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Overview of Negative dn

Problem. Thermal Lensing in AdLIGOSurface Thermal LensingSubstrate Thermal LensingEffect on AdLIGO Operation. Thermal Lensing in Arm Cavity. Reflected . z. . . wo. . . . . . RITM=2076 m. . . . Incident. . . Test Mass. Sur. Rth. Reflected (Red) beam sees a defocusing lens upon reflectionFor a 6

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Overview of Negative dn

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    2. Problem Thermal Lensing in AdLIGO Surface Thermal Lensing Substrate Thermal Lensing Effect on AdLIGO Operation

    3. Thermal Lensing in Arm Cavity Reflected (Red) beam sees a defocusing lens upon reflection For a 6.0 cm beam size, Sur. Rth = 106 km (sagitta ˜ 17 nm) Reduces the beam size to 5.45 mm Changes the Eigen mode of the cavity No appreciable higher order modes Either requires ROC compensation on ITMs or Adaptive Mode Matching Telescope

    4. Surface Deformation Thermal Deformation of Cavity mirror surface Model (Hello-Vinet Theory):

    5. Thermal Lensing in Recycling Cavity Transmitted (Black ) beam sees a focusing lens upon transmission For a 6 cm beam size, Sub. Rth = 6.8 km (Sagitta ˜ 270 nm) If ITM ROC is kept constant, substrate thermal lensing introduces 20% mode mismatch At least 15% higher order losses in the recycling cavity

    6. Substrate Deformation Thermal Deformation of Cavity mirror substrate Model (Hello-Vinet Theory):

    7. Effect on LIGO Operation

    8. Solution Substrate Thermal Compensation Philosophy Design Options

    10. Ideal Compensation Example

    11. Annular & Ring Heater Compensation

    12. Negative dn/dT Compensation

    13. One Particular Solution Negative dn/dT Compensation Negative dn/dT Approach Materials Issues and concerns

    14. Active Compensation Requirements: CO2 laser system for surface heating Availability in large size Purity/homogeneity Advantages: Works without any doubt, trick is to use non-spherical lens to combat the higher order modes Requires very low power (< 2W) Highly efficient/adaptive Ideal for spot correction Disadvantages: New material A lot of Data and tests needed Requires coating on both surfaces Active / Passive Compensation Passive Compensation Requirements: Availability in large size Purity/homogeneity Exact thickness, absorption values Advantages: No heating laser needed Highly efficient Analogous to compensation in Faraday Rotator Disadvantages: New material A lot of data and tests needed Less Adaptive Requires coating on both surfaces

    15. Negative Lens Formation in CP What it takes to Create a Negative Lens: A Figure of merit:

    16. Notes on Various Materials for Thermal Compensation CaF2 Normally used in IR Spectrometers Available in large sizes Not unknown to LIGO community Good Q value Thermal depolarization May be difficult to handle due to crystalline nature Appropriate coatings have to be developed Requires relatively large power

    17. Material Properties

    18. A Typical Example 6.8 km Thermal Lens in Substrate

    19. Experimental Demonstration (Positive Lens)

    20. Summary Current Status One sample of CaF2 has been purchased from Crystran Ltd., UK. Three samples of N-FK 51 has been purchased from Schott Glass Models show excellent performance in terms of compensation (less than 0.01% Higher Order losses) A table Top experiment is underway

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