Ultrastable highpower laser systems Patrick Kwee on behalf

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

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

Current Laser Systems

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

Laser Power & Quantum Noise rad iat ion pre ss ure no ise shot noise higher laser power • First generation used ~10 W • Second generation ~100 W • Third generation asks for ~1 k. W Patrick Kwee Ultra-stable, high-power laser systems 5

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

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

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 Kwee Ultra-stable, high-power laser systems 8

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

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

Third generation λ=? 1 k. W

Many Options… More power, k. W class • Interesting for high-frequency sensitivity • All reflective interferometers could handle higher powers Different wavelength • Longer wavelength (1550 nm) for silicon optics, cryogenic interferometers Spatial beam profile • Lag 33 mode • Flat-top profiles [LIGO DCC T 1000416] Patrick Kwee Ultra-stable, high-power laser systems 12

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

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 Kwee Ultra-stable, high-power laser systems 14

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

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

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 Kwee Ultra-stable, high-power laser systems 17