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 20160051:35014502-6 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 Asso￾ciates, 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 con￾trol 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 prob￾lems due to the uncontrolled length between the IMC and IFO15 (a parasitic interferometer). Initial and Enhanced LIGO never encountered any major problems with insufficient op￾tical 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 scat￾tered 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