AIPI Review of Scientific Instruments The advanced LIGO input optics Chris L.Mueller,Muzammil A.Arain,Giacomo Ciani,Ryan.T.DeRosa,Anamaria Effler,David Feldbaum, Valery V.Frolov,Paul Fulda,Joseph Gleason,Matthew Heintze,Keita Kawabe,Eleanor J.King,Keiko Kokeyama,William Z.Korth,Rodica M.Martin,Adam Mullavey,Jan Peold,Volker Quetschke,David H. Reitze,David B.Tanner,Cheryl Vorvick,Luke F.Williams,and Guido Mueller Citation:Review of Scientific Instruments 87,014502(2016);doi:10.1063/1.4936974 View online:http://dx.doi.org/10.1063/1.4936974 View Table of Contents:http://scitation.aip.org/content/aip/journal/rsi/87/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in ADL ORVIS:An air-delay-leg,line-imaging optically recording velocity interferometer system Rev.Sci.Instrum.85,045118(2014:10.1063/1.4871588 Thermal effects in the Input Optics of the Enhanced Laser Interferometer Gravitational-Wave Observatory interferometers Rev.Sci.Instrum.83,033109(2012:10.1063/1.3695405 Optical Design of the LISA Interferometric Metrology System AIP Conf..Proc.873,389(2006:10.1063/1.2405075 Precision alignment of the LIGO 4 km arms using the dual-frequency differential global positioning system Rev.Sci.Instrum.72,3086(2001);10.1063/1.1376138 The status of LIGO A1 Conf..Proc.523,101(2000);10.1063/1.1291847 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@aip.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 2016005135
The advanced LIGO input optics Chris L. Mueller, Muzammil A. Arain, Giacomo Ciani, Ryan. T. DeRosa, Anamaria Effler, David Feldbaum, Valery V. Frolov, Paul Fulda, Joseph Gleason, Matthew Heintze, Keita Kawabe, Eleanor J. King, Keiko Kokeyama, William Z. Korth, Rodica M. Martin, Adam Mullavey, Jan Peold, Volker Quetschke, David H. Reitze, David B. Tanner, Cheryl Vorvick, Luke F. Williams, and Guido Mueller Citation: Review of Scientific Instruments 87, 014502 (2016); doi: 10.1063/1.4936974 View online: http://dx.doi.org/10.1063/1.4936974 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/87/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in ADL ORVIS: An air-delay-leg, line-imaging optically recording velocity interferometer system Rev. Sci. Instrum. 85, 045118 (2014); 10.1063/1.4871588 Thermal effects in the Input Optics of the Enhanced Laser Interferometer Gravitational-Wave Observatory interferometers Rev. Sci. Instrum. 83, 033109 (2012); 10.1063/1.3695405 Optical Design of the LISA Interferometric Metrology System AIP Conf. Proc. 873, 389 (2006); 10.1063/1.2405075 Precision alignment of the LIGO 4 km arms using the dual-frequency differential global positioning system Rev. Sci. Instrum. 72, 3086 (2001); 10.1063/1.1376138 The status of LIGO AIP Conf. Proc. 523, 101 (2000); 10.1063/1.1291847 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 00:51:35
rossMark REVIEW OF SCIENTIFIC INSTRUMENTS 87.014502 (2016) The advanced LIGO input optics Chris L.Mueller,1.a)Muzammil A.Arain,1.b)Giacomo Ciani,1 Ryan.T.DeRosa,2 Anamaria Effler,2 David Feldbaum,1 Valery V.Frolov,3 Paul Fulda,1 Joseph Gleason,1 Matthew Heintze,1 Keita Kawabe,4 Eleanor J.King,5 Keiko Kokeyama,2 William Z.Korth,6 Rodica M.Martin,1 Adam Mullavey,3 Jan Peold,7 Volker Quetschke,8 David H.Reitze,1.c) David B.Tanner,1 Cheryl Vorvick,4 Luke F.Williams,1 and Guido Mueller1 University of Florida,Gainesville.Florida 32611,USA Louisiana State University,Baton Rouge,Louisiana 70803,USA 3LIGO Livingston Observatory,Livingston,Louisiana 70754,USA LIGO Hanford Observatory,Richland,Washington 99352,USA SUniversity of Adelaide,Adelaide.SA 5005,Australia LIGO,California Institute of Technology,Pasadena,California 91125,USA Max-Planck-Institut fuir Gravitationsphysik,30167 Hannover.Germany University of Texas at Brownsville,Brownsville,Texas 78520,USA (Received 27 July 2015:accepted 19 November 2015;published online 22 January 2016) The advanced LIGO gravitational wave detectors are nearing their design sensitivity and should begin taking meaningful astrophysical data in the fall of 2015.These resonant optical interferometers will have unprecedented sensitivity to the strains caused by passing gravitational waves.The input optics play a significant part in allowing these devices to reach such sensitivities.Residing between the pre-stabilized laser and the main interferometer,the input optics subsystem is tasked with preparing the laser beam for interferometry at the sub-attometer level while operating at continuous wave input power levels ranging from 100 mW to 150 W.These extreme operating conditions required every major component to be custom designed.These designs draw heavily on the experience and understanding gained during the operation of Initial LIGO and Enhanced LIGO.In this article,we report on how the components of the input optics were designed to meet their stringent requirements and present measurements showing how well they have lived up to their design.2016 AlP Publishing LLC.[http://dx.doi.org/10.1063/1.4936974] I.INTRODUCTION inject it into the main IFO.The PSL consists of a master laser, an amplifier stage,and a 200 W slave laser which is injection A worldwide effort to directly detect gravitational radi- locked to the amplified master laser.The 200 W output beam ation in the 10Hz to a few kHz frequency range with large is filtered by a short optical ring cavity,the pre-mode cleaner, scale laser interferometers (IFOs)has been underway for the before it is turned over to the IO(see Figure 1).The PSL pre- past two decades.In the United States the Laser Interferometer stabilizes the laser frequency to a fixed spacer reference cavity Gravitational-Wave Observatories (LIGO)in Livingston,LA using a tunable sideband locking technique.The PSL also (LLO)and in Hanford,WA,(LHO)have been operating provides interfaces to further stabilize its frequency and power. since the early 2000's.Initial and Enhanced LIGO (eLIGO) The IFO is a dual-recycled,cavity-enhanced Michelson produced several significant upper limits but did not have the interferometer-as sketched in Figure 2.The field enters the sensitivity to make the first direct detection of gravitational 55 m folded power recycling cavity(PRC)through the power waves.During this time of operation a significant amount recycling mirror(PRM).Two additional mirrors(PR2.PR3) of effort was invested by the LIGO Scientific Collaboration within the PRC form a telescope to increase the beam size to research and design Advanced LIGO (aLIGO),the first from ~2 mm to ~50 mm (Gaussian beam radius)before the major upgrade of Initial LIGO.In 2011 the Initial LIGO large beam is split at the beam splitter and injected into the detectors were decommissioned and installation of these up- two 4 km arm cavities formed by the input and end test masses. grades started.The installation was completed in 2014 and The reflected fields recombine at the BS and send most of the the commissioning phase has begun for many of the upgraded light back to the PRM where it constructively interferes with subsystems at the LIGO observatories.This paper focuses on the injected field.3 This leads to a power enhancement inside the input optics (IO)of aLIGO. the power recycling cavity and provides additional spatial,fre- The main task of the IO subsystem is to take the laser beam quency,and amplitude filtering of the laser beam.The second from the pre-stabilized laser system(PSL)and prepare and output of the BS sends light into the 55 m long folded signal recycling cavity(SRC)which also consists of a beam reduc- ing telescope(SR2,SR3)and the partially reflective signal aElectronic mail:cmueller@phys.ufl.edu b)Present address:KLA-Tencor,Milpitas,California 95035.USA. recycling mirror(SRM). )Present address:LIGO Laboratory.California Institute of Technology, This paper is organized as follows:Section II gives an Pasadena,California 91125.USA. overview of the IO;its functions,components,and the 0034-6748/2016/87(1)/014502/16/$30.00 87,014502-1 2016 AIP Publishing LLC Reuse of AlP Publishing cor subject fo the terms at:https://publishing.aip.org/authors/rights-and-permissions.Download to IP 183195.2516 On:Fri.22Ap1 20160051:35
REVIEW OF SCIENTIFIC INSTRUMENTS 87, 014502 (2016) The advanced LIGO input optics Chris L. Mueller, 1,a) Muzammil A. Arain, 1,b) Giacomo Ciani, 1 Ryan. T. DeRosa, 2 Anamaria Effler, 2 David Feldbaum, 1 Valery V. Frolov, 3 Paul Fulda, 1 Joseph Gleason, 1 Matthew Heintze, 1 Keita Kawabe, 4 Eleanor J. King, 5 Keiko Kokeyama, 2 William Z. Korth, 6 Rodica M. Martin, 1 Adam Mullavey, 3 Jan Peold, 7 Volker Quetschke, 8 David H. Reitze, 1,c) David B. Tanner, 1 Cheryl Vorvick, 4 Luke F. Williams, 1 and Guido Mueller1 1University of Florida, Gainesville, Florida 32611, USA 2Louisiana State University, Baton Rouge, Louisiana 70803, USA 3LIGO Livingston Observatory, Livingston, Louisiana 70754, USA 4LIGO Hanford Observatory, Richland, Washington 99352, USA 5University of Adelaide, Adelaide, SA 5005, Australia 6LIGO, California Institute of Technology, Pasadena, California 91125, USA 7Max-Planck-Institut für Gravitationsphysik, 30167 Hannover, Germany 8University of Texas at Brownsville, Brownsville, Texas 78520, USA (Received 27 July 2015; accepted 19 November 2015; published online 22 January 2016) The advanced LIGO gravitational wave detectors are nearing their design sensitivity and should begin taking meaningful astrophysical data in the fall of 2015. These resonant optical interferometers will have unprecedented sensitivity to the strains caused by passing gravitational waves. The input optics play a significant part in allowing these devices to reach such sensitivities. Residing between the pre-stabilized laser and the main interferometer, the input optics subsystem is tasked with preparing the laser beam for interferometry at the sub-attometer level while operating at continuous wave input power levels ranging from 100 mW to 150 W. These extreme operating conditions required every major component to be custom designed. These designs draw heavily on the experience and understanding gained during the operation of Initial LIGO and Enhanced LIGO. In this article, we report on how the components of the input optics were designed to meet their stringent requirements and present measurements showing how well they have lived up to their design. C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4936974] I. INTRODUCTION A worldwide effort to directly detect gravitational radiation in the 10 Hz to a few kHz frequency range with large scale laser interferometers (IFOs) has been underway for the past two decades. In the United States the Laser Interferometer Gravitational-Wave Observatories (LIGO) in Livingston, LA (LLO) and in Hanford, WA, (LHO) have been operating since the early 2000’s. Initial and Enhanced LIGO (eLIGO) produced several significant upper limits but did not have the sensitivity to make the first direct detection of gravitational waves. During this time of operation a significant amount of effort was invested by the LIGO Scientific Collaboration to research and design Advanced LIGO (aLIGO), the first major upgrade of Initial LIGO. In 2011 the Initial LIGO detectors were decommissioned and installation of these upgrades started. The installation was completed in 2014 and the commissioning phase has begun for many of the upgraded subsystems at the LIGO observatories. This paper focuses on the input optics (IO) of aLIGO. The main task of the IO subsystem is to take the laser beam from the pre-stabilized laser system1 (PSL) and prepare and a)Electronic mail: cmueller@phys.ufl.edu b)Present address: KLA-Tencor, Milpitas, California 95035, USA. c)Present address: LIGO Laboratory, California Institute of Technology, Pasadena, California 91125, USA. inject it into the main IFO. The PSL consists of a master laser, an amplifier stage, and a 200 W slave laser which is injection locked to the amplified master laser. The 200 W output beam is filtered by a short optical ring cavity, the pre-mode cleaner, before it is turned over to the IO (see Figure 1). The PSL prestabilizes the laser frequency to a fixed spacer reference cavity using a tunable sideband locking technique. The PSL also provides interfaces to further stabilize its frequency and power. The IFO is a dual-recycled, cavity-enhanced Michelson interferometer2 as sketched in Figure 2. The field enters the 55 m folded power recycling cavity (PRC) through the power recycling mirror (PRM). Two additional mirrors (PR2, PR3) within the PRC form a telescope to increase the beam size from ∼2 mm to ∼50 mm (Gaussian beam radius) before the large beam is split at the beam splitter and injected into the two 4 km arm cavities formed by the input and end test masses. The reflected fields recombine at the BS and send most of the light back to the PRM where it constructively interferes with the injected field.3 This leads to a power enhancement inside the power recycling cavity and provides additional spatial, frequency, and amplitude filtering of the laser beam. The second output of the BS sends light into the 55 m long folded signal recycling cavity4 (SRC) which also consists of a beam reducing telescope (SR2, SR3) and the partially reflective signal recycling mirror (SRM). This paper is organized as follows: Section II gives an overview of the IO; its functions, components, and the 0034-6748/2016/87(1)/014502/16/$30.00 87, 014502-1 © 2016 AIP Publishing LLC 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 00:51:35
014502-2 Mueller et al. Rev.Sci.Instrum.87,014502(2016) Slave Laser AO EO VCO Ref Ampl. to Input Optics ML:Master Laser FI:Faraday Isolator EOM:Electro-optic modulator Ref.:Reference Cavity AOM:Acousto-optic modulator PMC:Pre-mode cleaner VCO:Voltage Controlled Osc. Ampl.:Amplifier FIG.1.Sketch of the pre-stabilized laser (PSL)system.Red:Main beam,Green:Pick-off beam.The figure shows the low power master laser,the phase-correcting EOM,the amplifier stage,a high power Faraday isolator,and the high power slave laser.The pre-mode cleaner suppresses higher order spatial modes of the laser beam.The VCO drives the AOM which shifts the frequency of the pick-off beam,allowing both the reverence cavity and the PMC to be simultaneously resonant. general layout.Section III discusses the requirements for discusses the expected and measured in-vacuum performance the IO.Section IV presents the core of this paper;it will as known by the time of writing.The final integrated testing describe individual IO components,their performance in pre- of the IO subsystem at design sensitivity requires the main installation tests and the detailed layout of the IO.Section V interferometer to be nearly fully commissioned to act as a ETM 4km ~16m from IO PRM PR2 ITM ITM 4km ETM 7 PR3 ~25m √MBS SR2 1O:Input Optics h PRM:Power Recycling Mirror ~25m ~16m BS:Beam Splitter ITM:Input Test Mass I ETM:End Test Mass SR3 SRM SRM:Signal Recycling Mirror to output optics FIG.2.Sketch of the main interferometer which consists of two 4 km arm cavities,the beam splitter,and the folded 55 m long power and signal recycling cavities.The input optics is located between this system and the PSL shown in Figure 1. 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 20160051:35
014502-2 Mueller et al. Rev. Sci. Instrum. 87, 014502 (2016) FIG. 1. Sketch of the pre-stabilized laser (PSL) system. Red: Main beam, Green: Pick-off beam. The figure shows the low power master laser, the phase-correcting EOM, the amplifier stage, a high power Faraday isolator, and the high power slave laser. The pre-mode cleaner suppresses higher order spatial modes of the laser beam. The VCO drives the AOM which shifts the frequency of the pick-off beam, allowing both the reverence cavity and the PMC to be simultaneously resonant. general layout. Section III discusses the requirements for the IO. Section IV presents the core of this paper; it will describe individual IO components, their performance in preinstallation tests and the detailed layout of the IO. Section V discusses the expected and measured in-vacuum performance as known by the time of writing. The final integrated testing of the IO subsystem at design sensitivity requires the main interferometer to be nearly fully commissioned to act as a FIG. 2. Sketch of the main interferometer which consists of two 4 km arm cavities, the beam splitter, and the folded 55 m long power and signal recycling cavities. The input optics is located between this system and the PSL shown in Figure 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 00:51:35
014502-3 Mueller et al. Rev.Sci.Instrum.87,014502(2016) reference for many of the required measurements;this will be range of the beam (~13 m)is too large to allow easy access discussed in Section VI. to the second degree of freedom with a second piezo-actuated mirror. Between the lenses is a wedge to pick off a small fraction II.OVERVIEW OF THE INPUT OPTICS of the laser beam for diagnostic purposes.A fast photode- Figure 3 shows a sketch of the first part of the input optics. tector monitors the residual amplitude modulation at the phase This part is co-located with the PSL on the same optical table modulation frequencies while a second photodetector moni- inside the laser enclosure,outside of the vacuum system.It tors the DC power.A fraction of the main beam also transmits prepares the laser beam for the injection into the vacuum sys- through the bottom periscope mirror and is used to monitor the tem.The beam from the PSL is first routed through a half-wave power going into the vacuum system as well as the size,shape, plate and a polarizing beam splitter.These two elements form and quality of the beam. a manual power control stage which is used mainly during Following the periscope,the main beam is sent through a alignment processes on the optical table.The following mirror metal tube which includes a mechanical shutter and through transmits 2.5%of the light.This light is used by the arm length HAM16 into HAM2;all in-vacuum IO components are stabilization (ALS)system during lock acquisition of the main mounted on seismically isolated optical tables inside HAM2 interferometer.3 and HAM3.As shown in Figure 4,the beam passes over the Most of the light is sent through an electro-optic modu- Faraday isolator to a second periscope which lowers the beam lator which modulates the phase of the laser field with three to the in-vacuum beam height.The next element in the IO different modulation frequencies.Two of these frequencies are is the suspended IMC,a 33 m long (round-trip)triangular used by the interferometer sensing and control (ISC)system cavity.The two flat input and output mirrors,named MCI to sense most of the longitudinal and alignment degrees of and MC3,respectively,are located in HAM2 while the third freedom of the mirrors inside the IFO and to stabilize the curved mirror,MC2,is located in HAM3.Following MC3 are laser frequency and the alignment of the laser beam into the two suspended mirrors,IMI and IM2.which steer the beam interferometer.The third frequency is used to control the input through the Faraday isolator.IM3 and IM4 are used to steer mode cleaner.The two lenses LI and L2 mode match the beam the beam into the PRC.IM2 and IM3 are curved to mode to the in-vacuum input mode cleaner (IMC).The next steering match the output mode of the IMC to the mode of the main mirror directs the beam through another half-wave plate inside interferometer. a motorized rotation stage in front of two thin film polarizers. Two of the steering mirrors,IMI and IM4,transmit a This second power control stage is used during operations to small fraction of the light creating three different auxiliary adjust the power to the requested level.The periscope raises beams which are used to monitor the power and spatial mode the height of the beam and steers it into the vacuum system. of the IMC transmitted beam,of the beam going into the IFO, The top mirror is mounted on a piezo-actuated mirror mount to and of the beam which is reflected from the IFO.The latter fine tune the alignment of the beam into the vacuum chamber. two beams are routed to IOT2R,7 an optical table on the right A single piezo-actuated mirror is used because the Rayleigh side of HAM2,while the first beam and the field which is to ALS EOM PBS HWP RWP DCPD from PSL REPD Diag. BD DCPD to Vacuum Periscope ALS:Arm length stabilization system L1,L2:Lenses BD:Beam dump RFPD:Fast photo-detector DCPD:Photo-detector RWP:Rotating half-wave plate Diag:to Diagnostics TFP:Thin film polarizer EOM:Electro-optic modulator ∠：Water cooled beam dump HWP:Half-wave plate FIG.3.The IO on the in-air PSL table modulates the phase of the laser beam with the EOM,mode matches the light into the input mode cleaner (located inside the vacuum system),and controls the power injected into vacuum system. 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 20160051:35
014502-3 Mueller et al. Rev. Sci. Instrum. 87, 014502 (2016) reference for many of the required measurements; this will be discussed in Section VI. II. OVERVIEW OF THE INPUT OPTICS Figure 3 shows a sketch of the first part of the input optics. This part is co-located with the PSL on the same optical table inside the laser enclosure, outside of the vacuum system. It prepares the laser beam for the injection into the vacuum system. The beam from the PSL is first routed through a half-wave plate and a polarizing beam splitter. These two elements form a manual power control stage which is used mainly during alignment processes on the optical table. The following mirror transmits 2.5% of the light. This light is used by the arm length stabilization (ALS) system during lock acquisition of the main interferometer.5 Most of the light is sent through an electro-optic modulator which modulates the phase of the laser field with three different modulation frequencies. Two of these frequencies are used by the interferometer sensing and control (ISC) system to sense most of the longitudinal and alignment degrees of freedom of the mirrors inside the IFO and to stabilize the laser frequency and the alignment of the laser beam into the interferometer. The third frequency is used to control the input mode cleaner. The two lenses L1 and L2 mode match the beam to the in-vacuum input mode cleaner (IMC). The next steering mirror directs the beam through another half-wave plate inside a motorized rotation stage in front of two thin film polarizers. This second power control stage is used during operations to adjust the power to the requested level. The periscope raises the height of the beam and steers it into the vacuum system. The top mirror is mounted on a piezo-actuated mirror mount to fine tune the alignment of the beam into the vacuum chamber. A single piezo-actuated mirror is used because the Rayleigh range of the beam (∼13 m) is too large to allow easy access to the second degree of freedom with a second piezo-actuated mirror. Between the lenses is a wedge to pick off a small fraction of the laser beam for diagnostic purposes. A fast photodetector monitors the residual amplitude modulation at the phase modulation frequencies while a second photodetector monitors the DC power. A fraction of the main beam also transmits through the bottom periscope mirror and is used to monitor the power going into the vacuum system as well as the size, shape, and quality of the beam. Following the periscope, the main beam is sent through a metal tube which includes a mechanical shutter and through HAM16 into HAM2; all in-vacuum IO components are mounted on seismically isolated optical tables inside HAM2 and HAM3. As shown in Figure 4, the beam passes over the Faraday isolator to a second periscope which lowers the beam to the in-vacuum beam height. The next element in the IO is the suspended IMC, a 33 m long (round-trip) triangular cavity. The two flat input and output mirrors, named MC1 and MC3, respectively, are located in HAM2 while the third curved mirror, MC2, is located in HAM3. Following MC3 are two suspended mirrors, IM1 and IM2, which steer the beam through the Faraday isolator. IM3 and IM4 are used to steer the beam into the PRC. IM2 and IM3 are curved to mode match the output mode of the IMC to the mode of the main interferometer. Two of the steering mirrors, IM1 and IM4, transmit a small fraction of the light creating three different auxiliary beams which are used to monitor the power and spatial mode of the IMC transmitted beam, of the beam going into the IFO, and of the beam which is reflected from the IFO. The latter two beams are routed to IOT2R,7 an optical table on the right side of HAM2, while the first beam and the field which is FIG. 3. The IO on the in-air PSL table modulates the phase of the laser beam with the EOM, mode matches the light into the input mode cleaner (located inside the vacuum system), and controls the power injected into vacuum system. 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 00:51:35
014502-4 Mueller et al. Rev.Sci.Instrum.87,014502(2016) HAM2 to IOT2L HAM 3 IMI QPD to HAM ISC-Sled o HAM rom PSL R PR2 to BS MC:Mode Cleaner mirror(suspended) IM:Input Optics Mirror (suspended) QPD:Quadrant Photo Detector PSL-PD FI:Faraday Isolator to IOT2R BS:Beam Splitter (Core Optics) FIG.4.A sketch of the in-vacuum components and beam directions within the input optics in HAM 2 and HAM 3.The red beam is the forward going main beam while the green beams are auxiliary beams.The main items in the in-vacuum input optics are the input mode cleaner(IMC)which is formed by the three mirrors MC1,MC2,and MC3;the Faraday isolator (FD);and the four suspended steering mirrors IM1-4 of which IM2 and 3 match the spatial mode of the IMC into the main interferometer.The recycling cavity mirrors PRM,PR2,and PR3 are not part of the input optics.The ISC sled in HAM3 belongs to the interferometer sensing and control subsystem and provides alignment signals for the recycling cavity. reflected from MC1 are routed to IOT2L on the left side of III.INPUT OPTICS REQUIREMENTS HAM2.The position of the forward going beam through IM4 The aLIGO interferometer can be operated in different is monitored with an in-vacuum quadrant photodetector while a large fraction of this beam is also sent to an in-vacuum modes to optimize the sensitivity for different sources.10 These photodetector array which is used to monitor and stabilize the modes are characterized by the input power and the micro- laser power before it is injected into the IFO.Most of the IFO scopic position and reflectivity of the signal recycling mirror. reflected field goes back to the Faraday isolator where it is The requirements for the aLIGO input optics are specified separated from the incoming beam.This field is routed into to simultaneously meet the requirements for all anticipated HAMI where it is detected to generate length and alignment science modes and address all degrees of freedom of the laser field.Requirements in aLIGO are defined for three distinct sensing signals. frequency ranges:DC,the control band up to 10 Hz,and the In HAM3,a small fraction of the intra-mode cleaner field transmits through MC2 onto a quadrant photodetector to signal or detection band from 10 Hz to a few kHz.The require- ments in the detection band are defined in terms of linear monitor the beam position on MC2.The forward and back- wards traveling waves inside the PRC partly transmit through spectral densities and include a safety factor of ten such that all PR2 and are routed into HAMI and to an optical breadboard technical noise sources are an order of magnitude less than the sum of the fundamental limiting noise sources.To first order. inside HAM3,respectively.These beams are used by ISC for sensing and control of the interferometer and for diagnostic a perfectly symmetric Michelson interferometer is insensitive to all input noise sources which is an often overlooked reason purposes.The breadboard uses a lens to image the beam with for its use in the first place.However,all degrees of freedom orthogonal Gouy phases onto two quadrant photodetectors to of the injected laser field couple via some asymmetry to the monitor beam position and pointing inside the power recy- output signal.This drives the requirements in the control band cling cavity.IOT2R and IOT2L host photodetectors and digital cameras to monitor the power and beam sizes in each of the which are usually defined as RMS values.The more critical picked-off beams.IOT2L also hosts the photodetectors which requirements for the IO are as follows. are used by the interferometer sensing and control system to generate length and alignment sensing signals for the input A.Power mode cleaner. While the figure shows all key components in the correct The high power science modes require to inject 125 W of sequence,we intentionally left out the detailed beam routing, mode matched light into the interferometer with less than an the baffles used to suppress scattered light and protect all additional 5%in higher order modes.The PSL has to deliver components from the laser beam in case of misalignments,and 165 W of light in an appropriate TEMoo mode.Consequently, the beam dumps to capture all ghost beams. the net efficiency of TEMoo optical power transmission from A complete document tree which contains all design and the PSL output to the main interferometer has to be above as-built layouts as well as drawings of all components is avail- 75%.This sets limits on accumulated losses in all optical able within the LIGO Document Control Center(DCC)under components but also limits the allowed thermal lensing in document number E1201013.9 the EOM,the Faraday isolator,and the power control stages; Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/rights nd-pem sions.Download to 183195.251.60nFi22A1 20160051:35
014502-4 Mueller et al. Rev. Sci. Instrum. 87, 014502 (2016) FIG. 4. A sketch of the in-vacuum components and beam directions within the input optics in HAM 2 and HAM 3. The red beam is the forward going main beam while the green beams are auxiliary beams. The main items in the in-vacuum input optics are the input mode cleaner (IMC) which is formed by the three mirrors MC1, MC2, and MC3; the Faraday isolator (FI); and the four suspended steering mirrors IM1-4 of which IM2 and 3 match the spatial mode of the IMC into the main interferometer. The recycling cavity mirrors PRM, PR2, and PR3 are not part of the input optics. The ISC sled in HAM3 belongs to the interferometer sensing and control subsystem and provides alignment signals for the recycling cavity. reflected from MC1 are routed to IOT2L on the left side of HAM2. The position of the forward going beam through IM4 is monitored with an in-vacuum quadrant photodetector while a large fraction of this beam is also sent to an in-vacuum photodetector array which is used to monitor and stabilize the laser power before it is injected into the IFO. Most of the IFO reflected field goes back to the Faraday isolator where it is separated from the incoming beam. This field is routed into HAM1 where it is detected to generate length and alignment sensing signals. In HAM3, a small fraction of the intra-mode cleaner field transmits through MC2 onto a quadrant photodetector to monitor the beam position on MC2. The forward and backwards traveling waves inside the PRC partly transmit through PR2 and are routed into HAM1 and to an optical breadboard inside HAM3, respectively. These beams are used by ISC for sensing and control of the interferometer and for diagnostic purposes. The breadboard uses a lens to image the beam with orthogonal Gouy phases onto two quadrant photodetectors to monitor beam position and pointing inside the power recycling cavity. IOT2R and IOT2L host photodetectors and digital cameras to monitor the power and beam sizes in each of the picked-off beams. IOT2L also hosts the photodetectors which are used by the interferometer sensing and control system to generate length and alignment sensing signals for the input mode cleaner. While the figure shows all key components in the correct sequence, we intentionally left out the detailed beam routing, the baffles used to suppress scattered light and protect all components from the laser beam in case of misalignments, and the beam dumps to capture all ghost beams. A complete document tree which contains all design and as-built layouts as well as drawings of all components is available within the LIGO Document Control Center8 (DCC) under document number E1201013.9 III. INPUT OPTICS REQUIREMENTS The aLIGO interferometer can be operated in different modes to optimize the sensitivity for different sources.10 These modes are characterized by the input power and the microscopic position and reflectivity of the signal recycling mirror. The requirements for the aLIGO input optics are specified to simultaneously meet the requirements for all anticipated science modes and address all degrees of freedom of the laser field. Requirements in aLIGO are defined for three distinct frequency ranges: DC, the control band up to 10 Hz, and the signal or detection band from 10 Hz to a few kHz. The requirements in the detection band are defined in terms of linear spectral densities and include a safety factor of ten such that all technical noise sources are an order of magnitude less than the sum of the fundamental limiting noise sources. To first order, a perfectly symmetric Michelson interferometer is insensitive to all input noise sources which is an often overlooked reason for its use in the first place. However, all degrees of freedom of the injected laser field couple via some asymmetry to the output signal. This drives the requirements in the control band which are usually defined as RMS values. The more critical requirements for the IO are as follows. A. Power The high power science modes require to inject 125 W of mode matched light into the interferometer with less than an additional 5% in higher order modes. The PSL has to deliver 165 W of light in an appropriate TEM00 mode. Consequently, the net efficiency of TEM00 optical power transmission from the PSL output to the main interferometer has to be above 75%. This sets limits on accumulated losses in all optical components but also limits the allowed thermal lensing in the EOM, the Faraday isolator, and the power control stages; 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 00:51:35