Thermal Compensation Review

1. Thermal Compensation Review David Ottaway LIGO Laboratory MIT

2. Overview • Overview of Problem • Road map for design choices (Set by other systems) • Summary of current results from subscale tests and modeling • Current known unresolved issues • Conclusions LIGO Laboratory

3. Thermal Distortion • Absorption in coatings and substrates => Temperature Gradients • Temperature Gradients => Optical path distortions • 3 Types of distortions, relative strengths of which are shown below: LIGO Laboratory

4. Thermal Comparison of Advanced LIGO to LIGO 1 LIGO Laboratory

5. Effect on Advanced LIGO Interferometers LIGO Laboratory

6. Adaptive Thermal Compensation • Compensate for distortions in the substrates • Essential for Advanced LIGO sensitivity to be realized • Two parts to thermal compensation: 1. Coarse compensation of thermal lensing using heating ring and shielding 2. Small scale compensation using scanning CO2 laser • Accurate measurement of sapphire and fused silica thermal mechanical properties enable accurate models • Good propagation models to set design requirements (Melody and FFT Code) LIGO Laboratory

7. Requirements that flow from other systems • Core Optics (Down select) Sapphire -Significant possible inhomogeneous absorption -> Small spatial scale correction (scanning laser) -Large thermal conductivity -> Small amount of coarse compensation (ring heater) on compensation plates Fused Silica -Poor thermal conductivity and homogenous absorption (ring heater) • DC or RF read out scheme (Down select) -Reduces dependence on sidebands, might affect design requirements • Wavefront Sensing (LIGO 1 experience, not fully understood) -High spatial quality sidebands are probably necessary for accurate alignment control, may negate the effect of read out scheme LIGO Laboratory

8. Summary of Subscale Experiments and Modeling • Accurate measurements of fused silica and sapphire material properties • Experimental demonstration of shielded heater ring coarse spatial correction • Experimental demonstration of scanning CO2 laser fine spatial scale correction • Accurate models of Advanced LIGO Interferometers style interferometer using Melody and finite element analysis (Femlab), (Thermal modeling without SRM) • Scaling from subscale to full scale understood • Work done by Ryan Lawrence LIGO Laboratory

9. Thermophysical Parameters Measurement (295-320 K) LIGO Laboratory

10. Heater Ring Thermal Compensation LIGO Laboratory

11. Sub Scale Scanning Laser Test LIGO Laboratory

12. Scanning Laser Test Result Uncorrected Optic (6712 ppm scatter from TEM00) Corrected Optic (789 ppm scattered from TEM00) LIGO Laboratory

13. Predicted Effected of Thermal Compensation on Advanced LIGO LIGO Laboratory

14. Current Unresolved Issues • Gravitational wave sideband distortion and its effect on sensitivity. Generated within the cavity no distortion nulling due to prompt reflection. Greater understanding through incorporation in Melody (Ray Beausoleil) • Fabry-Perot mode size change due to input test mass surface deformation => Spot size change (actuate on arm cavity faces) • Accurate 2D absorption maps of Sapphire to aid in actuator selection (negative or positive dN/dT actuator plates) LIGO Laboratory

15. Conclusions • Subscale proof of principal experiment have been completed • Two different methods of compensation explored • Good agreement between theory and experiment • Reliable cavity modelling code (Melody) will allow accurate setting of final design requirements • Final design requirements require decisions from other sub-systems but should be achievable, regardless of these decisions LIGO Laboratory