014502-8 Mueller et al. Rev.Sci.Instrum.87,014502(2016) commissioning experience indicates that the modulation The softer calcite polarizers and the deuterated potassium indices are sufficient for the aLIGO length and alignment dihydrogen phosphate(DKDP)crystal were procured from the sensing scheme and it was decided to use the EOM as is manufacturers with their standard polishings and coatings.The for now and potentially improve the resonant circuits later if two calcite polarizers each have a thickness of~5 mm and necessary. are wedged at 8.5 to allow the orthogonally polarized beams The residual amplitude modulation20(RFAM)produced to separate sufficiently.The calcite wedges have an extinction by the EOM was also characterized.The AM/PM ratio for ratio of at least 105 and more than 99%optical efficiency. each of the three sidebands was measured to be 1.0.10-4, The magnetic field is created by a stack of seven magne- 1.2.10-5.and4.1.10-5 for the9.1MHz.24.0MHz.and45.5 tized Fe-Nb magnetic disks25 each having a bore of 24 mm MHz sidebands.All three measurements come out to be at or and a thickness of 19.7 mm.This stack produces a maximum below the requirement of 10-derived by Kokeyama et al.12 axial field of 1.16 T (LLO)and 1.55 T(LHO)near its center Temporal variation of the RFAM generation was found to which falls off towards the end.The difference in the magnetic be due primarily to temperature dependence which is able to field is caused by the selection of the magnetic materials and push the AM/PM ratio at 9.1 MHz as high as 3.10-4.This the thermal treatment of the individual magnets.26 The TGG may need to be addressed with a temperature stabilization crystals and quartz rotator are installed about 3 cm apart from servo in the future if RFAM is found to be an issue during each other before being fine tuned to produce 22.5 of rotation detector commissioning,but the design of the modulator was by adjusting their depth in the magnet.The entire FI is mounted left unchanged until such an issue arises. on a 648 mm x 178 mm breadboard for convenient transfer Detailed design drawings,assembly instructions,and into the horizontal access module (HAM)chamber after out- test reports are available under LIGO document number of-vacuum optimization. T1300084.21 After undergoing a thorough cleaning procedure,the FI was assembled and aligned with the main PSL beam in the laser enclosure.The optical table in the enclosure is made B.Faraday isolator from stainless steel while the optical table in HAM2 is made The Faraday isolator is a much more complicated optical from aluminum.The differences in magnetic susceptibility are device compared to the EOM.It is more susceptible to thermal significant enough to require the FI to be raised with an~11 cm lensing and its location after the mode cleaner amplifies the thick granite block visible in the bottom picture in Figure 7. requirement to maintain a good spatial mode.The FI has to The bottom periscope mirror in Figure 3 was removed and the handle between 20 and 130 W of laser power without signifi- beam was sent via several mirrors through the Faraday isolator. cantly altering the beam profile or polarization of the beam. This setting ensured that the beam parameters,beam size, Like the EOM,the aLIGO FI is also very similar to the FI and divergence angle,are very similar to the ones expected used ineLIGO.18 Both were designed to minimize and mitigate in-vacuum. thermal lensing and thermal stress induced depolarization by The thermal lensing of the Faraday isolator was deter- compensating these effects in subsequent crystals.22.23 mined from beam-scan measurements of a sample of the beam The aLIGO FI design consists of a Faraday rotator,a pair after it was transmitted through the isolator for incident powers of calcite-wedge polarizers,an element with a negative dn/dT as high as 120 W at LLO and 140 W at LHO.At both sites. for thermal-lens compensation,and a picomotor-controlled the diagnostic beam was focused with a lens of 1 m focal half-wave plate for restoring the optical isolation in-situ.In length and the beam profile was recorded with CCD or rotating addition,a heat sink is connected to the holders of the magneto- slit beam scans as a function of power for different DKDP optical crystals to drain excess heat into the FI breadboard. crystals.The thermal lens at the location of the Faraday was The Faraday rotator is based on an arrangement developed by then computed using an ABCD matrix algorithm. Khazanov et al.,24 that uses a pair of~1 cm long Terbium Figure 8 shows the thermal lensing measurements for the Gallium Garnet(TGG)crystals as magneto-optical elements, TGG crystals and different DKDP crystals at LHO and LLO. each nominally producing a 22.5 rotation of the electric field The magnitude of the thermal lensing in the DKDP is a nearly when placed in a magnetic field of about IT.They are separated linear function of the incident power if all other parameters are by a ~1 cm long piece of quartz that rotates the polarization equal.In reality the absorption varies from sample to sample field reciprocally by67.5°±0.6°.This arrangement(shown and causes the selection of the DKDP to be somewhat stochas- schematically at the top of Figure 7)allows thermally induced tic,a fact which is evident in the small difference between the birefringence produced in the first magneto-optical element to 3.0 mm and 3.5 mm measurements at LLO. be mostly compensated in the second one.The HWP is a zero- The length of the DKDP crystal was chosen to compen- order epoxy-free quartz half-wave plate.It is set to rotate the sate the a priori unknown thermal lensing in the TGG crys- polarization by an additional 22.5 to have 0 net rotation in tals.Based on experience from Initial and Enhanced LIGO. the forward going and 90 in the backward going direction. the expectation was that DKDP crystals between 3.5 mm and All crystals were selected to minimize absorption,ther- 5.5 mm would be needed to compensate the thermal lensing mal beam distortion and surface roughness.Those made of in the TGG crystals.However,the absorption in the newly harder and non-hygroscopic materials,the half-wave plate, purchased TGG crystals was lower than expected and even our quartz rotator,and TGG crystals,are all super-polished (sur- shortest crystals overcompensated.While the low absorption face roughness below 0.5 nm)and received a custom low loss in TGG is obviously good,it required to shorten the originally IBS AR coating with a rest reflectivity of less than 300 ppm. ordered DKDP.We choose 3.5 mm for both isolators instead Reuse of AlP Publishing c tent is subject to the te at:https//publis .aip.org/authors/rights wnload to IP. 183.195.251.60nFi22A 2016005135014502-8 Mueller et al. Rev. Sci. Instrum. 87, 014502 (2016) commissioning experience indicates that the modulation indices are sufficient for the aLIGO length and alignment sensing scheme and it was decided to use the EOM as is for now and potentially improve the resonant circuits later if necessary. The residual amplitude modulation20 (RFAM) produced by the EOM was also characterized. The AM/PM ratio for each of the three sidebands was measured to be 1.0 · 10−4 , 1.2 · 10−5 , and 4.1 · 10−5 for the 9.1 MHz, 24.0 MHz, and 45.5 MHz sidebands. All three measurements come out to be at or below the requirement of 10−4 derived by Kokeyama et al.12 Temporal variation of the RFAM generation was found to be due primarily to temperature dependence which is able to push the AM/PM ratio at 9.1 MHz as high as 3 · 10−4 . This may need to be addressed with a temperature stabilization servo in the future if RFAM is found to be an issue during detector commissioning, but the design of the modulator was left unchanged until such an issue arises. Detailed design drawings, assembly instructions, and test reports are available under LIGO document number T1300084.21 B. Faraday isolator The Faraday isolator is a much more complicated optical device compared to the EOM. It is more susceptible to thermal lensing and its location after the mode cleaner amplifies the requirement to maintain a good spatial mode. The FI has to handle between 20 and 130 W of laser power without signifi￾cantly altering the beam profile or polarization of the beam. Like the EOM, the aLIGO FI is also very similar to the FI used in eLIGO.18Both were designed to minimize and mitigate thermal lensing and thermal stress induced depolarization by compensating these effects in subsequent crystals.22,23 The aLIGO FI design consists of a Faraday rotator, a pair of calcite-wedge polarizers, an element with a negative dn/dT for thermal-lens compensation, and a picomotor-controlled half-wave plate for restoring the optical isolation in-situ. In addition, a heat sink is connected to the holders of the magneto￾optical crystals to drain excess heat into the FI breadboard. The Faraday rotator is based on an arrangement developed by Khazanov et al., 24 that uses a pair of ∼1 cm long Terbium Gallium Garnet (TGG) crystals as magneto-optical elements, each nominally producing a 22.5 ◦ rotation of the electric field when placed in a magnetic field of about 1T. They are separated by a ∼1 cm long piece of quartz that rotates the polarization field reciprocally by 67.5 ◦ ± 0.6 ◦ . This arrangement (shown schematically at the top of Figure 7) allows thermally induced birefringence produced in the first magneto-optical element to be mostly compensated in the second one. The HWP is a zero￾order epoxy-free quartz half-wave plate. It is set to rotate the polarization by an additional 22.5 ◦ to have 0◦ net rotation in the forward going and 90◦ in the backward going direction. All crystals were selected to minimize absorption, ther￾mal beam distortion and surface roughness. Those made of harder and non-hygroscopic materials, the half-wave plate, quartz rotator, and TGG crystals, are all super-polished (sur￾face roughness below 0.5 nm) and received a custom low loss IBS AR coating with a rest reflectivity of less than 300 ppm. The softer calcite polarizers and the deuterated potassium dihydrogen phosphate (DKDP) crystal were procured from the manufacturers with their standard polishings and coatings. The two calcite polarizers each have a thickness of ∼5 mm and are wedged at 8.5 ◦ to allow the orthogonally polarized beams to separate sufficiently. The calcite wedges have an extinction ratio of at least 105 and more than 99% optical efficiency. The magnetic field is created by a stack of seven magne￾tized Fe-Nb magnetic disks25 each having a bore of 24 mm and a thickness of 19.7 mm. This stack produces a maximum axial field of 1.16 T (LLO) and 1.55 T (LHO) near its center which falls off towards the end. The difference in the magnetic field is caused by the selection of the magnetic materials and the thermal treatment of the individual magnets.26 The TGG crystals and quartz rotator are installed about 3 cm apart from each other before being fine tuned to produce 22.5 ◦ of rotation by adjusting their depth in the magnet. The entire FI is mounted on a 648 mm × 178 mm breadboard for convenient transfer into the horizontal access module (HAM) chamber after out￾of-vacuum optimization. After undergoing a thorough cleaning procedure, the FI was assembled and aligned with the main PSL beam in the laser enclosure. The optical table in the enclosure is made from stainless steel while the optical table in HAM2 is made from aluminum. The differences in magnetic susceptibility are significant enough to require the FI to be raised with an ∼11 cm thick granite block visible in the bottom picture in Figure 7. The bottom periscope mirror in Figure 3 was removed and the beam was sent via several mirrors through the Faraday isolator. This setting ensured that the beam parameters, beam size, and divergence angle, are very similar to the ones expected in-vacuum. The thermal lensing of the Faraday isolator was deter￾mined from beam-scan measurements of a sample of the beam after it was transmitted through the isolator for incident powers as high as 120 W at LLO and 140 W at LHO. At both sites, the diagnostic beam was focused with a lens of 1 m focal length and the beam profile was recorded with CCD or rotating slit beam scans as a function of power for different DKDP crystals. The thermal lens at the location of the Faraday was then computed using an ABCD matrix algorithm. Figure 8 shows the thermal lensing measurements for the TGG crystals and different DKDP crystals at LHO and LLO. The magnitude of the thermal lensing in the DKDP is a nearly linear function of the incident power if all other parameters are equal. In reality the absorption varies from sample to sample and causes the selection of the DKDP to be somewhat stochas￾tic, a fact which is evident in the small difference between the 3.0 mm and 3.5 mm measurements at LLO. The length of the DKDP crystal was chosen to compen￾sate the a priori unknown thermal lensing in the TGG crys￾tals. Based on experience from Initial and Enhanced LIGO, the expectation was that DKDP crystals between 3.5 mm and 5.5 mm would be needed to compensate the thermal lensing in the TGG crystals. However, the absorption in the newly purchased TGG crystals was lower than expected and even our shortest crystals overcompensated. While the low absorption in TGG is obviously good, it required to shorten the originally ordered DKDP. We choose 3.5 mm for both isolators instead 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