033109-7 Dooley et al. Rev.Sci.Instrum.83,033109(2012) Thus,the combination of CWPs and a TFP combines We present in this section detailed measurements of the the best of each to provide a high extinction ratio (from the IO performance during Enhanced LIGO.Specific measure- CWPs)and ease of reflected beam extraction(from the TFP). ments and results presented in figures and the text come from The downsides that remain when using both polarizers are Livingston;performance at Hanford was similar and is in- that there is still some thermal drift from the CWPs.Also the cluded in tables summarizing the results. transmission is reduced due to the TFP and to the fact that there are 16 surfaces from which light can scatter. A.Optical efficiency 4.Heat conduction The optical efficiency of the Enhanced LIGO IO from Faraday isolators operating in a vacuum environment suf- EOM to recycling mirror was 75%,a marked improvement fer from increased heating with respect to those operating in over the approximate 60%that was measured for Initial LIGO.A substantial part of the improvement came from the air.Convective cooling at the faces of the optical components is no longer an effective heat removal channel,so proper heat discovery and subsequent correction of a 6.5%loss at the sec- sinking is essential to minimize thermal lensing and depo- ond of the in-vacuum steering mirrors directing light into the larization.It has been shown that Faraday isolators carefully MC (refer to Fig.3).A 45 reflecting mirror had been used for aligned in air can experience a dramatic reduction in isola- a beam with an 8 angle of incidence.Losses attributable to tion ratio(>10-15 dB)when placed in vacuum.28 The dom- the MC and FI are described in Subsections V A 1 and V A 2. inant cause is the coupling of the photoelastic effect to the A summary of the IO power budget is found in Table II. temperature gradient induced by laser beam absorption.Also of importance is the temperature dependence of the Verdet 1.Mode cleaner losses constant-different spatial parts of the beam experience dif- ferent polarization rotations in the presence of a temperature The MC was the greatest single source of power loss in gradient.29 both Initial and Enhanced LIGO.The MC visibility, To improve heat conduction away from the Faraday rota- V= Pin -Prefl tor optical components,we designed a housing for the TGG (1) and quartz crystals that provided improved heat sinking to the Pin Faraday rotator.We wrapped the TGGs with indium foil that where Pin is the power injected into the MC and Prea the made improved contact with the housing and we cushioned power reflected,was 92%.Visibility reduction is the result the DKDP and the HWP with indium wire in their aluminum of higher order mode content of Pin and mode mismatch into holders.This has the additional effect of avoiding the devel- the MC.The visibility was constant within 0.04%up to 30 W opment of thermal stresses in the crystals,an especially im- input power at both sites,providing a positive indication that portant consideration for the very fragile DKDP. thermal aberrations in the MC and upstream were negligible. 88%of the light coupled into the MC was transmitted. D.Mode-matching telescope design 2.6%of these losses were caused by poor AR coatings on the second surfaces of the 45 MC mirrors.The measured surface The mode matching into the interferometer (at microroughness of o<0.4 nm 31 caused scatter losses of Livingston)was measured to be at best 90%in Initial [4mms<22 ppm per mirror inside the MC,or a total of LIGO.Because of the stringent requirements placed on the 2.7%losses in transmission. LIGO vacuum system to reduce phase noise through scat- Another source of MC losses is via absorption of heat tering by residual gas,standard opto-mechanical translators by particulates residing on the mirror's surface.We measured are not permitted in the vacuum;it is therefore not possible the absorption with a technique that makes use of the fre- to physically move the mode matching telescope mirrors quency shift of the thermally driven drumhead eigenfrequen- while operating the interferometer.Through a combination cies of the mirror substrate.32 The frequency shift directly of needing to move the MMTs in order to fit the new FI correlates with the MC absorption via the substrate's change on the in-vacuum optics table and additional measurements and models to determine how to improve the coupling,a new set of MMT positions was chosen for Enhanced LIGO. TABLE II.Enhanced LIGO IO power budget.Errors are +1%,except for the TFP loss whose error is +0.1%.The composite MC transmission is the Fundamental design considerations are discussed in Ref.30. percentage of power after the MC to before the MC and is the product of the MC visibility and transmission.Initial LIGO values,where known,are V.PERFORMANCE OF THE ENHANCED LIGO INPUT included in parentheses and have errors of several percent. OPTICS Livingston Hanford The most convincing figure of merit for the IO perfor- mance is that the Enhanced LIGO interferometers achieved MC visibility 92% 97% low-noise operation with 20 W input power without thermal MC transmission 88% 90% Composite MC transmission 81%(72%) 87% issues from the IO.Additionally,the IO were operated suc- FI transmission 93%(86%) 94%(86%) cessfully up to the available 30 W of power.(Instabilities with TFP loss 4.0% 2.7% other interferometer subsystems limited the Enhanced LIGO IO efficiency (PSL to RM) 75%(60%) 82% science run operation to 20 W.) Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/rights-and-permissions. D0wmlo8 d to IP:183.195251.60m:Fi.22Apr2016 00:54:10033109-7 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) Thus, the combination of CWPs and a TFP combines the best of each to provide a high extinction ratio (from the CWPs) and ease of reflected beam extraction (from the TFP). The downsides that remain when using both polarizers are that there is still some thermal drift from the CWPs. Also the transmission is reduced due to the TFP and to the fact that there are 16 surfaces from which light can scatter. 4. Heat conduction Faraday isolators operating in a vacuum environment suf￾fer from increased heating with respect to those operating in air. Convective cooling at the faces of the optical components is no longer an effective heat removal channel, so proper heat sinking is essential to minimize thermal lensing and depo￾larization. It has been shown that Faraday isolators carefully aligned in air can experience a dramatic reduction in isola￾tion ratio (>10-15 dB) when placed in vacuum.28 The dom￾inant cause is the coupling of the photoelastic effect to the temperature gradient induced by laser beam absorption. Also of importance is the temperature dependence of the Verdet constant—different spatial parts of the beam experience dif￾ferent polarization rotations in the presence of a temperature gradient.29 To improve heat conduction away from the Faraday rota￾tor optical components, we designed a housing for the TGG and quartz crystals that provided improved heat sinking to the Faraday rotator. We wrapped the TGGs with indium foil that made improved contact with the housing and we cushioned the DKDP and the HWP with indium wire in their aluminum holders. This has the additional effect of avoiding the devel￾opment of thermal stresses in the crystals, an especially im￾portant consideration for the very fragile DKDP. D. Mode-matching telescope design The mode matching into the interferometer (at Livingston) was measured to be at best 90% in Initial LIGO. Because of the stringent requirements placed on the LIGO vacuum system to reduce phase noise through scat￾tering by residual gas, standard opto-mechanical translators are not permitted in the vacuum; it is therefore not possible to physically move the mode matching telescope mirrors while operating the interferometer. Through a combination of needing to move the MMTs in order to fit the new FI on the in-vacuum optics table and additional measurements and models to determine how to improve the coupling, a new set of MMT positions was chosen for Enhanced LIGO. Fundamental design considerations are discussed in Ref. 30. V. PERFORMANCE OF THE ENHANCED LIGO INPUT OPTICS The most convincing figure of merit for the IO perfor￾mance is that the Enhanced LIGO interferometers achieved low-noise operation with 20 W input power without thermal issues from the IO. Additionally, the IO were operated suc￾cessfully up to the available 30 W of power. (Instabilities with other interferometer subsystems limited the Enhanced LIGO science run operation to 20 W.) We present in this section detailed measurements of the IO performance during Enhanced LIGO. Specific measure￾ments and results presented in figures and the text come from Livingston; performance at Hanford was similar and is in￾cluded in tables summarizing the results. A. Optical efficiency The optical efficiency of the Enhanced LIGO IO from EOM to recycling mirror was 75%, a marked improvement over the approximate 60% that was measured for Initial LIGO. A substantial part of the improvement came from the discovery and subsequent correction of a 6.5% loss at the sec￾ond of the in-vacuum steering mirrors directing light into the MC (refer to Fig. 3). A 45◦ reflecting mirror had been used for a beam with an 8◦ angle of incidence. Losses attributable to the MC and FI are described in Subsections VA1 and VA2. A summary of the IO power budget is found in Table II. 1. Mode cleaner losses The MC was the greatest single source of power loss in both Initial and Enhanced LIGO. The MC visibility, V = Pin − Prefl Pin , (1) where Pin is the power injected into the MC and Prefl the power reflected, was 92%. Visibility reduction is the result of higher order mode content of Pin and mode mismatch into the MC. The visibility was constant within 0.04% up to 30 W input power at both sites, providing a positive indication that thermal aberrations in the MC and upstream were negligible. 88% of the light coupled into the MC was transmitted. 2.6% of these losses were caused by poor AR coatings on the second surfaces of the 45◦ MC mirrors. The measured surface microroughness of σrms < 0.4 nm 31 caused scatter losses of [4πσrms/λ] 2 < 22 ppm per mirror inside the MC, or a total of 2.7% losses in transmission. Another source of MC losses is via absorption of heat by particulates residing on the mirror’s surface. We measured the absorption with a technique that makes use of the fre￾quency shift of the thermally driven drumhead eigenfrequen￾cies of the mirror substrate.32 The frequency shift directly correlates with the MC absorption via the substrate’s change TABLE II. Enhanced LIGO IO power budget. Errors are ±1%, except for the TFP loss whose error is ±0.1%. The composite MC transmission is the percentage of power after the MC to before the MC and is the product of the MC visibility and transmission. Initial LIGO values, where known, are included in parentheses and have errors of several percent. Livingston Hanford MC visibility 92% 97% MC transmission 88% 90% Composite MC transmission 81% (72%) 87% FI transmission 93% (86%) 94% (86%) TFP loss 4.0% 2.7% IO efficiency (PSL to RM) 75% (60%) 82% 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