The LISA phase measurement system (PMS) will provide interferometric phase readout of the primary heterodyne signal at microcycle sensitivity, ranging measurements at sub-meter accuracy and data communication at rates of several kilobits per seconds. Our investigations are focused on inter-spacecraft laser ranging and data transfer for LISA using Direct Sequence Spread Spectrum (DS/SS) modulation onto the laser links. We present the setup of an optical experimental to test the levels of performance achievable with a single laser link as well as a new hardware prototype based on FPGA (Field Programmable Gate Array) processing to perform high-accuracy phase readout of the optical signal, ranging measurements, data communication and is suitable for clock noise demodulation and digital laser-phase locking

Bykov, Iouri

Danzmann, Karsten

Eichholz, Johannes

Esteban, Juan Jose

García, Antonio F.

Heinzel, Gerhard

Peinado, Antonio M.

Esteban Delgado, J.

Garcia, A.

Eichholz, J.

Peinado, A.

Bykov, I.

Heinzel, G.

Danzmann, K.

MPG.PuRe

Ranging and phase measurement for LISA

Institutionelles Repositorium der Leibniz Universität Hannover

This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 194.95.157.184This content was downloaded on 26/04/2017 at 13:06Please note that terms and conditions apply.Ranging and phase measurement for LISAView the table of contents for this issue, or go to the journal homepage for more2010 J. Phys.: Conf. Ser. 228 012045(http://iopscience.iop.org/1742-6596/228/1/012045)Home Search Collections Journals About Contact us My IOPscienceYou may also be interested in:Auxiliary functions of the LISA laser linkGerhard Heinzel, Juan José Esteban, Simon Barke et al.Verification of time-delay interferometry techniquesShawn J Mitryk, Vinzenz Wand and Guido MuellerImproved optical ranging for space based gravitational wave detectionAndrew J Sutton, Kirk McKenzie, Brent Ware et al.Fiber modulators and fiber amplifiers for LISAM Tröbs, S Barke, J Möbius et al.Phasemeter core for intersatellite laser heterodyne interferometry: modelling, simulations andexperimentsOliver Gerberding, Benjamin Sheard, Iouri Bykov et al.LISA long-arm interferometryBenjamin Sheard, Gerhard Heinzel and Karsten DanzmannA demonstration of LISA laser communicationS E Pollack and R T StebbinsProgress in Interferometry for LISA at JPLRobert Spero, Brian Bachman, Glenn de Vine et al.TDI and clock noise removal for the split interferometry configuration of LISAMarkus Otto, Gerhard Heinzel and Karsten DanzmannRanging and phase measurement for LISAJuan Jose´ Esteban1, Antonio F. Garc´ıa1, Johannes Eichholz1, AntonioM. Peinado2, Iouri Bykov1, Gerhard Heinzel1, Karsten Danzmann1.1Max-Planck-Institute for Gravitational Physics (Albert-Einstein-Institute) and LeibnizUniversity of Hannover, Germany2Department of signal theory, networking and communications, University of Granada, SpainE-mail: juan.jose.esteban@aei.mpg.deAbstract.The LISA phase measurement system (PMS) will provide interferometric phase readout ofthe primary heterodyne signal at microcycle sensitivity, ranging measurements at sub-meteraccuracy and data communication at rates of several kilobits per seconds. Our investigationsare focused on inter-spacecraft laser ranging and data transfer for LISA using Direct SequenceSpread Spectrum (DS/SS) modulation onto the laser links. We present the setup of an opticalexperimental to test the levels of performance achievable with a single laser link as well as a newhardware prototype based on FPGA (Field Programmable Gate Array) processing to performhigh-accuracy phase readout of the optical signal, ranging measurements, data communicationand is suitable for clock noise demodulation and digital laser-phase locking.1. IntroductionThe Laser Interferometer Space Antenna, LISA, will be a huge unequal armlength interferometerin space for gravitational wave detection [1]. LISA will consist of three spacecraft separated by5 million kilometers forming a triangular formation and communicating via three bidirectionallaser links. As consequence of variations in the spacecraft-to-spacecraft distance, the LISA armsare unequal and time-varying (change by approximately ±1% over one year orbit, ' 50.000 km),resulting in a limitation of the interferometric sensitivity due to laser frequency noise. In orderto suppress this effect and make LISA virtually work like an equal arm interferometer, one of thetechniques planned is a post-processing algorithm on ground, known as time-delay interferometry(TDI) [2]. It combines onboard ranging measurements with phase information of the laserbeams to suppress the effect of laser frequency noise by up to seven orders of magnitude. Thistechnique relies on interspacecraft distance tracking with resolution of the order of a few metersand therefore any error in the ranging measurements will turn into an error in the outcome ofthe post-processing algorithm [3]. In order to achieve inter-spacecraft laser ranging and datacommunication, the main carrier of the laser link is phase modulated with pseudo-random noise(PRN) and the travel time can be measured via the correlation of the local and incoming PRNcodes. The phase measurement system (PMS) processes the interferometric signal coming fromthe photodetectors on the optical bench to measure its phase with microcycle accuracy, performsranging at sub-meter resolutions and enables data transfer at several kilobits per seconds.8th Edoardo Amaldi Conference on Gravitational Waves IOP PublishingJournal of Physics: Conference Series 228 (2010) 012045 doi:10.1088/1742-6596/228/1/012045c© 2010 IOP Publishing Ltd 1In order to test the modulation scheme in a single LISA arm, an optical experiment hasbeen built in our laboratories. Figure 1 shows the experimental setup: two offset phase-lockedlasers are modulated using fibre-coupled electro-optic modulators (EOM) and injected onto amonolithic optical bench for interference. The beatnote is digitised and processed in a custom-designed PMS breadboard which implements the phase readout via a phase-lock loop (PLL)architecture and the ranging capability via a delay-lock loop (DLL) architecture.EPP DLLVariabledelayDigital PLLCorrelator ADCPRNgenerationDAQmaster laserslave laserincoupling opticselectro-optic phasemodulators opticalbenchphotodetectorPMS breadboardspectrum analyzerfunctiongeneratorAnalog PLLcomputerPolarizing Beam SplitterBeam SplitterMirrorLensλ/4 Waveplateλ/2 WaveplateFaraday IsolatorBeam DumpFiber CouplerEOM 1EOM 2Figure 1. General schematic of the experiment setup to test the laser modulation scheme and PMSperformance: two phase-locked lasers are modulated using an EOM and injected onto a Mach-Zehnderinterferometer. The beatnote is detected and processed in the PMS breadboard in order to obtain phasereadout, ranging measurement and data transfer.2. Ranging and data communicationRanging is based on the correlation properties of the pseudo-random noise (PRN) codes. Thereare a total of six laser beams exchanged between the LISA satellites, and therefore a PRN codehas been designed for each one of them. In the LISA topology, each laser is used simultaneouslyin different interferometric measurements producing a beatnote modulated with more than onecode. The main design driver for the codes is that after interference between any given two laser,a single PRN can be tracked separately from each other and without incurring in significant mu-tual interference. The set of six PRN sequences was designed by numerical optimisation andwith an even length of 1024 chips1. The chipping rate is about 1.5 Mbps, limited by the pho-todectector bandwidth, and in which the data sequences are encoded at 97 Kbps. This produces1 The size of the code family is a compromise between efficient use in a digital processor and the correlationproperties achieved at the running chipping rate8th Edoardo Amaldi Conference on Gravitational Waves IOP PublishingJournal of Physics: Conference Series 228 (2010) 012045 doi:10.1088/1742-6596/228/1/0120452a periodicity of the code every 200 km over the 5 million kilometer armlength, and therefore, aninitial positioning system is required. The deep-space network (DNS), combined with the startracker onboard each satellite, will provide a positioning uncertainty of about 25 km [4]. Afterthis initial positioning, a more accurate distance determination will be achieved using the DLLarchitecture proposed. Note that only when the cross-correlation is made by the same PRNcode with the same delay, does a peak appear in the correlation (see Figure 2). This way, thecorrelation peak serves as a timestamp if the start of the PRN is synchronised with the clock ofthe remote spacecraft.-400-200 0 200 400-0.2 0 0.2 0.4 0.6 0.8 1Autocorrelation of 6 pseudo-random bit codes-400-200 0 200 400-0.2 0 0.2 0.4 0.6 0.8 1Selected  cross-correlations of 6 possible pseudo-random bit codesFigure 2. Autocorrelation of each pseudocode (left) and cross-correlation between a possible set oftwo different pseudocodes (right).The presence of sudden phase changes modulated to the laser beam could compromise thephase stability of the LISA fringe signal. However, recent experiments pointed out that mediumphase modulation could be applied without degradation of the phase fidelity of the sciencesignal [5]. Consequently, a low-index phase modulation has been chosen using about 1% of thecarrier power for ranging and data communication. Figure 3 shows the spectrum of the beatnotemeasured on the optical laboratory where an equivalent modulation index of m=0.16 rad wasphase modulated using a EOMs.3. Phase measurement systemThe main task to be implemented by the LISA phasemeter is the phase readout of the beat-note at a required sensitivity of 2pi × 10−6 rad/√Hz on the frequency range from 0.1 mHz to100 mHz. This task corresponds with the core processing of the PMS and is referred as themain science measurement, since the gravitational wave information is contained here. Themost suitable architecture to implement this measurement is based on a digital phase-lockedloop (DPLL)[6, 7] scheme and the general block diagram is represented in Figure 4: the beat-note is fed into an in-phase/quadrature (I/Q) demodulator to acquire its phase. A control looplocks the phase of a Numerically Controlled Oscillator (NCO) to the incoming beatnote. Thephase measurement is formed in a floating-point unit as the sum of a raw phase estimationfrom the NCO and the arctangent of I and Q component. Once the phase is being processed,the PRN modulated onto the phase of the main carrier can be tracked in back-end processingusing the fast residual phase error as input signal to the delay-locked loop (DLL)[8] architecture.8th Edoardo Amaldi Conference on Gravitational Waves IOP PublishingJournal of Physics: Conference Series 228 (2010) 012045 doi:10.1088/1742-6596/228/1/0120453Figure 3. Spectrum of the electrical beat signal of two offset-phase locked lasers modulated by twoPRN sequence with a modulation index of m=0.16 rad.Figure 4. General PMS block diagram. FPGA: Field Programmable Gate array, ADC: AnalogDigital Converter, DAC: Digital Analog Converter, LPF: Low Pass Filter, NCO: Numerically ControlledOscillator, DLL: Delay-locked loop, EPP: Enhanced Parallel port, DIOB: Digital Input/Output board.The DLL correlates the incoming phase signal with three versions of the same reference PRN:a punctual, an early and a late one (see block diagram in Figure 5). The early and late versionsof the reference code2 are delayed in the current implementation by plus and minus half a chiprespectively, and the punctual version is not delayed with respect to the transmitted PRN. Thepunctual correlator is responsible for data recovery and peak detection, whereas the differencebetween early and late correlators is used as the error signal in a control loop to update thedelay of the code generator to the input signal, thus providing tracking between the incomingand the local PRN. The control logic switches between two different modes of working:2 “early” (shifted by +∆/2) and “late” (shifted by −∆/2) where ∆ ≈ 0.1Tc . . . Tc8th Edoardo Amaldi Conference on Gravitational Waves IOP PublishingJournal of Physics: Conference Series 228 (2010) 012045 doi:10.1088/1742-6596/228/1/0120454• Acquisition mode: determines delay between the local and incoming PRN sequences at µsaccuracy (one chip length). The local PRN is shifted with a coarse resolution of one codeperiod until a peak of correlation is detected at the correct delay. Lower code period couldbe used, but it would increase the acquisition time. The acquition time for the currentimplementation is about 0.67 s.• Tracking mode: once the acquisition is finished, it determines the timing delay with higherresolution (ns accuracy) and enables data transfer. The estimated delay at measurementsrate of 1.5 kHz updates the pseudocode generator to produce the three copies of the localPRN at a resolution of 20 ns.Figure 5. General block diagram of the DLL implemented in the same board as the phasemeter. Thephase signal coming from the DPLL was internally simulated to provide data encoding, known shot noiseand variable delay as input to the DLL.4. HardwareThe hardware implementation of the PMS breadboard (see Picture 6) is based on a space-compatible FPGA processor running at 50 MHz, in which the DPLL and DLL architectureshave been programmed and integrated. The breadboard has been designed with four indepen-dent A/D channels being able to measure in parallel the phase of each quadrant photodiodes.Additionally, the breadboard implements two independent D/A channels for digital offset phase-locking purposes and two EOM drivers for PRN modulation.The EOMs and Mach-Zehnder interferometer used in the optical experiment are shown inFigure 7. Two Fibre-coupled EOM with frequencies up to 3 GHz have been tested in our Institutefor clock noise sideband modulation[9] and are currently used for low-phase PRN modulation.The Mach-Zehnder interferometer bonded on a Zerodur-baseplate provides a stable optical benchenvironment with beam extraction at five points and is suitable for future testing of laser rangingwith weak-light optical powers.8th Edoardo Amaldi Conference on Gravitational Waves IOP PublishingJournal of Physics: Conference Series 228 (2010) 012045 doi:10.1088/1742-6596/228/1/0120455Figure 6. PMS breadboard. Main processor unit: ProAsic3E Actel FPGA with 3 Million systemgates. Four input channels (AD9446-100), two output channels (AD9744-210), output interfaces: DigitalInput/Output at 2 GB/s and parallel port at 1 MB/s.Figure 7. Fibre coupled electro-optic modulator (left), Mach-Zehnder interferometer (centre), lasersand optics (right)..The functionality of our PMS breadboard has been verified and its performance is beingtested in the laboratory[7]. The functionality of the implemented DLL architecture proposed insection 3 has been also verified achieving an ideal ranging resolution of 20 ns at a measurementrate of 1.5 kHz[10]. The system is still under active development, however simulated results showan expected distance determination of sub-meter precision with lower measurement rates andin the presence of realistic LISA-like noise sources, i.e., interference with a second PRN, dataencoded and an equivalent shot noise of about 60µrad/√Hz. The effective ranging accuracy canbe improved by post-processing, taking into account optical Doppler shifts measurements in thePMS and orbit integration with Kalman filters3.5. ConclusionsThis paper presents the optical experiments and relevant hardware components involved inthe design of a delay-locked loop scheme for tracking the absolute armlength in LISA. Thecustom-designed Phase Measurement System breadboard can fit the architecture proposed. Itfunctionality with optics signal is being confirmed with respect to the previous electrical testing.We thus conclude that the current experimental setup can be expected to be used to verify3 This technique is similar to the applied in the Global Positioning System (GPS)8th Edoardo Amaldi Conference on Gravitational Waves IOP PublishingJournal of Physics: Conference Series 228 (2010) 012045 doi:10.1088/1742-6596/228/1/0120456in a more realistic environment how the different modulation schemes affect the phase fidelityof the main science measurement. Besides, it can also be used to verify the phase redout ofperformance in a single LISA arm.6. References[1] The LISA Study Team 1998 Laser Interferometer Space Antenna for the Detection and Observation ofGravitational Waves: Pre-Phase A Report 2nd edn MPG233. Max-Planck-Institute for Quantum Optics.[2] Massimo T, Shaddock D, Sylvestre J, Armstrong J 2003 Implementation of time-delay interferometry for LISAPhys. Rev. D67.[3] The LISA Frequency Control Team. LISA Frequency Control White Paper. Not published yet. July 2009[4] Folkner W. M 2009 LISA orbit selection and stability 2001 Class. Quamtum Grav. 18 4053-4057.[5] Pollack S E, Stebbins R T 2006 A demonstration of LISA laser communication Class. Quantum Grav. 234201-4213.[6] Shaddock D, Ware B, Halverson P, Spero R, Klipstein B 2006 Overview of the LISA Phasemeter AIP ConfProc. 873 654-660.[7] Bykov I, Esteban J.J, Garc´ıa Mar´ın A.F, Heinzel G, Danzmann K 2008 LISA phasemeter development:Advanced prototyping J. Phys.: Conf. Series. 154.[8] Esteban J.J, Bykov I, Garc´ıa Mar´ın A.F, Heinzel G, Danzmann K 2008 Optical ranging and data transferdevelopment for LISA J. Phys.: Conf. Series. 154.[9] Simon B, Troebs M, Sheard B, Heinzel G, Danzmann K 2009 Phase noise contribution of EOMs and HFcables J. Phys.: Conf. Series. 154.[10] Esteban J.J, Garc´ıa Mar´ın A.F, Bykov I, Heinzel G, Danzmann K 2009 Free-space laser ranging and datacommunication IEEE Conf Proc. 275-281.8th Edoardo Amaldi Conference on Gravitational Waves IOP PublishingJournal of Physics: Conference Series 228 (2010) 012045 doi:10.1088/1742-6596/228/1/0120457

Ranging and phase measurement for LISA

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Institutionelles Repositorium der Leibniz Universität Hannover