Stabilized high-power lar 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.Wesls,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
3Lar 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 lar system,devel-
oped for the ground-bad gravitational wave detector Advanced LIGO
洋葱羊肉(Lar Interferometer Gravitational-Wave Obrvatory),was comprehen-
sively characterized.Lar 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)Lar stabilization;(120.3180)Interferometry.
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1.Introduction
Interferometric gravitational wave detectors[1,2]perform one of the most preci differential length measurements ever.Their goal is to directly detect the faint signals of gravitational waves emitted by astrophysical sources.The Advanced LIGO(Lar Interferometer Gravitational-Wave Obrvatory)[3]project is currently installing three cond-generation,ground-bad detectors at two obrvatory sites in the USA.The4kilometer-long baline Michelson inter-ferometers have an anticipated tenfold better nsitivity than theirfirst-generation counterparts (Inital LIGO)and will presumably reach a strain nsitivity between10−24and10−23Hz−1/2.
One key technology necessary to reach this extreme nsitivity are ultra-stable high-power lar systems[4,5].A high lar output power is required to reach a high signal-to-quantum-noi ratio,since the effect of quantum noi at high frequencies in the gravitational wave readout is reduced with increasing circulating lar power in the interferometer.In addition to quantum noi,tec
hnical lar noi coupling to the gravitational wave channel is a major noi source[6].Thus it is important to reduce the coupling of lar by optical design or by exploiting symmetries,and to reduce lar noi itlf by various active and passive stabilization schemes.
In this article,we report on the pre-stabilized lar(PSL)of the Advanced LIGO detector. The PSL is bad on a high-power solid-state lar that is comprehensively stabilized.One lar system was t 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 cond #161737 - $15.00 USD Received 18 Jan 2012; revid 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 obrvatory PSL.For this we measured various beam parameters and noi 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 lar beam at1064nm wavelength with a maximum power of150W with99.5%in the TEM00mode.The active and passive stabilization schemes efficiently re-duced the technical lar noi by veral orders of magnitude such that most design require-ments[5,7]were fulfilled.In the gravitational wave detection frequency band the relative power noi 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 noi was consistent with the maximum acceptable frequency noi 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 obrved no instabilities due to cross couplings.
This stabilized lar system is an indispensable part of Advanced LIGO and fulfilled nearly all design goals concerning the maximum acceptable noi levels of the different beam pa-rameters right after installation.Furthermore all or a subt of the implemented stabilization schemes might be of interest for many other high-precision optical experiments that are limited by lar noi.Besides gravitational wave detectors,stabilized lar systems are in the field of optical frequency standards,macroscopic quantum objects,precision spectroscopy and optical traps.
In the following ction the lar system,the stabilization scheme and the characterization methods are described(Section2).Then,the results of the characterization(Section3)and the conclusions(Section4)are prented.
2.Lar system and stabilization
The PSL consists of the lar,developed and fabricated by Lar 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 obrvatory sites the optical table is located in an acoustically isolated cleanroom.Most of the required electronics,the lar diodes for pumping the lar,and water chillers for cooling components on the optical table are placed outside of this cleanroom.
The lar itlf consists of three stages(Fig.1).An almostfinal version of the lar,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 lar and the minor modifications implemented between engineering prototype and reference system are given in the following.
Thefirst stage,the master lar,is a commercial non-planar ring-oscillator[9,10](NPRO) manufactured by InnoLight GmbH in Hannover,Germany.This solid-state lar us a Nd:Y AG crystal as the lar medium and resonator at the same time.The NPRO is pumped by lar diodes at808nm and delivers an output power of2W.An internal power stabilization,called the noi eater,suppress the relaxation oscillation at around1MHz.Due to its monolithic res-onator,the lar has exceptional intrinsic frequency stability.The two subquent lar stages, ud for power scaling,inherit the frequency stability of the master lar.
The cond stage(medium-power amplifier)is a single-pass amplifier[11]with an output power of35W.The ed lar beam from the NPRO stage pass through four Nd:YVO4crys-#161737 - $15.00 USD Received 18 Jan 2012; revid 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 lar system of Advanced LIGO.The three-staged lar(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 cloly 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 lar 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 ud as the active media. Each is longitudinally pumped by venfiber-coupled lar diodes at808nm.The oscillator is injection-locked[12]to the previous lar stage using a feedback control loop.A broadband EOM(electro-optic modulator)placed between the NPRO and the medium-power amplifier is ud to generate the required pha 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 lar 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 lar.
In general the PSL is designed to be operated in two different power modes.In high-power mode all three lar 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 lar resonator is clod.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 ud in the early commissioning pha and in the low-frequency-optimized operation mode of Advanced LIGO and is not discusd further in this article.
The stabilization has three ctions(Fig.1:PMC,PD2,reference cavity):A passive resonator, the so called pre-mode-cleaner(PMC),is ud tofilter the lar beam spatially and temporally (e subction2.1).Two pick-off beams at the PMC are ud for the active power stabilization (e subction2.2)and the active frequency pre-stabilization,respectively(e subction2.3).
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; revid 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-cess are ud to control different aspects of the lar system.The Experimental Physics and Industrial Control System(EPICS)[14]and its associated ur tools are ud 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 ud to analyze two different beams,pick-off beams of the medium power stage and of the HPO.Two shutters are ud to multiplex the to the DBB.We are able to measurefluctuations in power,frequency and beam pointin
g 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 ud this instrument during the characterization of the lar system to measure the mentioned lar 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 transver modes,to improve the beam quality and the pointing stability of the lar 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 subquent subsystems of the gravitational wave detector.
We developed and ud 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 confi
guration 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 lar 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 afines 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 pha was about1.7rad which is equivalent to0.27FSR.
One TEM00resonance frequency of the PMC is stabilized to the lar frequency.The Pound-Drever-Hall(PDH)[20,21]nsing scheme is ud to generate error signals,reusing the pha 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 ud 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 ud to stabilize the PMC resonance frequency to the lar frequency.
In addition,the electronics is able to automatically bring the PMC into resonance with the lar(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 clos the analog control loop.This lock acquisition proce-#161737 - $15.00 USD Received 18 Jan 2012; revid 27 Feb 2012; accepted 4 Mar 2012; published 24 Apr 2012 (C) 2012 OSA7 May 2012 / Vol. 20, No. 10 / OPTICS EXPRESS 10621
