Lasers for Advanced Interferometers Benno Willke ILIAS WG3 Hannover, October 2004 opening remarks: Laser not limiting noise source beside quantum noise, laser not limited by technology or fundamental problems, however R&D and developement is required to meet demanding stability and reliability requirements

Requirements - Topology Sagnac: broadband source to reduce scattered light noise power control recycled Michelson: coherence control spatial control squeezed light IFOs different wavelength Prestabilized Laser System (PSL) Laser design depends strongly on topology which is not driven by available lasers as laser is not one of the limiting noise sources.

Requirement – High Power Laser Limits IFO Limits stress fracture birefringence / depolarization spatial distortions cavity stability / thermal lens spurious oscillations in high gain material thermal problems due to heating of surface/bulk (cavity stability, limit in cooling for cryogenic detectors) scattered light noise radiation pressure fluctuations in IFO and in frequency reference (low f performance) trade off: high input power high power recycling gain easier control problem easier lock acquisition (radiation pressure dynamics) spatial and temporal filtering good frequency reference easier lock acquisition: smaller dynamic effects due to rad pressure of build up light

Requirement – Noise Frequency Noise Coupling Limits via arm asymmetry options: change IFO readout (DC readout / external modulation) sensing noise shot noise in PDH signal discriminator slope Intensity Noise Coupling Limits via radiation pressure noise technical RIN sees common mode rejection (same pendulum TFs) in IFO quantum noise can be reduced by squeezing techniques and QND via RIN on photo detectors non perfect dark fringe contrast requires passive filtering for rf readout via rad. press. in frequency reference sensing noise shot noise pointing combined with detector inhomogeneity

Requirement – Noise / Design Spatial Fluctuations Coupling Limits via RIN caused by cavities via higher order mode “waste light” on IFO photodiodes additional shot noise on PDH detector fluctuations in spurious interferometers finesse of modecleaners thermal problems in modecleaner noise introduced by modecleaner 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 Laser design depends strongly on topology which is not driven by available lasers as laser is not one of the limiting noise sources.

Laser Design common concept: different power stage concepts: 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

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)

High Power Stage main problem: thermal design solutions 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

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

End pumped slab geometry 808nm Pump undoped end signal OUT 3.33cm 1.51cm 1.51cm 0.6% Nd:YAG signal IN undoped end Motivation -> Higher efficiency Near total absorption of pump light. Confinement of pump radiation leads to better mode overlap 808nm Pump 1.1mm X 0.9mm Stanford High Power Laser Lab

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

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

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

Fiber Lasers courtesy H. Zellmer

Fiber Laser Result of Jena Group Backscattered signal NPRO 9.4 m Yb-doped LMA-fiber Dichr. mirror 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)

Advanced LIGO Laser – Requirements 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

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

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 loops with highest bandwidth up to 1MHz UGF

frequency noise requirement PSL same as LIGOI

intensity noise requirement

Adv LIGO - PSL optical layout NPRO 1W GEO typ ring laser 15W high power ring laser 200W spatial filter resonator (PMC) AOM frequency reference resonator possible solution, conceptual design not finished and not reviewed

PSL – stabilization scheme intensity stabilization outer loop injection locking intensity stabilization inner loop PMC loop frequency stabilization inner loop what is the current status? frequency stabilization outer loop

Power Noise Reduction

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

Birefringence Compensation The figure shows some experiments on the optimized double had design. The green curve shows the non polarized output power of the system without QR in place. It can be seen that the bifocusing acts at a pump power level of nearly 350 W and the resonator gets unstable in one of the polarization directions. With QR the bifocusing can be compensated and the output power can further increased ( blue curve). To achieve linear polarisation the third experiment was done with BP inside the resonator. The output power was limited below 60W (red curve). With QR in place the linear polarized output power reached the same level as in non polarized operation ( black curve ). * The BFC works very well *

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

high power stage status Feb 2004 linear polarized with birefringence compensation

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