014502-5 Mueller et al. Rev.Sci.Instrum.87,014502(2016) the reflective optics and fused silica lenses are much less The IO does not provide any active element to change susceptible to thermal lensing.Efficient power coupling is or stabilize the laser power within the control or the detec- also dependent on good mode matching between the recycling tion band.The PSL uses a first loop which stabilizes the cavities and the arm cavities in the main interferometer. laser power measured with a photodetector on the PSL table to 2x 10-8/VHz between 20 and 100 Hz and meeting the aforementioned requirements above 100 Hz.The PSL further B.Power control stabilizes the injected power in the 20 Hz-100 Hz band with The injected power into the interferometer has to be the photodetector array shown(PSL-PD)in Figure 4 which adjustable from the control room from minimum to full power is placed after the IMC.The IO has to supply the auxiliary for diagnostic and operational purposes,to acquire lock of the beam for this array and maintain a sufficiently high correlation main interferometer,and to operate between different science with the injected beam and minimize the chances of additional modes.The rate of power change(dP/dt)has to be sufficiently power fluctuations within any of these two beams. small to limit the radiation pressure kick inside the IMC and the main interferometer to a level that can be handled by the length and alignment control system.It has to be sufficiently D.Frequency fluctuations fast to not limit the time to transition to full power after lock In the detection band,the laser frequency will ultimately acquisition,i.e.,it should be possible to change from minimum be stabilized to the common mode of the two arm cavities to maximum power within a few seconds. which are the most stable references available in this frequency Note that minimum power here cannot mean zero power range.At lower frequencies the arm cavities are not a good because of the limited extinction ratio of polarizers.Going to reference and are made to follow the frequency reference zero power requires actuation of the aforementioned mechan- inside of the PSL.The input mode cleaner acts as a frequency ical shutter which can only be accessed manually between the reference during lock acquisition and as an intermediate fre- laser enclosure and HAM1.The emergency shutter is part of quency reference during science mode.It is integrated into the PSL laser system and cuts the laser power at the source. the complex and nested laser frequency stabilization system. Furthermore,the power control system within the IO is not Based on the expected common mode servo gain the frequency used for actively stabilizing the laser power within the control noise requirements for the IMC are set to or the detection band. oy(f=10Hz)<50 mHz/VHz. C.Power fluctuations 6v(f 100Hz)<1 mHz/VHz. Fluctuations in the laser power can couple through many These requirements can be expressed equivalently as different channels to the error signal used to detect the differ- length fluctuations of the IMC: ential length of the interferometer arms,i.e.,the gravitational wave detection signal.The noise scales with the asymmetries 6(f=10Hz)<3.10-15m/H五， in the interferometer.Two different mechanisms are expected to dominate the susceptibility of the interferometer to power 6(f≥100Hz)<6.10-17m/VHz. fluctuations.The optical power inside the arm cavities will push the test masses outwards.Any change in power will E.RF modulation frequencies cause fluctuations in that pressure which can lead to displace- ment noise at low frequencies.The susceptibility to radiation The main laser field consists of a carrier and multiple pressure noise scales with differences in the power build up pairs of sidebands.The carrier has to be resonant in the arm inside the arm cavities and it is assumed that these differences cavities and the power recycling cavity:the resonance condi- are below 1%.At high frequencies,direct coupling of power tion in the signal recycling cavity depends on the tuning and fluctuations to the gravitational wave signal limits the allowed specific science mode.One pair of sidebands must be resonant power fluctuations.When the interferometer is held at its oper- in the power recycling cavity,while the second pair must ating point the two arm cavities are detuned by a few pm which resonate in both the power and signal recycling cavity.The causes some light to leak out to the dark port.Gravitational modulation signals of f1=9.1 MHz and f2 =45.5 MHz are waves will modulate these offsets causing the light power at provided by the interferometer sensing and control system. the dark port to fluctuate.Obviously,power fluctuations in the A third modulation frequency of f3=24.1 MHz is required laser itself.although highly filtered by the interferometer.will to sense and control the input mode cleaner.The last pair of cause similar fluctuations.The relative intensity noise in the sidebands should be rejected by the input mode cleaner so as detection band has to be below 2 x 10-/VHz at 10 Hz increas- not to interfere with the sensing and control system of the main ing with f to 2 x 10-8/VHz at 100 Hz and remaining flat after interferometer. this.Furthermore,the expected seismically excited motion of the test masses limits the allowed radiation pressure noise in the control band to 10-2/VHz below 0.2 Hz.Above 0.2 Hz,the F.RF modulation depth requirements follow two power laws;initially f-7 then f-3, The required modulation depths depend on the final length before connecting with the detection band requirement at and alignment sensing and control scheme.This scheme is 10Hz. likely to evolve over the commissioning time but the current Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/rights -and-perm ssions.Download to 183195.251.60nFri22A1 2016005135
014502-5 Mueller et al. Rev. Sci. Instrum. 87, 014502 (2016) the reflective optics and fused silica lenses are much less susceptible to thermal lensing. Efficient power coupling is also dependent on good mode matching between the recycling cavities and the arm cavities in the main interferometer. B. Power control The injected power into the interferometer has to be adjustable from the control room from minimum to full power for diagnostic and operational purposes, to acquire lock of the main interferometer, and to operate between different science modes. The rate of power change (dP/dt) has to be sufficiently small to limit the radiation pressure kick inside the IMC and the main interferometer to a level that can be handled by the length and alignment control system. It has to be sufficiently fast to not limit the time to transition to full power after lock acquisition, i.e., it should be possible to change from minimum to maximum power within a few seconds. Note that minimum power here cannot mean zero power because of the limited extinction ratio of polarizers. Going to zero power requires actuation of the aforementioned mechanical shutter which can only be accessed manually between the laser enclosure and HAM1. The emergency shutter is part of the PSL laser system and cuts the laser power at the source. Furthermore, the power control system within the IO is not used for actively stabilizing the laser power within the control or the detection band. C. Power fluctuations Fluctuations in the laser power can couple through many different channels to the error signal used to detect the differential length of the interferometer arms, i.e., the gravitational wave detection signal. The noise scales with the asymmetries in the interferometer. Two different mechanisms are expected to dominate the susceptibility of the interferometer to power fluctuations. The optical power inside the arm cavities will push the test masses outwards. Any change in power will cause fluctuations in that pressure which can lead to displacement noise at low frequencies. The susceptibility to radiation pressure noise scales with differences in the power build up inside the arm cavities and it is assumed that these differences are below 1%. At high frequencies, direct coupling of power fluctuations to the gravitational wave signal limits the allowed power fluctuations. When the interferometer is held at its operating point the two arm cavities are detuned by a few pm which causes some light to leak out to the dark port.11 Gravitational waves will modulate these offsets causing the light power at the dark port to fluctuate. Obviously, power fluctuations in the laser itself, although highly filtered by the interferometer, will cause similar fluctuations. The relative intensity noise in the detection band has to be below 2 × 10−9 / √ Hz at 10 Hz increasing with f to 2 × 10−8 / √ Hz at 100 Hz and remaining flat after this. Furthermore, the expected seismically excited motion of the test masses limits the allowed radiation pressure noise in the control band to 10−2 / √ Hz below 0.2 Hz. Above 0.2 Hz, the requirements follow two power laws; initially f −7 then f −3 , before connecting with the detection band requirement at 10 Hz. The IO does not provide any active element to change or stabilize the laser power within the control or the detection band. The PSL uses a first loop which stabilizes the laser power measured with a photodetector on the PSL table to 2 × 10−8 / √ Hz between 20 and 100 Hz and meeting the aforementioned requirements above 100 Hz. The PSL further stabilizes the injected power in the 20 Hz–100 Hz band with the photodetector array shown (PSL-PD) in Figure 4 which is placed after the IMC. The IO has to supply the auxiliary beam for this array and maintain a sufficiently high correlation with the injected beam and minimize the chances of additional power fluctuations within any of these two beams. D. Frequency fluctuations In the detection band, the laser frequency will ultimately be stabilized to the common mode of the two arm cavities which are the most stable references available in this frequency range. At lower frequencies the arm cavities are not a good reference and are made to follow the frequency reference inside of the PSL. The input mode cleaner acts as a frequency reference during lock acquisition and as an intermediate frequency reference during science mode. It is integrated into the complex and nested laser frequency stabilization system. Based on the expected common mode servo gain the frequency noise requirements for the IMC are set to δν( f = 10 Hz) < 50 mHz/ √ Hz, δν( f ≥ 100 Hz) < 1 mHz/ √ Hz. These requirements can be expressed equivalently as length fluctuations of the IMC: δℓ( f = 10 Hz) < 3 · 10−15 m/ √ Hz, δℓ( f ≥ 100 Hz) < 6 · 10−17 m/ √ Hz. E. RF modulation frequencies The main laser field consists of a carrier and multiple pairs of sidebands. The carrier has to be resonant in the arm cavities and the power recycling cavity; the resonance condition in the signal recycling cavity depends on the tuning and specific science mode. One pair of sidebands must be resonant in the power recycling cavity, while the second pair must resonate in both the power and signal recycling cavity. The modulation signals of f1 = 9.1 MHz and f2 = 45.5 MHz are provided by the interferometer sensing and control system. A third modulation frequency of f3 = 24.1 MHz is required to sense and control the input mode cleaner. The last pair of sidebands should be rejected by the input mode cleaner so as not to interfere with the sensing and control system of the main interferometer. F. RF modulation depth The required modulation depths depend on the final length and alignment sensing and control scheme. This scheme is likely to evolve over the commissioning time but the current 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
014502-6 Mueller et al. Rev.Sci.Instrum.87,014502 (2016) assumption is that a modulation index of 0.4 for a 10 Vpp signal for a long time without a Faraday isolator between the mode driving the EOM is more than sufficient.Note that this only cleaner and the main interferometer and encountered prob- applies to the two modulation frequencies which are used for lems due to the uncontrolled length between the IMC and sensing and control of the main interferometer:the modulation IFO5(a parasitic interferometer).Initial and Enhanced LIGO index for the third frequency needs only to be large enough to never encountered any major problems with insufficient op- control the IMC. tical isolation in the Faraday isolator.The requirements of The classic phase modulation/demodulation sensing 30 dB for the optical isolation in the Faraday isolator were set scheme for a single optical cavity measures how much the based on the experience in Initial LIGO,taking into account cavity converts phase modulation into amplitude modulation the higher injected power. when near resonance.Unfortunately all phase modulators also modulate the amplitude of the laser field.This amplitude J.Additional requirements modulation can saturate the RF amplifiers and mixers in the detection chain and generate offsets in the error signals which It is well known that parasitic interferometers and scat- have to be compensated.aLIGO requires that the amplitude tered light together with mechanically excited surfaces can add modulation index is less than 10-4 of the phase modulation frequency and amplitude noise to a laser beam.The IO adopted index.12 a policy to limit the added noise to 10%of the maximum allowed noise (based on the main interferometer sensitivity); note that the allowed frequency and amplitude noise prior to G.RF modulation noise the input mode cleaner is significantly higher than after the Changes in the amplitude and phase of the RF modulation mode cleaner.This drives requirements on the residual motion signals can pollute the gravitational wave detection signal by of the optical components,the surface quality of all optical changing the power buildup of the carrier in the arm cavities components and their coatings,and on the placement and or through cross coupling in the length and alignment sensing efficiency of the optical baffes.The requirement to align the IO and control schemes.These effects were analyzed by the ISC drives requirements on actuation ranges for all optics and,last group.10 The analysis uses specifications from a commercial but not the least,the IO has to meet the stringent cleanliness crystal oscillator manufacturer produced by Wenzel Asso- and vacuum requirements of aLIGO.These requirements are ciates,Inc.as the expected oscillator phase and amplitude discussed throughout the paper when relevant. noise.These specifications for phase noise are 10-5 rad/VHz at 10 Hz falling with 1/f3/2 to3 x 10-7rad/VHz at 100 Hz and then a little faster than 1/f to 2x 10-rad/VHz at a kHz above IV.INPUT OPTICS COMPONENTS which they stay constant.The specifications for amplitude AND FINAL LAYOUT noise are 10-7/VHz at 10 Hz falling with 1/f between 10 and This section will first discuss the individual components 100 Hz and then with 1/f until 1 kHz above which they and their measured performance.This will be followed by a stay constant at 3x 10-9/VHz.These specifications have been description of the optical layout which includes a discussion adopted as requirements although the analysis shows that they of beam parameters and mode matching between the various could be relaxed at higher frequencies. areas. H.Beam jitter A.Electro-optic modulators Changes in the location and direction of the injected beam The electro-optic modulators must use a material capable can be described as scattering light from the TEMoo into a of withstanding CW optical powers of up to 200 W and TEMio mode.This light scatters back into the TEMoo mode intensities up to 25kW/cm2.At these power levels the induced inside a misaligned interferometer and creates noise in the thermal lensing,stress induced depolarization,and damage gravitational wave signal.13 This is an example where noise in threshold of the electro-optic material must be taken into the detection band,here beam jitter,couples to noise in the con- consideration.Rubidium titanyl phosphate (RTP)was chosen trol band,here tilt of the input test masses.It is expected that many years ago over other electro-optic materials,such as the test masses will all be aligned to better than 2 nrad RMS rubidium titanyl arsenate (RbTiOAsO4 or RTA)and lithium with respect to the nominal optical axis of the interferometer. niobate (LiNb03),as the most promising modulator material Under this assumption,the relative amplitude of the injected after a literature survey,discussions with various vendors, 10-mode has to stay below 10-/VHz at 10 Hz falling with and corroborating lab experiments.16.17 RTP has a very high 1/f2 until 100 Hz above which the requirement stays constant damage threshold,low optical absorption,and a fairly high at 10-8/VHz. electro-optical coefficient.Enhanced LIGO allowed for testing of the material and design over a one-year period at 30 W input I.Optical isolation power.18 The aLIGO EOM uses a patented design19 which is The Faraday isolator isolates the IMC from back reflected very similar to the one used in eLIGO:both consist of a light from the main interferometer.The requirements for the 4 x 4 x 40 mm long wedged RTP crystal(see Figure 5).The isolation ratio are based on experience gained during the initial 2.85 wedges prohibit parasitic interferometers from building years of operating LIGO and also VIRGO.4 Virgo operated up inside the crystal and allow for separation of the two Reuse of AlP Publishing conte nt is subiect to the temms at:httos //oub hing.aip.org/authors/rights-and-p Download to 183.195.251.60nFi22A1 20160051:35
014502-6 Mueller et al. Rev. Sci. Instrum. 87, 014502 (2016) assumption is that a modulation index of 0.4 for a 10 Vpp signal driving the EOM is more than sufficient. Note that this only applies to the two modulation frequencies which are used for sensing and control of the main interferometer; the modulation index for the third frequency needs only to be large enough to control the IMC. The classic phase modulation/demodulation sensing scheme for a single optical cavity measures how much the cavity converts phase modulation into amplitude modulation when near resonance. Unfortunately all phase modulators also modulate the amplitude of the laser field. This amplitude modulation can saturate the RF amplifiers and mixers in the detection chain and generate offsets in the error signals which have to be compensated. aLIGO requires that the amplitude modulation index is less than 10−4 of the phase modulation index.12 G. RF modulation noise Changes in the amplitude and phase of the RF modulation signals can pollute the gravitational wave detection signal by changing the power buildup of the carrier in the arm cavities or through cross coupling in the length and alignment sensing and control schemes. These effects were analyzed by the ISC group.10 The analysis uses specifications from a commercial crystal oscillator manufacturer produced by Wenzel Associates, Inc. as the expected oscillator phase and amplitude noise. These specifications for phase noise are 10−5 rad/ √ Hz at 10 Hz falling with 1/ f 3/2 to 3 × 10−7 rad/ √ Hz at 100 Hz and then a little faster than 1/ f to 2 × 10−8 rad/ √ Hz at a kHz above which they stay constant.10 The specifications for amplitude noise are 10−7 / √ Hz at 10 Hz falling with 1/f between 10 and 100 Hz and then with 1/ f until 1 kHz above which they stay constant at 3 × 10−9 / √ Hz. These specifications have been adopted as requirements although the analysis shows that they could be relaxed at higher frequencies. H. Beam jitter Changes in the location and direction of the injected beam can be described as scattering light from the TEM00 into a TEM10 mode. This light scatters back into the TEM00 mode inside a misaligned interferometer and creates noise in the gravitational wave signal.13 This is an example where noise in the detection band, here beam jitter, couples to noise in the control band, here tilt of the input test masses. It is expected that the test masses will all be aligned to better than 2 nrad RMS with respect to the nominal optical axis of the interferometer. Under this assumption, the relative amplitude of the injected 10-mode has to stay below 10−6 / √ Hz at 10 Hz falling with 1/ f 2 until 100 Hz above which the requirement stays constant at 10−8 / √ Hz. I. Optical isolation The Faraday isolator isolates the IMC from back reflected light from the main interferometer. The requirements for the isolation ratio are based on experience gained during the initial years of operating LIGO and also VIRGO.14 Virgo operated for a long time without a Faraday isolator between the mode cleaner and the main interferometer and encountered problems due to the uncontrolled length between the IMC and IFO15 (a parasitic interferometer). Initial and Enhanced LIGO never encountered any major problems with insufficient optical isolation in the Faraday isolator. The requirements of 30 dB for the optical isolation in the Faraday isolator were set based on the experience in Initial LIGO, taking into account the higher injected power. J. Additional requirements It is well known that parasitic interferometers and scattered light together with mechanically excited surfaces can add frequency and amplitude noise to a laser beam. The IO adopted a policy to limit the added noise to 10% of the maximum allowed noise (based on the main interferometer sensitivity); note that the allowed frequency and amplitude noise prior to the input mode cleaner is significantly higher than after the mode cleaner. This drives requirements on the residual motion of the optical components, the surface quality of all optical components and their coatings, and on the placement and efficiency of the optical baffles. The requirement to align the IO drives requirements on actuation ranges for all optics and, last but not the least, the IO has to meet the stringent cleanliness and vacuum requirements of aLIGO. These requirements are discussed throughout the paper when relevant. IV. INPUT OPTICS COMPONENTS AND FINAL LAYOUT This section will first discuss the individual components and their measured performance. This will be followed by a description of the optical layout which includes a discussion of beam parameters and mode matching between the various areas. A. Electro-optic modulators The electro-optic modulators must use a material capable of withstanding CW optical powers of up to 200 W and intensities up to 25 kW/cm2 . At these power levels the induced thermal lensing, stress induced depolarization, and damage threshold of the electro-optic material must be taken into consideration. Rubidium titanyl phosphate (RTP) was chosen many years ago over other electro-optic materials, such as rubidium titanyl arsenate (RbTiOAsO4 or RTA) and lithium niobate (LiNb03), as the most promising modulator material after a literature survey, discussions with various vendors, and corroborating lab experiments.16,17 RTP has a very high damage threshold, low optical absorption, and a fairly high electro-optical coefficient. Enhanced LIGO allowed for testing of the material and design over a one-year period at 30 W input power.18 The aLIGO EOM uses a patented design19 which is very similar to the one used in eLIGO; both consist of a 4 × 4 × 40 mm long wedged RTP crystal (see Figure 5). The 2.85◦ wedges prohibit parasitic interferometers from building up inside the crystal and allow for separation of the two 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