033109-8 Dooley et al. Rev.Sci.Instrum.83,033109(2012) 0.6 0.4 0 f=28164Hz(MC1) -0.4 f-28209Hz(MC2) 0.6 f-28237Hz(MC3) 0 20 25 Time(h) 0.5 0.5 5 10 15 20 25 5 10 15 20 25 Time (h) FIG.6.Data from the MC absorption measurement post drag-wiping.Power into the MC was cycled between0.9 W and 5.1 Wat 3-h intervals (bottom frame) and the change in frequency of the drumhead mode of each mirror was recorded (top frame).The ambient temperature(middle frame)was also recorded in order to correct for its effects. in Young's modulus with temperature,dy/dT.A finite ele- 2.Faraday isolator losses ment model(COMSOL Ref.33)was used to compute the ex- pected frequency shift from a temperature change of the sub- The FI was the second greatest source of power loss with strate resulting from the mirror coating absorption.The mea- its transmission of 93%.This was an improvement over the sured eigenfrequencies for each mirror at room temperature 86%transmission of the Initial LIGO FI.The most lossy ele- ment in the FI is the thin film polarizer,accounting for 4%of are 28164 Hz,28209 Hz,and 28237 Hz,respectively. We cycled the power into the MC between 0.9 W and total losses.The integrated losses from AR coatings and ab- 5.1 W at 3-h intervals,allowing enough time for a thermal sorption in the TGGs,CWPs,HWP,and DKDP account for characteristic time constant to be reached.At the same time, the remaining 3%of missing power. we recorded the frequencies of the high Q drumhead mode peaks as found in the mode cleaner frequency error signal, heterodyned down by 28 kHz(see Figure 6).Correcting for B.Faraday isolation ratio ambient temperature fluctuations,we find a frequency shift The isolation ratio is defined as the ratio of power in- of 0.043,0.043,and 0.072 Hz/W.As a result of drag-wiping cident on the FI in the reverse direction (the light reflected the mirrors,the absorption decreased for all but one mirror,as from the interferometer)to the power transmitted in the re- shown for both Hanford and Livingston in Table III. verse direction and is often quoted in decibels:isolation ratio =10log10(Pin-reverse/Pout-reverse).We measured the isolation ra- tio of the FI as a function of input power both in air prior to TABLE III.Absorption values for the Livingston and Hanford mode installation and in situ during Enhanced LIGO operation. cleaner mirrors before (in parentheses)and after drag wiping.The precision To measure the in-vacuum isolation ratio,we misaligned is土10%. the interferometer arms so that the input beam would be promptly reflected off of the 97%reflective recycling mirror. Mirror Livingston Hanford This also has the consequence that the FI is subjected to twice MCI 2.1ppm(18.7Ppm) 5.8(6.1Ppm) the input power.Our isolation monitor was a pick-off of the MC2 2.0 ppm (5.5 ppm) 7.6(23.9Ppm) backwards transmitted beam taken immediately after trans- MC3 3.4 ppm (12.8 ppm) 15.6(12.5ppm) mission through the FI that we sent out of a vacuum chamber viewport.Refer to the "isolation check beam"in Fig.3.The Reuse of AlP 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 00:54:10033109-8 Dooley et al. Rev. Sci. Instrum. 83, 033109 (2012) 0 5 10 15 20 25 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 Mode Freq Shift (Hz) Time (h) 0 5 10 15 20 25 0.5 0 0.5 BSC1&3 dT (F) 0 5 10 15 20 25 0 2 4 6 PMC ( W) Time (h) f=28164 Hz (MC1) f=28209 Hz (MC2) f=28237 Hz (MC3) FIG. 6. Data from the MC absorption measurement post drag-wiping. Power into the MC was cycled between 0.9 W and 5.1 W at 3-h intervals (bottom frame) and the change in frequency of the drumhead mode of each mirror was recorded (top frame). The ambient temperature (middle frame) was also recorded in order to correct for its effects. in Young’s modulus with temperature, dY/dT. A finite ele￾ment model (COMSOL Ref. 33) was used to compute the ex￾pected frequency shift from a temperature change of the sub￾strate resulting from the mirror coating absorption. The mea￾sured eigenfrequencies for each mirror at room temperature are 28164 Hz, 28209 Hz, and 28237 Hz, respectively. We cycled the power into the MC between 0.9 W and 5.1 W at 3-h intervals, allowing enough time for a thermal characteristic time constant to be reached. At the same time, we recorded the frequencies of the high Q drumhead mode peaks as found in the mode cleaner frequency error signal, heterodyned down by 28 kHz (see Figure 6). Correcting for ambient temperature fluctuations, we find a frequency shift of 0.043, 0.043, and 0.072 Hz/W. As a result of drag-wiping the mirrors, the absorption decreased for all but one mirror, as shown for both Hanford and Livingston in Table III. TABLE III. Absorption values for the Livingston and Hanford mode cleaner mirrors before (in parentheses) and after drag wiping. The precision is ±10%. Mirror Livingston Hanford MC1 2.1 ppm (18.7 ppm) 5.8 (6.1 ppm) MC2 2.0 ppm (5.5 ppm) 7.6 (23.9 ppm) MC3 3.4 ppm (12.8 ppm) 15.6 (12.5 ppm) 2. Faraday isolator losses The FI was the second greatest source of power loss with its transmission of 93%. This was an improvement over the 86% transmission of the Initial LIGO FI. The most lossy ele￾ment in the FI is the thin film polarizer, accounting for 4% of total losses. The integrated losses from AR coatings and ab￾sorption in the TGGs, CWPs, HWP, and DKDP account for the remaining 3% of missing power. B. Faraday isolation ratio The isolation ratio is defined as the ratio of power in￾cident on the FI in the reverse direction (the light reflected from the interferometer) to the power transmitted in the re￾verse direction and is often quoted in decibels: isolation ratio = 10log10(Pin-reverse/Pout-reverse). We measured the isolation ra￾tio of the FI as a function of input power both in air prior to installation and in situ during Enhanced LIGO operation. To measure the in-vacuum isolation ratio, we misaligned the interferometer arms so that the input beam would be promptly reflected off of the 97% reflective recycling mirror. This also has the consequence that the FI is subjected to twice the input power. Our isolation monitor was a pick-off of the backwards transmitted beam taken immediately after trans￾mission through the FI that we sent out of a vacuum chamber viewport. Refer to the “isolation check beam” in Fig. 3. The 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