The advanced scheme of “signal recycling” is to be used in the British-German GEO 600 project, in addition to “power recycling” (which has become standard for all laser-interferometer gravitational wave detector projects). This combination, “dual recycling,” has been demonstrated for the first time on a fully suspended interferometer, the Garching prototype with 30 m arm length. Signal enhancement and power buildup were as predicted; operation was reliable, and a significant contrast enhancement was observed. A control system that can be extended to full scale gravitational wave detectors was employed

Danzmann, K.

Heinzel, G.

Mizuno, J.

Rüdiger, A.

Schilling, R.

Skeldon, K.

Strain, K.

Willke, B.

Winkler, W.

English

Heinzel, Gerhard

Strain, Kenneth A.

Skeldon, K. D.

Willke, Benno

Winkler, Walter

Schilling, Roland

Rüdiger, Albrecht

Danzmann, Karsten

Max Planck Society eDoc Server

Experimental Demonstration of a Suspended Dual Recycling Interferometer for Gravitational Wave Detection

MPG.PuRe

VOLUME 81, NUMBER 25 P H Y S I C A L R E V I E W L E T T E R S 21 DECEMBER 1998ymmanyct,alimeentmentwasExperimental Demonstration of a Suspended Dual Recycling Interferometerfor Gravitational Wave DetectionG. Heinzel,1,* K. A. Strain,2 J. Mizuno,1,† K. D. Skeldon,2 B. Willke,3 W. Winkler,1R. Schilling,1 A. Rüdiger,1 and K. Danzmann1,31Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, German2Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdo3Institut für Atom- und Molekülphysik, Universität Hannover, Callinstrasse 38, D-30167 Hannover, Ger(Received 1 September 1998)The advanced scheme of “signal recycling” is to be used in the British-German GEO 600 projein addition to “power recycling” (which has become standard for all laser-interferometer gravitationwave detector projects). This combination, “dual recycling,” has been demonstrated for the first ton a fully suspended interferometer, the Garching prototype with 30 m arm length. Signal enhancemand power buildup were as predicted; operation was reliable, and a significant contrast enhancewas observed. A control system that can be extended to full scale gravitational wave detectorsemployed. [S0031-9007(98)07923-X]PACS numbers: 04.80.Nn, 07.60.Ly, 95.55.Ymthn, cn tntlyerEOueingorkighy artethsufd bisehtetex-ngors,8]veteandngingalth)brytheedsant,orfn ine ofingofans isne-het isisOne of the most exciting developments in science incoming decade is expected to be the direct observatiogravitational waves from sources such as supernovaealescing neutron stars or black holes, pulsars, or evebig bang (see Ref. [1] for a review). There are currefive projects [2–6] constructing gravitational wave obsvatories that employ laser interferometry. Of these, G600 will be a forerunner in employing the novel techniqof “dual recycling,” with several of the others considerthis technique for future stages of their projects.Laser-interferometric gravitational wave detectors wby sensing the differential phase shift imposed on the lin the two (orthogonal) arms of an interferometer bpassing gravitational wave. This phase shift is conveinto an observable light-power change at the output ofinterferometer. When other noise sources have beenciently reduced, the smallest detectable signal is limitethe variance of the photon-counting statistics (shot noThe shot-noise limited sensitivity is improved if the ligenergy (number of photons) stored in the interferomcan be increased.A sketch of the optical layout of the dual recycling eperiment is shown in Fig. 1. Mirrors for power recycliand signal recycling, denoted by MPR and MSR, are addedto the basic Michelson interferometer formed by mirrM1, M2, and a beam splitter (BS). Power recycling [7enhances the circulating light power and hence improthe signal-to-noise ratio. The interferometer is operaso that its output is held at an interference minimumthe power recycling mirror MPR is chosen and positioneto maximize the light amplitude in the “power recyclicavity,” formed by the Michelson interferometer and MPR.The performance can also be improved by optimizthe time of interaction of the light with the gravitationwave (i.e., by choosing the optimal detector bandwidThis can be realized by using either resonant (Fa0031-9007y98y81(25)y5493(4)$15.00eofo-he-tdefi-y).rsdd.-Perot) cavities or nonresonant optical delay lines inarms of the interferometer. The Fabry-Perot system neadditional control loops to keep the arm cavities resonwhile the delay-line scheme is limited by the large mirrsize required and by stray light problems.If signal recycling[9] is incorporated into the design othe interferometer, however, the detector bandwidth caprinciple be made as narrow as desired without the usdelay lines or arm cavities. Furthermore, signal recyclis a flexible and efficient way to enhance the sensitivityan interferometer within a restricted bandwidth aboutarbitrary user-selectable center frequency [9,10]. Thiuseful in searching for continuous radiation from knownarrow band sources such as pulsars.FIG. 1. The optical layout of the dual recycling interferomter. The power recycling mirror MPR and signal recyclingmirror MSR are shown. The distance of each mirror from tcenter of the beam splitter (BS) surface in our experimenmarked. The power buildup in the power recycling cavityindicated by the heavy gray lines.© 1998 The American Physical Society 5493VOLUME 81, NUMBER 25 P H Y S I C A L R E V I E W L E T T E R S 21 DECEMBER 1998vitaeamrrornceity”ginnchs.ntsfroanmert oac-tersmdu-tive, thonhethedoweth,sonure(anl oh inasencythobav-ckeaowea-clingthrolallyplieiththoncf thz)en-mistoz)terkelsao-n-ndss atrk-eu-theed].nyeecthisthbyheithla-terpli-aled-ac-The signal sidebands produced by a passing grational wave will be separated from the carrier at the bsplitter and directed towards the signal recycling miMSR. The amplitudes of these sidebands are enhawhen they are resonant in the “signal recycling cavformed by the Michelson interferometer and MSR, lead-ing to an equivalent enhancement of the light emerthrough MSR and detected by PD3.Dual recycling was first demonstrated on a small, betop interferometer [11] with rigidly mounted mirrorHowever, full-scale detectors will use optical componethat are suspended as pendulums to isolate themseismic vibration. We report here on the constructionoperation of a fully suspended dual recycled interferoter, carried out at the Garching 30 m prototype as pathe GEO 600 project.To minimize unwanted motion and effects of refrtive index fluctuations of the air, the whole interferomesystem is enclosed in a vacuum chamber. For seiisolation, the mirrors and the beam splitter are indivially suspended as pendulums. Each component is acdamped with respect to local references. In additionrelative positions and orientations of the mirrors are ctrolled to maintain the desired operating condition. Tlongitudinal control is described in this Letter, andautoalignment system for the angular degrees of freewill be the subject of a forthcoming paper [12].The longitudinal degrees of freedom are the porecycling cavity length, the signal recycling cavity lengand the differential length of the two arms of the Michelinterferometer. In practice, these lengths are measrelative to the wavelength of the light from the sourceargon ion laser in our experiment). Thus the contro1 degree of freedom, the power recycling cavity lengtour experiment, can be replaced by the control of the lfrequency.The signal for the power recycling and laser frequecontrol was obtained using the Pound-Drever-Hall me[13] with the phase modulation frequency chosen to12 MHz (see Fig. 2). An additional prestabilization city was employed [14] which makes acquisition of losimpler. At Fourier frequencies within the intended msurement band the laser frequency was locked to the precycling cavity length. At frequencies below the msurement band the laser frequency and the power recycavity length were locked to the (relatively stable) lenof the rigid reference cavity.The signals for the remaining two longitudinal contloops were obtained by a variation of the method initiproposed by Schnupp [15]. Phase modulation was apto the laser light before it enters the interferometer. Wthe parameters used in our 30 m experiment (and alsoin GEO 600), the coupled cavity has a series of resonaseparated approximately by the free spectral ranges opower and signal recycling cavities (both near 4.85 MHThe second such resonance, near 9.7 MHz, was chosthe modulation frequencyfm in our experiment. An inten5494-dg-md-ficlye-mrdfrde-ergdseese.asFIG. 2. Simplified diagram of the longitudinal control systefor the dual recycling experiment. Light from the laserphase modulated at 12 MHz, before a fraction is divertedthe reference cavity, RC. The Schnupp modulation (9.7 MHis then applied before the light is directed to the interferomethrough a single mode optical fiber. PC represents a Poccell, PD a photodiode, Mix an electronic mixer, VCO200 MHz voltage-controlled oscillator, and AOM an acoustoptic modulator.tional asymmetry in the arm lengths of the Michelson iterferometer causes a fraction of the modulation sidebato appear at the output port, even when the Michelson ian interference minimum.Any deviation of the Michelson phase from the dafringe condition will then produce a proportional component atfm in the photocurrent detected by PD3. ThMichelson error signal is obtained by coherently demodlating this photocurrent and is also used to extractgravitational wave information from the system.The signal required to control the position of MSR wasderived by a novel method employing thesameSchnuppmodulation as used for the dark-fringe lock, first testin tabletop experiments without power recycling [16,17There is some sideband light incident on MSR and reflectedback into the arms of the interferometer at all times. Adisplacement of MSR will then change the phase of thsideband field in the arms of the interferometer with respto the carrier field (which does not reach MSR in thedesired operating condition). It can be shown that tprovides a suitable locking signal for dual recycling wipeak response at low frequency.In our experiment (see Fig. 2), this signal is acquiredsplitting off a small fraction (,0.1%) of the light in onearm at the rear surface of BS and directing it to PD4. Tphotocurrent from PD4 was demodulated coherently wthe 9.7 MHz modulation, using an appropriate demodution phase.Each of the signals used to control the interferomewas taken from the relevant demodulator, suitably amfied and filtered, and fed back to control the longitudinpositions of the 4 mirrors and the laser frequency. Feback to the mirrors was accomplished via coil-magnettuators that apply forces directly to the mirrors.VOLUME 81, NUMBER 25 P H Y S I C A L R E V I E W L E T T E R S 21 DECEMBER 1998ms,pendTeedcanthaamerr aamheonedanwaeringngf thondtofednainalemalingndaseti-sfac-ornctedemioningtlyefectrere toaseedthe-anthatde-areout-te”ealecttoith-gthehatny-asalen-ceingion3utnottruenrrorene inThe complexity of this asymmetrical optical systemakes it difficult to predict its behavior intuitively. Thua method of simulating the error signals was develo[18] and used to design our experiment.While a full description of the control system is beyothe scope of a Letter, a few key points are emphasized.“unconditional” locking of the laser light frequency to thlength of the power recycling cavity was much simplifiby a slight asymmetry of the beam splitter’s reflectanto transmittance (51.5:48.5) [8]. This has no significeffect on the signal at the output port, but ensuresthere is always some light reflected back from the besplitter towards the input port. This allows the powrecycling cavity to be resonant with the incident light fobroad range of conditions of the interferometer, which wessential for lock acquisition of the dual recycling systeIt was found that, at least during acquisition of lock, tgain and bandwidth of the loop controlling MSR shouldbe greater than that of the control loop for the Michelsdark fringe. In this condition the system normally settlto the desired operating state within a few seconds,remained locked for several tens of minutes.Tests were carried out to ensure that dual recyclingoperating correctly with maximum response close to zfrequency. Power recycling was confirmed by monitorthe light power in the region of the beam splitter usicalibrated photodetectors that sense a small fraction olight inside the power recycling cavity, and also a fractiof the illuminating light from the laser. The observepower enhancement of50 6 5 is just what is expectedwith a 7% transmittance power recycling mirror.Coil-magnet actuators on M2 were used to apply to thamirror a sinusoidal longitudinal position modulation8.8 3 10213 m (rms) at 375 Hz. The signal thus producat the output (visible on the spectrum of the output sigshown in Fig. 3) was measured with dual recyclingoperation. Subsequently the signal recycling mirror wmisaligned (by,3 mrad) so that it was acting as a simpattenuator. Then the signal was seen to be 1800 tiFIG. 3. A power spectrum of the typical output from the durecycling system. The noise is technical in nature originatmainly from insufficiently suppressed laser frequency aamplitude fluctuations.dheetts.dsoelsessmaller than before. Taking into account thes3.9 6 0.2d%transmittance of MSR, the corrected enhancement w70 6 4.This value is in reasonable agreement with the theorcally expected value of91 6 5, derived assuming losslesoptical components. The enhancement consists of twotors: a signal recycling gain of10.1 6 0.5 (correspondingto the transmittance of 3.9% of the signal recycling mirrMSR) and an improved coupling of Schnupp modulatiosidebands to the output that should increase the detesignal by a factor of 9. (The parameters of the systwere chosen to maximize the transfer of the modulatsidebands to the main photodetector with dual recyclworking.) This agreement is possible only with correcworking dual recycling, and the shortfall is probably duto the losses at the optical components and by imperalignment of the signal recycling cavity.An important feature of dual recycling is themodecleaning effect. The “dark fringe” of an interferometeis in practice not completely dark, due to higher ordtransverse modes produced by imperfect optics and duthe modulation sidebands, which are necessary for phdetection. In order to reach the ideal shot-noise limitsensitivity, the modulation sidebands must dominatelight power at the photodetector [19].By “closing” the output with the signal recycling mirror the whole interferometer becomes a (complex) resoncavity. If the mirror curvatures are chosen to ensure tthe predominantly excited higher order modes are nongenerate with the fundamental mode, their amplitudesstrongly suppressed both inside the system and at theput port. This mode cleaning effect reduces the “waslight and therefore allows a closer approach to the idshot-noise limited performance. The mode cleaning effwas clearly observed in our experiment.A charge-coupled device (CCD) camera was usedrecord the shape of the emerging light beam with and wout signal recycling. The addition of the signal recyclinmirror suppressed most of the structure from the light atoutput port, as shown in Fig. 4. It was also observed tthe influence of intentional misalignment of the Michelsointerferometer was considerably smaller with dual reccling. While manual alignment of the interferometer wvery difficult (we relied on the automatic system), the durecycling system was tolerant of a larger degree of inttional misalignment before any degradation in performanwas seen.For a more quantitative description of the mode-cleaneffect, we use the term “contrast” to represent the fractof the light power leaking out towards photodetector PDrelative to that incident on the beam splitter from the inpside. The conventional concept of fringe contrast canreadily be applied to a power-recycled system, where a“bright fringe” never exists. Note that with this definitiothe contrast could be made perfect with an opaque miMSR, and thus an increased contrast is useful only what the same time the signal is enhanced, as is the cas5495VOLUME 81, NUMBER 25 P H Y S I C A L R E V I E W L E T T E R S 21 DECEMBER 1998mwethlosohefteatltasuewealcheeontionticbthtengoerS.itymicaly,s,.,9494-,-rk,onler-ofFIG. 4. CCD images of the output light from the 30interferometer. The left image shows the best result with porecycling only, while the right image is a typical result widual recycling. In this latter case the beam shape is very cto the profile of a fundamental Gaussian mode (with mostthe power in Schnupp modulation sidebands).our experiment. Note, further, that the light power at tphotodetector includes the modulation sidebands usedphase detection. This contribution must be appropriataken into account to obtain the “effective contrast” thindicates the quality of interference.In practice, the contrast could not be measured direcbut was deduced from observed data. The contrasthe interferometer without the signal recycling mirror wmeasured to be no better than 1:1000, even with optimalignment. This includes a small contribution from thmodulation sideband power, but is dominated by the poin higher order modes.With the signal recycling mirror in place and durecycling in operation, the contrast could easily rea1:3700, now dominated by the modulation sidebands, tpower being enhanced by a factor ofø81 due to signalrecycling. If we subtract the computed sideband powfrom the total power at the photodetector, the effective ctrast of the interferometer would be1:104 or even better.The experiment described was the first demonstraof dual recycling with suspended optical components, athe results were in reasonable agreement with theorepredictions. A significant mode-cleaning effect was oserved. Dual recycling proved to be fully compatible withe autoalignment system. The complete control sysused in our demonstration will form the basis of the cotrol system for the GEO 600 interferometer.The authors thank A. Freise and colleagues at Glasfor their help and H. Klein, H. Emme, and A. Weidnfor their excellent technical support. K. A. S. and K. D.5496reforlyty,ofmrhirr-ndal-m-wacknowledge support from PPARC and the Universof Glasgow; K. D. S. also acknowledges support frothe Royal Society of Edinburgh through a BPyRSEfellowship.*Email address: ghh@mpq.mpg.de†Present address: The Institute of Physical and ChemResearch (RIKEN), Optical Engineering LaboratorHirosawa 2-1, Wako, Saitama 351-0198, Japan.[1] K. S. Thorne, in300 Years of Gravitation,edited by S. W.Hawking and W. Israel (Cambridge University PresCambridge, 1987), pp. 330–458.[2] A. Abramovici et al., Science256, 325–333 (1992).[3] C. Bradaschiaet al.,Nuclear Instrum. Methods Phys. ResSect. A289, 518–525 (1990).[4] K. Danzmannet al., in First Edoardo Amaldi Confer-ence on Gravitational Wave Experiments, Frascati, 19(World Scientific, Singapore, 1995), pp. 100–111.[5] K. Tsubono et al., in First Edoardo Amaldi Confer-ence on Gravitational Wave Experiments, Frascati, 19(Ref. [4]), pp. 112–114.[6] D. Blair et al., in Gravitational Wave Detection, Proceedings of the TAMA Workshop, Saitama, Japan, 1996,editedby K. Tsubonoet al. (Universal Academy Press, Tokyo1997).[7] R. W. P. Dreveret al., in Quantum Optics, Experimental Gravitation, and Measurement Theory,edited byP. Meystre and M. O. Scully (Plenum Press, New Yo1983), pp. 503–514.[8] D. Schnieret al., Phys. Lett. A225, 210–216 (1997).[9] B. J. Meers, Phys. Rev. D38, 2317–2326 (1988).[10] B. J. Meers, Phys. Lett. A142, 465–470 (1989).[11] K. A. Strain and B. J. Meers, Phys. Rev. Lett.66, 1391–1394 (1991).[12] G. Heinzelet al., Opt. Commun. (to be published).[13] R. W. P. Dreveret al., Appl. Phys. B31, 97–105 (1983).[14] J. Hough et al., in The Detection of GravitationalRadiation, edited by D. G. Blair (Cambridge UniversityPress, Cambridge, 1991), pp. 329–351.[15] L. Schnupp, contribution to the European CollaboratiMeeting on Interferometric Detection of GravitationaWaves, Sorrento, 1988 (unpublished).[16] D. Maass, Diploma-thesis, University of Hannov(1995); A. Barthel, Diploma-thesis, University of Hannover (1997); A. Freise, Diploma-thesis, UniversityHannover (1998).[17] M. B. Gray et al., Appl. Opt. (to be published).[18] J. Mizunoet al. (to be published).[19] B. J. Meers and K. A. Strain, Phys. Rev. D43, 3117–3130(1991).

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Experimental Demonstration of a Suspended Dual Recycling Interferometer for Gravitational Wave Detection

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