1. Ultra-stable, high-power laser systems Patrick Kwee on behalf of AEI Hannover and LZH Advanced detectors session, 26. March 2011 Albert-Einstein-Institut Hannover

2. 22 Motivation Laser noise couples to GW read out channel Ultra-stable, high-power laser systemsPatrick Kwee Reduce coupling Null instrument,... Exploiting symmetries,... Reduce noise High laser power → reduces quantum noise Ultra stable laser → reduces technical noise

3. Current Laser Systems

4. 44 NPRO Patrick Kwee Non-planar ring oscillator (NPRO) Solid state Nd:YAG laser, 1064nm Diode pumped at 808nm Perfect laser for GWD... Exceptional frequency stability Low power noise, low pointing noise Good beam quality... but too low output power (max. ~2W) [Kane, Byer, "Monolithic, unidirectional…" Opt. Lett. 10, 65-67 (1985)] Ultra-stable, high-power laser systems

5. 55 Laser Power & Quantum Noise Patrick Kwee First generation used ~10W Second generation ~100W Third generation asks for ~1kW shot noise radiation pressure noise higher laser power Ultra-stable, high-power laser systems

6. 66 Power Scaling to 200W Patrick Kwee Single pass Nd:YVO amplifier, 35W, used at eLIGO, GEO HF Nd:YAG ring oscillator, 200W, injection locked, depolarization compensated, used at aLIGO [Winkelmann et al., App. Phys. B, 102:529-538, 2011] Ultra-stable, high-power laser systems

7. 77 Laser Stabilization Patrick Kwee [Kwee, ”Laser characterization and stabilization for…”, Ph.D. thesis (2010)] Active stabilization Sensors Actuators Feedback control loops Passive filtering Optical resonators Birefringent crystals... Ultra-stable, high-power laser systems

8. 88 Beam Quality Laser power required in fundamental mode of IF, TEM 00 Higher order modes cause additional shot noise and noise couplings Rigid spacer ring cavity (called PMC) Transmits only TEM 00 mode TEM 00 fraction increased from ~95% to >99% Patrick KweeUltra-stable, high-power laser systems

9. 99 Power Noise Patrick Kwee Coupling via radiation pressure and dark fringe offset between ~10Hz... ~10kHz Active pre-stabilization, photodetector as sensor, AOM as actuator Ultra-stable, high-power laser systems

10. 10 Frequency Noise Patrick Kwee High frequency stability required for interferometer lock acquisition Active pre-stabilization, rigid reference resonator as sensor, several high-bandwidth actuators Ultra-stable, high-power laser systems

11. Third generation λ=? 1kW

12. 12 Many Options… More power, kW class Interesting for high-frequency sensitivity All reflective interferometers could handle higher powers Different wavelength Longer wavelength (1550nm) for silicon optics, cryogenic interferometers Spatial beam profile Lag 33 mode Flat-top profiles Patrick KweeUltra-stable, high-power laser systems [LIGO DCC T1000416]

13. 13 Current Developments 1kW @ 1064nm –2W NPRO as seed –photonic crystal fiber amplifier for ~500W regime (current record is ~500W, but military research) –Subsequent single-pass Nd:YAG amplifier for ~1kW –Coherent beam combining for ~1kW 100W @ 1550nm –1-2W DFB fiber laser as seed –Erbium fiber amplifier for ~100W (currently ~50-60W) 100W @ 532nm –200W @ 1064nm solid-state laser system –High-power frequency doubling with LBO (currently ~130W) Patrick KweeUltra-stable, high-power laser systems

14. 14 Stabilization Frequency pre-stabilization ✓ (only necessary for lock acquisition) Beam quality ✓ (rigid filter resonators work fine) Power noise at radio frequencies ✓ (filter resonators could be cascaded) Beam pointing ( ✓ ) (only passive until now, active stabilization possible) Power stabilization ( ✓ ) (already limited, but new concepts on hand) Patrick KweeUltra-stable, high-power laser systems

15. 15 Summary Patrick Kwee Laser system of Advanced LIGO Power scaling Stabilization methods Developments for third generation Ultra-stable, high-power laser systems

16. 16 Power Noise at RF Coupling via interferometer heterodyne readout, alignment readout, … Modulation frequencies between 10MHz...100MHz Rigid spacer ring cavity acts as low-pass filter Patrick KweeUltra-stable, high-power laser systems

17. 17 Fiber Amplifers Substantial progress in recent years Very good intrinsic beam quality Very compact laser systems High effiency All-fiber systems possible But... Same reliability as solid-state systems? Non-linear effects due to high intensities Sensitive to acoustics Patrick KweeUltra-stable, high-power laser systems