Laser Interferometer Gravitational Wave Observatory

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Laser Interferometer Gravitational Wave Observatory

LIGO Laboratory / LIGO Scientific Collaboration

LIGO-T060083-01-D ADVANCED LIGO 06/01/07

Auxiliary Optics Support System

Conceptual Design Document, Vol. 1
Thermal Compensation System

Michael Smith, Phil Willems

Distribution of this document:

LIGO Scientific Collaboration

This is an internal working note

of the LIGO Project.

California Institute of Technology

LIGO Project – MS 18-34

1200 E. California Blvd.

Pasadena, CA 91125

Phone (626) 395-2129

Fax (626) 304-9834


Massachusetts Institute of Technology

LIGO Project – NW17-161

175 Albany St

Cambridge, MA 02139

Phone (617) 253-4824

Fax (617) 253-7014


LIGO Hanford Observatory

P.O. Box 1970

Mail Stop S9-02

Richland, WA 99352

Phone 509-372-8106

Fax 509-372-8137

LIGO Livingston Observatory

P.O. Box 940

Livingston, LA 70754

Phone 225-686-3100

Fax 225-686-7189

Table of Contents

1 Version History for this Document 5

2 Past Experience with Thermal Compensation 6

3 Overall Design 7

3.1 Design Philosophy 7

3.2 Overall Layout of TCS 7

4 TCS Elements 11

4.1 Actuators 11

4.1.1 Test Mass Ring Heater 11

4.1.2 Compensation Plate 15

4.1.3 CO2 Laser Projector 18 Fixed-mask Staring Projector 18 Scanned Projector 19 In-vacuum Optics 20

4.1.4 Folded IFO TCS Actuator Design 21

4.1.5 Simultaneous Compensation on the ITM and CP 22

4.1.6 Beamsplitter Compensation 22

4.2 TCS Sensors 22

4.2.1 Dedicated Sensors 22 Probe Beam Layout 22 Dedicated Sensor Conceptual Design 27

4.2.2 Phase Camera Design 29

5 Notes on Sensor and Actuator Beam Distortion 30

6 Notes on Thermal Depolarization 31

7 Installation, Commissioning, and Control of TCS 32

7.1 Installation and Commissioning 32

7.2 Differential Control 34

8 Appendices 36

8.1 Coupling of Test Mass Flexure Noise to Displacement Noise 36

8.2 Scanned CO2 Laser Projector Noise 37

Table of Figures

Figure 1: layout of thermal compensators and thermal compensation sensors. Red dots: shielded ring heaters. Blue arrows: optical path sensors (Hartmann sensors). Green projections: carbon dioxide laser heaters. 9

Figure 2: Block diagram of the TCS system. 10

Figure 3: TM surface deformation due to interferometer self-heating, and resultant arm cavity mode intensity profile. In the arm cavity mode plot, the simulation is in green, and the best fit gaussian in red. 11

Figure 4: Arm cavity mode intensity profile resulting from the deformation in Figure 3. The simulation is in green, and the best fit gaussian in red. 12

Figure 5: suspended test mass with shielded ring heater in position. The ring heater is inside the parabolic shield and not visible- neither is the mount holding the ring and shield to the suspension cage. 12

Figure 6: ring compensated test mass thermoelastic HR surface deformation. 13

Figure 7: Arm cavity mode intensity profile resulting from surface deformation in Figure 6. As before, the FFT simulation is in green, and the best-fit Gaussian (with 6 cm spot size at ITM) in red. 14

Figure 8: Phase profiles through directly heated ITM, without (left) and with (right) test mass ring heater compensation. 15

Figure 9: Radial phase profile of an uncompensated ITM at full IFO power. 16

Figure 10: Compensation heating profile from optimized shielded ring heater.TCS must also minimize the stray light at the dark port, in concert with the output mode cleaner. 17

Figure 11: ITM phase profile after compensation by the heating pattern in Figure 11. 18

Figure 12: Fixed-mask staring CO2 projector 19

Figure 13: In-vacuum optics of the CO2 laser projectors. 21

Figure 14: Configuration of dedicated sensor probe insertion points at beamsplitter. The solid lines show the paths of the main IFO beam. The dotted lines show the paths of the injected probe beams (except where they overlap with the main IFO beam. 24

Figure 15: Steering mirror and telescope layout for the ITM CP sensors. The probe beam enters vacuum at the lower right, is raised by a periscope in the vacuum, is expanded by the telescope above the SRM as it travels to the left, and is steered down by a periscope to slightly below the optical table for injection to the BS AR face. 25

Figure 16: On-axis beamsplitter phase maps- the color scale indicates radians of phase. On the left, 25 mW bulk heating from homogeneous absorption. On the right, 25 mW uniform HR surface absorption. 26

Figure 17: Dedicated beamsplitter sensor probe beam optical path. The ghost beams from unwanted reflections are also shown. 26

Figure 18: Layout of ITM HR surface sensors. 27

Figure 19: IFO performance with thermal compensation and no SRM. From Lawrence's thesis. 35

Figure 20: Thermoelastic deformation from 1 second of 100 W of barrel heating. 36

Figure 21: Sample injected noise spectrum from scanned carbon dioxide laser projector. 37

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