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Cutting-Edge Autoalignment Techniques in Optical Systems

Explore autoalignment methods for precise control and improved performance in optical systems, maximizing light power, reducing noise coupling, and achieving stable operation. Learn about differential wavefront sensing and spot position feedback for effective beam management.

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Cutting-Edge Autoalignment Techniques in Optical Systems

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  1. Alignment Control of GEO 600 Hartmut Grote for the GEO600 team Institut für Atom- und Molekülphysik University of Hannover Max-Planck-Institut für Gravitationsphysik 9. July 2003

  2. Autoalignment: Why ? • Superimpose beam axes • Maximize light power • Stabilze optical gain • Center beam spots on mirrors • Minimize angular to longitudinal noise coupling

  3. Autoalignment: How ? • Differrential wavefront sensing (analog feedback for 14 DOF in GEO) • Spot position sensing (digital feedback for 20 DOF in GEO)

  4. Modecleaner Autoalignment Bandwidth: 1kHz

  5. Modecleaner Reflected Power Pitch DWS feedback ... and yaw DWS feedback Reflected power [Arb.] Time [s]

  6. PR und MI Autoalignment

  7. Double – Triple Pendulum Blades Stack Magnet-Coil Actuator Range: 100 µm, Typ. DC-10Hz Intermediate mass Mirror Elektrostatic Actuator Range: 1 µm (DC), Typ. 10-100Hz 3mm

  8. Electrostatic Drive Processing square root with analog electronics Peak force: 30 µN Needed for acquisition Maximal mirror speed with PR gain 300 is 100 nm/s

  9. Intermediate Mass Actuator Alignment transfer function Amplitude [Abs.] Frequency [Hz]

  10. Michelson Power Levels Dark port Light power [Arb.] Time [s] Light power [Arb.] PR cavity Time [s]

  11. Spectral Density of MI-DWS Feedback Differential MCe/MCn alignmrnt [rad/sqrt(Hz)] Error signal Frequency [Hz]

  12. DWS Feedback Noise • Noise coupling by spot position deviations Induced length noise RMS deviation from center of pitch / yaw RMS angular noise Angular noise in GW band (e.g. caused by DWS feedback)

  13. Michelson Strain Sensitivity S1 Sqrt-noise HV-amplifier noise Strain sensitivity [1/sqrt(Hz)] Noise limit Michelson DWS feedback noise (1mm) Frequency [Hz]

  14. PR and MI Autoalignment Matrix diagonalization for 4 dimensions

  15. Computer Control

  16. Operation during S1 • 98% duty cycle, almost no human interaction required BS pitch FB [µrad] Time [days] Temp. [Deg. C] Time [days] Time

  17. Summary and Outlook • Autoalignment for modecleaners and power recycled Michelson complete (14 DOF DWS, 20 DOF spot pos.) • Long term stable operation achieved (~98% duty cycle during S1, longest lock >121 hours) • Michelson DWS feedback needs 10-20dB more gain at 0.5-2 Hz (reach 10nradRMS for MI arms) • Signal recycling DWS feedback in near future

  18. Differential Wavefront Sensing Detect angle between wavefronts in “near-“ and “far“ field • Sensitivity typically 10 p rad / sqrt(Hz)

  19. Modelliertes Alignment am PR MI

  20. A larger mismatch in the radii of curvature makes the system more sensitive to misalignment. Michelson Error Signal

  21. Autoalignment Experimental setup for 1 cavity

  22. Autoalignment Experimental setup for 1 cavity Differential wavefront sensing Orthogonalization < 1/10 Bandwith 0.2 or 6 Hz

  23. Autoalignment Experimental setup for 1 cavity Local beam centering servos Bandwidth ~ 1kHz

  24. Autoalignment Experimental setup for 1 cavity Spot position control

  25. Autoalignment setup PR + MI cavity

  26. Misalignment eines Resonators Achse der Eigenmode Achse des eintretenden Strahls Messe Winkel zwischen Wellenfronten an zwei verschiedenen Positionen

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