Sub-pm${{\sqrt{{\rm Hz}}^{-1}}}$ non-reciprocal noise in the LISA backlink fiber

Abstract

The future space-based gravitational wave detector Laser Interferometer Space Antenna (LISA) requires bidirectional exchange of light between its two optical benches on board of each of its three satellites. The current baseline foresees a polarization-maintaining single-mode fiber for this backlink connection.Phase changes which are common in both directions do not enter the science measurement, but differential ("non-reciprocal") phase fluctuations directly do and must thus be guaranteed to be small enough.We have built a setup consisting of a Zerodur™ baseplate with fused silica components attached to it using hydroxide-catalysis bonding and demonstrated the reciprocity of a polarization-maintaining single-mode fiber at the 1 pm/ √ Hz level as is required for LISA. We used balanced detection to reduce the influence of parasitic optical beams on the reciprocity measurement and a fiber length stabilization to avoid nonlinear effects in our phase measurement system (phase meter). For LISA, a different phase meter is planned to be used that does not show this nonlinearity. We corrected the influence of beam angle changes and temperature changes on the reciprocity measurement in post-processing.

Fiber backscatter under increasing exposure to ionizing radiation

“…However, Rayleigh scattering within the fibers was found to be a critical factor as the scattered light travels along the beam in the backward direction and produces a spurious interference with fluctuating phase and, therefore, limits the performance. Previous experiments suggested a linear dependency between the backscattered power and the fiber length in the order of 4 ppm•m −1 for short fiber lengths [7,8]. The understanding of the backscatter induced noise and the previous experimental results make it clear that a backscatter level on the same order of magnitude can be accounted for in a direct fiber connection backlink design [7].…”

“…Previous experiments suggested a linear dependency between the backscattered power and the fiber length in the order of 4 ppm•m −1 for short fiber lengths [7,8]. The understanding of the backscatter induced noise and the previous experimental results make it clear that a backscatter level on the same order of magnitude can be accounted for in a direct fiber connection backlink design [7]. However, in case of a significant further increase, leading to noise predictions that cannot be tuned below the picometer level requirement by changing design parameters such as cable length, one would need to implement another, more complex backlink implementation [6].…”

“…This connection is often referred to as "the backlink". A straightforward implementation of this is a bidirectional fiber connection [6,7]. However, Rayleigh scattering within the fibers was found to be a critical factor as the scattered light travels along the beam in the backward direction and produces a spurious interference with fluctuating phase and, therefore, limits the performance.…”

The LISA interferometer: impact of stray light on the phase of the heterodyne signal

“…For the sake of simplicity we did not consider a balanced detection and omitted again the √ I 0 e iωt factor. Hence, the heterodyne signal is S(t) = 1 + a 2 + u 2 + 2u cos(Ω 2 t + ψ) + 2a cos(Ω 1 t + φ) + 2au cos(∆ Ω t + φ − ψ), (10) where ∆ Ω = Ω 1 − Ω 2 .…”

Ghost Beam Suppression in Deep Frequency Modulation Interferometry for Compact on-Axis Optical Heads

“…The presence of ghost beams is well known to cause non-linear, cyclic phase noise in heterodyne and homodyne interferometers and this especially includes parasitic reflections from fibers, which often have significantly higher phase dynamics than ghost beams generated within the often ultra-stable optical heads [20]. Reducing assembly efforts is also crucial when aiming for optimal performance at very low readout frequencies, which often implies the use of quasi-monolithic mounting structures.…”