High power lasers and their stabilization Benno Willke ET Workshop, Cascina 2008

B. Willke / ET workshop 08 2 Lasers for future GWDs - Objectives fundamental noise sources related to laser shot noise in interferometer readout radiation pressure noise technical noise sources related to the laser arm in-balance couples power noise and frequency noise pointing drives MC requirements technical laser design requirements single frequency (1064nm, 1550nm) single spatial mode ( HG, LAG, …) linear polarized low noise / good actuators low free running noise actuators with large range and high bandwidth passive noise filtering AdvLIGO © David Shoemaker

B. Willke / ET workshop 08 3 Higher power levels higher power reduces sensing noise and increases radiation pressure noise current detectors do not see radiation pressure noise power increase helps for Advanced detectors for third generation useful power dependents on observation frequency further power increase helps if mirror mass is increased together with power – SQL lower sub-SQL readout techniques like variational readout are used how much power can IFO handle for technical reasons? © Yanbei Chen

B. Willke / ET workshop 08 4 State of the art lasers in first and second generation detectors slab or rod design 10W for initial detectors up to 60W in eLIGO and Virgo+ 200W in advanced detectors 10kW class lasers are available but they are not single-frequency polarized low noise single-mode fiber amplifier – several hundred Watts

B. Willke / ET workshop 08 5 Which path to higher power ? key task: manage thermo-optical effects reduce the deposited heat and the thermal gradient 885nm pumping Yb:YAG high brightness mode selective pumping undoped end-caps, segmented rods increase length average over thermal gradient – zig-zag slabs compensation of thermal aberrations depolarization compensation adaptive optics techniques mode cleaner Upconversion-process Relaxation Laser transition Relaxation Energy Excitation Non-radiating transition

B. Willke / ET workshop 08 6 Laser head design

B. Willke / ET workshop 08 7 Depolarization compensation

B. Willke / ET workshop W prototype - layout

B. Willke / ET workshop 08 9 Fiber amplifier Hildebrandt et al., Optics Express, 14 (2006) W, 98% 148W, 92.6%

B. Willke / ET workshop Low noise operation - stabilization design goals a)reduce free running noise b)low noise sensing c)high bandwidth actuators (coupling into RIN)

B. Willke / ET workshop Low noise operation – frequency noise Heurs et al., Opt. Lett, 29 (2004) 2148 design goals a)reduce free running noise b)low noise sensing c)high bandwidth actuators frequency noise a)driven by NPRO pump LD RIN b)simple due to steep discriminator slope c)600kHz loop UGF is state of the art a)passive filtering with ring resonators (coupling into RIN) (Rollins 2008)

B. Willke / ET workshop Low noise operation – power noise design goals a)reduce free running noise b)low noise sensing c)high bandwidth actuators power noise a)use low noise pump source, reduce coupling from vibration and pointing b)dynamic range limited (1/f), optical AC coupling c)500kHz loops state of the art, high power actuators Seifert et al., Opt. Lett. 31 (2006) 2000

B. Willke / ET workshop optical AC coupling – transfer function Transfer function of relative power fluctuations from A to B: G = B/A Kwee et al, Opt. Lett. 33 (2008) 1509f

B. Willke / ET workshop Power noise measurement Detected photo current: 3mA Equivalent photo current: 702mA Kwee et al, Opt. Lett. 33 (2008) 1509f

B. Willke / ET workshop Low noise operation - Stabilization design goals a)reduce free running noise b)low noise sensing c)high bandwidth actuators spatial noise a)rigid laser design, stationary thermal lenses, no turbulent water flow (conductive cooling) b)only for pointing available (TEM01,TEM10), Hartman sensors ? c)passive filtering (coupling into RIN), adaptive optics ?

B. Willke / ET workshop Low noise operation - Stabilization design goals a)reduce free running noise b)low noise sensing c)high bandwidth actuators frequency noise a)driven by NPRO pump RIN b)ok c)600kHz loops state of the art power noise a)low noise pump source, reduce coupling from vibration and pointing b)dynamic range limited (1/f), optical AC coupling c)500kHz loops state of the art, high power actuators spatial noise a)rigid laser design, stationary thermal lenses, no turbulent water flow (conductive cooling) b)only for pointing available (TEM01,TEM10), Hartman sensors ? c)passive filtering (coupling into RIN), adaptive optics ? polarization a)polarizer in laser design (rigid mounting, no or stationary depolarization b) c)passive filtering with ring resonators (coupling into RIN) Ф

B. Willke / ET workshop ET design study - lasers exploit what is possible in terms of power power levels of up to 1kW at 1064nm high power laser development at 1550nm started AEI / LZH goal for next 5 years 1kW at 1064nm 50W at 1550nm which stability levels seem possible RIN in /sqrt(Hz) range squeezed vacuum sources suggest iterative approach start with optimistic estimates of what is achievable exploit IFO layout, control and noise coupling in these limits trade off options based on costs (money and time) and associated risk define requirements