033109-4 Dooley et al. Rev.Sci.Instrum.83,033109(2012) Interferometer Sensing and Control table ④ Photodiode ⊕ Quadrant photodiode Interferometer RF length detector feck beam reflected beam RF alignment detector Pre-stabilized laser table 12.2m LASER Electro-optic Mode Mode cleaner Mode cleahe modulator matching reflected beam lenses Input Optics table 卤卤 FIG.3.Enhanced LIGO Input Optics optical and sensing configuration.The HAMI(horizontal access module)vacuum chamber is featured in the center,with locations of all major optics superimposed.HAM2 is shown on the right,with its components.These tables are separated by 12 m.The primary beam path, beginning at the pre-stabilized laser and going to the power recycling mirror,is shown in red as a solid line,and auxiliary beams are different colors and dotted. The MMTs,MCs,and steering mirror(SM)are suspended;all other optics are fixed to the seismically isolated table.The laser and sensing and diagnostic photodiodes are on in-air tables. IO,and accommodated up to four times the power of Ini- crystal dimensions are 4 x 4 x 40 mm and their faces are tial LIGO.Also,the design was a prototype for handling the wedged by 2.85 and anti-reflection (AR)coated.The wedge 180 W laser planned for Advanced LIGO.Because the ad- serves to separate the polarizations and prevents an etalon ef- verse thermal properties of the Initial LIGO IO(beam drift, fect,resulting in a suppression of amplitude modulation.Only birefringence,and lensing)are all attributable primarily to ab- one crystal is used in the EOM in order to reduce the number sorption of laser light by the optical elements,the primary de- of surface reflections.Three separate pairs of electrodes,each sign consideration was finding optics with lower absorption.19 with its own resonant LC circuit,are placed across the crystal Both the EOM and the FI were replaced for Enhanced LIGO. in series,producing the three required sets of RF sidebands: Only minor changes were made to the MC and MMT.A de- 24.5 MHz,33.3 MHz,and 61.2 MHz.A diagram is shown in tailed layout of the Enhanced LIGO IO is shown in Figure 3. Fig.4.Reference 23 contains further details about the modu- lator architecture. A.Electro-optic modulator design We replaced the commercially made New Focus 4003 B.Mode cleaner design resonant phase modulator of Initial LIGO with an in-house The MC is a suspended 12.2 m long triangular ring cavity EOM design and construction.Both a new crystal choice and with finesseF=1280 and free spectral range of 12.243 MHz. architectural design change allow for superior performance. The three mirror architecture was selected over the standard The Enhanced LIGO EOM design uses a crystal of ru- two mirror linear filter cavity because it acts as a polarization bidium titanyl phosphate (RTP),which has at most 1/10 the absorption coefficient at 1064 nm of the lithium nio- bate (LiNbO3)crystal from Initial LIGO.At 200 W the RTP TABLE I.Comparison of selected properties of the Initial and Enhanced should produce a thermal lens of 200 m and higher order LIGO EOM crystals,LiNbO3,and RTP,respectively.RTP was preferred for Enhanced LIGO because of its lower absorption,superior thermal properties, mode content of less than 1%,compared to the 3.3 m lens and similar electro-optic properties. the LiNbO3 produces at 10 W.The RTP has a minimal risk of damage,because it has both twice the damage threshold of Units LiNbO3 RTP LiNbOa and is subjected to a beam twice the size of that in Ini- tial LIGO.RTP and LiNbO3 have similar electro-optic coeffi- Damage threshold MW/cm2 280 >600 <5000 cients.Also,RTP's dn/dT anisotropy is 50%smaller.Table I Absorption coeff.at 1064 nm ppm/cm <500 Electro-optic coeff.(n2r33) pm/V 306 239 compares the properties of most interest of the two crystals. dnldT 10-6K 5.4 2.79 We procured the RTP crystals from Raicol and packaged dn-ldT 10-6K 37.9 9.24 them into specially designed,custom-built modulators.The Reuse of AlP Publishing content is subject to the terms at:https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 183.195251.60:Fi.22Apr2016 00:54:10033109-4 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) FIG. 3. Enhanced LIGO Input Optics optical and sensing configuration. The HAM1 (horizontal access module) vacuum chamber is featured in the center, with locations of all major optics superimposed. HAM2 is shown on the right, with its components. These tables are separated by 12 m. The primary beam path, beginning at the pre-stabilized laser and going to the power recycling mirror, is shown in red as a solid line, and auxiliary beams are different colors and dotted. The MMTs, MCs, and steering mirror (SM) are suspended; all other optics are fixed to the seismically isolated table. The laser and sensing and diagnostic photodiodes are on in-air tables. IO, and accommodated up to four times the power of Ini￾tial LIGO. Also, the design was a prototype for handling the 180 W laser planned for Advanced LIGO. Because the ad￾verse thermal properties of the Initial LIGO IO (beam drift, birefringence, and lensing) are all attributable primarily to ab￾sorption of laser light by the optical elements, the primary de￾sign consideration was finding optics with lower absorption.19 Both the EOM and the FI were replaced for Enhanced LIGO. Only minor changes were made to the MC and MMT. A de￾tailed layout of the Enhanced LIGO IO is shown in Figure 3. A. Electro-optic modulator design We replaced the commercially made New Focus 4003 resonant phase modulator of Initial LIGO with an in-house EOM design and construction. Both a new crystal choice and architectural design change allow for superior performance. The Enhanced LIGO EOM design uses a crystal of ru￾bidium titanyl phosphate (RTP), which has at most 1/10 the absorption coefficient at 1064 nm of the lithium nio￾bate (LiNbO3) crystal from Initial LIGO. At 200 W the RTP should produce a thermal lens of 200 m and higher order mode content of less than 1%, compared to the 3.3 m lens the LiNbO3 produces at 10 W. The RTP has a minimal risk of damage, because it has both twice the damage threshold of LiNbO3 and is subjected to a beam twice the size of that in Ini￾tial LIGO. RTP and LiNbO3 have similar electro-optic coeffi- cients. Also, RTP’s dn/dT anisotropy is 50% smaller. Table I compares the properties of most interest of the two crystals. We procured the RTP crystals from Raicol and packaged them into specially designed, custom-built modulators. The crystal dimensions are 4 × 4 × 40 mm and their faces are wedged by 2.85◦ and anti-reflection (AR) coated. The wedge serves to separate the polarizations and prevents an etalon ef￾fect, resulting in a suppression of amplitude modulation. Only one crystal is used in the EOM in order to reduce the number of surface reflections. Three separate pairs of electrodes, each with its own resonant LC circuit, are placed across the crystal in series, producing the three required sets of RF sidebands: 24.5 MHz, 33.3 MHz, and 61.2 MHz. A diagram is shown in Fig. 4. Reference 23 contains further details about the modu￾lator architecture. B. Mode cleaner design The MC is a suspended 12.2 m long triangular ring cavity with finesse F = 1280 and free spectral range of 12.243 MHz. The three mirror architecture was selected over the standard two mirror linear filter cavity because it acts as a polarization TABLE I. Comparison of selected properties of the Initial and Enhanced LIGO EOM crystals, LiNbO3, and RTP, respectively. RTP was preferred for Enhanced LIGO because of its lower absorption, superior thermal properties, and similar electro-optic properties.19 Units LiNbO3 RTP Damage threshold MW/cm2 280 >600 Absorption coeff. at 1064 nm ppm/cm <5000 <500 Electro-optic coeff. (n3 zr33) pm/V 306 239 dny/dT 10−6/K 5.4 2.79 dnz/dT 10−6/K 37.9 9.24 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