上海交通大学：《探索物质微观世界》课程教学资源（补充材料）引力波_Thermal effects in the Input Optics of the Enhanced Laser Interferometer Gravitational Wave Observatory interferometers

AIP Review of Scientific Instruments Thermal effects in the Input Optics of the Enhanced Laser Interferometer Gravitational- Wave Observatory interferometers Katherine L.Dooley,Muzammil A.Arain,David Feldbaum,Valery V.Frolov,Matthew Heintze,Daniel Hoak, Efim A.Khazanov,Antonio Lucianetti,Rodica M.Martin,Guido Mueller,Oleg Palashov,Volker Quetschke, David H.Reitze,R.L.Savage,D.B.Tanner,Luke F.Williams,and Wan Wu Citation:Review of Scientific Instruments 83,033109(2012);doi:10.1063/1.3695405 View online:http://dx.doi.org/10.1063/1.3695405 View Table of Contents:http://scitation.aip.org/content/aip/journal/rsi/83/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigation of vacuum system requirements for a 5 km baseline gravitational-wave detector J.Vac.Sci.Technol..A25,763(2007):10.1116/1.2743645 Probing anisotropies of gravitational-wave backgrounds with a space-based interferometer A1 PConf.Proc.873,494(2006):10.1063/1.2405090 Grating Fabrication for Gravitational-Wave Interferometers and LISA GRS AIP Conf..Proc.873,359(2006:10.1063/1.2405069 Laser Interferometer Gravitational Wave Detectors-the Challenges A1 PConf.Proc.782,264(2005);10.1063/1.2032734 Mode-cleaning and injection optics of the gravitational-wave detector GEO600 Rev.Sci.Instrum.74,3787(2003;10.1063/1.1589160 Recognize Those Utilizing Science to Innovate American Business Call for Nominate Proven Leaders for the 2016 A/P General Prize for Industrial Applications of Physics Motors Nominations More Information /www.aip.org/industry/prize Deadline∥July1,2016 AIP Questions /assoc@alp.org Reuse of AlP 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 0054:10

Thermal effects in the Input Optics of the Enhanced Laser Interferometer Gravitational￾Wave Observatory interferometers Katherine L. Dooley, Muzammil A. Arain, David Feldbaum, Valery V. Frolov, Matthew Heintze, Daniel Hoak, Efim A. Khazanov, Antonio Lucianetti, Rodica M. Martin, Guido Mueller, Oleg Palashov, Volker Quetschke, David H. Reitze, R. L. Savage, D. B. Tanner, Luke F. Williams, and Wan Wu Citation: Review of Scientific Instruments 83, 033109 (2012); doi: 10.1063/1.3695405 View online: http://dx.doi.org/10.1063/1.3695405 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/83/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigation of vacuum system requirements for a 5 km baseline gravitational-wave detector J. Vac. Sci. Technol. A 25, 763 (2007); 10.1116/1.2743645 Probing anisotropies of gravitational‐wave backgrounds with a space‐based interferometer AIP Conf. Proc. 873, 494 (2006); 10.1063/1.2405090 Grating Fabrication for Gravitational‐Wave Interferometers and LISA GRS AIP Conf. Proc. 873, 359 (2006); 10.1063/1.2405069 Laser Interferometer Gravitational Wave Detectors—the Challenges AIP Conf. Proc. 782, 264 (2005); 10.1063/1.2032734 Mode-cleaning and injection optics of the gravitational-wave detector GEO600   Rev. Sci. Instrum. 74, 3787 (2003); 10.1063/1.1589160 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

REVIEW OF SCIENTIFIC INSTRUMENTS 83.033109(2012) Thermal effects in the Input Optics of the Enhanced Laser Interferometer Gravitational-Wave Observatory interferometers Katherine L.Dooley,1.a)Muzammil A.Arain,1.b)David Feldbaum,1 Valery V.Frolov,2 Matthew Heintze,1 Daniel Hoak,2.c)Efim A.Khazanov,3 Antonio Lucianetti,1.d) Rodica M.Martin,1 Guido Mueller,1 Oleg Palashov,3 Volker Quetschke,1.) David H.Reitze,1.R.L.Savage,4 D.B.Tanner,1 Luke F.Williams,1 and Wan Wu1.9) University of Florida,Gainesville,Florida 32611.USA 2LIGO,Livingston Observatory,Livingston,Louisiana 70754.USA 3Institute of Applied Physics,Nizhny Novgorod 603950.Russia ALIGO.Hanford Observatory,Richland,Washington 99352.USA (Received 9 December 2011;accepted 23 January 2012;published online 23 March 2012) We present the design and performance of the LIGO Input Optics subsystem as implemented for the sixth science run of the LIGO interferometers.The Initial LIGO Input Optics experienced thermal side effects when operating with 7 W input power.We designed,built,and implemented improved versions of the Input Optics for Enhanced LIGO,an incremental upgrade to the Initial LIGO inter- ferometers,designed to run with 30 W input power.At four times the power of Initial LIGO,the Enhanced LIGO Input Optics demonstrated improved performance including better optical isolation, less thermal drift,minimal thermal lensing,and higher optical efficiency.The success of the Input Optics design fosters confidence for its ability to perform well in Advanced LIGO.2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.3695405] I.INTRODUCTION the arm lengths,producing signal at the AS port proportional The field of ground-based gravitational-wave (GW) to the GW strain and the input power.The Fabry-Perot cavi- physics is rapidly approaching a state with a high likelihood ties in the Michelson arms and a power recycling mirror(RM) of detecting GWs for the first time in the latter half of this at the symmetric port are two modifications to the Michelson interferometer that increase the laser power in the arms and decade.Such a detection will not only validate part of Ein- stein's general theory of relativity,but also initiate an era therefore improve the detector's sensitivity to GWs. of astrophysical observation of the universe through GWs. A network of first generation kilometer scale laser in- Gravitational waves are dynamical strains in space-time,h terferometer gravitational-wave detectors completed an in- =AL/L,that travel at the speed of light and are gener- tegrated 2-year data collection run in 2007,called Science Run 5(S5).The instruments were:the American Laser Inter- ated by non-axisymmetric acceleration of mass.A first de- ferometer Gravitational-Wave Observatory (LIGO).one in tection is expected to witness an event such as a binary black hole/neutron star merger. Livingston,LA with 4 km long arms and two in Hanford, WA with 4 km and 2 km long arms;the 3 km French-Italian The typical detector configuration used by current gen- eration gravitational-wave observatories is a power-recycled detector VIRGO (Ref.3)in Cascina,Italy;and the 600 m Fabry-Perot Michelson laser interferometer featuring sus- German-British detector GEO(Ref.4)located near Hannover, pended test masses in vacuum as depicted in Figure 1.A Germany.Multiple separated detectors increase detection confidence through signal coincidence and improve source lo- diode-pumped,power amplified,and intensity and frequency stabilized Nd:YAG laser emits light at =1064 nm.The calization via waveform reconstruction. The first generation of LIGO,now known as Initial laser is directed to a Michelson interferometer whose two arm LIGO,achieved its design goal of sensitivity to GWs in the lengths are set to maintain destructive interference of the re- combined light at the anti-symmetric(AS)port.An appropri- 40-7000 Hz band,including a record strain sensitivity of ately polarized gravitational wave will differentially change 2 x 10-23/Hz at 155 Hz.However,only nearby sources produce enough GW strain to appear above the noise level of Initial LIGO and no gravitational wave has yet been found aAuthor to whom correspondence should be addressed.Electronic mail: in the S5 data.A second generation of LIGO detectors,Ad- kate.dooley@aei.mpg.de.Present address:Albert-Einstein-Institut,Max- Planck-Institut fur Gravitationsphysik,D-30167 Hannover,Germany. vanced LIGO,has been designed to be at least an order of b)Present address:KLA-Tencor,Milpitas.California95035,USA. magnitude more sensitive at several hundred Hz and above c)Present address:University of Massachusetts-Amherst,Amherst, Massachusetts 01003.USA. and to give an impressive increase in bandwidth down to d)Present address:Ecole Polytechnique,91128 Palaiseau Cedex,France. 10 Hz.Advanced LIGO is expected to open the field of GW e)Present address:The University of Texas at Brownsville,Brownsville, astronomy through the detection of many events per year.To Texas 78520.USA. test some of Advanced LIGO's new technologies and to in- DPresent address:LIGO Laboratory,Califoria Institute of Technology. Pasadena,California 91125,USA. crease the chances of detection through a more sensitive data g)Present address:NASA Langley Research Center,Hampton,Virginia taking run,an incremental upgrade to the detectors was car- 23666.USA. ried out after S5.5 This project,Enhanced LIGO,culminated 0034-6748/2012/83(3)/033109/12/S30.00 83,033109-1 2012 American Institute of Physics Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/nghts-and-permi D0wmlo8 d to IP:183.195251.60Fi.22Apr2016 00:54:10

REVIEW OF SCIENTIFIC INSTRUMENTS 83, 033109 (2012) Thermal effects in the Input Optics of the Enhanced Laser Interferometer Gravitational-Wave Observatory interferometers Katherine L. Dooley,1,a) Muzammil A. Arain,1,b) David Feldbaum,1 Valery V. Frolov,2 Matthew Heintze,1 Daniel Hoak,2,c) Efim A. Khazanov,3 Antonio Lucianetti,1,d) Rodica M. Martin,1 Guido Mueller,1 Oleg Palashov,3 Volker Quetschke,1,e) David H. Reitze,1,f) R. L. Savage,4 D. B. Tanner,1 Luke F. Williams,1 and Wan Wu1,g) 1University of Florida, Gainesville, Florida 32611, USA 2LIGO, Livingston Observatory, Livingston, Louisiana 70754, USA 3Institute of Applied Physics, Nizhny Novgorod 603950, Russia 4LIGO, Hanford Observatory, Richland, Washington 99352, USA (Received 9 December 2011; accepted 23 January 2012; published online 23 March 2012) We present the design and performance of the LIGO Input Optics subsystem as implemented for the sixth science run of the LIGO interferometers. The Initial LIGO Input Optics experienced thermal side effects when operating with 7 W input power. We designed, built, and implemented improved versions of the Input Optics for Enhanced LIGO, an incremental upgrade to the Initial LIGO inter￾ferometers, designed to run with 30 W input power. At four times the power of Initial LIGO, the Enhanced LIGO Input Optics demonstrated improved performance including better optical isolation, less thermal drift, minimal thermal lensing, and higher optical efficiency. The success of the Input Optics design fosters confidence for its ability to perform well in Advanced LIGO. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3695405] I. INTRODUCTION The field of ground-based gravitational-wave (GW) physics is rapidly approaching a state with a high likelihood of detecting GWs for the first time in the latter half of this decade. Such a detection will not only validate part of Ein￾stein’s general theory of relativity, but also initiate an era of astrophysical observation of the universe through GWs. Gravitational waves are dynamical strains in space-time, h = L/L, that travel at the speed of light and are gener￾ated by non-axisymmetric acceleration of mass. A first de￾tection is expected to witness an event such as a binary black hole/neutron star merger.1 The typical detector configuration used by current gen￾eration gravitational-wave observatories is a power-recycled Fabry-Perot Michelson laser interferometer featuring sus￾pended test masses in vacuum as depicted in Figure 1. A diode-pumped, power amplified, and intensity and frequency stabilized Nd:YAG laser emits light at λ = 1064 nm. The laser is directed to a Michelson interferometer whose two arm lengths are set to maintain destructive interference of the re￾combined light at the anti-symmetric (AS) port. An appropri￾ately polarized gravitational wave will differentially change a)Author to whom correspondence should be addressed. Electronic mail: kate.dooley@aei.mpg.de. Present address: Albert-Einstein-Institut, Max￾Planck-Institut für Gravitationsphysik, D-30167 Hannover, Germany. b)Present address: KLA-Tencor, Milpitas, California 95035, USA. c)Present address: University of Massachusetts–Amherst, Amherst, Massachusetts 01003, USA. d)Present address: École Polytechnique, 91128 Palaiseau Cedex, France. e)Present address: The University of Texas at Brownsville, Brownsville, Texas 78520, USA. f)Present address: LIGO Laboratory, California Institute of Technology, Pasadena, California 91125, USA. g)Present address: NASA Langley Research Center, Hampton, Virginia 23666, USA. the arm lengths, producing signal at the AS port proportional to the GW strain and the input power. The Fabry-Perot cavi￾ties in the Michelson arms and a power recycling mirror (RM) at the symmetric port are two modifications to the Michelson interferometer that increase the laser power in the arms and therefore improve the detector’s sensitivity to GWs. A network of first generation kilometer scale laser in￾terferometer gravitational-wave detectors completed an in￾tegrated 2-year data collection run in 2007, called Science Run 5 (S5). The instruments were: the American Laser Inter￾ferometer Gravitational-Wave Observatory (LIGO),2 one in Livingston, LA with 4 km long arms and two in Hanford, WA with 4 km and 2 km long arms; the 3 km French-Italian detector VIRGO (Ref. 3) in Cascina, Italy; and the 600 m German-British detector GEO (Ref. 4) located near Hannover, Germany. Multiple separated detectors increase detection confidence through signal coincidence and improve source lo￾calization via waveform reconstruction. The first generation of LIGO, now known as Initial LIGO, achieved its design goal of sensitivity to GWs in the 40–7000 Hz band, including a record strain sensitivity of 2 × 10−23/ √Hz at 155 Hz. However, only nearby sources produce enough GW strain to appear above the noise level of Initial LIGO and no gravitational wave has yet been found in the S5 data. A second generation of LIGO detectors, Ad￾vanced LIGO, has been designed to be at least an order of magnitude more sensitive at several hundred Hz and above and to give an impressive increase in bandwidth down to 10 Hz. Advanced LIGO is expected to open the field of GW astronomy through the detection of many events per year.1 To test some of Advanced LIGO’s new technologies and to in￾crease the chances of detection through a more sensitive data taking run, an incremental upgrade to the detectors was car￾ried out after S5 .5 This project, Enhanced LIGO, culminated 0034-6748/2012/83(3)/033109/12/$30.00 © 2012 American Institute of Physics 83, 033109-1 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

033109-2 Dooley et al Rev.Sci.Instrum.83,033109(2012) End tion System,13 the Alignment Sensing and Control,14 and the Input Optics (IO)were modified. Mirror This paper reports on the design and performance of the LIGO Input Optics subsystem in Enhanced LIGO,focusing specifically on its operational capabilities as the laser power is increased to 30 W.Substantial improvements in the IO power handling capabilities with respect to Initial LIGO per- Input formance are seen.The paper is organized as follows.First, Test in Sec.II.we define the role of the IO subsystem and detail Mass the function of each of the major IO subcomponents.Then,in LASER Sec.III we describe thermal effects which impact the opera- Beam Input Test End Test tion of the IO and summarize the problems experienced with Power Splitter Mass Mirror Mass Mirror Recycling the IO in Initial LIGO.In Sec.IV we present the IO design for Mirror Advanced LIGO in detail and describe how it addresses these ④ detection photodiode problems.Sect.V presents the performance of the prototype Advanced LIGO IO design as tested during Enhanced LIGO. FIG.1.Optical layout of a Fabry-Perot Michelson laser interferometer, Finally,we extrapolate from these experiences in Sec.VI to showing primary components.The four test masses,beam splitter,and power recycling mirror are physically located in an ultrahigh vacuum system and discuss the expected IO performance in Advanced LIGO.The are seismically isolated.A photodiode at the anti-symmetric port detects dif- paper concludes with a summary in Sec.VII. ferential arm length changes. II.FUNCTION OF THE INPUT OPTICS The Input Optics is one of the primary subsystems of the with the S6 science run from July 2009 to October 2010.Cur- LIGO interferometers.Its purpose is to deliver an aligned, rently,construction of Advanced LIGO is underway.Simul- spatially pure,mode-matched beam with phase-modulation taneously,VIRGO and GEO are both undergoing their own upgrades.3.6 sidebands to the power-recycled Fabry-Perot Michelson in- terferometer.The IO also prevents reflected or backscattered The baseline Advanced LIGO design?improves upon light from reaching the laser and distributes the reflected field Initial LIGO by incorporating improved seismic isolation,8 from the interferometer(designated the reflected port)to pho- the addition of a signal recycling mirror at the output port, todiodes for sensing and controlling the length and alignment homodyne readout,and an increase in available laser power of the interferometer.In addition,the IO provides an interme- from 8 W to 180 W.The substantial increase in laser power diate level of frequency stabilization and must have high over- improves the shot-noise-limited sensitivity,but introduces a all optical efficiency.It must perform these functions without multitude of thermally induced side effects that must be ad- limiting the strain sensitivity of the LIGO interferometer.Fi- dressed for proper operation. nally,it must operate robustly and continuously over years of Enhanced LIGO tested portions of the Advanced LIGO operation.The conceptual design is found in Ref.15. designs so that unforeseen difficulties could be addressed and As shown in Fig.2,the IO subsystem consists of four so that a more sensitive data taking run could take place.An principle components located between the pre-stabilized laser output mode cleaner was designed,built and installed,and dc readout of the GW signal was implemented.10 An Advanced and the power recycling mirror: LIGO active seismic isolation table was also built,installed. electro-optic modulator(EOM) and tested(Chapter 5 of Ref.11).In addition,the 10 W Initial mode cleaner cavity (MC) LIGO laser was replaced with a 35 W laser.12 Accompanying 。Faraday isolator(F) the increase in laser power,the test mass Thermal Compensa- mode-matching telescope (MMT) Input Optics Electro-optic Faraday modulator Mode cleaner Isolator Pre-stabilized laser Mode-matching telescope FIG.2.Block diagram of the Input Optics subsystem.The IO is located between the pre-stabilized laser and the recycling mirror and consists of four principle components:electro-optic modulator,mode cleaner,Farday isolator,and mode-matching telescope.The electro-optic modulator is the only IO component outside of the vacuum system.Diagram is not to scale. Reuse of AlP 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

033109-2 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) FIG. 1. Optical layout of a Fabry-Perot Michelson laser interferometer, showing primary components. The four test masses, beam splitter, and power recycling mirror are physically located in an ultrahigh vacuum system and are seismically isolated. A photodiode at the anti-symmetric port detects dif￾ferential arm length changes. with the S6 science run from July 2009 to October 2010. Cur￾rently, construction of Advanced LIGO is underway. Simul￾taneously, VIRGO and GEO are both undergoing their own upgrades.3, 6 The baseline Advanced LIGO design7 improves upon Initial LIGO by incorporating improved seismic isolation,8 the addition of a signal recycling mirror at the output port,9 homodyne readout, and an increase in available laser power from 8 W to 180 W. The substantial increase in laser power improves the shot-noise-limited sensitivity, but introduces a multitude of thermally induced side effects that must be ad￾dressed for proper operation. Enhanced LIGO tested portions of the Advanced LIGO designs so that unforeseen difficulties could be addressed and so that a more sensitive data taking run could take place. An output mode cleaner was designed, built and installed, and dc readout of the GW signal was implemented.10 An Advanced LIGO active seismic isolation table was also built, installed, and tested (Chapter 5 of Ref. 11). In addition, the 10 W Initial LIGO laser was replaced with a 35 W laser.12 Accompanying the increase in laser power, the test mass Thermal Compensa￾tion System,13 the Alignment Sensing and Control ,14 and the Input Optics (IO) were modified. This paper reports on the design and performance of the LIGO Input Optics subsystem in Enhanced LIGO, focusing specifically on its operational capabilities as the laser power is increased to 30 W. Substantial improvements in the IO power handling capabilities with respect to Initial LIGO per￾formance are seen. The paper is organized as follows. First, in Sec. II, we define the role of the IO subsystem and detail the function of each of the major IO subcomponents. Then, in Sec. III we describe thermal effects which impact the opera￾tion of the IO and summarize the problems experienced with the IO in Initial LIGO. In Sec. IV we present the IO design for Advanced LIGO in detail and describe how it addresses these problems. Sect. V presents the performance of the prototype Advanced LIGO IO design as tested during Enhanced LIGO. Finally, we extrapolate from these experiences in Sec. VI to discuss the expected IO performance in Advanced LIGO. The paper concludes with a summary in Sec. VII. II. FUNCTION OF THE INPUT OPTICS The Input Optics is one of the primary subsystems of the LIGO interferometers. Its purpose is to deliver an aligned, spatially pure, mode-matched beam with phase-modulation sidebands to the power-recycled Fabry-Perot Michelson in￾terferometer. The IO also prevents reflected or backscattered light from reaching the laser and distributes the reflected field from the interferometer (designated the reflected port) to pho￾todiodes for sensing and controlling the length and alignment of the interferometer. In addition, the IO provides an interme￾diate level of frequency stabilization and must have high over￾all optical efficiency. It must perform these functions without limiting the strain sensitivity of the LIGO interferometer. Fi￾nally, it must operate robustly and continuously over years of operation. The conceptual design is found in Ref. 15. As shown in Fig. 2, the IO subsystem consists of four principle components located between the pre-stabilized laser and the power recycling mirror: electro-optic modulator (EOM) mode cleaner cavity (MC) Faraday isolator (FI) mode-matching telescope (MMT) FIG. 2. Block diagram of the Input Optics subsystem. The IO is located between the pre-stabilized laser and the recycling mirror and consists of four principle components: electro-optic modulator, mode cleaner, Farday isolator, and mode-matching telescope. The electro-optic modulator is the only IO component outside of the vacuum system. Diagram is not to scale. 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

033109-3 Dooley et al Rev.Sci.Instrum.83,033109(2012) Each element is a common building block of many opti- ferometer.The mode-matching telescope is a set of three sus- cal experiments and not unique to LIGO.However,their pended concave mirrors between the MC and interferometer roles specific to the successful operation of interferometry for that expand the beam from a radius of 1.6 mm at the MC gravitational-wave detection are of interest and demand fur- waist to a radius of 33 mm at the arm cavity waist.The MMT ther attention.Here,we briefly review the purpose of each of should play a passive role by delivering properly shaped light the IO components;further details about the design require- to the interferometer without introducing beam jitter or any ments are in Ref.16 significant aberration that can reduce mode coupling. A.Electro-optic modulator III.THERMAL PROBLEMS IN INITIAL LIGO The Length Sensing and Control (LSC)and Angular The Initial LIGO interferometers were equipped with a Sensing and Control(ASC)subsystems require phase modu- 10 W laser,yet operated with only 7 W input power due lation of the laser light at RF frequencies.This modulation is produced by an EOM,generating sidebands of the laser light to power-related problems with other subsystems.The EOM was located in the 10 W beam and the other components expe- which act as references against which interferometer length and angle changes are measured.17 The sideband light must rienced anywhere up to 7 W power.The 7 W operational limit was not due to the failure of the IO;however,many aspects of be either resonant only in the recycling cavity or not resonant the IO performance did degrade with power. in the interferometer at all.The sidebands must be offset from One of the primary problems of the Initial LIGO IO the carrier by integer multiples of the MC free spectral range (Ref.18)was thermal deflection of the back propagating beam to pass through the MC due to thermally induced refractive index gradients in the FI. A significant beam drift between the interferometer's locked B.Mode cleaner and unlocked states led to clipping of the reflected beam on Stably aligned cavities,limited non-mode-matched the photodiodes used for length and alignment control (see (junk)light,and a frequency and amplitude stabilized laser Fig.3.Our measurements determined a deflection of approx- are key features of any ultra sensitive laser interferometer.The imately 100 urad/W in the FI.This problem was mitigated at MC,at the heart of the IO,plays a major role the time by the design and implementation of an active beam A three-mirror triangular ring cavity,the MC suppresses steering servo on the beam coming from the isolator. laser output not in the fundamental TEMoo mode,serving two There were also known limits to the power the IO could major purposes.It enables the robustness of the ASC because sustain.Thermal lensing in the FI optics began to alter signif- higher order modes would otherwise contaminate the angu- icantly the beam mode at powers greater than 10 W,leading lar sensing signals of the interferometer.Also,all non-TEMoo to a several percent reduction in mode matching to the in- light on the length sensing photodiodes,including those used terferometer.19 Additionally,absorptive FI elements would for the GW readout,contributes shot noise but not signal and create thermal birefringence,degrading the optical efficiency therefore diminishes the signal to noise ratio.The MC is thus and isolation ratio with power.20 The Initial LIGO New Focus largely responsible for achieving an aligned,minimally shot- EOMs had an operational power limit of around 10 W.There noise-limited interferometer. was a high risk of damage to the crystals under the stress of The MC also plays an active role in laser frequency the 0.4 mm radius beam.Also,anisotropic thermal lensing stabilization,17 which is necessary for ensuring that the signal with focal lengths as severe as 3.3 m at 10 W made the EOMs at the anti-symmetric port is due to arm length fluctuations unsuitable for much higher power.Finally,the MC mirrors rather than laser frequency fluctuations.In addition,the MC exhibited high absorption(as much as 24 ppm per mirror) passively suppresses beam jitter at frequencies above 10 Hz. enough that thermal lensing of the MC optics at enhanced LIGO powers would induce higher order modal frequency C.Faraday isolator degeneracy and result in a power-dependent mode mismatch into the interferometer.21.22 In fact,as input power increased Faraday isolators are four-port optical devices which uti- from 1 W to 7 W the mode matching decreased from 90% lize the Faraday effect to allow for non-reciprocal polarization to83%. switching of laser beams.Any backscatter or reflected light In addition to the thermal limitations of the Initial LIGO from the interferometer (due to impedance mismatch,mode IO,optical efficiency in delivering light from the laser into mismatch,non-resonant sidebands,or signal)needs to be di- the interferometer was not optimal.Of the light entering the verted to protect the laser from back propagating light,which IO chain,only 60%remained by the time it reached the power can introduce amplitude and phase noise.This diversion of recycling mirror.Moreover,because at best only 90%of the the reflected light is also necessary for extracting length and light at the recycling mirror was coupled into the arm cavity angular information about the interferometer's cavities.The mode,room was left for improvement in the implementation FI fulfills both needs of the MMT. D.Mode-matching telescope IV.ENHANCED LIGO INPUT OPTICS DESIGN The lowest order MC and arm cavity spatial eigenmodes The Enhanced LIGO IO design addressed the thermal ef- need to be matched for maximal power buildup in the inter- fects that compromised the performance of the Initial LIGO Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/rights-and-per D0wmlo8doP:183.195251.60:Fi.22Apr2016 00:54:10

033109-3 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) Each element is a common building block of many opti￾cal experiments and not unique to LIGO. However, their roles specific to the successful operation of interferometry for gravitational-wave detection are of interest and demand fur￾ther attention. Here, we briefly review the purpose of each of the IO components; further details about the design require￾ments are in Ref. 16. A. Electro-optic modulator The Length Sensing and Control (LSC) and Angular Sensing and Control (ASC) subsystems require phase modu￾lation of the laser light at RF frequencies. This modulation is produced by an EOM, generating sidebands of the laser light which act as references against which interferometer length and angle changes are measured. 17 The sideband light must be either resonant only in the recycling cavity or not resonant in the interferometer at all. The sidebands must be offset from the carrier by integer multiples of the MC free spectral range to pass through the MC. B. Mode cleaner Stably aligned cavities, limited non-mode-matched (junk) light, and a frequency and amplitude stabilized laser are key features of any ultra sensitive laser interferometer. The MC, at the heart of the IO, plays a major role. A three-mirror triangular ring cavity, the MC suppresses laser output not in the fundamental TEM00 mode, serving two major purposes. It enables the robustness of the ASC because higher order modes would otherwise contaminate the angu￾lar sensing signals of the interferometer. Also, all non-TEM00 light on the length sensing photodiodes, including those used for the GW readout, contributes shot noise but not signal and therefore diminishes the signal to noise ratio. The MC is thus largely responsible for achieving an aligned, minimally shot￾noise-limited interferometer. The MC also plays an active role in laser frequency stabilization,17 which is necessary for ensuring that the signal at the anti-symmetric port is due to arm length fluctuations rather than laser frequency fluctuations. In addition, the MC passively suppresses beam jitter at frequencies above 10 Hz. C. Faraday isolator Faraday isolators are four-port optical devices which uti￾lize the Faraday effect to allow for non-reciprocal polarization switching of laser beams. Any backscatter or reflected light from the interferometer (due to impedance mismatch, mode mismatch, non-resonant sidebands, or signal) needs to be di￾verted to protect the laser from back propagating light, which can introduce amplitude and phase noise. This diversion of the reflected light is also necessary for extracting length and angular information about the interferometer’s cavities. The FI fulfills both needs. D. Mode-matching telescope The lowest order MC and arm cavity spatial eigenmodes need to be matched for maximal power buildup in the inter￾ferometer. The mode-matching telescope is a set of three sus￾pended concave mirrors between the MC and interferometer that expand the beam from a radius of 1.6 mm at the MC waist to a radius of 33 mm at the arm cavity waist. The MMT should play a passive role by delivering properly shaped light to the interferometer without introducing beam jitter or any significant aberration that can reduce mode coupling. III. THERMAL PROBLEMS IN INITIAL LIGO The Initial LIGO interferometers were equipped with a 10 W laser, yet operated with only 7 W input power due to power-related problems with other subsystems. The EOM was located in the 10 W beam and the other components expe￾rienced anywhere up to 7 W power. The 7 W operational limit was not due to the failure of the IO; however, many aspects of the IO performance did degrade with power. One of the primary problems of the Initial LIGO IO (Ref. 18) was thermal deflection of the back propagating beam due to thermally induced refractive index gradients in the FI. A significant beam drift between the interferometer’s locked and unlocked states led to clipping of the reflected beam on the photodiodes used for length and alignment control (see Fig. 3. Our measurements determined a deflection of approx￾imately 100 μrad/W in the FI. This problem was mitigated at the time by the design and implementation of an active beam steering servo on the beam coming from the isolator. There were also known limits to the power the IO could sustain. Thermal lensing in the FI optics began to alter signif￾icantly the beam mode at powers greater than 10 W, leading to a several percent reduction in mode matching to the in￾terferometer. 19 Additionally, absorptive FI elements would create thermal birefringence, degrading the optical efficiency and isolation ratio with power.20 The Initial LIGO New Focus EOMs had an operational power limit of around 10 W. There was a high risk of damage to the crystals under the stress of the 0.4 mm radius beam. Also, anisotropic thermal lensing with focal lengths as severe as 3.3 m at 10 W made the EOMs unsuitable for much higher power. Finally, the MC mirrors exhibited high absorption (as much as 24 ppm per mirror)— enough that thermal lensing of the MC optics at enhanced LIGO powers would induce higher order modal frequency degeneracy and result in a power-dependent mode mismatch into the interferometer.21, 22 In fact, as input power increased from 1 W to 7 W the mode matching decreased from 90% to 83%. In addition to the thermal limitations of the Initial LIGO IO, optical efficiency in delivering light from the laser into the interferometer was not optimal. Of the light entering the IO chain, only 60% remained by the time it reached the power recycling mirror. Moreover, because at best only 90% of the light at the recycling mirror was coupled into the arm cavity mode, room was left for improvement in the implementation of the MMT. IV. ENHANCED LIGO INPUT OPTICS DESIGN The Enhanced LIGO IO design addressed the thermal ef￾fects that compromised the performance of the Initial LIGO 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

033109-4 Dooley et al. Rev.Sci.Instrum.83,033109(2012) Interferometer Sensing and Control table ④ Photodiode ⊕ Quadrant photodiode Interferometer RF length detector feck beam reflected beam RF alignment detector Pre-stabilized laser table 12.2m LASER Electro-optic Mode Mode cleaner Mode cleahe modulator matching reflected beam lenses Input Optics table 卤卤 FIG.3.Enhanced LIGO Input Optics optical and sensing configuration.The HAMI(horizontal access module)vacuum chamber is featured in the center,with locations of all major optics superimposed.HAM2 is shown on the right,with its components.These tables are separated by 12 m.The primary beam path, beginning at the pre-stabilized laser and going to the power recycling mirror,is shown in red as a solid line,and auxiliary beams are different colors and dotted. The MMTs,MCs,and steering mirror(SM)are suspended;all other optics are fixed to the seismically isolated table.The laser and sensing and diagnostic photodiodes are on in-air tables. IO,and accommodated up to four times the power of Ini- crystal dimensions are 4 x 4 x 40 mm and their faces are tial LIGO.Also,the design was a prototype for handling the wedged by 2.85 and anti-reflection (AR)coated.The wedge 180 W laser planned for Advanced LIGO.Because the ad- serves to separate the polarizations and prevents an etalon ef- verse thermal properties of the Initial LIGO IO(beam drift, fect,resulting in a suppression of amplitude modulation.Only birefringence,and lensing)are all attributable primarily to ab- one crystal is used in the EOM in order to reduce the number sorption of laser light by the optical elements,the primary de- of surface reflections.Three separate pairs of electrodes,each sign consideration was finding optics with lower absorption.19 with its own resonant LC circuit,are placed across the crystal Both the EOM and the FI were replaced for Enhanced LIGO. in series,producing the three required sets of RF sidebands: Only minor changes were made to the MC and MMT.A de- 24.5 MHz,33.3 MHz,and 61.2 MHz.A diagram is shown in tailed layout of the Enhanced LIGO IO is shown in Figure 3. Fig.4.Reference 23 contains further details about the modu- lator architecture. A.Electro-optic modulator design We replaced the commercially made New Focus 4003 B.Mode cleaner design resonant phase modulator of Initial LIGO with an in-house The MC is a suspended 12.2 m long triangular ring cavity EOM design and construction.Both a new crystal choice and with finesseF=1280 and free spectral range of 12.243 MHz. architectural design change allow for superior performance. The three mirror architecture was selected over the standard The Enhanced LIGO EOM design uses a crystal of ru- two mirror linear filter cavity because it acts as a polarization bidium titanyl phosphate (RTP),which has at most 1/10 the absorption coefficient at 1064 nm of the lithium nio- bate (LiNbO3)crystal from Initial LIGO.At 200 W the RTP TABLE I.Comparison of selected properties of the Initial and Enhanced should produce a thermal lens of 200 m and higher order LIGO EOM crystals,LiNbO3,and RTP,respectively.RTP was preferred for Enhanced LIGO because of its lower absorption,superior thermal properties, mode content of less than 1%,compared to the 3.3 m lens and similar electro-optic properties. the LiNbO3 produces at 10 W.The RTP has a minimal risk of damage,because it has both twice the damage threshold of Units LiNbO3 RTP LiNbOa and is subjected to a beam twice the size of that in Ini- tial LIGO.RTP and LiNbO3 have similar electro-optic coeffi- Damage threshold MW/cm2 280 >600 <5000 cients.Also,RTP's dn/dT anisotropy is 50%smaller.Table I Absorption coeff.at 1064 nm ppm/cm <500 Electro-optic coeff.(n2r33) pm/V 306 239 compares the properties of most interest of the two crystals. dnldT 10-6K 5.4 2.79 We procured the RTP crystals from Raicol and packaged dn-ldT 10-6K 37.9 9.24 them into specially designed,custom-built modulators.The Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 183.195251.60:Fi.22Apr2016 00:54:10

033109-4 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) FIG. 3. Enhanced LIGO Input Optics optical and sensing configuration. The HAM1 (horizontal access module) vacuum chamber is featured in the center, with locations of all major optics superimposed. HAM2 is shown on the right, with its components. These tables are separated by 12 m. The primary beam path, beginning at the pre-stabilized laser and going to the power recycling mirror, is shown in red as a solid line, and auxiliary beams are different colors and dotted. The MMTs, MCs, and steering mirror (SM) are suspended; all other optics are fixed to the seismically isolated table. The laser and sensing and diagnostic photodiodes are on in-air tables. IO, and accommodated up to four times the power of Ini￾tial LIGO. Also, the design was a prototype for handling the 180 W laser planned for Advanced LIGO. Because the ad￾verse thermal properties of the Initial LIGO IO (beam drift, birefringence, and lensing) are all attributable primarily to ab￾sorption of laser light by the optical elements, the primary de￾sign consideration was finding optics with lower absorption.19 Both the EOM and the FI were replaced for Enhanced LIGO. Only minor changes were made to the MC and MMT. A de￾tailed layout of the Enhanced LIGO IO is shown in Figure 3. A. Electro-optic modulator design We replaced the commercially made New Focus 4003 resonant phase modulator of Initial LIGO with an in-house EOM design and construction. Both a new crystal choice and architectural design change allow for superior performance. The Enhanced LIGO EOM design uses a crystal of ru￾bidium titanyl phosphate (RTP), which has at most 1/10 the absorption coefficient at 1064 nm of the lithium nio￾bate (LiNbO3) crystal from Initial LIGO. At 200 W the RTP should produce a thermal lens of 200 m and higher order mode content of less than 1%, compared to the 3.3 m lens the LiNbO3 produces at 10 W. The RTP has a minimal risk of damage, because it has both twice the damage threshold of LiNbO3 and is subjected to a beam twice the size of that in Ini￾tial LIGO. RTP and LiNbO3 have similar electro-optic coeffi- cients. Also, RTP’s dn/dT anisotropy is 50% smaller. Table I compares the properties of most interest of the two crystals. We procured the RTP crystals from Raicol and packaged them into specially designed, custom-built modulators. The crystal dimensions are 4 × 4 × 40 mm and their faces are wedged by 2.85◦ and anti-reflection (AR) coated. The wedge serves to separate the polarizations and prevents an etalon ef￾fect, resulting in a suppression of amplitude modulation. Only one crystal is used in the EOM in order to reduce the number of surface reflections. Three separate pairs of electrodes, each with its own resonant LC circuit, are placed across the crystal in series, producing the three required sets of RF sidebands: 24.5 MHz, 33.3 MHz, and 61.2 MHz. A diagram is shown in Fig. 4. Reference 23 contains further details about the modu￾lator architecture. B. Mode cleaner design The MC is a suspended 12.2 m long triangular ring cavity with finesse F = 1280 and free spectral range of 12.243 MHz. The three mirror architecture was selected over the standard two mirror linear filter cavity because it acts as a polarization TABLE I. Comparison of selected properties of the Initial and Enhanced LIGO EOM crystals, LiNbO3, and RTP, respectively. RTP was preferred for Enhanced LIGO because of its lower absorption, superior thermal properties, and similar electro-optic properties.19 Units LiNbO3 RTP Damage threshold MW/cm2 280 >600 Absorption coeff. at 1064 nm ppm/cm <5000 <500 Electro-optic coeff. (n3 zr33) pm/V 306 239 dny/dT 10−6/K 5.4 2.79 dnz/dT 10−6/K 37.9 9.24 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