The Laser Interferometer Space Antenna (LISA) and other space based gravitational wave detector designs require a laser communication subsystem to, among other things, transfer clock signals between spacecraft (SC) in order to cancel clock noise in post-processing. The original LISA baseline design requires frequency synthesizers to convert each SC clock into a 2 GHz signal, and electro-optic modulators (EOMs) to modulate this 2 GHz clock signal onto the laser light. Both the frequency synthesizers and the EOMs must operate with a phase fidelity of . In this paper we present measurements of the phase fidelity of frequency synthesizers and EOMs. We found that both the frequency synthesizers and the EOMs meet the requirement when tested independently and together. We also performed an electronic test of the clock noise transfer using frequency synthesizers and the University of Florida LISA Interferometry (UFLIS) phasemeter. We found that by applying a time varying fractional delay filter we could suppress the clock noise to a level below our measurement limit, which is currently determined by timing jitter and is less than an order of magnitude above the LISA requirement for phase measurements.
© 2012 OSA
Several designs for space based gravitational wave detectors have been proposed [1–3]. Of these the Laser Interferometer Space Antenna (LISA), a proposed joint mission between NASA and ESA, is the most studied. Even though the future status of LISA is uncertain, its design specifications will be used here as the interferometry of the other proposed designs are similar to that of LISA.
LISA is designed to observe gravitational radiation at frequencies around 1 mHz. The LISA mission uses heterodyne laser interferometry to measure fluctuations in the path length between three spacecraft (SC) caused by gravitational waves. In order to reach the precision required to observe gravitational waves, the laser frequency noise must be reduced using frequency stabilization and time delay interferometry (TDI). TDI is a technique in which the phase measurements on each SC are combined with time delayed versions of the same signals in a way that cancels the laser frequency noise . For LISA to be successful the SC must be able to communicate with each other in order to transfer clock signals, measure the light travel time between the SC, and to share recorded data. All of these auxiliary functions will be accomplished using the laser links between the SC.
Each SC measures the phase of the heterodyne signals relative to its own clock. Since TDI requires measurements from each SC, the clock noise enters the TDI combinations  and is non-negligible. Each phase measurement is made in comparison to an oscillator of the same frequency of the beat note being measured. This oscillator is derived from the local clock and has a phase noise of where ν is the frequency of the merasured laser beat note and νclk and Φclk(t) are the frequency and phase noise of the clock. To cancel this noise, the clock signals on each SC will be frequency up-converted to a GHz signal using a frequency synthesizer, modulated onto the laser signals using an Electro-Optic Modulator (EOM), and sent to the other SC. There the sidebands will beat against similar sidebands generated by the clock of the local SC. These beat signals will be recorded and used in post-processing to measure and then cancel the clock noise . Since the sidebands will only contain ten percent of the laser power their signal to noise ratio will be lower than that of the main carrier beat note. Up-converting multiplies the phase noise of the clock signals by the up-conversion factor reducing the precision required to measure the clock phase noise. The clock noise is measured by subtracting the upper sideband-sideband measurement from the lower sideband-sideband measurement. This cancels the common laser phase noise and doubles the clock noise signal which is contained in both upper and lower sideband beat notes. This results in a measured clock noise signal of S = 2αΦclk(t), where α is the frequency up-conversion factor.
The LISA science team has set a maximum noise requirement of1]. In order to cancel the clock terms to the same precision, the error in must be less than the limitation in Eq. (1). This leads to a requirement on the phase noise of the measured clock noise signals of
The clock transfer requires frequency synthesizers, high frequency electronic cables, and EOMs. All of these components must be investigated to ensure that they do not add phase noise beyond the requirement in Eq. (3). Other groups investigated EOMs at 8 GHz , EOMs at 2 GHz , and high frequency cables at 2 GHz . The 8 GHz experiment measured the differential phase noise added between two EOMs using a single laser source. The 2 GHz experiment measured the phase noise added by a single EOM in a set-up with two lasers utilizing a low frequency LISA Technology Package (LTP) type phasemeter to measure the phase fluctuations. We present results on a differential measurement of the phase noise added by two frequency synthesizers, a measurement of the phase noise added by a single EOM at a modulation of 2 GHz using a LISA like phasemeter, a differential measurement of the phase noise added by the combination of two frequency synthesizers and two EOMs, and an electronic test of the clock noise transfer concept using the frequency synthesizers.
2. Experimental results
The experiments in this report make use of the University of Florida LISA Interferometry Simulator (UFLIS) phasemeter . The UFLIS phasemeter is an IQ tracking phasemeter programmed onto the FPGA of a Pentek digital signal processing board. The UFLIS phasemeter is capable of measuring the phase of a 20 MHz signal to an accuracy approximately at 1 mHz. This limitation is due to timing jitter and improves as the signal frequency is lowered. All of the experiments in this report were performed with the phasemeter clocked at 50 MHz and the phase data down sampled to 11.9 Hz.
2.1. Frequency synthesizers
Figure 1 is a diagram of the experimental set up to measure the differential noise between two frequency synthesizers. The tested synthesizers were manufactured by Rupptronik (www.rupptronik.de) and are capable of converting a 50 MHz signal to a frequency between 1.980 and 2.020 GHz in steps of 100 kHz. A 50 MHz signal was split and sent to two frequency synthesizers to produce two 2.001 GHz signals. Each of these signals was mixed with a common 2.000 GHz signal. The outputs of those mixers were low pass filtered and the resulting 1 MHz signals were measured with the UFLIS phasemeter. The difference of the two measurements is plotted in Fig. 2. Also plotted for comparison is the result of the same experiment using the output of two Stanford Research Systems CG635 clock generators phase locked to a common 10 MHz signal. The frequency synthesizers meet the requirement for all frequencies while the clock generators do not. The differential phase noise generated by the two synthesizers declines with dependence towards higher frequencies. It has ample margin at all frequencies except around 3 mHz where LISA and most other space based gravitational wave proposals reach their maximum strain sensitivity.
2.2. Electro-Optic modulators
Figure 3 is a diagram of the experiment to determine the noise added to the clock noise transfer by the EOM when it is modulated at 2 GHz. The input and output of the tested EOM (Jenoptik, model PM1060HF, www.jenoptik.com) are coupled to polarization maintaining fibers. This EOM was also tested by a separate group of researchers . A 2 GHz signal is modulated onto Laser 2 which is offset phase locked to a reference laser at 1.999 GHz. The reference laser also passes through an EOM because the same set up was used for other experiments in which both lasers were modulated. In this experiment the reference laser only serves to create a beat note with laser 2 and any phase noise added by the EOM will be canceled along with the intrinsic phase noise of the reference laser. The bandwidth of the phase-locked loop is 10 kHz, which is much smaller than the GHz sideband signals. Thus the sidebands are safely outside of the servo bandwidth and will not be suppressed by the phase-locked loop. The modulation results in three signals at the photodetector; the lower sideband at 1 MHz (aliased from −1 MHz), the carrier at 1.999 GHz, and the upper sideband at 3.999 GHz (which is well outside of the bandwidth of the photodetector). The phase of the lower sideband and the carrier areFig. 4. The EOM meets the requirement at all frequencies.
2.3. Frequency synthesizer and EOM combination
In space based gravitational wave detectors the frequency synthesizers and EOMs will be used together to transmit the clock noise signals. An experiment was performed to test both devices at the same time in a single experiment. Figure 5 is a differential measurement of the combined phase noise of the frequency synthesizers and the EOMs. A 50 MHz signal was frequency up-converted by two frequency synthesizers, one to 2.000 GHz and the other to 2.009 GHz. This same 50 MHz signal was used to clock the phasemeter. Each signal was modulated onto separate lasers using EOMs. The beat note between the lasers was formed as in section 2.2, and Laser 2 was offset phase locked to the reference laser at 10 MHz. Two signals from the photodetector were measured with the phasemeter, the 10 MHz carrier-carrier signal, and the 1 MHz lower sideband-sideband signal. If νclk and Φclk(t) are the frequency and phase noise of the 50 MHz signal, α1, and α2 are the up-conversion factors of the frequency synthesizers, and β1, and β2 are the combined phase noises of the frequency synthesizer and EOM combinations, then the phase noise of the carrier-carrier beat note is measured asFig. 6. The differential noise between the two synthesizer-EOM combinations is less than the requirement for all frequencies.
2.4. Electronic test of clock noise transfer
The previous experiments used a single clock to measure the phase noise added by the frequency synthesizers and the EOMs. LISA and other proposed space based gravitational wave detectors will measure the different signals with independent clocks. The differential clock noise and clock rate must be measured and included in the TDI combinations. Figure 7 is a diagram of an electronic test of the clock noise transfer using frequency synthesizers. A 20 MHz signal was split and measured with two separate phasemeters, each clocked by independent 50 MHz oscillators. Our digital signal processing hardware allows for two separate pairs of analog to digital converters (ADCs) and FPGAs to be externally clocked by different sources. Channels 1 and 2 were clocked by clock 1, while channels 3 and 4 were clocked by clock 2. Both clocks were run independently of each other and set to 50 MHz. The clock signals were both frequency up-converted with the frequency synthesizers; clock 1 to 2.00 GHz, and clock 2 to 2.001 GHz. The up-converted signals were mixed, low pass filtered, and measured by the phasemeter on channel 2. Let f and ϕ(t) be the common 20 MHz oscillator’s frequency and phase noise, F1, Φ1(t) and F2, Φ2(t) be the frequency and phase noise of clocks 1 and 2 respectively, and α1 and α2 be the up-conversion factor of the frequency synthesizers. The signals measured by the phasemeter are
Figure 8 shows the results of the clock noise transfer. The initial 20 MHz signal is plotted in red, while the contribution due to the clock noise is found from S1 − S3 and is plotted in blue. The combination in Eq. (13) is plotted in cyan. Since the measurements of S1 and S3 are made relative to independent clocks, there is a delay that is smaller than the sampling rate (a fractional delay) between the signals which amplifies the noise. A traditional fractional delay filter cannot be used to remove this delay as the clocks run at slightly different frequencies causing the fractional delay between the signals to slowly grow larger. Instead a time varying fractional delay filter was implemented by recalculating the filter coefficients for every data point. The change in the fractional delay was approximated to be linear. The initial fractional delay was found by applying a traditional fractional delay filter to the first ten minutes of data and searching for the fractional delay value that minimized Eq. (13). The difference in the frequency between the clocks was measured by subtracting the means of the measured frequencies of S1 and S3. This value was used to calculate the rate of change of the fractional delay. The results of Eq. (13) with the time varying fractional delay filter are plotted in magenta. The measurement was limited by the expected phasemeter system noise for a 20 MHz signal. This limitation, at its worst, is slightly less than a factor of six above the requirement.
We have performed several experiments testing the phase fidelity of both frequency synthesizers and EOMs. These components are necessary for the clock noise transfer in space based gravitational wave detectors including LISA. We found that the differential noise between two frequency synthesizers was less than the requirement set by the LISA science team. The differential measurement suffers from two drawbacks, that there may be some common noise between the two synthesizers that is canceled in the differential measurement, and that the frequency synthesizers were both operated at the same frequency up-conversion factor. In LISA the up-conversion factor must be different in order to produce sideband-sideband beat notes at frequencies separate from the main carrier-carrier beat note. The experimental test of the EOMs did not suffer from these drawbacks as they were tested individually. They were found to meet the requirement at a modulation frequency of 2 GHz.
The frequency synthesizers and the EOMs were also tested in a differential measurement of their combined phase noise. The differential phase noise of the frequency synthesizer and EOM combination was found to be lower than the requirement. The frequency synthesizers were also used in an electronic test of the clock noise transfer. This experiment tested the ability of two synthesizers, operating at different up-conversion factors, to cancel the clock noise incurred by measuring the phase of a 20 MHz signal with two independently running clocks. The clock noise was measured and removed to the performance limitation of our phasemeter system by employing a time varying fractional delay filter. This limitation is at worst a factor of six higher than the requirement for clock noise cancelation.
This work is supported by NASA Grant NNX09AF99G.
References and links
1. LISA International Science Team (LIST), “LISA assessment study report: yellow book,” Eur. Space Agency , 1–141 (2011).
2. NGO science working team, “NGO assessment study report: yellow book,” Eur. Space Agency , 1–153 (2011).
3. “Concepts for the NASA gravitational-wave mission,” nSPIRES-NASA Request for Information: NNH11ZDA019L, (2011). http://pcos.gsfc.nasa.gov/studies/gravitational-wave-mission.php
4. M. Tinto, D. A. Shaddock, J. Sylvestre, and J. W. Armstrong, “Implementation of time-delay interferometry for LISA,” Phys. Rev. D 67(12), 122003 (2003). [CrossRef]
5. F. B. Estabrook, M. Tinto, and J. W. Armstrong, “Time delay interferometry with moving spacecraft arrays,” Phys. Rev. D 69(8), 082001 (2004). [CrossRef]
6. D. A. Shaddock, B. Ware, R.E. Spero, and M. Vallisneri, “Postprocessed time-delay interferometry for LISA,” Phys. Rev. D 70(8), 1101–1106 (2004). [CrossRef]
7. W. Klipstein, P. G. Halverson, R. Peters, R. Cruz, and D. A. Shaddock, “Clock noise removal in LISA,” in AIP Conf. Proc. 873, pp. 312–318 (2006). [CrossRef]
8. S. Barke, M. Tröbs, B. Sheard, G. Heinzel, and K. Danzmann, “EOM sideband phase characteristics for the spaceborne gravitational wave detector LISA,” Appl. Phys. B: Lasers Opt. 98(1), 33–39 (2010). [CrossRef]
9. S. Mitryk, V. Wand, and G. Mueller, “Verification of time-delay interferometry techniques using the University of Florida LISA interferometry simulator,” Class. Quantum Grav. 27(8), 084012 (2010). [CrossRef]