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 2016005135014502-5 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 mechan￾ical 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 differ￾ential 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 displace￾ment 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 oper￾ating 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 increas￾ing 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 detec￾tion 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 fre￾quency 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 condi￾tion 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