Picometer-Stable Hexagonal Optical Bench to Verify LISA Phase Extraction Linearity and Precision

The Laser Interferometer Space Antenna (LISA) and its metrology chain have to fulfill stringent performance requirements to enable the space-based detection of gravitational waves. This implies the necessity of performance verification methods. In particular, the extraction of the interferometric phase, implemented by a phasemeter, needs to be probed for linearity and phase noise contributions. This Letter reports on a hexagonal quasimonolithic optical bench implementing a three-signal test for this purpose. Its characterization as sufficiently stable down to picometer levels is presented as well as its usage for a benchmark phasemeter performance measurement under LISA conditions. These results make it a candidate for the core of a LISA metrology verification facility.

Figure 1

Schematic of the hexagonal quasimonolithic optical bench implementing a three-signal linearity test. Complementary interferometer outputs are utilized for diagnostic $\pi$ pretests.

Figure 2

Measurement (3 h) with moderate phase noise input conditions. The three-signal combination (red) fulfills the LISA requirement scaled for three signals. Its fundamental noise can be found in the $\pi$ pretests (green), suggesting sufficient stability of the hexagonal optical bench itself.

Figure 3

Measurements with LISA-like input phase noise and heterodyne frequencies. A dynamic range of 8–11 orders of magnitude can be computed from the three input signals (top) and the three-signal combinations (red: fixed heterodyne frequencies, 3 h; brown dashed: heterodyne frequency sweep over 90 h). The LISA three-signal requirement was fulfilled except between 0.4–20 mHz, with the photoreceivers being a major noise contributor (noise projection in blue).

Figure 4

Time series of input phase fluctuations and resulting three-signal combination, illustrating the high dynamic range essential for TDI.