033109-6 Dooley et al. Rev.Sci.Instrum.83,033109(2012) rejected reverse to ISC table (s-pol) B TFP DKDP QR magnet aperture in (p-pol) out 888 (p-pol) aperture CWP TGG HWP CWP 36in. rejected input (s-pol) beam dump FIG.5.Faraday isolator photograph and schematic.The FI preserves the polarization of the light in the forward-going direction and rotates it by 90 in the reverse direction.Light from the MC enters from the left and exits at the right towards the interferometer.It is ideally p-polarized,but any s-polarization contamination is promptly diverted ~10 mrad by the CWP and then reflected by the TFP and dumped.The p-polarized reflected beam from the interferometer enters from the right and is rotated to s-polarized light which is picked-off by the TFP and sent to the Interferometer Sensing and Control (ISC)table.Any imperfections in the Faraday rotation of the interferometer return beam results in p-polarized light traveling backwards along the original input path. long and placed in housing 155 mm in diameter and 161 mm reflected direction.and thermal beam drift.The CWPs have long.The TGG diameter is 20 mm. very high extinction ratios (>10)and high transmission (>99%)contributing to good optical efficiency and isola- 2.Thermal lensing tion performance.However,the angle separating the exiting Thermal lensing in the FI is addressed by including orthogonal polarizations of light is very small,on the order DKDP,a negative dn/dT material,in the beam path.Ab- of 10 mrad.This small angle requires the light to travel rela- sorption of light in the DKDP results in a de-focusing of tively large distances before we can pick off the beams needed the beam,which partially compensates for the thermal fo- for interferometer sensing and control.In addition,thermally cusing induced by absorption in the TGGs.26.27 The optical induced index of refraction gradients due to the 4.95 wedge path length (thickness)of the DKDP is chosen to slightly angle of the CWPs result in thermal drift.However,the CWPs over-compensate the positive thermal lens induced in the for the Enhanced LIGO FI have a measured low absorption of TGG crystals,anticipating other positive thermal lenses in the 0.0013 cm-1 with an expected thermal lens of 60 m at 30 W system. and drift of less than 1.3 urad/W.19 The advantages of the thin film polarizer over the calcite 3.Polarizers wedge polarizer are that it exhibits negligible thermal drift when compared with CWPs and it operates at the Brewster The polarizers used (two CWPs and one TFP)each of- angle of 55,thus diverting the return beam in an easily ac- fer advantages and disadvantages related to optical efficiency cessible way.However,the TFP has a lower transmission than in the forward-propagating direction,optical isolation in the the CWP.about 96%,and an extinction ratio of only 103. 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 00:54:10033109-6 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) FIG. 5. Faraday isolator photograph and schematic. The FI preserves the polarization of the light in the forward-going direction and rotates it by 90◦ in the reverse direction. Light from the MC enters from the left and exits at the right towards the interferometer. It is ideally p-polarized, but any s-polarization contamination is promptly diverted ∼10 mrad by the CWP and then reflected by the TFP and dumped. The p-polarized reflected beam from the interferometer enters from the right and is rotated to s-polarized light which is picked-off by the TFP and sent to the Interferometer Sensing and Control (ISC) table. Any imperfections in the Faraday rotation of the interferometer return beam results in p-polarized light traveling backwards along the original input path. long and placed in housing 155 mm in diameter and 161 mm long. The TGG diameter is 20 mm. 2. Thermal lensing Thermal lensing in the FI is addressed by including DKDP, a negative dn/dT material, in the beam path. Ab￾sorption of light in the DKDP results in a de-focusing of the beam, which partially compensates for the thermal fo￾cusing induced by absorption in the TGGs.26, 27 The optical path length (thickness) of the DKDP is chosen to slightly over-compensate the positive thermal lens induced in the TGG crystals, anticipating other positive thermal lenses in the system. 3. Polarizers The polarizers used (two CWPs and one TFP) each of￾fer advantages and disadvantages related to optical efficiency in the forward-propagating direction, optical isolation in the reflected direction, and thermal beam drift. The CWPs have very high extinction ratios (>105) and high transmission (> 99%) contributing to good optical efficiency and isola￾tion performance. However, the angle separating the exiting orthogonal polarizations of light is very small, on the order of 10 mrad. This small angle requires the light to travel rela￾tively large distances before we can pick off the beams needed for interferometer sensing and control. In addition, thermally induced index of refraction gradients due to the 4.95◦ wedge angle of the CWPs result in thermal drift. However, the CWPs for the Enhanced LIGO FI have a measured low absorption of 0.0013 cm−1 with an expected thermal lens of 60 m at 30 W and drift of less than 1.3 μrad/W.19 The advantages of the thin film polarizer over the calcite wedge polarizer are that it exhibits negligible thermal drift when compared with CWPs and it operates at the Brewster angle of 55◦, thus diverting the return beam in an easily ac￾cessible way. However, the TFP has a lower transmission than the CWP, about 96%, and an extinction ratio of only 103. 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