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Lasers for Advanced Interferometers

This study outlines the design requirements and challenges for high-power laser interferometers, focusing on stability, noise reduction, and efficient power control. Various laser designs and topologies are discussed, including Prestabilized Laser Systems and Master-Laser NPRO concepts. The text delves into the spatial and frequency noise aspects, birefringence compensation, and the use of different laser stages like rods, slabs, and fibers. Furthermore, it addresses cooling solutions, beam propagation techniques, and advanced laser concepts applied in institutes like the Stanford High Power Laser Lab and LIGO.

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Lasers for Advanced Interferometers

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  1. Lasers for Advanced Interferometers Benno Willke ILIAS WG3 Hannover, October 2004

  2. Requirements - Topology • Sagnac: • broadband source to reduce scattered light noise • power control • recycled Michelson: • coherence control • power control • spatial control • squeezed light IFOs • different wavelength Prestabilized Laser System (PSL)

  3. Requirement – High Power trade off:

  4. Requirement – Noise Frequency Noise Intensity Noise

  5. Requirement – Noise / Design Spatial Fluctuations Design Requirements • stability / reliability • soft failure mode • easy to maintain / rare maintenance interval • good efficiency • good stationarity / low glitch rate • high bandwidth / large range actuators

  6. Laser Design • common concept: • laser diode pumped solid state lasers • transfer frequency stability of low power master laser to high power stage • Maser Laser Power Amplifier (MOPA) • injection locked oscillator • different power stage concepts: • rods • zig-zag slabs • fibers • thin disc lasers / active mirror laser

  7. Nd:YAG Master-Laser NPRO (non-planar ring oscillator) • output power: 800mW • frequency noise: [ 10kHz/f ] Hz/sqrt(Hz) • power noise: 10-6 /sqrt(Hz)

  8. High Power Stage • main problem: thermal design • stress fracture • thermal lensing – spatial profile • birefringence with tangential and radial principle axis • solutions • reduce deposited heat – Yb:YAG, high efficiency • propagate beam perpendicular to temperature gradient – zig-zag, thin disc lasers • increase interaction length – fiber lasers • compensate birefringence

  9. Face-pumping - Edge-pumping Pumping zig-zag plane Face- pumping zig-zag slab Cooling Edge- pumping zig-zag plane Pumping Cooling Stanford High Power Laser Lab Adelaide University

  10. End pumped slab geometry 808nm Pump undoped end signal OUT 3.33cm 1.51cm 1.51cm 0.6% Nd:YAG signal IN undoped end 808nm Pump 1.1mm X 0.9mm Stanford High Power Laser Lab

  11. Mode-matching optics 10W LIGO MOPA System 20 W Amplifier ISOLATOR Lightwave Electronics Mode-matching optics 2-pass End Pumped Slab #2 Pump Power = 430 W Expected TEM00 Output Power = 160W Stanford High Power Concept Pump Power = 130 Output TEM00Power = 50 W 2-pass End Pumped Slab #1 TO PRE MODE CLEANER

  12. End Pumped Rods Nd:YAG - GEO600 Laser (14W) Nd:YVO4 - Virgo Laser (20W) Laser Zentrum Hannover

  13. output QR f f BP from Master QR f f HR@1064 f 2f f HT@808 LZH High Power Concept

  14. Fiber Lasers courtesy H. Zellmer

  15. Fiber Laser Result of Jena Group Backscattered signal Dichr. mirror 9.4 m Yb-doped LMA-fiber NPRO Fiber coupled laser diode Isolator To experiment Input-output diagram Yb-doped LMA-Fiber Core:  = 28.5 µm, NA = 0.06 MFD 23 µm Doping. 700 ppm (mol) Yb2O3 Pumpc.:  = 400 µm, NA = 0.38, D-Form Seed: 800 mW Diffraction limited (M2 = 1.1) Polarization 82% (10:1)

  16. Power / Beamprofile: 165W in gausian TEM00 mode less than 5W in non- TEM00 modes Drift: 1% power drift over 24hr. 2% pointing drift Control: tidal frequency acuator +/- 50 MHz, time constant < 30min power actuator 10kHz BW, +/-1% range frequency actuatot BW:<20o lag at 100kHz, range: DC-1Hz: 1MHz, 1Hz-100kHz: 10kHz Advanced LIGO Laser – Requirements

  17. key elements: undoped bonded end-caps birefringence compensation pumplight homogenization NPRO output QR f f EOM FI BP FI modemaching YAG / Nd:YAG 3x2x6 optics QR f f BP YAG / Nd:YAG / YAG HR@1064 f 2f f 3x 7x40x7 HT@808 20 W Master High Power Slave Injection Locked Oscillators - Hannover

  18. Prestabilized Laser PSL • frequency stability: • stabilize master laser to rigid or suspended-mirror cavity • power stability: • feed-back to pump source of high power stage • passive filtering at rf • spatial profile • passiver modecleaning • active mode compesation

  19. frequency noise requirement

  20. intensity noise requirement

  21. high power ring laser 200W GEO typring laser15W spatial filterresonator (PMC) NPRO1W frequencyreferenceresonator AOM Adv LIGO - PSL optical layout

  22. PSL – stabilization scheme intensity stabilizationouter loop injection locking intensity stabilizationinner loop PMC loop frequency stabilizationinner loop frequency stabilizationouter loop

  23. Power Noise Reduction

  24. Relock Time relock time < 500 ms faster relock possible depending on piezo ramp

  25. Birefringence Compensation

  26. output QR f f BP from Master QR f f HR@1064 f 2f f HT@808 End-Pumped Rods

  27. high power stage status Feb 2004 linear polarized with birefringencecompensation

  28. Summary • different high power stages: • end-pumped slabs • end-pumped rods • fiber amplifier • different topologies: • MOPA • injection locking • Advanced LIGO pre-stabilized laser system • status of laser development • possible stabilization schemes

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