roll-off (above few Hz)
Finally, the contribution of noise coming from the actuation system (coil driver and DACs) have been reduced below the measured sensitivity by implementing new coil control systems and optimizing the dynamic range with respect to the needed forces. This has been obtained by implementing shaping filters to reduce the high frequency (above 10 Hz) contribution of
DAC noise.
Diffused light and acoustic couplings
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. A more detailed description of the device is given in the corresponding publication (see publication list at the end of this report). Figure 4 shows a measurement of the performance of the scattering suppression device.
The blue trace shows the sensitivity of the GEO detector in a setup, where scattered light from the optical bench behind one of the end mirrors was artificially enhanced.
The scatter suppression device (labeled as 'modulator' in the legend) is switched off in this case. The red trace shows the suppression of the scattered light effect by the device, while the green trace shows the reference sensitivity with no artificial scatter source.

It is shown that the scattered light can be suppressed by about one order of magnitude with the new device.
Figure 4: The effect of the modulation device, that was developed to suppress scattered light.
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 backscattering of the dark port optical components to the main interferometer.
Diffused light mitigation on the Virgo external benches
In order to improve the Virgo sensitivity at low frequency we carried out a campaign during
April-May 2008 on the Virgo external benches to reduce the level of diffused light. This light introduces noise on the dark fringe signal by re-coupling of the seismic motion of the optical benches.
In order to help this mitigation work, some studies were realized in the EGO optics lab to identify through the most common Virgo optical components, which were the most diffusing.
We set up a test bench that enabled to compare the diffusion of very different components as mirrors, lenses, wave-plates, photodiodes, beam dumps…
These results were used to determine which components should be avoided or replaced on the external Virgo benches, depending on their position and quantity of light that they receive.
One of the most critical components identified during this study are photodiodes which have a high average diffusion rate :1000-2000 ppm, and some of them are worse by more than one order of magnitude. We could also show concerning beam dumps that three orders of magnitude were separating an optimized beam dump realized with absorbing glass and razor blade beam dumps commonly used in Virgo. Similar differences were observed for mirrors

with total diffused light values going from 10 ppm to a few thousands ppm for some widely used commercial mirrors (CVI).
This study was the base for the diffused light mitigation. This was carried out by injecting some seismic lines with a shaker placed on the benches. The extra seismic noise was therefore creating some direct and up-converted noise measurable on the dark fringe trough the phasemodulation of diffused light re-introduced into the interferometer. By making some modifications on the benches, the improvement could be monitored by looking at the level of this up-converted noise.
Figure 5 shows the dark fringe noise without any seismic excitation (purple), with excitation before (red) and after (dark) improvement. After the mitigation we can see that diffused light on this bench (the External Injection Bench) is much reduced.
A similar work was carried out on all Virgo external benches. It mainly consisted in dumping correctly with absorbing glass some spurious reflections, substituting or removing some photodiodes and mirrors. A special care was also given to dump properly all powerful beams with dedicated beam traps. Positions of some optics on the benches were also modified in order to lower their back-scattering rate (photodiodes).
As shown on fig. 6, after this mitigation campaign a clear gain was obtained on the Virgo sensitivity at low frequencies (10- 100Hz) noise injection before/ modifications after detection studies
Figure 5: Dark fringe signal. Red curve shows
seismic noise re-introduced by diffused light on the EIB.
Figure 6: Virgo sensitivity before/ after mitigation on all Virgo External benches.
Thermal compensation
In parallel to VIRGO, GEO has also begun to investigate a thermal lensing compensation system. Both groups follow a similar approach in term of simulations and informal interactions often take place.
Even if the thermal compensation goals for VIRGO and GEO are the same, important differences must be highlighted:
• The GEO interferometer does not have arm cavities, as a result the thermal compensation will act on the beam splitter (instead of the input mirror for VIRGO).
•
Since inside the GEO beam splitter, the laser beam is not perpendicular to the surface, the heating pattern required to compensate thermal lensing cannot be circularly symmetric.

•
The absorbed power in the beamsplitter is much lower than inside the VIRGO input mirror
(around 8 times lower for the current configurations), therefore a lower compensation power is required, so we can possibly use a thermal radiative heat source instead of a CO2 laser for compensation.
The simulation of the thermal lensing compensation is done using the ANSYS program, interfaced with Matlab. Using the advanced optimization functions of Matlab, a suitable heating pattern can be found automatically. Such pattern is shown in figure 7.
Figure 7: optimal surface heating pattern to compensate the thermal lensing in the GEO beamsplitter.
The optimal heating pattern is found by minimizing the wavefront distortion induced by the beamsplitter. Numerically this is done by minimizing an overlap integral between the beam, distorted by thermal lensing, and the original cavity mode. The wavefront distortion before and after the compensation is shown in Figure 8.
Since the GEO detector requires a rather low heating power to compensate thermal lensing, we plan to implement thermally radiative elements instead of a CO2 laser beam to create the heating pattern. Radiative elements are much cheaper and more robust than a laser but the accurate focusing of wavelengths between 5µm and 12µm may pose a problem.
Currently the research is focusing on how to create the desired heating pattern accurately, and how it could be possible to tune its shape and position in a controlled manner. One of the explored designs consists of 24 small radiative elements (with each having a focusing reflector) arranged in an almost circular shape. Ray tracing simulations are used to determine the feasibility of this approach.

Figure 8: wavefront distortion induced by the optical absorption in the GEO beam splitter without (left) and with (right) thermal compensation. Using thermal compensation, the wavefront distortions are reduced is the central part (at the location of the laser beam) of the beam splitter.
Thermal Compensation System on the Virgo input mirrors
Since the main change of Virgo+ is an increase of the input power from 8 to 25 Watt, a lot of the commissioning effort is related to coping with the thermo-optic effects caused by the spurious absorption of the high power beams.
The main problem caused by the increase of power is thermal lensing in the input mirrors of the Fabry-Perot cavities in the long arms, which prevents achieving a stable lock of the interferometer. To solve this problem, we implemented a Thermal Compensation System
(TCS), which consist of 2 CO2 lasers that can heat the input mirrors from the side with a chosen pattern. A similar solution was implemented at LIGO several years ago. The positive thermal lens caused by the main YAG laser can be compensated by illuminating an annulusshaped pattern around the main beam. A second beam, which has a similar size as the YAG beam, can be switched on when the interferometer unlocks. This should always keep the mirrors in the same thermal condition, which should allow for a fast relock. The installation of the system was finished recently. Preliminary results show that using the TCS, we can indeed move relevant parameters of the interferometer towards a 'cold state'. A lot more commissioning time will be required to fully commission the system and use it to increase the input power.

Figure 9: Thermal image of the TCS beam with both the annulus and the 'central spot' switched on
Injection system
Thermal issues have also become more pronounced in the optics of the injection system.
During the last shut-down, an extra optical amplifier was installed on the laser bench which can produce a power of up to 60 Watt. One problem observed already a few years ago was the loss of the attenuation factor of the Faraday Isolator when going from air to vacuum, which is now understood using simulations. The problem has been solved by adding a remotely tunable wave-plate in the vacuum system. Other effects observed where a thermally induced misalignment and thermal lensing of the mirrors of the Input Mode Cleaner. These effects have now largely been understood by simulations and have been solved by a careful alignment.
Phase camera
A new analysis tool that was recently installed is the 'phase camera'. It is able to simultaneously demodulate the individual sidebands and carrier and to make images by scanning the beam over a pinhole detector. Although originally designed for characterizing the whole interferometer in standard conditions, we were able to use it in a simple configuration in which only a single arm-cavity is locked. In this way it is possible to directly image the thermal lens developing in a single input mirror. This method has already been used successfully for the alignment of the TCS beam and will be used further for the calibration of the TCS system.

Figure 10: Amplitude images (top) and phase images (bottom) obtained with the phase camera during a TCS test. The blue spot is a thermal lens caused by a heating the input mirror with a Gaussian shaped CO2 beam.
Simulations
To better understand all the thermal issues mentioned above, a large simulation effort was undertaken. Simulations were performed using ANSYS, Matlab and optical software. For special cases we even have a complete analytical solution. Quantitative predictions, such as time-constants and induced wavefront tilts have been accurately predicted. Also for complex problems, such as misalignments of the TCS system, the simulations have helped to get a better understanding of experimental results. One important parameter which we are trying to extract by combining experiments with simulations is is the absorption coefficient of the input mirrors, which is currently known only with a large uncertainty.

Figure 11: Simulation of input mirror heated by annulus shaped TCS beam.
Graphs on the left show the thermal profile, the one on the right shows the transmitted optical wavefront and a fit of wavefront curvature.
DC detection studies
Even if GEO as part of the Astrowatch period had to maximize the collection of data over the last year, some time has been dedicated to explore new ways to read the gravitational wave signal from the interferometer. The most promising scheme is called DC readout and is planned to be used by the next generation of interferometers (including Advanced Virgo). DC readout requires a small offset from the dark fringe, such that at the output of the detector a small part of the carrier is present and can be used as a local oscillator beam.
At the same time as going to DC readout, we also switch the signal recycling mirror cavity from a detuning of 550 Hz to a tune cavity case (i.e. detuning at 0 Hz).
Switching from the current heterodyne readout to tuned DC readout is partially automated and can be accomplished within 15 minutes within any loss of lock. A sensitivity comparison between the two configurations is shown in Figure 12.
Although DC readout is still an experimental technique and requires further commissioning, it is absolutely encouraging that we manage to get similar sensitivity in the area of 100 Hz- 300
Hz with DC readout, compared to the standard heterodyne technique. One of the current problems with DC readout is that the autoalignment system is more prone to instability and that there is some unknown excess noise in the area around 500 Hz. We are confident that these problems will be solved in the coming months as GEO will be released from its
Astrowatch duty during spring 2009, allowing some extended commissioning.

Figure 12: initial comparison between heterodyne readout (blue curve) and the tuned DC readout (red curve). It is worth to the note the different slope of the shot noise at high frequency, above 1 kHz.
A dedicated calibration procedure has been created and tested for the DC readout configuration. The different calibration scripts for the different configurations can be easily switched between each other and it does not affect the way the data is recorded (so it is completely transparent for the user).
Comparison of characterization methodologies and tools:
Noise non stationarity monitor
HACR 1 glitches analysis
A long term analysis of HACR glitches was carried out for the GEO detector. The time period of analysis was taken to be from February 2007 to April 2008. In this analysis we see that there are three frequency regions where glitches accumulate; one at low frequencies below
250 Hz (the main source being laser power), another one between 250 to 700 Hz, and variable frequency bands at frequencies above 700 Hz (which we have named “the owl”, and which are still of unknown origin). We show a representative plot of these glitch bands in Fig. 13 where the time and central frequency of the h-channel (G1:DER_DATA_H) HACR-detected glitches are plotted for 3 weeks of January 2008.
1
Hierarchical Algorithm for Curves and Ridges, it is the event trigger generator used in GEO600 to detect transientburst event. [1, 2]

Figure 13: Time and central frequency plot of the G1:DER_DATA_H channel HACR glitches for the month of January 2008
It was noticed that the two lower frequency bands of glitch accumulation (the one below 250
Hz and the one between 250 and 700 Hz) affected appreciably GEO600 sensitivity in the frequency range between 90 and 600 Hz. The higher the glitch density in these bands the lower GEO600’s sensitivity, it being more affected by the lower frequency band.
Because we have already carried out several coincidence analyses between glitches in channel
H and the glitches in the main auxiliary channels, we decided to look at a new approach to see if we could gain more information. This approach compares not the glitches themselves but the glitch rates in the three formerly mentioned frequency bands between the H channel and the main auxiliary channels. This was done to see if it could give a new idea of the origins of the owl or if we could get an explanation of the changes in glitch density.
The glitch rate analysis consisted in determining the number of glitches per hour (we tried with different resolutions and the hour resolution was the best compromise), for channels H and each of the most relevant auxiliary channels (we chose those in GEO600 summary pages
2
, 40 in total). The glitch rate was obtained for each of the above frequency bands and, in order to avoid misleading effects in the posterior comparisons, we only analysed those 1 hour segments where the detector was locked for more than 90% of the segment time and which was not adjacent to unlocked segments. We further filtered the analysed segments by rejecting those with no glitches in any of the frequency bands for the channel H.
The comparison of the glitch rates, between channel H and the auxiliary channels, was done by a cross-correlation of the glitch rate curves (normalized respect to their maximum glitch rate value for each channel) of each auxiliary channel with channel H. Then we also looked at the periodicity of this cross-correlation by a coherence analysis of these normalized glitch rates.
Due to the huge amount of data obtained it was impractical to complete every possible analysis, however a few conclusions could be reached:
1.
The analysis provided time identification of some important hardware changes on
GEO600, as for instance the installation of new transformers.
2 GEO600 summary pages are a group of web pages with reports of the most relevant detector characterization information of GEO updated in a per day basis. They also monitor the current detector status. [3]

2.
An interesting peak (lasting for more than 1 week) of high cross-correlation in the middle bandwidth (250-700 Hz) was observed in most of the seismic isolation channels (SEI), magnetometer channels, LSC_MID channels, POWERGRID channel, AA channels. Note that while it is very obvious in the SEI channels in TFN and TFE, this effect does not appear on the SEI channels at TCC as shown in fig. 14. This is an interesting feature because all the seismometers are in the inner foundation where the mirrors are located, this isolates the mirrors from activity within the GEO buildings but it does not isolate them from seismic activity. All seismometers should be similarly sensitive to seismic activity (similar to channel H), however the isolation of the foundations of the East and
North buildings is not as good as in the central building, therefore activity in the N and E buildings appears more easily in seismometers and channel H than activity in the central building. However this does not explain why seismometers in central building do not show the high cross-correlation peak for this period of time as the other seismometers do.
Notice that during this time there were some changes made to the laser power. The experiments consisted of adjusting the half-wave-plate angle between 70 and 75 degrees, which controls the optical input power to the system.
Figure 14: Glitch rate cross-correlation between channel H and seismometer channels.
Notice that each point in the cross-correlation plots corresponds to 24 hours, therefore this phenomenon lasts for around a week.
It is interesting to point out that the cross-correlation does not have a relationship with the coincidences between the investigated channels and H. The cross-correlation is based on glitch rate information, so high cross-correlation means that a high number of glitches in
H changes similarly to the number of glitches in the auxiliary channel but this does not mean that those glitches interact.
3.
The high frequency band (above 700 Hz) glitch rate for channel H is mainly dominated by the glitch bands associated with the owl. Therefore looking at the glitch rate plot of channel H allows a rapid identification of the presence of the owl in time.

HACR at Virgo
The Virgo part of HACR code is running on-line since VSR1 and provides continuously triggers for the main gravitational wave channels. It is now being integrated inside an automatic web interface to noise monitoring.
One of the most useful application of HACR to the commissioning activity was linked to the test of new demodulation and ADC electronics. This new electronic has been installed on one of the two photodiodes used for the dark fringe read-out. HACR has been used to compare the glitchiness of the two output signals. An excess of glitches has been found in the new electronic and this has helped a lot the board debugging.
Fig. 15. Triggers detected by VirgoHACR in the two channels reading the dark fringe. The left one was acquired with the standard electronic, the right one with one under test.
Automatic noise budget
Since the automatic noise budget has been developed and tested during the previous three years, it is now in a pretty mature and robust state. For GEO600, a set of new scripts have been written for the tuned DC readout configuration. In general, tuned DC readout allows a simpler automatic noise budget since the gravitational-wave data is no longer spread over two quadratures which have to be optimally recombined as in the case of detuned heterodyne readout.
The noise budget for DC readout is still preliminary and incomplete as some noises have to be injected by different means for technical reasons. The current DC readout noise budget is shown in Figure 16.
Automatic noise budget tools are widely used also in Virgo. One example is shown in fig. 3, displaying the control noise contributions. In fig. 17 the overall noise budget concerning the
WSR11 (Weekly Science Run 11) recently completed is shown.

Figure 16: Preliminary DC readout noise budget. Other traces will be added during the planned extensive commissioning of this configuration in the middle of 2009.
Figure 17: Preliminary Virgo noise budget during WSR11 (Feb 14-16 2009)

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. We intend to continue this networking in the years to come, with both detectors increasing sensitivity and taking part in science runs in this rapidly growing and ever more exciting field of research.
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,
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