The German-British laser-interferometric gravitational wave detector GEO 600 is in its 13th year of operation since its first lock in 2001. After participating in science runs with other first generation detectors, GEO 600 has continued collecting data as an astrowatch instrument with a duty cycle of 62% during the time when the other detectors have gone offline to undergo substantial upgrades. Less invasive upgrades to demonstrate advanced technologies and improve the GEO 600 sensitivity at high frequencies as part of the GEO-HF program have additionally been carried out in parallel to data taking. We report briefly on the status of GEO 600

Dooley, K.L.

et al.

LIGO Scientific Collaboration

Institutionelles Repositorium der Leibniz Universität Hannover

Journal of Physics: Conference SeriesPAPER • OPEN ACCESSStatus of GEO 600To cite this article: K L Dooley and LIGO Scientific Collaboration 2015 J. Phys.: Conf. Ser. 610012015 View the article online for updates and enhancements.Related contentRecent results on the search forcontinuous sources with LIGO and GEO600Alicia M Sintes and the Ligo Scientificcollaboration)-Silica suspension and coatingdevelopments for Advanced LIGOCagnoli G, Armandula H, Cantley C A etal.-Thermal correction of the radii of curvatureof mirrors for GEO 600H Lück, A Freise, S Goßler et al.-Recent citationsBroadband sensitivity enhancement ofdetuned dual-recycled Michelsoninterferometers with EPR entanglementDaniel D. Brown et al-Statistical measures of Planck scale signalcorrelations in interferometersCraig J Hogan and Ohkyung Kwon-First Measurements of High FrequencyCross-Spectra from a Pair of LargeMichelson InterferometersAaron S. Chou et al-This content was downloaded from IP address 194.95.158.72 on 27/09/2017 at 09:39Status of GEO600K L Dooley 1 for the LIGO Scientific CollaborationMax-Planck-Institut fu¨r Gravitationsphysik (Albert-Einstein-Institut) und Leibniz Universita¨tHannover, Callinstr. 38, D-30167 Hannover, GermanyE-mail: kate.dooley@aei.mpg.deAbstract. The German-British laser-interferometric gravitational wave detector GEO 600 isin its 13th year of operation since its first lock in 2001. After participating in science runswith other first generation detectors, GEO 600 has continued collecting data as an astrowatchinstrument with a duty cycle of 62% during the time when the other detectors have gone oﬄineto undergo substantial upgrades. Less invasive upgrades to demonstrate advanced technologiesand improve the GEO 600 sensitivity at high frequencies as part of the GEO-HF program haveadditionally been carried out in parallel to data taking. We report briefly on the status ofGEO 600.1. BackgroundGEO 600 [1] is the German-British laser-interferometric gravitational wave (GW) detector thatwas built in the late 1990s and started operation in the early 2000s [2, 3] in conjunction withother first generation GW detectors. GEO 600 uses a dual-recycled Michelson configuration with1200 m long arms folded inside 600 m long beam tubes. From the start, the project focused onresearching, incorporating, and demonstrating riskier technologies compared to the designs ofits longer-arm counterparts, Virgo (3 km) [4] and LIGO (4 km) [5]. Early design features thatwere unique to GEO 600 included multi-chain suspensions with a monolithic final stage and areaction mass chain, electro-static actuators, and signal recycling [6].Following the era of first generation detector science runs when the other observatories begangoing oﬄine for substantial upgrades [7, 8], GEO 600 opted to serve the role of being thesole detector continuing to collect data through a program called Astrowatch. Simultaneously,starting in 2009, GEO 600 began a program called GEO-HF [9] which has the aim of carrying outa set of upgrades to improve the detector’s high frequency sensitivity above approximately 600 Hzwhere quantum shot noise is the limiting noise source. The primary techniques implementedinclude the injection of squeezed vacuum, a change in signal recycling bandwidth and operatingpoint, DC readout with an output mode cleaner, and higher laser power combined with a thermalcompensation system.2. GEO-HF statusFigure 1 shows the results to date of how the GEO-HF upgrades have affected the GEO 600strain sensitivity. The most notable effect is above 600 Hz where as much as a factor of 4improvement in sensitivity is achieved. The dominant contributing factor to this improvement1 Current address: The University of Mississippi, University, MS 38677, USA10th International LISA Symposium (LISAX) IOP PublishingJournal of Physics: Conference Series 610 (2015) 012015 doi:10.1088/1742-6596/610/1/012015Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd 1102 10310−2210−2110−2010−19strain [Hz−1/2 ]frequency [Hz]  20092013Figure 1. Strain sensitivity of GEO 600 today compared to the sensitivity at the start ofthe GEO-HF upgrade [3]. A new signal recycling mirror, squeezed vacuum injection, and DCreadout contribute to the substantial improvement in high-frequency sensitivity. An effort toincrease the laser power is ongoing. The large cluster of lines centered around 650 Hz are theviolin modes of the suspension fibers.is the decrease in the finesse of the signal recycling cavity (SRC). This was accomplished in 2010by exchanging the signal recycling mirror from one with reflectivity of R = 98% to one withR = 90%, thus reducing the storage time of the GW signal in the interferometer and thereforealtering the detector’s frequency response. The next most significant factor is squeezing, whichcreates (in its current state) a factor 1.5 improvement in sensitivity above approximately 1 kHzand was first demonstrated in 2010 [10]. Finally, a change in 2009 from heterodyne to homodyne(DC) readout of the GW signal [11] renders a fundamental factor√3/2 improvement in signal-to-noise ratio (SNR) [12]. An increase in laser power is not reflected in the 2013 sensitivity curveand is a topic of ongoing work.The marked degradation in strain sensitivity below 100 Hz arises from technical side effectsof both DC readout and the new signal recycling mirror. The limiting noise sources at thesefrequencies are alignment feedback noise and signal recycling length feedback noise. First,the dark-fringe offset required for DC readout creates a TEM00 carrier field on the Michelsonalignment sensors. Jitter of this field caused by output optics creates an alignment sensingoffset which is impressed on the mirrors. Second, the new signal recycling cavity’s lower finessedecreased the SNR of the SRC length signal, creating higher feedback noise.3. Ongoing researchOngoing research at GEO 600 has several fronts. For one, there are still some technical challengesand open research questions related to realizing the full potential of DC readout and squeezing.Second, the GEO-HF goal of increasing the laser power by a factor of 4 – 6 must still be achievedand requires the design and commissioning of a new thermal compensation subsystem.10th International LISA Symposium (LISAX) IOP PublishingJournal of Physics: Conference Series 610 (2015) 012015 doi:10.1088/1742-6596/610/1/01201523.1. Squeezed vacuum integrationInjection of squeezed vacuum to the interferometer’s output port is a novel technique employedto suppress quantum noise [13]. For nearly the last 3 years, GEO 600 has demonstrated thefirst long-term application of squeezed vacuum in a gravitational wave detector, achieving a90% duty cycle during science times and improving the shot-noise-limited strain sensitivity upto a factor of 1.5 (3.7 dB) [10, 14]. In order to achieve even more stable and better squeezinglevels, recent work at GEO 600 has focused on the development and commissioning of squeezeralignment and phase control systems [6]. Ongoing work includes the commissioning of a newwavefront sensing scheme in reflection of the OMC to ensure maximum overlap of the squeezedfield with the interferometer output field to achieve higher squeezing levels. In addition, a newerror signal for sensing the relative phase of the squeezed field with the interferometer output isbeing studied in order to eliminate lock point offsets and create more stable squeezing [15].3.2. Higher laser powerOne unique feature of GEO 600 is that it does not have Fabry-Perot arm cavities. Instead, asignal recycling mirror alone is used together with the power recycling mirror to amplify theGW signal. A side effect is that all of the stored power in the interferometer is transmittedthrough the substrate of the beam splitter (BS). The current operation of GEO 600 uses 2–3 kW at the BS, which leads to thermal lensing problems of the same magnitude that will befaced by advanced detectors when operated at full design power. GEO 600 currently does nothave any thermal compensation at the BS and this limits the ability to operate with higherlaser power. During commissioning periods, stable operation with as much as 7 kW can beachieved, but there is excess noise below 600 Hz in this high power state. Design of an arrayof heating elements to project optimized patterns of radiation onto the BS to compensate thelens represents ongoing work. Other challenges related to higher power which have already beenaddressed are the correction of astigmatism at one of the end mirrors using segmented thermalcompensation [16] and the elimination of stray light coupling to shadow sensors on each of themirrors through a modulation-demodulation technique [17].3.3. Alternative OMC alignment techniquesThe upgrade from RF to DC readout involved the installation of an output mode cleaner (OMC)in the interferometer’s output port directly before the GW readout photodiode. Its purpose isto filter out all light that does not contribute to the GW signal. Higher order modes make itchallenging to develop an alignment scheme with sufficient bandwidth that maximizes the SNR ofthe light transmitted through the OMC without introducing beam jitter coupling. Ongoing workat GEO 600 is the commissioning of a new alignment technique which uses wavefront sensorsin reflection of the OMC. The wavefront sensors sense the relative alignment between fieldsrepresenting the GW mode and the OMC eigenmode. The fields used are the radio frequencysidebands which are resonant in the interferometer and audio sidebands which are generatedfrom a length dither of the OMC [6]. This scheme both eliminates low frequency dithering ofthe output steering optics and facilitates an increase in the alignment control bandwidth up toseveral Hz.4. AstrowatchStarting in 2007, GEO 600 has served as the community’s Astrowatch detector. Astrowatch modeis a science run from the standpoint of detector operations such that GEO 600 is maintainedat its highest possible sensitivity in a full interferometer lock and no experiments are allowedwhich could reduce its sensitivity. Furthermore, a realtime system with interferometer parametertracking assures valid strain calibration [6]. A search in the Astrowatch data is triggered by anexternal event such as a gamma ray burst [18], neutrino detection, or an optical detection10th International LISA Symposium (LISAX) IOP PublishingJournal of Physics: Conference Series 610 (2015) 012015 doi:10.1088/1742-6596/610/1/0120153of a supernova. Upon operating in science mode mostly on nights and weekends, GEO 600has maintained an average science time duty cycle of 62% over the last several years. Activecommissioning and upgrade activities are carried out during the remainder of the time.5. Summary and outlookGEO 600 continues to actively pursue the goals of the GEO-HF upgrade program and todedicate significant time to collecting Astrowatch data during the time when the other GWdetectors are being upgraded. The techniques of signal recycling, homodyne (DC) readout, andsqueezed vacuum injection have been demonstrated to the benefit of the GEO 600 high frequencysensitivity. Research into better integration of squeezed vacuum and improved OMC alignmentschemes are active areas of continued work on these topics. At the same time, a new thermalcompensation system for the BS is being developed which will enable the use of higher laserpowers. GEO 600 will continue to collect Astrowatch data and research advanced techniques inconjuction with furthering the improvement of its high-frequency sensitivity.AcknowledgementsThe authors are grateful for support from the Science and Technology Facilities Council (STFC),the University of Glasgow in the UK, the Bundesministerium fu¨r Bildung und Forschung(BMBF), and the state of Lower Saxony in Germany. This work was partly supported by DFGgrant SFB/Transregio 7 Gravitational Wave Astronomy. This document has been assignedLIGO document number P1400172.References[1] Willke B et al. 2002 Classical and Quantum Gravity 19 1377+[2] Grote H 2008 Classical and Quantum Gravity 25 114043+[3] Grote H and the LIGO Scientific Collaboration 2010 Classical and Quantum Gravity 27 084003+[4] Accadia T et al. 2012 Journal of Instrumentation 7 P03012+[5] Abbott B P et al. 2009 Reports on Progress in Physics 72 076901+[6] Affeldt C et al. 2014 Classical and Quantum Gravity 31 224002+[7] Harry G M and the LIGO Scientific Collaboration 2010 Classical and Quantum Gravity 27 084006+[8] The Virgo Collaboration 2009 Advanced Virgo Baseline Design Tech. Rep. VIR-027A-09[9] Lu¨ck H et al. 2010 Journal of Physics: Conference Series 228 012012+[10] The LIGO Scientific Collaboration 2011 Nat Phys 7 962–965[11] Hild S et al. 2009 Classical and Quantum Gravity 26 055012+[12] Niebauer T M, Schilling R, Danzmann K, Ru¨diger A and Winkler W 1991 Physical Review A 43 5022–5029[13] Caves C M 1980 Physical Review Letters 45 75–79[14] Grote H, Danzmann K, Dooley K L, Schnabel R, Slutsky J and Vahlbruch H 2013 Physical Review Letters110 181101+[15] Dooley K L, Schreiber E, Vahlbruch H, Affeldt C, Leong J R, Wittel H, and Grote H 2014 submitted to OSAarXiv:1411.3454[16] Wittel H et al. 2014 Classical and Quantum Gravity 31 065008+[17] Affeldt C 2014 Laser Power Increase for GEO600 Ph.D. thesis Leibniz Universita¨t Hannover[18] The LIGO Scientific Collaboration and the Virgo Collaboration 2014 Physical Review D 89 122004+10th International LISA Symposium (LISAX) IOP PublishingJournal of Physics: Conference Series 610 (2015) 012015 doi:10.1088/1742-6596/610/1/0120154

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