033109-11 Dooley et al Rev.Sci.Instrum.83,033109(2012) The Enhanced LIGO EOM showed no thermal lensing, design,and performance of the IO.Several improvements to degraded transmission,nor damage in over 17000 h of sus- the design and implementation of the Enhanced LIGO IO tained operation at 30 W of laser power.Measurements of over the Initial LIGO IO have led to improved optical ef- the thermal lensing in RTP at powers up to 160 W show a ficiency and coupling to the main interferometer through a relative power loss of <0.4%,indicating that thermal lensing substantial reduction in thermo-optical effects in the major should be negligible in Advanced LIGO.Peak irradiances in IO optical components,including the electro-optic modula- the EOM will be approximately four times that of Enhanced tors,mode cleaner,and Faraday isolator.The IO performance LIGO(a 45%larger beam diameter will somewhat offset the in Enhanced LIGO enables us to infer its performance in Ad- increased power).Testing of RTP at 10 times the expected vanced LIGO,and indicates that high power interferometry Advanced LIGO irradiance over 100 hours show no signs of will be possible without severe thermal effects. damage or degraded transmission. The MC showed no measurable change in operational state as a function of input power.This bodes well for the ACKNOWLEDGMENTS Advanced LIGO mode cleaner.Compared with the Enhanced LIGO MC,the Advanced LIGO MC is designed with a lower The authors thank R.Adhikari for his wisdom and guid- finesse (520)than Initial LIGO (1280).For 150 W input ance.B.Bland for providing lessons to K.Dooley and power,the Advanced LIGO MC will operate with 3 times D.Hoak on how to handle the small optics suspensions,K. greater stored power than Initial LIGO.The corresponding Kawabe and N.Smith-Lefebvre for their support at LHO, peak irradiance is 400 kW/m2,well below the continuous- T.Fricke for engaging in helpful discussions,and V.Ze- wave coating damage threshold.Absorption in the Advanced lenogorsky and D.Zheleznov for their assistance in prepar- LIGO MC mirror optical coatings has been measured at ing for the Enhanced LIGO IO installation.Additionally,the 0.5 ppm,roughly four times less than the best mirror coating authors thank the LIGO Scientific Collaboration for access to absorption in Enhanced LIGO,so the expected thermal load- the data.This work was supported by the National Science ing due to coating absorption should be reduced in Advanced Foundation (Grant Nos.PHY-0855313 and PHY-0555453). LIGO.The larger Advanced LIGO MC mirror substrates and LIGO was constructed by the California Institute of Tech- higher input powers result in a significantly higher contribu- nology and Massachusetts Institute of Technology with fund- tion to bulk absorption,roughly 20 times Enhanced LIGO, ing from the National Science Foundation and operates under however the expected thermal lensing leads to small change cooperative agreement PHY-0757058.This paper has LIGO (<0.5%)in the output mode.22 Document Number LIGO-P1100056. The Enhanced LIGO data obtained from the FI allows us to make several predictions about how it will perform in J.Abadie.B.P.Abbott,R.Abbott,M.Abernathy,T.Accadia,F.Acernese, Advanced LIGO.The measured isolation ratio decrease of C.Adams,R.Adhikari,P.Ajith,B.Allen et al..Classical Quant.Grav.27 0.02 dB/W will result in a loss of 3 dB for a 150 W power 173001+(2010). level expected for Advanced LIGO relative to its cold state. 2B.P.Abbott.R.Abbott.R.Adhikari,P.Ajith,B.Allen,G.Allen, R.S.Amin.S.B.Anderson,W.G.Anderson,M.A.Arain et al.,Rep. However,the Advanced LIGO FI will employ an in situ Prog.Phys.72,076901+(2009). adjustable half wave plate which will allow for a partial 3F.Acernese,P.Amico,M.Alshourbagy.F.Antonucci,S.Aoudia, restoration of the isolation ratio.In addition,a new FI scheme P.Astone.S.Avino,L.Baggio,G.Ballardin,F.Barone et al,J.Opt.A. to better compensate for thermal depolarization and thus Pure Appl..Opt.10,064009+(2008). H.Luick.M.Hewitson,P.Ajith,B.Allen,P.Aufmuth,C.Aulbert,S.Babak yield higher isolation ratios will be implemented.35 The R.Balasubramanian,B.W.Barr.S.Berukoff et al.,Classical Quant.Grav maximum thermally induced angular steering expected is 480 23.S71(2006. urad (using a drift rate of 3.2 urad/W),approximately equal R.Adhikari,P.Fritschel,and S.Waldman,"Enhanced LIGO,"Technical Report T060156,LIGO Laboratory,2006. to the beam divergence angle.This has some implications 6H.Luick,C.Affeldt,J.Degallaix,A.Freise,H.Grote,M.Hewitson,S.Hild, for the Advanced LIGO length and alignment sensing and J.Leong.M.Prijatelj.K.A.Strain.B.Willke.H.Wittel,and K.Danzmann, control system,as the reflected FI beam is used as a sensing J.Phys.:Conf.Ser.228.012012+(2010). beam.Operation of Advanced LIGO at high powers will Advanced LIGO Systems Group,"Advanced LIGO Systems Design." Technical Report T010075,LIGO Laboratory,2009. likely require the use of a beam stabilization servo to lock the 8N.A.Robertson.B.Abbott,R.Abbott,R.Adhikari.G.S.Allen, position of the reflected beam on the sensing photodiodes. H.Armandula,S.M.Aston,A.Baglino,M.Barton,B.Bland et al.,Proc. Although no measurable thermal lensing was observed (no SPE5500,pp.81-91(2004). change in the beam waist size or position),the measured 9B.J.Meers,Phys.Rev.D38,2317(1988). 10T.Fricke,N.Smith-Lefebvre,R.Abbott,R.Adhikari,K.Dooley,M.Evans, presence of higher order modes in the FI at high powers is P.Fritschel,V.Frolov,K.Kawabe,J.Kissel,B.Slagmolen,and S. suggestive of imperfect thermal lens compensation by the Waldman,Classical Quant.Grav.29,065005(2012). DKDP.This fault potentially can be reduced by a careful 1J.S.Kissel,"Calibrating and Improving the Sensitivity of the LIGO Detec- selection of the thickness of the DKDP to better match the tors,"Ph.D.dissertation (Louisiana State University.2010). absorbed power in the TGG crystals. 12M.Frede,B.Schulz.R.Wilhelm,P.Kwee,F.Seifert,B.Willke,and D.Kracht,Opt.Express 15,459(2007). 13p.Willems,A.Brooks,M.Mageswaran,V.Sannibale,C.Vorvick, D.Atkinson,R.Amin,and C.Adams,"Thermal compensation in enhanced VIl.SUMMARY LIGO,"Technical Report G0900182,LIGO Laboratory,2009. 14K.Dooley,"Design and performance of high laser power interferometers In summary,we have presented a comprehensive inves- for gravitational-wave detection,"Ph.D.dissertation (University of Florida. tigation of the Enhanced LIGO IO,including the function. 2011)- Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/nghts-and-perm D0wmlo8 d to IP:183.195251.60Fi.22Apr2016 00:54:10033109-11 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) The Enhanced LIGO EOM showed no thermal lensing, degraded transmission, nor damage in over 17 000 h of sus￾tained operation at 30 W of laser power. Measurements of the thermal lensing in RTP at powers up to 160 W show a relative power loss of <0.4%, indicating that thermal lensing should be negligible in Advanced LIGO. Peak irradiances in the EOM will be approximately four times that of Enhanced LIGO (a 45% larger beam diameter will somewhat offset the increased power). Testing of RTP at 10 times the expected Advanced LIGO irradiance over 100 hours show no signs of damage or degraded transmission. The MC showed no measurable change in operational state as a function of input power. This bodes well for the Advanced LIGO mode cleaner. Compared with the Enhanced LIGO MC, the Advanced LIGO MC is designed with a lower finesse (520) than Initial LIGO (1280). For 150 W input power, the Advanced LIGO MC will operate with 3 times greater stored power than Initial LIGO. The corresponding peak irradiance is 400 kW/m2, well below the continuous￾wave coating damage threshold. Absorption in the Advanced LIGO MC mirror optical coatings has been measured at 0.5 ppm, roughly four times less than the best mirror coating absorption in Enhanced LIGO, so the expected thermal load￾ing due to coating absorption should be reduced in Advanced LIGO. The larger Advanced LIGO MC mirror substrates and higher input powers result in a significantly higher contribu￾tion to bulk absorption, roughly 20 times Enhanced LIGO, however the expected thermal lensing leads to small change (<0.5%) in the output mode .22 The Enhanced LIGO data obtained from the FI allows us to make several predictions about how it will perform in Advanced LIGO. The measured isolation ratio decrease of 0.02 dB/W will result in a loss of 3 dB for a 150 W power level expected for Advanced LIGO relative to its cold state. However, the Advanced LIGO FI will employ an in situ adjustable half wave plate which will allow for a partial restoration of the isolation ratio. In addition, a new FI scheme to better compensate for thermal depolarization and thus yield higher isolation ratios will be implemented.35 The maximum thermally induced angular steering expected is 480 μrad (using a drift rate of 3.2 μrad/W), approximately equal to the beam divergence angle. This has some implications for the Advanced LIGO length and alignment sensing and control system, as the reflected FI beam is used as a sensing beam. Operation of Advanced LIGO at high powers will likely require the use of a beam stabilization servo to lock the position of the reflected beam on the sensing photodiodes. Although no measurable thermal lensing was observed (no change in the beam waist size or position), the measured presence of higher order modes in the FI at high powers is suggestive of imperfect thermal lens compensation by the DKDP. This fault potentially can be reduced by a careful selection of the thickness of the DKDP to better match the absorbed power in the TGG crystals. VII. SUMMARY In summary, we have presented a comprehensive inves￾tigation of the Enhanced LIGO IO, including the function, design, and performance of the IO. Several improvements to the design and implementation of the Enhanced LIGO IO over the Initial LIGO IO have led to improved optical ef- ficiency and coupling to the main interferometer through a substantial reduction in thermo-optical effects in the major IO optical components, including the electro-optic modula￾tors, mode cleaner, and Faraday isolator. The IO performance in Enhanced LIGO enables us to infer its performance in Ad￾vanced LIGO, and indicates that high power interferometry will be possible without severe thermal effects. ACKNOWLEDGMENTS The authors thank R. Adhikari for his wisdom and guid￾ance, B. Bland for providing lessons to K. Dooley and D. Hoak on how to handle the small optics suspensions, K. Kawabe and N. Smith-Lefebvre for their support at LHO, T. Fricke for engaging in helpful discussions, and V. Ze￾lenogorsky and D. Zheleznov for their assistance in prepar￾ing for the Enhanced LIGO IO installation. Additionally, the authors thank the LIGO Scientific Collaboration for access to the data. This work was supported by the National Science Foundation (Grant Nos. PHY-0855313 and PHY-0555453). LIGO was constructed by the California Institute of Tech￾nology and Massachusetts Institute of Technology with fund￾ing from the National Science Foundation and operates under cooperative agreement PHY-0757058. This paper has LIGO Document Number LIGO-P1100056. 1J. Abadie, B. P. Abbott, R. Abbott, M. Abernathy, T. Accadia, F. Acernese, C. Adams, R. Adhikari, P. Ajith, B. Allen et al., Classical Quant. Grav. 27, 173001+ (2010). 2B. P. Abbott, R. Abbott, R. Adhikari, P. Ajith, B. Allen, G. Allen, R. S. Amin, S. B. Anderson, W. G. Anderson, M. A. Arain et al., Rep. Prog. .Phys. 72, 076901+ (2009). 3F. Acernese, P. Amico, M. Alshourbagy, F. Antonucci, S. Aoudia, P. Astone, S. Avino, L. Baggio, G. Ballardin, F. Barone et al., J. Opt. A, Pure Appl. Opt. 10, 064009+ (2008). 4H. Lück, M. Hewitson, P. Ajith, B. Allen, P. Aufmuth, C. Aulbert, S. Babak, R. Balasubramanian, B. W. Barr, S. Berukoff et al., Classical Quant. Grav. 23, S71 (2006). 5R. Adhikari, P. Fritschel, and S. Waldman, “Enhanced LIGO,” Technical Report T060156, LIGO Laboratory, 2006. 6H. Lück, C. Affeldt, J. Degallaix, A. Freise, H. Grote, M. Hewitson, S. Hild, J. Leong, M. Prijatelj, K. A. Strain, B. Willke, H. Wittel, and K. Danzmann, J. Phys.: Conf. Ser. 228, 012012+ (2010). 7Advanced LIGO Systems Group, “Advanced LIGO Systems Design,” Technical Report T010075, LIGO Laboratory, 2009. 8N. A. Robertson, B. Abbott, R. Abbott, R. Adhikari, G. S. Allen, H. Armandula, S. M. Aston, A. Baglino, M. Barton, B. Bland et al., Proc. SPIE 5500, pp. 81–91 (2004). 9B. J. Meers, Phys. Rev. D 38, 2317 (1988). 10T. Fricke, N. Smith-Lefebvre, R. Abbott, R. Adhikari, K. Dooley, M. Evans, P. Fritschel, V. Frolov, K. Kawabe, J. Kissel, B. Slagmolen, and S. Waldman, Classical Quant. Grav. 29, 065005 (2012). 11J. S. Kissel, “Calibrating and Improving the Sensitivity of the LIGO Detec￾tors,” Ph.D. dissertation (Louisiana State University, 2010). 12M. Frede, B. Schulz, R. Wilhelm, P. Kwee, F. Seifert, B. Willke, and D. Kracht, Opt. Express 15, 459 (2007). 13P. Willems, A. Brooks, M. Mageswaran, V. Sannibale, C. Vorvick, D. Atkinson, R. Amin, and C. Adams, “Thermal compensation in enhanced LIGO,” Technical Report G0900182, LIGO Laboratory, 2009. 14K. Dooley, “Design and performance of high laser power interferometers for gravitational-wave detection,” Ph.D. dissertation (University of Florida, 2011). Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 183.195.251.6 On: Fri, 22 Apr 2016 00:54:10