B. Ware

Overview of the LISA Phasemeter

The LISA phasemeter is required to measure the phase of an electrical signal with an error less than 3 μcycles/Hz over times scales from 1 to 1000 seconds. This phase sensitivity must be achieved in the presence of laser phase fluctuations 108 times larger than the target sensitivity. Other challenging aspects of the measurement are that the heterodyne frequency varies from 2 to 20 MHz and the signal contains multiple frequency tones that must be measured. The phasemeter architecture uses high‐speed analog to digital conversion followed by a digital phase locked loop. An overview of the phasemeter architecture is presented along with results for the breadboard LISA Phasemeter demonstrating that critical requirements are met.

Postprocessed time-delay interferometry for LISA

High-precision interpolation of LISA phase measurements allows signal reconstruction and formulation of time-delay interferometry (TDI) combinations to be conducted in postprocessing. The reconstruction is based on phase measurements made at approximately 10 Hz (for a 1 Hz signal bandwidth) at regular intervals independent of the TDI delay times. Interpolation introduces an error less than 1x10{sup -8} with continuous data segments as short as 2 s in duration. The 10 Hz sampling rate represents an increase from the 2 Hz sampling rate needed for the original implementation of TDI. The advantages of this technique include increased flexibility of the data analysis and significantly simplified hardware.

Experimental Demonstration of Time-Delay Interferometry for the Laser Interferometer Space Antenna

We report on the first demonstration of time-delay interferometry (TDI) for LISA, the Laser Interferometer Space Antenna. TDI was implemented in a laboratory experiment designed to mimic the noise couplings that will occur in LISA. TDI suppressed laser frequency noise by approximately 10(9) and clock phase noise by 6×10(4), recovering the intrinsic displacement noise floor of our laboratory test bed. This removal of laser frequency noise and clock phase noise in postprocessing marks the first experimental validation of the LISA measurement scheme.

Coherent range-gated laser displacement metrology with compact optical head

We describe a new architecture for laser displacement metrology with a drastic reduction in the size and complexity of the optical head. Connected by a single optical fiber, the compact heads are easy to integrate and readily multiplexed to support applications requiring large numbers of sensors. The approach is made possible by modulating the outgoing laser light with a binary random noise code, allowing the detected signals to be discriminated based on their propagation delay. We demonstrate a displacement resolution of 1.1 nm rms.

Laser ranging and communications for LISA

The Laser Interferometer Space Antenna (LISA) will use Time Delay Interferometry (TDI) to suppress the otherwise dominant laser frequency noise. The technique uses sub-sample interpolation of the recorded optical phase measurements to form a family of interferometric combinations immune to frequency noise. This paper reports on the development of a Pseudo-Random Noise laser ranging system used to measure the sub-sample interpolation time shifts required for TDI operation. The system also includes an optical communication capability that meets the 20 kbps LISA requirement. An experimental demonstration of an integrated LISA phase measurement and ranging system achieved a ≈ 0.19 m rms absolute range error with a 0.5Hz signal bandwidth, surpassing the 1 m rms LISA specification. The range measurement is limited by mutual interference between the ranging signals exchanged between spacecraft and the interaction of the ranging code with the phase measurement.

Progress in interferometry for LISA at JPL

Recent advances at JPL in experimentation and design for LISA interferometry include the demonstration of time delay interferometry using electronically separated end stations, a new arm-locking design with improved gain and stability, and progress in flight readiness of digital and analog electronics for phase measurements.

Planet Signal Extraction for the Terrestrial Planet Finder Interferometer

Signal extraction is a vital link between science and systems-engineering requirements. In this paper we present a Terrestrial Planet Finder (TPF) interferometer planet-signal-extraction algorithm and demonstrate the performance of several nulling-interferometer designs on canonical TPF astronomical scenes. We create the output response of a linear phase-chopping dual Bracewell nulling interferometer and a matrix version of the correlation method employed to generate dirty images. We derive general and specific map parameters, such as signal-to-noise ratio, signal-to-artifact ratio, and detection confidence, used for individual maps or comparing array architectures. We implement a matrix form of CLEAN that removes map artifacts, produces reconstructed images, and retrieves planetary signals. Monte Carlo simulations show that some fixed-length structurally connected interferometer configurations can detect Earth-like planets for systems at 10 pc in the presence of stellar Poisson noise. Since angular resolution depends on baseline length, a design that can vary array configuration for each specific scene is superior to an interferometer with a fixed array length. Thus, a flexible free-flying architecture should satisfy the science requirements for more TPF candidates, compared to a fixed-length structurally connected architecture.

Flight phasemeter on the Laser Ranging Interferometer on the GRACE Follow-On mission

As the first inter-spacecraft laser interferometer, the Laser Ranging Interferometer (LRI) on the GRACE Follow-On Mission will demonstrate interferometry technology relevant to the LISA mission. This paper focuses on the completed LRI Laser Ranging Processor (LRP), which includes heterodyne signal phase tracking at precision, differential wavefront sensing, offset frequency phase locking and Pound-Drever-Hall laser stabilization. The LRI design has characteristics that are similar to those for LISA: 1064 nm NPRO laser source, science bandwidth in the mHz range, MHz-range intermediate frequency and Doppler shift, detected optical power of tens of picoWatts. Laser frequency stabilization has been demonstrated at a level below , better than the LISA requirement of . The LRP has completed all performance testing and environmental qualification and has been delivered to the GRACE Follow-On spacecraft. The LRI is poised to test the LISA techniques of tone-assisted time delay interferometry and arm-locking. GRACE Follow-On launches in 2017.