Stabilized high-power laser system for
the gravitational wave detector advanced
LIGO
P.Kwee,1,∗C.Bogan,2K.Danzmann,1,2M.Frede,4H.Kim,1P.King,5
J.P¨o ld,1O.Puncken,3R.L.Savage,5F.Seifert,5P.Wessels,3
L.Winkelmann,3and B.Willke2
1Max-Planck-Institut f¨u r Gravitationsphysik(Albert-Einstein-Institut),Hannover,Germany
2Leibniz Universit¨a t Hannover,Hannover,Germany
3Laser Zentrum Hannover e.V.,Hannover,Germany
4neoLASE GmbH,Hannover,Germany
5LIGO Laboratory,California Institute of Technology,Pasadena,California,USA
*patrick.kwee@aei.mpg.de
Abstract:An ultra-stable,high-power cw Nd:Y AG laser system,devel-
oped for the ground-based gravitational wave detector Advanced LIGO
(Laser Interferometer Gravitational-Wave Observatory),was comprehen-
sively characterized.Laser power,frequency,beam pointing and beam
quality were simultaneously stabilized using different active and passive
schemes.The output beam,the performance of the stabilization,and the
cross-coupling between different stabilization feedback control loops were
characterized and found to fulfill most design requirements.The employed
stabilization schemes and the achieved performance are of relevance to
many high-precision optical experiments.
©2012Optical Society of America
OCIS codes:(140.3425)Laser stabilization;(120.3180)Interferometry.
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1.Introduction
seifertInterferometric gravitational wave detectors[1,2]perform one of the most precise differential length measurements ever.Their goal is to directly detect the faint signals of gravitational waves emitted by astrophysical sources.The Advanced LIGO(Laser Interferometer Gravitational-Wave Observatory)[3]project is currently installing three second-generation,ground-based detectors at two observatory sites in the USA.The4kilometer-long baseline Michelson inter-ferometers have an anticipated tenfold better sensitivity than theirfirst-generation counterparts (Inital LIGO)and will presumably reach a strain sensitivity between10−24and10−23Hz−1/2.
One key technology necessary to reach this extreme sensitivity are ultra-stable high-power laser systems[4,5].A high laser output power is required to reach a high signal-to-quantum-noise ratio,since the effect of quantum noise at high frequencies in the gravitational wave readout is reduced with increasing circulating laser power in the interferometer.In addition to quantum noise,tec
hnical laser noise coupling to the gravitational wave channel is a major noise source[6].Thus it is important to reduce the coupling of laser by optical design or by exploiting symmetries,and to reduce laser noise itself by various active and passive stabilization schemes.
In this article,we report on the pre-stabilized laser(PSL)of the Advanced LIGO detector. The PSL is based on a high-power solid-state laser that is comprehensively stabilized.One laser system was set up at the Albert-Einstein-Institute(AEI)in Hannover,Germany,the so called PSL reference system.Another identical PSL has already been installed at one Advanced LIGO site,the one near Livingston,LA,USA,and two more PSLs will be installed at the second #161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20,  No. 10 / OPTICS EXPRESS  10618
site at Hanford,WA,USA.We have characterized the reference PSL and thefirst observatory PSL.For this we measured various beam parameters and noise levels of the output beam in the gravitational wave detection frequency band from about10Hz to10kHz,measured the performance of the active and passive stabilization schemes,and determined upper bounds for the cross coupling between different control loops.
At the time of writing the PSL reference system has been operated continuously for more than18months,and continues to operate reliably.The reference system delivered a continuous-wave,single-frequency laser beam at1064nm wavelength with a maximum power of150W with99.5%in the TEM00mode.The active and passive stabilization schemes efficiently re-duced the technical laser noise by several orders of magnitude such that most design require-ments[5,7]were fulfilled.In the gravitational wave detection frequency band the relative power noise was as low as2×10−8Hz−1/2,relative beam pointingfluctuations were as low as1×10−7Hz−1/2,and an in-loop measurement of the frequency noise was consistent with the maximum acceptable frequency noise of about0.1HzHz−1/2.The cross couplings between the control loops were,in general,rather small or at the expected levels.Thus we were able to optimize each loop individually and observed no instabilities due to cross couplings.
This stabilized laser system is an indispensable part of Advanced LIGO and fulfilled nearly all design goals concerning the maximum acceptable noise levels of the different beam pa-rameters right after installation.Furthermore all or a subset of the implemented stabilization schemes might be of interest for many other high-precision optical experiments that are limited by laser noise.Besides gravitational wave detectors,stabilized laser systems are in the field of optical frequency standards,macroscopic quantum objects,precision spectroscopy and optical traps.
In the following section the laser system,the stabilization scheme and the characterization methods are described(Section2).Then,the results of the characterization(Section3)and the conclusions(Section4)are presented.
2.Laser system and stabilization
The PSL consists of the laser,developed and fabricated by Laser Zentrum Hannover e.V.(LZH) and neoLASE,and the stabilization,developed and integrated by AEI.
The optical components of the PSL are on a commercial optical table,occupying a space of about1.5×3.5m2,in a clean,dust-free environment.At the observatory sites the optical table is located in an acoustically isolated cleanroom.Most of the required electronics,the laser diodes for pumping the laser,and water chillers for cooling components on the optical table are placed outside of this cleanroom.
The laser itself consists of three stages(Fig.1).An almostfinal version of the laser,the so-called engineering prototype,is described in detail in[8].The primary focus of this article is the stabilization and characterization of the PSL.Thus only a rough overview of the laser and the minor modifications implemented between engineering prototype and reference system are given in the following.
Thefirst stage,the master laser,is a commercial non-planar ring-oscillator[9,10](NPRO) manufactured by InnoLight GmbH in Hannover,Germany.This solid-state laser uses a Nd:Y AG crystal as the laser medium and resonator at the same time.The NPRO is pumped by laser diodes at808nm and delivers an output power of2W.An internal power stabilization,called the noise eater,suppresses the relaxation oscillation at around1MHz.Due to its monolithic res-onator,the laser has exceptional intrinsic frequency stability.The two subsequent laser stages, used for power scaling,inherit the frequency stability of the master laser.
The second stage(medium-power amplifier)is a single-pass amplifier[11]with an output power of35W.The seed laser beam from the NPRO stage passes through four Nd:YVO4crys-#161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20,  No. 10 / OPTICS EXPRESS  10619
power stabilization
Fig.1.Pre-stabilized laser system of Advanced LIGO.The three-staged laser(NPRO,
medium power amplifier,high power oscillator)and the stabilization scheme(pre-mode-
cleaner,power and frequency stabilization)are shown.The input-mode-cleaner is not part
of the PSL but closely related.NPRO,non-planar ring oscillator;EOM,electro-optic mod-
ulator;FI,Faraday isolator;AOM,acousto-optic modulator.
tals which are longitudinally pumped byfiber-coupled laser diodes at808nm.
The third stage is an injection-locked ring oscillator[8]with an output power of about220W, called the high-power oscillator(HPO).Four Nd:Y AG crystals are used as the active media. Each is longitudinally pumped by sevenfiber-coupled laser diodes at808nm.The oscillator is injection-locked[12]to the previous laser stage using a feedback control loop.A broadband EOM(electro-optic modulator)placed between the NPRO and the medium-power amplifier is used to generate the required phase modulation sidebands at35.5MHz.Thus the high output power and good beam quality of this last stage is combined with the good frequency stability of the previous stages.
The reference system features some minor modifications compared to the engineering proto-type[8]concerning the optics:The external halo aperture was integrated into the laser system permanently improving the beam quality.Additionally,a few minor designflaws related to the mechanical structure and the optical layout were engineered out.This did not degrade the output performance,nor the characteristics of the locked laser.
In general the PSL is designed to be operated in two different power modes.In high-power mode all three laser stages are engaged with a power of about160W at the PSL output.In low-power mode the high-power oscillator is turned off and a shutter inside the laser resonator is closed.The beam of the medium-power stage is reflected at the output coupler of the high power stage leaving a residual power of about13W at the PSL output.This low-power mode will be used in the early commissioning phase and in the low-frequency-optimized operation mode of Advanced LIGO and is not discussed further in this article.
The stabilization has three sections(Fig.1:PMC,PD2,reference cavity):A passive resonator, the so called pre-mode-cleaner(PMC),is used tofilter the laser beam spatially and temporally (see subsection2.1).Two pick-off beams at the PMC are used for the active power stabilization (see subsection2.2)and the active frequency pre-stabilization,respectively(see subsection2.3).
In general most stabilization feedback control loops of the PSL are implemented using analog electronics.A real-time computer system(Control and Data Acquisition Systems,CDS,[13]) which is common to many other subsystems of Advanced LIGO,is utilized to control and mon-itor important parameters of the analog electronics.The lock acquisition of various loops,a few #161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20,  No. 10 / OPTICS EXPRESS  10620
slow digital control loops,and the data acquisition are implemented using this computer sys-tem.Many signals are recorded at different sampling rates ranging from16Hz to33kHz for diagnostics,monitoring and vetoing of gravitational wave signals.In total four real-time pro-cesses are used to control different aspects of the laser system.The Experimental Physics and Industrial Control System(EPICS)[14]and its associated user tools are used to communicate with the real-time software modules.
The PSL contains a permanent,dedicated diagnostic instrument,the so called diagnostic breadboard(DBB,not shown in Fig.1)[15].This instrument is used to analyze two different beams,pick-off beams of the medium power stage and of the HPO.Two shutters are used to multiplex these to the DBB.We are able to measurefluctuations in power,frequency and beam pointing in an automated
way with this instrument.In addition the beam quality quantified by the higher order mode content of the beam was measured using a modescan technique[16].The DBB is controlled by one real-time process of the CDS.In contrast to most of the other control loops in the PSL,all DBB control loops were implemented digitally.We used this instrument during the characterization of the laser system to measure the mentioned laser beam parameters of the HPO.In addition we temporarily placed an identical copy of the DBB downstream of the PMC to characterize the output beam of the PSL reference system.
2.1.Pre-mode-cleaner
A key component of the stabilization scheme is the passive ring resonator,called the pre-mode-cleaner(PMC)[17,18].It functions to suppress higher-order transverse modes,to improve the beam quality and the pointing stability of the laser beam,and tofilter powerfluctuations at radio frequencies.The beam transmitted through this resonator is the output beam of the PSL, and it is delivered to the subsequent subsystems of the gravitational wave detector.
We developed and used a computer program[19]to model thefilter effects of the PMC as a function of various resonator parameters in order to aid its design.This led to a resonator with a bow-tie configur
ation consisting of four low-loss mirrors glued to an aluminum spacer. The optical round-trip length is2m with a free spectral range(FSR)of150MHz.The inci-dence angle of the horizontally polarized laser beam is6◦.Theflat input and output coupling mirrors have a power transmission of2.4%and the two concave high reflectivity mirrors(3m radius of curvature)have a transmission of68ppm.The measured bandwidth was,as expected, 560kHz which corresponds to afinesse of133and a power build-up factor of42.The Gaussian input/output beam had a waist radius of about568µm and the measured acquired round-trip Gouy phase was about1.7rad which is equivalent to0.27FSR.
One TEM00resonance frequency of the PMC is stabilized to the laser frequency.The Pound-Drever-Hall(PDH)[20,21]sensing scheme is used to generate error signals,reusing the phase modulation sidebands at35.5MHz created between NPRO and medium power amplifier for the injection locking.The signal of the photodetector PD1,placed in reflection of the PMC, is demodulated at35.5MHz.This photodetector consists of a1mm InGaAs photodiode and a transimpedance amplifier.A piezo-electric element(PZT)between one of the curved mirrors and the spacer is used as a fast actuator to control the round-trip length and thereby the reso-nance frequencies of the PMC.With a maximum voltage of382V we were able to change the round-trip length by about2.4µm.An analog feedback control loop with a bandwidth of about 7kHz is used to stabilize the PMC resonance frequency to the laser frequency.
In addition,the electronics is able to automatically bring the PMC into resonance with the laser(lock acquisition).For this process a125ms period ramp signal with an amplitude cor-responding to about one FSR is applied to the PZT of the PMC.The average power on pho-todetector PD1is monitored and as soon as the power drops below a given threshold the logic considers the PMC as resonant and closes the analog control loop.This lock acquisition proce-#161737 - $15.00 USD Received 18 Jan 2012; revised 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20,  No. 10 / OPTICS EXPRESS  10621

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